Control of Air Pollution From New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Final rule.

SUMMARY:

The Environmental Protection Agency (EPA) is finalizing a program to further reduce air pollution, including ozone and particulate matter (PM), from heavy-duty engines and vehicles across the United States. The final program includes new emission standards that are significantly more stringent and that cover a wider range of heavy-duty engine operating conditions compared to today's standards; further, the final program requires these more stringent emissions standards to be met for a longer period of when these engines operate on the road. Heavy-duty vehicles and engines are important contributors to concentrations of ozone and particulate matter and their resulting threat to public health, which includes premature death, respiratory illness (including childhood asthma), cardiovascular problems, and other adverse health impacts. The final rulemaking promulgates new numeric standards and changes key provisions of the existing heavy-duty emission control program, including the test procedures, regulatory useful life, emission-related warranty, and other requirements. Together, the provisions in the final rule will further reduce the air quality impacts of heavy-duty engines across a range of operating conditions and over a longer period of the operational life of heavy-duty engines. The requirements in the final rule will lower emissions of NO_X and other air pollutants (PM, hydrocarbons (HC), carbon monoxide (CO), and air toxics) beginning no later than model year 2027. We are also finalizing limited amendments to the regulations that implement our air pollutant emission standards for other sectors ( e.g., light-duty vehicles, marine diesel engines, locomotives, and various other types of nonroad engines, vehicles, and equipment).

DATES:

This final rule is effective on March 27, 2023. The incorporation by reference of certain material listed in this rule is approved by the Director of the Federal Register as of March 27, 2023.

ADDRESSES:

Docket: EPA has established a docket for this action under Docket ID No. EPA-HQ-OAR-2019-0055. Publicly available docket materials are available either electronically at www.regulations.gov or in hard copy at Air and Radiation Docket and Information Center, EPA Docket Center, EPA/DC, EPA WJC West Building, 1301 Constitution Ave., NW, Room 3334, Washington, DC. Out of an abundance of caution for members of the public and our staff, the EPA Docket Center and Reading Room are open to the public by appointment only to reduce the risk of transmitting COVID-19. Our Docket Center staff also continues to provide remote customer service via email, phone, and webform. Hand deliveries and couriers may be received by scheduled appointment only. For further information on EPA Docket Center services and the current status, please visit us online at www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT:

Brian Nelson, Assessment and Standards Division, Office of Transportation and Air Quality, Environmental Protection Agency, 2000 Traverwood Drive, Ann Arbor, MI 48105; telephone number: (734) 214-4278; email address: nelson.brian@epa.gov.

SUPPLEMENTARY INFORMATION:

Does this action apply to me?

This action relates to companies that manufacture, sell, or import into the United States new heavy-duty highway engines. Additional amendments apply for gasoline refueling facilities and for manufacturers of all sizes and types of motor vehicles, stationary engines, aircraft and aircraft engines, and various types of nonroad engines, vehicles, and equipment. Regulated categories and entities include the following:

This table is not intended to be exhaustive, but rather provides a guide for readers regarding entities likely to be regulated by this action. This table lists the types of entities that EPA is now aware could potentially be regulated by this action. Other types of entities not listed in the table could also be regulated. To determine whether your entity is regulated by this action, you should carefully examine the applicability criteria found in Sections XI and XII of this preamble. If you have questions regarding the applicability of this action to a particular entity, consult the person listed in the FOR FURTHER INFORMATION CONTACT section.

Public participation: Docket: All documents in the docket are listed on the www.regulations.gov website. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the internet and will be publicly available only in hard copy form through the EPA Docket Center at the location listed in the ADDRESSES section of this document.

What action is the agency taking?

The Environmental Protection Agency (EPA) is adopting a rule to reduce air pollution from highway heavy-duty vehicles and engines. The final rulemaking will promulgate new numeric standards and change key provisions of the existing heavy-duty emission control program, including the test procedures, regulatory useful life, emission-related warranty, and other requirements. Together, the provisions in the final rule will further reduce the air quality impacts of heavy-duty engines across a range of operating conditions and over a longer period of the operational life of heavy-duty engines. Heavy-duty vehicles and engines are important contributors to concentrations of ozone and particulate matter and their resulting threat to public health, which includes premature death, respiratory illness (including childhood asthma), cardiovascular problems, and other adverse health impacts. This final rule will reduce emissions of nitrogen oxides and other pollutants.

What is the agency's authority for taking this action?

Clean Air Act section 202(a)(1) requires that EPA set emission standards for air pollutants from new motor vehicles or new motor vehicle engines that the Administrator has found cause or contribute to air pollution that may endanger public health or welfare. See Sections I.D and XIII of this preamble for more information on the agency's authority for this action.

What are the incremental costs and benefits of this action?

Our analysis of the final standards shows that annual total costs for the final program relative to the baseline (or no action scenario) range from $3.9 billion in 2027 to $4.7 billion in 2045 (2017 dollars, undiscounted, see Table V-16). The present value of program costs for the final rule, and additional details are presented in Section V. Section VIII presents our analysis of the human health benefits associated with the final standards. We estimate that in 2045, the final rule will result in total annual monetized ozone- and PM_2.5 -related benefits of $12 and $33 billion at a 3 percent discount rate, and $10 and $30 billion at a 7 percent discount rate (2017 dollars, discount rate applied to account for mortality cessation lag, see Table VIII-3). These benefits only reflect those associated with reductions in NO_X emissions (a precursor to both ozone and secondarily-formed PM_2.5) and directly-emitted PM_2.5 from highway heavy-duty engines. The agency was unable to quantify or monetize all the benefits of the final program, therefore the monetized benefit values are underestimates. There are additional human health and environmental benefits associated with reductions in exposure to ambient concentrations of PM_2.5, ozone, and NO₂ that data, resource, or methodological limitations have prevented EPA from quantifying. There will also be benefits associated with reductions in air toxic pollutant emissions that result from the final program, but we did not attempt to monetize those impacts because of methodological limitations. More detailed information about the benefits analysis conducted for the final rule, including the present value of program benefits, is included in Section VIII and RIA Chapter 8. We compare total monetized health benefits to total costs associated with the final rule in Section IX. Our results show that annual benefits of the final rule will be larger than the annual costs in 2045, with annual net benefits of $6.9 and $29 billion assuming a 3 percent discount rate, and net benefits of $5.8 and $25 billion assuming a 7 percent discount rate. The benefits of the final rule also outweigh the costs when expressed in present value terms and as equalized annual values (see Section IX for these values). See Section VIII for more details on the net benefit estimates

Did EPA conduct a peer review before issuing this action?

This regulatory action was supported by influential scientific information. EPA therefore conducted peer review in accordance with OMB's Final Information Quality Bulletin for Peer Review. Specifically, we conducted peer review on five analyses: (1) Analysis of Heavy-Duty Vehicle Sales Impacts Due to New Regulation (Sales Impacts), (2) Exhaust Emission Rates for Heavy-Duty Onroad Vehicles in MOVES_CTI NPRM (Emission Rates), (3) Population and Activity of Onroad Vehicles in MOVES_CTI NPRM (Population and Activity), (4) Cost teardowns of Heavy-Duty Valvetrain (Valvetrain costs), and (5) Cost teardown of Emission Aftertreatment Systems (Aftertreatment Costs). All peer review was in the form of letter reviews conducted by a contractor. The peer review reports for each analysis are in the docket for this action and at EPA's Science Inventory (https://cfpub.epa.gov/si/).

Table of Contents

I. Executive Summary

A. Introduction

B. Overview of the Final Regulatory Action

C. Impacts of the Standards

D. EPA Statutory Authority for This Action

II. Need for Additional Emissions Control

A. Background on Pollutants Impacted by This Proposal

B. Health Effects Associated With Exposure to Pollutants Impacted by This Rule

C. Environmental Effects Associated With Exposure to Pollutants Impacted by This Rule

D. Environmental Justice

III. Test Procedures and Standards

A. Overview

B. Summary of Compression-Ignition Exhaust Emission Standards and Duty Cycle Test Procedures

C. Summary of Compression-Ignition Off-Cycle Standards and Off-Cycle Test Procedures

D. Summary of Spark-Ignition HDE Exhaust Emission Standards and Test Procedures

E. Summary of Spark-Ignition HDV Refueling Emission Standards and Test Procedures

IV. Compliance Provisions and Flexibilities

A. Regulatory Useful Life

B. Ensuring Long-Term In-Use Emissions Performance

C. Onboard Diagnostics

D. Inducements

E. Fuel Quality

F. Durability Testing

G. Averaging, Banking, and Trading

V. Program Costs

A. Technology Package Costs

B. Operating Costs

C. Program Costs

VI. Estimated Emissions Reductions From the Final Program

A. Emission Inventory Methodology

B. Estimated Emission Reductions From the Final Program

C. Estimated Emission Reductions by Engine Operations and Processes

VII. Air Quality Impacts of the Final Rule

A. Ozone

B. Particulate Matter

C. Nitrogen Dioxide

D. Carbon Monoxide

E. Air Toxics

F. Visibility

G. Nitrogen Deposition

H. Demographic Analysis of Air Quality

VIII. Benefits of the Heavy-Duty Engine and Vehicle Standards

IX. Comparison of Benefits and Costs

A. Methods

B. Results

X. Economic Impact Analysis

A. Impact on Vehicle Sales, Mode Shift, and Fleet Turnover

B. Employment Impacts

XI. Other Amendments

A. General Compliance Provisions (40 CFR Part 1068) and Other Cross-Sector Issues

B. Heavy-Duty Highway Engine and Vehicle Emission Standards (40 CFR Parts 1036 and 1037)

C. Fuel Dispensing Rates for Heavy-Duty Vehicles (40 CFR Parts 80 and 1090)

D. Refueling Interface for Motor Vehicles (40 CFR Parts 80 and 1090)

E. Light-Duty Motor Vehicles (40 CFR Parts 85, 86, and 600)

F. Large Nonroad Spark-Ignition Engines (40 CFR Part 1048)

G. Small Nonroad Spark-Ignition Engines (40 CFR Part 1054)

H. Recreational Vehicles and Nonroad Evaporative Emissions (40 CFR Parts 1051 and 1060)

I. Marine Diesel Engines (40 CFR Parts 1042 and 1043)

J. Locomotives (40 CFR Part 1033)

K. Stationary Compression-Ignition Engines (40 CFR Part 60, subpart IIII)

L. Nonroad Compression-Ignition Engines (40 CFR Part 1039)

XII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act (PRA)

C. Regulatory Flexibility Act (RFA)

D. Unfunded Mandates Reform Act (UMRA)

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

G. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use

I. National Technology Transfer and Advancement Act (NTTAA) and 1 CFR Part 51

J. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

K. Congressional Review Act

L. Judicial Review

XIII. Statutory Provisions and Legal Authority

I. Executive Summary

A. Introduction

1. Summary of the Final Criteria Pollutant Program

In this action, the EPA is finalizing a program to further reduce air pollution, including pollutants that create ozone and particulate matter (PM), from heavy-duty engines and vehicles across the United States. The final program includes new, more stringent emissions standards that cover a wider range of heavy-duty engine operating conditions compared to today's standards, and it requires these more stringent emissions standards to be met for a longer period of time of when these engines operate on the road.

This final rule is part of a comprehensive strategy, the “Clean Trucks Plan,” which lays out a series of clean air and climate regulations that the agency is developing to reduce pollution from large commercial heavy-duty trucks and buses, as well as to advance the transition to a zero-emissions transportation future. Consistent with President Biden's Executive Order (E.O.) 14037, this final rule is the first step in the Clean Trucks Plan. We expect the next two steps of the Clean Trucks Plan will take into consideration recent Congressional action, including the recent Inflation Reduction Act of 2022, that we anticipate will spur significant change in the heavy-duty sector. We are not taking final action at this time on the proposed targeted updates to the existing Heavy-Duty Greenhouse Gas Emissions Phase 2 program (HD GHG Phase 2); rather, we intend to consider potential changes to certain HD GHG Phase 2 standards as part of a subsequent rulemaking.

Across the United States, heavy-duty engines emit oxides of nitrogen (NO_X) and other pollutants that are significant contributors to concentrations of ozone and PM_2.5 and their resulting adverse health effects, which include death, respiratory illness (including childhood asthma), and cardiovascular problems. Without this final rule, heavy-duty engines would continue to be one of the largest contributors to mobile source NO_X emissions nationwide in the future, representing 32 percent of the mobile source NO_X emissions in calendar year 2045. Furthermore, we estimate that without this final rule, heavy-duty engines would represent 90 percent of the onroad NO_X inventory in calendar year 2045. Reducing NO_X emissions is a critical part of many areas' strategies to attain and maintain the National Ambient Air Quality Standards (NAAQS) for ozone and PM; many state and local agencies anticipate challenges in attaining the NAAQS, maintaining the NAAQS in the future, and/or preventing nonattainment. Some nonattainment areas have already been “bumped up” to higher classifications because of challenges in attaining the NAAQS.

In addition, emissions from heavy-duty engines can result in higher pollutant levels for people living near truck freight routes. Based on a study EPA conducted of people living near truck routes, an estimated 72 million people live within 200 meters of a truck freight route. Relative to the rest of the population, people of color and those with lower incomes are more likely to live near truck routes. This population includes children; childcare facilities and schools can also be in close proximity to freight routes.

The final rulemaking will promulgate new numeric standards and change key provisions of the existing heavy-duty emission control program, including the test procedures, regulatory useful life, emission-related warranty, and other requirements. Together, the provisions in the final rule will further reduce the air quality impacts of heavy-duty engines across a range of operating conditions and over a longer portion of the operational life of heavy-duty engines. The requirements in the final rule will lower emissions of NO_X and other air pollutants (PM, hydrocarbons (HC), carbon monoxide (CO), and air toxics) beginning no later than model year (MY) 2027. The emission reductions from the final rule will increase over time as more new, cleaner vehicles enter the fleet.

We estimate that the final rule will reduce NO_X emissions from heavy-duty vehicles in 2040 by more than 40 percent; by 2045, a year by which most of the regulated fleet will have turned over, heavy-duty NO_X emissions will be almost 50 percent lower than they would have been without this action. These emission reductions will result in widespread decreases in ambient concentrations of pollutants such as ozone and PM_2.5 . We estimate that in 2045, the final rule will result in total annual monetized ozone- and PM_2.5 -related benefits of $12 and $33 billion at a 3 percent discount rate, and $10 and $30 billion at a 7 percent discount rate. These widespread air quality improvements will play an important role in addressing concerns raised by state, local, and Tribal governments, as well as communities, about the contributions of heavy-duty engines to air quality challenges they face such as meeting their obligations to attain or continue to meet NAAQS, and to reduce other human health and environmental impacts of air pollution. This rule's emission reductions will reduce air pollution in close proximity to major roadways, where concentrations of many air pollutants are elevated and where people of color and people with low income are disproportionately exposed.

In EPA's judgment, our analyses in this final rule show that the final standards will result in the greatest degree of emission reduction achievable starting in model year 2027, giving appropriate consideration to costs and other factors, which is consistent with EPA's statutory authority under Clean Air Act (CAA) section 202(a)(3)(A).

CAA section 202(a)(1) requires the EPA to “by regulation prescribe (and from time to time revise) . . . standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines . . . , which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” CAA section 202(a)(3)(C) requires that NO_X , PM, HC, and CO (hereafter referred to as “criteria pollutants”) standards for certain heavy-duty vehicles and engines apply for no less than 3 model years and apply no earlier than 4 years after promulgation.

Although heavy-duty engines have become much cleaner over the last decade, catalysts and other technologies have evolved such that harmful air pollutants can be reduced even further. The final standards are based on technology improvements that have become available over the 20 years since the last major rule was promulgated to address emissions of criteria pollutants and toxic pollutants from heavy-duty engines, as well as projections of continued technology improvements that build on these existing technologies. The criteria pollutant provisions we are adopting in this final rule apply for all heavy-duty engine (HDE) classes: Spark-ignition (SI) HDE, as well as compression-ignition (CI) Light HDE, CI Medium HDE, and CI Heavy HDE.

As described in Section III, the final standards will reduce emissions during a broader range of operating conditions compared to the current standards, such that nearly all in-use operation will be covered. Available data indicate that emission levels demonstrated for certification are not currently achieved under the broad range of real-world operating conditions. In fact, less than ten percent of the data collected during a typical test while the vehicle is operated on the road is subject to EPA's current on-the-road emission standards. These testing data further show that NO_X emissions from heavy-duty CI engines are high during many periods of vehicle operation that are not subject to current on-the-road emission standards. For example, “low-load” engine conditions occur when a vehicle operates in stop-and-go traffic or is idling; these low-load conditions can result in exhaust temperature decreases that then lead to the diesel engine's selective catalytic reduction (SCR)-based emission control system becoming less effective or ceasing to function. Test data collected as part of EPA's manufacturer-run in-use testing program indicate that this low-load operation could account for more than half of the NO_X emissions from a vehicle during a typical workday. Similarly, heavy-duty SI engines also operate in conditions where their catalyst technology becomes less effective, resulting in higher levels of air pollutants; however, unlike CI engines, it is sustained medium-to-high load operation where emission levels are less certain. To address these concerns, as part of our comprehensive approach, the final standards include both revisions to our existing test procedures and new test procedures to reduce emissions from heavy-duty engines under a broader range of operating conditions, including low-load conditions.

Data also show that tampering and mal-maintenance of the engine's emission control system after the useful life period is projected to result in NO_X emissions that would represent a substantial part of the HD emissions inventory in 2045. To address this problem, as part of our comprehensive approach, the final rule includes longer regulatory useful life and emission-related warranty requirements to ensure the final emissions standards will be met through more of the operational life of heavy-duty vehicles. Further, the final rule includes requirements for manufacturers to better ensure that operators keep in-use engines and emission control systems working properly in the real world. We expect these final provisions to improve maintenance and serviceability will reduce incentives to tamper with the emission control systems on MY 2027 and later engines, which would avoid large increases in emissions that would impact the reductions projected from the final rule. For example, we estimate NO_X emissions will increase more than 3000 percent due to malfunction of the NO_X emissions aftertreatment on a MY 2027 and later heavy heavy-duty vehicle. To address this, the final rule requires manufacturers to meet emission standards with less frequent scheduled maintenance for emission-related parts and systems, and to provide more information on how to diagnose and repair emission control systems. In addition, the final rule requires manufacturers to demonstrate that they design their engines to limit access to electronic controls to prevent operators from reprogramming the engine to bypass or disable emission controls. The final rule also specifies a balanced approach for manufacturers to design their engines with features to ensure that operators perform ongoing maintenance to keep SCR emission control systems working properly, without creating a level of burden and corresponding frustration for operators that could increase the risk of operators completely disabling emission control systems. These provisions combined with the longer useful life and warranty periods will provide a comprehensive approach to ensure that the new, much more stringent emissions standards are met during in use operations.

The final standards and requirements are based on further consideration of the data included in the proposed rule, as well as additional supporting data from our own test programs, and consideration of the extensive public input EPA received in response to the proposed rule. The proposal was posted on the EPA website on March 7, 2022, and published in the Federal Register on March 28, 2022 (87 FR 17414, March 28, 2022). EPA held three virtual public hearings in April 2022. We received more than 260,000 public comments. A broad range of stakeholders provided comments, including state and local governments, heavy-duty engine manufacturers, emissions control suppliers and others in the heavy-duty industry, environmental organizations, environmental justice organizations, state, local, and Tribal organizations, consumer groups, labor groups, private citizens, and others. Some of the issues raised in comments included the need for new, more stringent NO_X standards, particularly in communities already overburdened by pollution; the feasibility and costs of more stringent NO_X standards combined with much longer useful life periods; the longer emissions-related warranty periods; a single- vs. two-step program; and various details on the flexibilities and other program design features of the proposed program. We briefly discuss several of these key issues in Section I.B, with more detail in later sections in this preamble and in the Response to Comments document that is available in the public docket for this rule.

This Section I provides an overview of the final program, the impacts of the final program, and how the final program is consistent with EPA's statutory requirements. The need for additional emissions control from heavy-duty engines is described in Section II. We describe the final standards and compliance flexibilities in detail in Sections III and IV. We discuss our analyses of estimated emission reductions, air quality improvements, costs, and monetized benefits of the final program in Sections V through X. Section XI describes limited amendments to the regulations that implement our air pollutant emission standards for other sectors ( e.g., light-duty vehicles, marine diesel engines, locomotives, and various types of nonroad engines, vehicles, and equipment).

2. EPA Will Address HD GHG Emissions in a Subsequent Rulemaking

Although we proposed targeted revisions to the MY2027 GHG Phase 2 standards as part of the same proposal in which we laid out more stringent NO_X standards, in this final rule we are not taking final action on updates to the GHG standards. Instead, we intend to consider potential changes to certain HD GHG Phase 2 standards as part of a subsequent rulemaking.

B. Overview of the Final Regulatory Action

We are finalizing a program that will begin in MY 2027, which is the earliest year that these new criteria pollutant standards can begin to apply under CAA section 202(a)(3)(C). The final NO_X standards are a single-step program that reflect the greatest degree of emission reduction achievable starting in MY2027, giving appropriate consideration to costs and other factors. The final rule establishes not only new, much more stringent NO_X standards compared to today's standards, but also requires lower NO_X emissions over a much wider range of testing conditions both in the laboratory and when engines are operating on the road. Further, the final standards include longer useful life periods, as well as significant increases in the emissions-related warranty periods. The longer useful life and emissions warranty periods are particularly important for ensuring continued emissions control when the engines are operating on the road. These final standards will result in significant reductions in emissions of NO_X, PM_2.5, and other air pollutants across the country, which we project will meaningfully decrease ozone concentrations across the country. We expect the largest improvements in both ozone and PM_2.5 to occur in areas with the worst baseline air quality. In a supplemental demographic analysis, we also found that larger numbers of people of color are projected to reside in these areas with the worst baseline air quality.

The final standards and requirements are based on further consideration of the data included in the proposed rule, as well as additional supporting data from our own test programs, and consideration of the extensive public input EPA received in response to the proposed rule. As required by CAA section 202(a)(3), the final new numeric NO_X standards will result in the greatest degree of emission reduction achievable for a national program starting in MY 2027 through the application of technology that the Administrator has determined will be available starting in MY 2027, after giving appropriate consideration to cost, energy, and safety factors associated with the application of such technology. The EPA proposal included two options for the NO_X program. Proposed Option 1 was the more stringent option, and it included new standards and other program elements starting in MY 2027, which were further strengthened in MY 2031. Proposed Option 2 was the less stringent option, with new standards and requirements implemented fully in MY 2027. The final numeric NO_X standards and testing requirements are largely consistent with the proposed Option 1 in MY 2027. The final numeric standards and regulatory useful life values will reduce NO_X emissions not only when trucks are new, but throughout a longer period of their operational life under real-world conditions. For the smaller engine service-class categories, we are finalizing the longest regulatory useful life and emissions warranty periods proposed, and for the largest engines we are finalizing requirements for useful life and emissions aftertreatment durability demonstration that are significantly longer than required today.

As previously noted in this Section I, we received a large number and wide range of comments on the proposed rule. Several comments raised particularly significant issues related to some fundamental components of the proposed program, including the level of the numeric standards and feasibility of lower numeric standards combined with longer useful life periods. We briefly discuss these key issues in this Section I.B, with more detail in later sections in this preamble. The Response to Comments document provides our responses to the comments we received; it is located in the docket for this rulemaking.

1. Key Changes From the Proposal

i. Feasibility of More Stringent NO_X Standards Combined With Much Longer Useful Life Periods

Many stakeholders commented on the proposed numeric NO_X standards, and the feasibility of maintaining those numeric standards over the proposed useful life periods. Environmental organizations and other commenters, including suppliers to the heavy-duty industry, generally urged EPA to adopt the most stringent standards proposed, or to finalize even more stringent standards by fully aligning with the California Air Resources Board (CARB) Low NO_X Omnibus program. In contrast, most engine manufacturers, truck dealers, fleets, and other members of the heavy-duty industry stated that even the less stringent proposed numeric standards and useful life periods would be extremely challenging to meet, particularly for the largest heavy-duty engines. Some of these commenters provided data that they stated showed the potential for large impacts on the purchase price of a new truck if EPA were to finalize the most stringent proposed numeric standards and useful life periods for the largest heavy-duty engines.

As summarized in I.B.2 and detailed in preamble Section III, we are finalizing numeric NO_X standards and useful life periods that are largely consistent with the most stringent proposed option for MY 2027. For all heavy-duty engine classes, the final numeric NO_X standards for medium- and high-load engine operations match the most stringent standards proposed for MY 2027; for low-load operations we are finalizing the most stringent standard proposed for any model year (see I.B.1.ii for discussion). For smaller heavy-duty engines ( i.e., light and medium heavy-duty engines CI and SI heavy-duty engines), the numeric standards are combined with the longest useful life periods we proposed. The final numeric NO_X emissions standards and useful life periods for smaller heavy-duty engines are based on further consideration of data included in the proposal from our engine demonstration programs that show the final NO_X emissions standards are feasible at the final useful life periods applicable to these smaller heavy-duty engines. Our assessment of the data available at the time of proposal is further supported by our evaluation of additional information and public comments stating that the proposed standards are feasible for these smaller engine categories. For the largest heavy-duty engines ( i.e., heavy heavy-duty engines), the final numeric standards are combined with the longest useful life mileage that we proposed for MY 2027. The final useful life periods for the largest heavy-duty engines are 50 percent longer than today's useful life periods, which will play an important role in ensuring continued emissions control while the engines operate on the road.

After further consideration of the data included in the proposal, as well as information submitted by commenters and additional data we collected since the time of proposal, we are finalizing two updates from our proposed testing requirements in order to ensure the greatest degree of emission reduction achievable are met throughout the final useful life periods; these updates are tailored to the larger engine classes (medium and heavy heavy-duty engines), which have longer useful life periods and more rigorous duty-cycles compared to the smaller engine classes. First, we are finalizing a requirement for manufacturers to demonstrate before heavy heavy-duty engines are in-use that the emissions control technology is durable through a period of time longer than the final useful life mileage. For these largest engines with the longest useful life mileages, the extended laboratory durability demonstration will better ensure the final standards will be met throughout the regulatory useful life under real-world operations where conditions are more variable. Second, we are finalizing an interim compliance allowance that applies when EPA evaluates whether the heavy or medium heavy-duty engines are meeting the final standards after these engines are in use in the real world. When combined with the final useful life values, we believe the interim compliance allowance will address concerns raised in comments from manufacturers that the more stringent proposed MY 2027 standards would not be feasible to meet over the very long useful life periods of heavy heavy-duty engines, or under the challenging duty-cycles of medium heavy-duty engines. This interim, in-use compliance allowance is generally consistent with our past practice (for example, see 66 FR 5114, January 18, 2001); also consistent with past practice, the interim compliance allowance is included as an interim provision that we may reassess in the future through rulemaking based on the performance of emissions controls over the final useful life periods for medium and heavy heavy-duty engines. To set standards that result in the greatest emission reductions achievable for medium and heavy heavy-duty engines, we considered additional data that we and others collected since the time of the proposal; these data show the significant technical challenge of maintaining very low NO_X emissions throughout very long useful life periods for heavy heavy-duty engines, and greater amounts of certain aging mechanisms over the long useful life periods of medium heavy-duty engines. In addition to these data, in setting these standards, we gave appropriate consideration to costs associated with the application of technology to achieve maximum emissions reductions in MY 2027 ( i.e., cost of compliance for manufacturers associated with the standards) and other factors. We determined that for heavy heavy-duty engines the combination of: (1) The most stringent MY 2027 standards proposed, (2) longer useful life periods compared to today's useful life periods, (3) targeted, interim compliance allowance approach to in-use compliance testing, and (4) the extended durability demonstration for emissions control technologies is appropriate, feasible, and consistent with our authority under the CAA to set technology-forcing NO_X pollutant standards for heavy-duty engines for their useful life. Similarly, for medium heavy-duty engines we determined that the combination of the first three elements ( i.e., most stringent MY 2027 standards proposed, increase in useful life periods, and interim compliance allowance for in-use testing) is appropriate, feasible, and consistent with our CAA authority to set technology-forcing NO_X pollutant standards for heavy-duty engines for their useful life.

ii. Test Procedures To Control Emissions Under a Broader Range of Engine Operations

Many commenters supported our proposal to update our test procedures to more accurately account for and control emissions across a broader range of engine operation, including in urban driving conditions and other operations that could impact communities already overburdened with pollution. Consistent with our proposal, we are finalizing several provisions to reduce emissions from a broader range of engine operating conditions. First, we are finalizing new standards for our existing test procedures to reduce emissions under medium- and high-load operations ( e.g., when trucks are traveling on the highway). Second, we are finalizing new standards and a corresponding new test procedure to measure emissions during low-load operations ( i.e., the low-load cycle, LLC). Third, we are finalizing new standards and updates to an existing test procedure to measure emissions over the broader range of operations that occur when heavy-duty engines are operating on the road ( i.e., off-cycle).

The new, more stringent numeric standards for the existing laboratory-based test procedures that measure emissions during medium- and high-load operations will ensure significant emissions reductions from heavy-duty engines. Without this final rule, these medium- and high-load operations are projected to contribute the most to heavy-duty NO_X emissions in 2045.

We are finalizing as proposed a new LLC test procedure, which will ensure demonstration of emission control under sustained low-load operations. After further consideration of data included in the proposal, as well as additional information from the comments summarized in this section, we are finalizing the most stringent numeric LLC standard proposed for any model year. As discussed in our proposal, data from our CI engine demonstration program showed that the lowest numeric NO_X standard proposed would be feasible for the LLC throughout a useful life period similar to the useful life period we are finalizing for the largest heavy-duty engines. After further consideration of this data, and additional support from data collected since the time of proposal, we are finalizing the most stringent standard proposed for any model year.

We are finalizing new numeric standards and revisions to the proposed off-cycle test procedure. We proposed updates to the current off-cycle test procedure that included binning emissions measurements based on the type of operation the engine is performing when the measurement data is being collected. Specifically, we proposed that emissions data would be grouped into three bins, based on whether the engine was operating in idle (Bin 1), low-load (Bin 2), or medium-to-high load (Bin 3). Given the different operational profiles of each of the three bins, we proposed a separate standard for each bin. Based on further consideration of data included in the proposal, as well as additional support from our consideration of data provided by commenters, we are finalizing off-cycle standards for two bins, rather than three bins; correspondingly, we are finalizing a two-bin approach for grouping emissions data collected during off-cycle test procedures. Our evaluation of available information shows that two bins better represent the differences in engine operations that influence emissions ( e.g., exhaust temperature, catalyst efficiency) and ensure sufficient data is collected in each bin to allow for an accurate analysis of the data to determine if emissions comply with the standard for each bin. Preamble Section 0 further discusses the final off-cycle standards with additional detail in preamble Section III.

iii. Lengthening Emissions-Related Warranty

EPA received general support from many commenters for the proposal to lengthen the emissions-related warranty beyond existing requirements. Some commenters expressed support for one of the proposed options, and one organization suggested a warranty period even longer than either proposed option. Several stakeholders also commented on the costs of lengthened warranty periods and potential economic impacts. For instance, one state commenter supported EPA's cost estimates and agreed that the higher initial cost will be offset by lower repair costs; further, the commenter expects the resale value of lengthened warranty will be maintained for subsequent owners. In contrast, stakeholders in the heavy-duty engine and truck industry ( e.g., engine and vehicle manufacturers, truck dealers, suppliers of emissions control technologies) commented that the proposed warranty periods would add costs to vehicles, and raised concerns about these cost impacts on first purchasers. Many commenters indicated that purchase price increases due to the longer warranty periods may delay emission reductions, stating that high costs could incentivize pre-buy and reduce fleet turnover from old technology.

After further consideration of data included in the proposal, and consideration of additional supporting information from the comments summarized in this Section I.B.1.iii, we are finalizing a single-step increase for new, longer warranty periods to begin in MY 2027. Several commenters recommended we pull ahead the longest proposed warranty periods to start in MY 2027. We agree with that approach for the smaller heavy-duty engine classes, and our final warranty mileages match the longest proposed warranty periods for these smaller engines ( i.e., Spark-ignition HDE, Light HDE, and Medium HDE). However, we are finalizing a different approach for the largest heavy-duty engines ( i.e., Heavy HDE). We are finalizing a warranty mileage that matches the MY 2027 step of the most stringent proposed option to maximize the emission control assurance and to cover a percentage of the final useful life that is more consistent with the warranty periods of the smaller engine classes. The final emissions warranty periods are approximately two to four times longer than today's emissions warranty periods. The durations of the final emissions warranty periods balance two factors: First, the expected improvements in engine emission performance from longer emissions warranty periods due to increases in maintenance and lower rates of tampering with emissions controls (see preamble Section IV.B for more discussion); and second, the potential, particularly for the largest heavy-duty engines, for very large increases in purchase price due to much longer warranty periods to slow fleet turnover through increases in pre- and low-buy, and subsequently result in fewer emissions reductions. We are finalizing emissions warranty periods that in our evaluation will provide a significant increase in the emissions warranty coverage while avoiding large increases in the purchase price of a new truck.

iv. Model Year 2027 Single-Step Program

Many stakeholders expressed support for a single-step program to implement new emissions standards and program requirements beginning in model year 2027, which is consistent with one of the proposed options. Stakeholders in the heavy-duty engine and truck industry, including suppliers of emissions controls technologies, truck dealers, and engine manufacturers, generally stated that a single-step program avoids technology disruptions and allows industry to focus on research and development for zero-emissions vehicle technologies for model years beyond 2027. Some of these commenters further noted that a two-step approach would result in gaps in available technology for some vehicle types and could exacerbate slower fleet turnover from pre- and low-buy associated with new standards. The trade association for truck dealers noted that a two-step approach would significantly compromise expected vehicle performance characteristics, including fuel economy. Other commenters also generally supported a single-step approach in order for the most stringent standards to begin as soon as possible, which would lead to larger emissions reductions earlier than a two-step approach. Several of these stakeholders noted the importance of early emissions reductions in communities already overburdened with pollution.

The final NO_X standards are a single-step program that reflect the greatest emission reductions achievable starting in MY 2027, giving appropriate consideration to costs and other factors. In this final rule, we are focused on achieving the greatest emission reductions achievable in the MY 2027 timeframe, and have applied our judgment in determining the appropriate standards for MY 2027 under our CAA authority for a national program. As the heavy-duty industry continues to transition to zero-emission technologies, EPA could consider additional criteria pollutant standards for model years beyond 2027 in future rules.

v. Averaging, Banking, and Trading of NO_X Emissions

The majority of stakeholders supported the proposed program to allow averaging, banking, and trading (ABT) of NO_X emissions, although several suggested adjustments for EPA to consider in the final rule. Stakeholders provided additional input on several specific aspects of the proposed ABT program, including the proposed family emissions limit (FEL) caps, the proposed Early Adoption Incentives, and the proposed allowance for manufacturers to generate NO_X emissions credits from Zero Emissions Vehicles (ZEVs). In this Section we briefly discuss stakeholder perspectives on these specific aspects of the proposed ABT program, as well as our approach for each in the final rule.

a. Family Emissions Limit Caps

A wide range of stakeholders urged EPA to finalize a lower FEL cap than proposed; there was broad agreement that the FEL cap in the final rule should be 100 mg/hp-hr or lower, with commenters citing various considerations, such as the magnitude of reduction between the current and proposed standards, as well as the desire to prevent competitive disruption.

After further consideration, including consideration of public comments, we are finalizing lower FEL caps than proposed. The FEL caps in the final rule are 65 mg/hp-hr for MY 2027 through 2030, and 50 mg/hp-hr for MY 2031 and later. Our rationale for the final FEL caps includes two main factors. First, we agree with commenters that the difference between the current standard (approximately 200 mg/hp-hr) and the standards we are finalizing for MY 2027 and later suggests that FEL caps lower than the current standard are appropriate to ensure that available emissions control technologies are adopted. This is consistent with our past practice when issuing rules for heavy-duty onroad engines or nonroad engines in which there was a substantial ( e.g., greater than 50 percent) difference between the numeric levels of the existing and new standards (69 FR 38997, June 29, 2004; 66 FR 5111, January 18, 2001). Specifically, by finalizing FEL caps below the current standards, we are ensuring that the vast majority of new engines introduced into commerce include updated emissions control technologies compared to the emissions control technologies manufacturers use to meet the current standards.

Second, finalizing FEL caps below the current standard is consistent with comments from manufacturers stating that a FEL cap of 100 mg/hp-hr or between 50 and 100 mg/hp-hr would help to prevent competitive disruptions ( i.e., require all manufacturers to make improvements in their emissions control technologies).

The FEL caps for the final rule have been set at a level to ensure sizeable emission reductions from the current 2010 standards, while providing manufacturers with flexibility in meeting the final standards. When combined with the other restrictions in the final ABT program ( i.e., credit life, averaging sets, expiration of existing credit balances), we determined the final FEL caps of 65 mg/hp-hr in MYs 2027 through 2030, and 50 mg/hp-hr in MY 2031 and later avoid potential adverse effects on the emissions reductions expected from the final program.

b. Encouraging Early Adoption of New Emissions Controls Technologies

Several stakeholders provided general comments on the proposed Early Adoption Incentive program, which included emissions credit multipliers of 1.5 or 2.0 for meeting all proposed requirements prior to the applicable model year. Although many of the stakeholders in the heavy-duty engine industry generally supported incentives such as emissions credit multipliers to encourage early investments in emissions reductions technology; other industry stakeholders were concerned that the multipliers would incentivize some technologies ( e.g., hybrid powertrains, natural gas engines) over others ( e.g., battery-electric vehicles). Environmental organizations and other commenters were concerned that the emissions credit multipliers would result in an excess of credits that would undermine some of the benefits of the rule.

After consideration of public comments, EPA is not finalizing the proposed Early Adoption Incentives program, and in turn we are not including emissions credit multipliers in the final program. Rather, we are finalizing an updated version of the proposed transitional credit program under the ABT program. As described in preamble Section IV.G.7, the transitional credit program that we are finalizing provides four pathways to generate straight NO_X emissions credits ( i.e., no credit multipliers) in order to encourage the early introduction engines with NO_X -reducing technology.

c. Heavy-Duty Zero Emissions Vehicles and NO_X Emissions Credits

Numerous stakeholders provided feedback on EPA's proposal to allow manufacturers to generate NO_X emissions credits from ZEVs. Environmental organizations and other commenters, as well as suppliers of heavy-duty engine and vehicle components, broadly oppose allowing manufacturers to generate NO_X emissions credits from ZEVs. These stakeholders present several lines of argument, including the potential for: (1) Substantial impacts on the emissions reductions expected from the proposed rule, which could also result in disproportionate impacts in disadvantaged communities already overburdened with pollution; and (2) higher emissions from internal combustion engines, rather than further incentives for additional ZEVs (further noting that other State and Federal actions are providing more meaningful and less environmentally costly HD ZEV incentives). In contrast, heavy-duty engine and vehicle manufacturers generally support allowing manufacturers to generate these credits. These stakeholders also provided several lines of argument, including: (1) The potential for ZEVs to help meet emissions reductions and air quality goals; (2) an assertion that ZEV NO_X credits are essential to the achievability of the standards for some manufacturers; and (3) ZEV NO_X credits allow manufacturers to manage investments across different products that may ultimately result in increased ZEV deployment.

After further consideration, including consideration of public comments, we are not finalizing the allowance for manufacturers to generate NO_X emissions credits from heavy-duty ZEVs. Our decision is based on two primary considerations. First, the standards in the final rule are technology-forcing, yet achievable for MY 2027 and later internal combustion engines without this flexibility. Second, because the final standards are not based on projected utilization of ZEV technology, and because we believe there will be increased penetration of ZEVs in the heavy-duty fleet by MY 2027 and later, we are concerned that allowing ZEVs to generate NO_X emissions credits would result in fewer emissions reductions than intended from this rule. For example, by allowing manufacturers to generate ZEV NO_X credits, EPA would be allowing higher emissions (through internal combustion engines using credits to emit up to the FEL cap) in MY 2027 and later, without requiring commensurate emissions reductions (through additional ZEVs beyond those already entering the market without this rule). This erosion of emissions benefits could have particularly adverse impacts in communities already overburdened by pollution. In addition, we continue to believe that testing requirements to ensure continued battery and fuel cell performance over the useful life of a ZEV may be important to ensure the zero-emissions tailpipe performance for which they are generating NO_X credits; however, after further consideration, including consideration of public comments, we believe it is appropriate to take additional time to work with industry and other stakeholders on any test procedures and other specifications for ZEV battery and fuel cell performance over the useful life period of the ZEV.

2. Summary of the Key Provisions in the Regulatory Action

i. Controlling Criteria Pollutant Emissions Under a Broader Range of Operating Conditions

The final rule provisions will reduce emissions from heavy-duty engines under a range of operating conditions through revisions to our emissions standards and test procedures. These revisions will apply to both laboratory-based standards and test procedures for both heavy-duty CI and SI engines, as well as the off-cycle standards and test procedures for heavy-duty CI engines. These final provisions are outlined immediately below and detailed in Section III.

a. Final Laboratory Standards and Test Procedures

For heavy-duty CI engines, we are finalizing new standards for laboratory-based tests using the current duty cycles, the transient Federal Test Procedure (FTP) and the steady-state Supplemental Emission Test (SET) procedure. These existing test procedures require CI engine manufacturers to demonstrate the effectiveness of emission controls when the engine is transitioning from low-to-high loads or operating under sustained high load, but do not include demonstration of emission control under sustained low-load operations. As proposed, we are finalizing a new, laboratory-based LLC test procedure for heavy-duty CI engines to demonstrate emission control when the engine is operating under low-load and idle conditions. The addition of the LLC will help ensure lower NO_X emissions in urban areas and other locations where heavy-duty vehicles operate in stop-and-go traffic or other low-load conditions. As stated in Section I.B.1, we are finalizing the most stringent standard proposed for any model year for low-load operations based on further evaluation of data included in the proposal, and supported by information received during the comment period. We are also finalizing as proposed the option for manufacturers to test hybrid engines and powertrains together using the final powertrain test procedure.

For heavy-duty SI engines, we are finalizing new standards for laboratory-based testing using the current FTP duty cycle, as well as updates to the current engine mapping procedure to ensure the engines achieve the highest torque level possible during testing. We are also finalizing the proposed addition of the SET duty-cycle test procedure to the heavy-duty SI laboratory demonstrations; it is currently only required for heavy-duty CI engines. Heavy-duty SI engines are increasingly used in larger heavy-duty vehicles, which makes it more likely for these engines to be used in higher-load operations covered by the SET.

Our final NO_X emission standards for all defined duty cycles for heavy-duty CI and SI engines are detailed in Table I-1. As shown, the final NO_X standards will be implemented with a single step in MY 2027 and reflect the greatest emission reductions achievable starting in MY 2027, giving appropriate consideration to costs and other factors. As discussed in I.B.1.i, for the largest heavy-duty engines we are finalizing two updates to our testing requirements to ensure the greatest emissions reductions technically achievable are met throughout the final useful life periods of the largest heavy-duty engines: (1) A requirement for manufacturers to demonstrate before heavy heavy-duty engines are in-use that the emissions control technology are durable through a period of time longer than the final useful mileage, and (2) a compliance allowance that applies when EPA evaluates whether medium or heavy heavy-duty engines are meeting the final standards after these engines are in-use in the real world. We requested comment on an interim compliance allowance, and it is consistent with our past practice (for example, see 66 FR 5114, January 18, 2001); the interim compliance allowance is shown in the final column of Table I-1. See Section III for more discussion on feasibility of the final standards. Consistent with our existing, MY 2010 standards for criteria pollutants, the final standards, presented in Table 1, are numerically identical for SI and CI engines.

b. Final On-the-Road Standards and Test Procedures

In addition to demonstrating emission control over defined duty cycles tested in a laboratory, heavy-duty CI engines must be able to demonstrate emission control over operations experienced while engines are in use on the road in the real world ( i.e., “off-cycle” testing). We are finalizing with revisions the proposed updates to the procedure for off-cycle testing, such that data collected during a wider range of operating conditions will be valid, and therefore subject to emission standards.

Similar to the current approach, emission measurements collected during off-cycle testing will be collected on a second-by-second basis. As proposed, we are finalizing that the emissions data will be grouped into 300-second windows of operation. Each 300-second window will then be binned based on the type of operation that the engine performs during that 300-second period. Specifically, the average power of the engine during each 300-second window will determine whether the emissions during that window are binned as idle (Bin 1), or non-idle (Bin 2).

Our final, two-bin approach covers a wide range of operations that occur in the real world—significantly more in-use operation than today's requirements. Bin 1 includes extended idle and other very low-load operations, where engine exhaust temperatures may drop below the optimal temperature where SCR-based aftertreatment works best. Bin 2 includes a large fraction of urban driving conditions, during which engine exhaust temperatures are generally moderate, as well as higher-power operations, such as on-highway driving, that typically results in higher exhaust temperatures and high catalyst efficiencies. Given the different operational profiles of each of these two bins, we are finalizing, as proposed, a separate standard for each bin. As proposed, the final structure follows that of our current not-to-exceed (NTE) off-cycle standards where testing is conducted while the engine operates on the road conducting its normal driving patterns, however, the final standards apply over a much broader range of engine operation.

Table I-2 presents our final off-cycle standards for NO_X emissions from heavy-duty CI engines. As discussed in I.B.1.i, for the medium and heavy heavy-duty engines we are also finalizing an interim compliance allowance that applies to non-idle (Bin 2) off-cycle standard after the engines are in-use. This interim compliance allowance is consistent with our past practice (for example, see 66 FR 5114, January 18, 2001) and is shown in the final column of Table I-2. See Section III for details on the final off-cycle standards for other pollutants.

In addition to the final standards for the defined duty cycle and off-cycle test procedures, the final standards include several other provisions for controlling emissions from specific operations in CI or SI engines. First, we are finalizing, as proposed, to allow CI engine manufacturers to voluntarily certify to idle standards using a new idle test procedure that is based on an existing California Air Resources Board (CARB) procedure.

We are also finalizing two options for manufacturers to control engine crankcase emissions. Specifically, manufacturers will be required to either: (1) As proposed, close the crankcase, or (2) measure and account for crankcase emissions using an updated version of the current requirements for an open crankcase. We believe that either will ensure that the total emissions are accounted for during certification testing and throughout the engine operation during useful life. See Section III.B for more discussion on both the final idle and crankcase provisions.

For heavy-duty SI, we are finalizing as proposed a new refueling emission standard for incomplete vehicles above 14,000 lb GVWR starting in MY 2027. The final refueling standard is based on the current refueling standard that applies to complete heavy-duty gasoline-fueled vehicles. Consistent with the current evaporative emission standards that apply for these same vehicles, we are finalizing a requirement that manufacturers can use an engineering analysis to demonstrate that they meet our final refueling standard. We are also adopting an optional alternative phase-in compliance pathway that manufacturers can opt into in lieu of being subject to this implementation date for all incomplete heavy-duty vehicles above 14,000 pounds GVWR (see Section III.E for details).

ii. Ensuring Standards Are Met Over a Greater Portion of an Engine's Operational Life

In addition to reducing emissions under a broad range of engine operating conditions, the final program also includes provisions to ensure emissions standards are met over a greater portion of an engine's operational life. These final provisions include: (1) Lengthened regulatory useful life periods for heavy-duty engines, (2) revised requirement for the largest heavy-duty engines to demonstrate that the emissions control technology is durable through a period of time longer than the final useful life mileage, (3) updated methods to more accurately and efficiently demonstrate the durability of emissions controls, (4) lengthened emission warranty periods, and (5) increased assurance that emission controls will be maintained properly through more of the service life of heavy-duty engines. Each of these final provisions is outlined immediately below and detailed in Section IV.

a. Final Useful Life Periods

Consistent with the proposal, the final useful life periods will cover a significant portion of the engine's operational life. The longer useful life periods, in combination with the durability demonstration requirements we are finalizing in this rule, are expected to lead manufacturers to further improve the durability of their emission-related components. After additional consideration of data included in the proposal, as well as additional data provided in public comments, we are modifying our proposed useful life periods to account for the combined effect of useful life and the final numeric standards on the overall stringency and emissions reductions of the program (see Section IV.A for additional details).

For smaller heavy-duty engines ( i.e., Spark-ignition HDE, Light HDE, and Medium HDE) we are finalizing the longest useful life periods proposed ( i.e., MY 2031 step of proposed option 1), to apply starting in MY 2027. The final useful life mileage for Heavy HDE, which has a distinctly longer operational life than the smaller engine classes, is approximately 50 percent longer than today's useful life mileage for these engines and matches the longest useful life we proposed for MY 2027. Our final useful life periods for all heavy-duty engine classes are presented in Table I-3. We are also increasing the years-based useful life from the current 10 years to values that vary by engine class and match the respective proposed options. After considering comments, we are also adding hours-based useful life values to all engine categories based on a 20 mile per hour speed threshold and the corresponding final mileage values.

b. Extended Laboratory Demonstration of Emissions Control Durability for the Largest Heavy-Duty Engines

As discussed in Section I.B.1.i, for the largest heavy-duty engines we are finalizing two updates to our proposed testing requirements in order to ensure the greatest emissions reductions technically achievable are met throughout the final useful life periods of these engines. One of the approaches (an in-use interim compliance allowance for medium and heavy heavy-duty engines) was noted in Section I.B.2.i; here we focus on the requirement for manufacturers to demonstrate before the largest heavy-duty engines are in use that the emissions control technology is durable through a period of time longer than the final useful mileage. Specifically, we are finalizing a requirement for manufacturers to demonstrate before the largest heavy-duty engines are in use that the emissions controls on these engines are durable ( e.g., capable of controlling NO_X emissions over the FTP duty-cycle at a level of 35 mg/hp-hr) through the equivalent of 750,000 miles. The extended durability demonstration in a laboratory environment will better ensure the final standards will be met throughout the longer final regulatory useful life mileage of 650,000 miles when these engines are operating in the real world where conditions are more variable. As discussed immediately below in Section I.B.2.ii.c, we are also finalizing provisions to improve the accuracy and efficiency of emissions control durability demonstrations for all heavy-duty engine classes.

c. Final Durability Demonstration

EPA regulations require manufacturers to include durability demonstration data as part of an application for certification of an engine family. Manufacturers typically complete this demonstration by following regulatory procedures to calculate a deterioration factor (DF). The final useful life periods outlined in Table I-4 will require manufacturers to extend their durability demonstrations to show that the engines will meet applicable emission standards throughout the lengthened useful life.

To address the need for accurate and efficient emission durability demonstration methods, EPA worked with manufacturers and CARB to address this concern through guidance for MY 2020 and later engines. Consistent with the recent guidance, we proposed three methods for determining DFs. We are finalizing two of the three proposed methods; we are not finalizing the option to perform a fuel-based accelerated DF determination, noting that it has been shown to underestimate emission control system deterioration. The two methods we are finalizing include: (1) Allowing manufacturers to continue the current practice of determining DFs based on engine dynamometer-based aging of the complete engine and aftertreatment system out to regulatory useful life, and (2) a new option to bench-age the aftertreatment system at an accelerated rate to limit the burden of generating a DF over the final lengthened useful life periods. If manufacturers choose the second option (accelerated bench-aging of the aftertreatment system), then they may also choose to use an accelerated aging test procedure that we are codifying in this final rule; the test procedure is, based on a test program that we introduced in the proposal to evaluate a rapid-aging protocol for diesel catalysts. We are also finalizing with revisions two of the three proposed DF verification options to confirm the accuracy of the DF values submitted by manufacturers for certification. After further consideration of data included in the proposal, as well as supported by information provided in public comments, we are finalizing that, upon EPA request, manufacturers would be required to provide confirmation of the DF accuracy through one of two options.

d. Final Emission-Related Warranty Periods

We are updating and significantly strengthening the emission-related warranty periods, for model year 2027 and later heavy-duty engines. We are finalizing most of the emission-related warranty provisions of 40 CFR 1036.120 as proposed. Following our approach for useful life, we are revising the proposed warranty periods for each primary intended service class to reflect the difference in average operational life of each class and in consideration of the information provided by commenters (see preamble Section IV and the Response to Comments document for details).

EPA's current emissions-related warranty periods for heavy-duty engines range from 22 percent to 54 percent of the current regulatory useful life. Notably, these percent values have decreased over time given that the warranty periods have not changed since 1983 even as the useful life periods were lengthened. The revised warranty periods are expected to result in better maintenance, including maintenance of emission-related components, and less tampering, which would help to ensure the benefits of the emission controls in-use. In addition, longer regulatory warranty periods may lead engine manufacturers to simplify repair processes and make them more aware of system defects that need to be tracked and reported to EPA.

Our final emission-related warranty periods for heavy-duty engines are presented in Table I-4. The final warranty mileages that apply starting in MY 2027 for Spark-ignition HDE, Light HDE, and Medium HDE match the longest warranty mileages proposed ( i.e., MY 2031 step of proposed Option 1) for these primary intended service classes. For Heavy HDE, which has a distinctly longer operational life, the final warranty mileage matches the longest warranty mileage proposed to apply in MY 2027 ( i.e., MY 2027 step of proposed Option 1), and is more than four times longer than today's warranty mileage for these engines. We are also increasing the years-based warranty from the current 5 years to 10 years for all engine classes. After considering comments, we are also adding hours-based warranty values to all primary intended service classes based on a 20 mile per hour speed threshold and the corresponding final mileage values. Consistent with current warranty provisions, the warranty period would be whichever warranty value ( i.e., mileage, hours, or years) occurs first.

e. Provisions To Ensure Long-Term Emissions Performance

We proposed several approaches for an enhanced, comprehensive strategy to increase the likelihood that emission controls will be maintained properly through more of the operational life of heavy-duty engines, including beyond their useful life periods. These approaches include updated maintenance provisions, revised requirements for the owner's manual and emissions label, codified engine derates or “inducements” regulations, and updated onboard diagnostics (OBD) regulations.

Our final updates to maintenance provisions include defining the type of maintenance manufacturers may choose to recommend to owners in maintenance instructions, updating minimum maintenance intervals for certain critical emission-related components, and outlining specific requirements for maintenance instructions provided in the owner's manual.

We are finalizing changes to the owner's manual and emissions label requirements to ensure access to certain maintenance information and improve serviceability. We expect this additional maintenance information to improve factors that contribute to mal-maintenance, which would result in better service experiences for independent repair technicians, specialized repair technicians, owners who repair their own equipment, and possibly vehicle inspection and maintenance technicians. We also believe improving owner experiences with operating and maintaining heavy-duty engines can reduce the likelihood of tampering.

In addition, we are adopting inducement regulations that are an update to and replace existing guidance regarding recommended methods for manufacturers to reduce engine performance to induce operators to maintain appropriate levels of high-quality diesel emission fluid (DEF) in their SCR-based aftertreatment systems and discourage tampering with such systems. See Section IV.D for details on the principles we followed to develop multi-step derate schedules that are tailored to different operating characteristics, as well as changes in the final rule inducement regulations from the proposal.

We are also finalizing updated OBD regulations both to better address newer diagnostic methods and available technologies, and to streamline provisions where possible. We are incorporating by reference the current CARB OBD regulations, updated in 2019, as proposed. Specifically, manufacturers must comply with OBD requirements as referenced in the CARB OBD regulations starting in model year 2027, with optional compliance based on the CARB OBD regulations for earlier model years. After considering comments, many of which included specific technical information and requests for clarification, we are finalizing certain provisions with revisions from proposal and postponing others for consideration in a future rulemaking (see Section IV.C for details).

iii. Averaging, Banking, and Trading of NO_X Emissions Credits

In addition the key program provisions, EPA is finalizing an averaging, banking, and trading (ABT) program for heavy-duty engines that provides manufacturers with flexibility in their product planning while encouraging the early introduction of emissions control technologies and maintaining the expected emissions reductions from the program. Several core aspects of the final ABT program are consistent with the proposal, but the final ABT program also includes several updates after consideration of public comments. In particular, EPA requested comment on and agrees with commenters that a lower family emission limit (FEL) cap than proposed is appropriate for the final rule. Further, after consideration of public comments, EPA is choosing not to finalize at this time the proposed Early Adoption Incentives program, and in turn we are not including emissions credit multipliers in the final program. Rather, we are finalizing an updated version of the proposed transitional credit program under the ABT program. The revised transitional credit program that we are finalizing provides four pathways to generate NO_X emissions credits in MYs 2022 through 2026 that are valued based on the extent to which the engines generating credits comply with the requirements we are finalizing for MY 2027 and later ( e.g., credits discounted at a rate of 40 percent for engines meeting a lower numeric standard but none of the other MY 2027 and later requirements). Specifically, the four transitional credit pathways in the final rule are: (1) In MY 2026, for heavy heavy-duty or medium heavy-duty engine service classes, certify all engines in the manufacturer's respective service class to a FEL of 50 mg/hp-hr or less and meet all other EPA requirements for MYs 2027 and later to generate undiscounted credits that have additional flexibilities for use in MYs 2027 and later (2026 Service Class Pull Ahead Credits); (2) starting in MY 2024, certify one or more engine family(ies) to a FEL below the current MY 2010 emissions standards and meet all other EPA requirements for MYs 2027 and later to generate undiscounted credits based on the longer UL periods included in the 2027 and later program (Full Credits); (3) starting in MY 2024, certify one or more engine family(ies) to a FEL below the current MY 2010 emissions standards and several of the key requirements for MYs 2027 and later, while meeting the current useful life and warranty requirements to generate undiscounted credits based on the shorter UL period (Partial Credits); (4) starting in MY 2022, certify one or more engine family(ies) to a FEL below the current MY 2010 emissions standards, while complying with all other MY2010 requirements, to generate discounted credits (Discounted Credits). We note that the transitional credit and main ABT program we are finalizing does not allow engines certified to state standards that are different than the Federal EPA standards to generate Federal EPA credits.

In addition, we are finalizing an optional production volume allowance for MYs 2027 through 2029 that is consistent with our request for comment in the proposal but different in several key aspects, including a requirement for manufacturers to use NO_X emissions credits to certify heavy heavy-duty engines compliant with MY 2010 requirements in MYs 2027 through 2029. Finally, we have decided not to finalize an allowance for manufacturers to generate NO_X emissions credits from heavy-duty ZEVs (see Section IV.G for details on the final ABT program).

iv. Migration From 40 CFR Part 86, Subpart A

Heavy-duty criteria pollutant regulations were originally codified into 40 CFR part 86, subpart A, in the 1980s. As discussed in the proposal, this rulemaking provides an opportunity to clarify and improve the wording of our existing heavy-duty criteria pollutant regulations in plain language and migrate them to 40 CFR part 1036. Part 1036, which was created for the Phase 1 GHG program, provides a consistent, updated format for our heavy-duty regulations, with improved organization. In general, this migration is not intended to change the compliance program specified in part 86, except as specifically stated in this final rulemaking. See our summary of the migration in Section III.A. The final provisions of part 1036 will generally apply for model years 2027 and later, unless noted, and manufacturers will continue to use part 86 in the interim.

v. Technical Amendments to Regulatory Provisions for Mobile Source Sectors

EPA has promulgated emission standards for highway and nonroad engines, vehicles, and equipment. Section XI of this final rule describes several amendments to correct, clarify, and streamline a wide range of regulatory provisions for many of those different types of engines, vehicles, and equipment. Section XI.A includes technical amendments to compliance provisions that apply broadly across EPA's emission control programs to multiple industry sectors, including light-duty vehicles, light-duty trucks, marine diesel engines, locomotives, and various other types of nonroad engines, vehicles, and equipment. Some of those amendments are for broadly applicable testing and compliance provisions in 40 CFR parts 1065, 1066, and 1068. Other cross-sector issues involve making the same or similar changes in multiple standard-setting parts for individual industry sectors. The rest of Section XI describes amendments we are finalizing that apply uniquely for individual industry sectors. Except as specifically identified in this rulemaking, EPA did not reopen any of the underlying provisions across these standard setting parts.

We are finalizing amendments in two areas of note for the general compliance provisions in 40 CFR part 1068. First, we are finalizing, with updates from proposal, a comprehensive approach for making confidentiality determinations related to compliance information that companies submit to or is collected by EPA. These provisions apply for highway, nonroad, and stationary engine, vehicle, and equipment programs, as well as aircraft and portable fuel containers.

Second, we are finalizing, with updates from proposal, provisions that include clarifying text to establish what qualifies as an adjustable parameter and to identify the practically adjustable range for those adjustable parameters. The adjustable-parameter provisions in the final rule also include specific provisions related to electronic controls that aim to deter tampering.

C. Impacts of the Standards

1. Projected Emission Reductions and Air Quality Improvements

Our analysis of the estimated emission reductions, air quality improvements, costs, and monetized benefits of the final rule is outlined in this section and detailed in Sections V through X. The final standards, which are described in detail in Sections III and IV, are expected to reduce emissions from highway heavy-duty engines in several ways. We project the final emission standards for heavy-duty CI engines will reduce tailpipe emissions of NO_X; the combination of the final low-load test cycle and off-cycle test procedure for CI engines will help to ensure that the reductions in tailpipe emissions are achieved in-use, not only under high-speed, on-highway conditions, but also under low-load and idle conditions. We also project reduced tailpipe emissions of NO_X from the final emission standards for heavy-duty SI engines, as well as reductions of CO, PM, VOCs, and associated air toxics, particularly under cold-start and high-load operating conditions. The final emissions warranty and regulatory useful life requirements for heavy-duty CI and SI engines will also help maintain emissions controls of all pollutants beyond the existing useful life periods, which will result in additional emissions reductions of all pollutants from both CI and SI engines, including primary exhaust PM_2.5 . The onboard refueling vapor recovery requirements for heavy-duty SI engines will reduce VOCs and associated air toxics. Table I-5 summarizes the projected reductions in heavy-duty emissions from the final standards in 2045 and shows the significant reductions in NO_X emissions. Section VI and Regulatory Impact Analysis (RIA) Chapter 5 provide more information on our projected emission reductions for the final rule.

The final standards will also reduce emissions of other pollutants. For instance, the final rule will result in a 28 percent reduction in benzene from highway heavy-duty engines in 2045. Leading up to 2045, emission reductions are expected to increase over time as the fleet turns over to new, compliant engines.

We expect this rule will decrease ambient concentrations of air pollutants, including significant improvements in ozone concentrations in 2045, as demonstrated in the air quality modeling analysis. We also expect reductions in ambient PM_2.5, NO₂ and CO due to this rule. The emission reductions provided by the final standards will be important in helping areas attain and maintain the NAAQS and prevent future nonattainment. This rule's emission reductions will also reduce air pollution in close proximity to major roadways, reduce nitrogen deposition and improve visibility.

Our consideration of environmental justice literature indicates that people of color and people with low income are disproportionately exposed to elevated concentrations of many pollutants in close proximity to major roadways. We also used our air quality data from the proposal to conduct a demographic analysis of human exposure to future air quality in scenarios with and without the rule in place. Although the spatial resolution of the air quality modeling is not sufficient to capture very local heterogeneity of human exposures, particularly the pollution concentration gradients near roads, the analysis does allow estimates of demographic trends at a national scale. To compare demographic trends, we sorted 2045 baseline air quality concentrations from highest to lowest concentration and created two groups: Areas within the contiguous United States with the worst air quality and the rest of the country. We found that in the 2045 baseline, the number of people of color living within areas with the worst air quality is nearly double that of non-Hispanic Whites. We also found that the largest predicted improvements in both ozone and PM_2.5 are estimated to occur in areas with the worst baseline air quality, where larger numbers of people of color are projected to reside. An expanded analysis of the air quality impacts experienced by specific race and ethnic groups found that non-Hispanic Blacks will receive the greatest improvement in PM_2.5 and ozone concentrations as a result of the standards. More details on our air quality modeling and demographic analyses are included in Section VII and RIA Chapter 6.

2. Summary of Costs and Benefits

Our estimates of reductions in heavy-duty engine emissions and the associated air quality impacts are based on manufacturers adding emissions-reduction technologies and making emission control components more durable in response to the final standards and longer regulatory useful life periods; our estimates of emissions reductions also account for improved repair of emissions controls by owners in response to the longer emissions-related warranty periods and other provisions in the final rule.

Our program cost analysis includes both the total technology costs ( i.e., manufacturers' costs to add or update emissions control technologies) and the operating costs ( i.e., owners' costs to maintain and operate MY 2027 and later vehicles) (see Section V and RIA Chapter 7). Our evaluation of total technology costs of the final rule includes direct costs ( i.e., cost of materials, labor costs) and indirect manufacturing costs ( e.g., warranty, research and development). The direct manufacturing costs include individual technology costs for emission-related engine components and for exhaust aftertreatment systems. Importantly, our analysis of direct manufacturing costs includes the costs of the existing emission control technologies, because we expect the emissions warranty and regulatory useful life provisions in the final standards to have some impact on not only the new technology added to comply with the standards, but also on any existing emission control components. The cost estimates thus account for existing engine hardware and aftertreatment systems for which new costs will be incurred due to the new warranty and useful life provisions, even absent any changes in the level of emission standards. The indirect manufacturing costs in our analysis include the additional costs—research and development, marketing, administrative costs, etc.—incurred by manufacturers in running the company.

As part of our evaluation of operating costs, we estimate costs truck owners incur to repair emission control system components. Our repair cost estimates are based on industry data showing the amount spent annually by truck owners on different types of repairs, and our estimate of the percentage of those repairs that are related to emission control components. Our analysis of this data shows that extending the useful life and emission warranty periods will lower emission repair costs during several years of operation for several vehicle types. More discussion on our emission repair costs estimates is included in Section V, with additional details presented in RIA Chapter 7.

We combined our estimates of emission repair costs with other operating costs ( i.e., urea/DEF, fuel consumption) and technology costs to calculate total program costs. Our analysis of the final standards shows that total costs for the final program relative to the baseline (or no action scenario) range from $3.9 billion in 2027 to $4.7 billion in 2045 (2017 dollars, undiscounted, see Table V-16). The present value of program costs for the final rule, and additional details are presented in Section V.

Section VIII presents our analysis of the human health benefits associated with the final standards. We estimate that in 2045, the final rule will result in total annual monetized ozone- and PM_2.5 -related benefits of $12 and $33 billion at a 3 percent discount rate, and $10 and $30 billion at a 7 percent discount rate. These benefits only reflect those associated with reductions in NO_X emissions (a precursor to both ozone and secondarily-formed PM_2.5) and directly-emitted PM_2.5 from highway heavy-duty engines.

There are additional human health and environmental benefits associated with reductions in exposure to ambient concentrations of PM_2.5, ozone, and NO₂ that EPA has not quantified due to data, resource, or methodological limitations. There will also be health benefits associated with reductions in air toxic pollutant emissions that result from the final program, but we did not attempt to quantify or monetize those impacts due to methodological limitations. Because we were unable to quantify and monetize all of the benefits associated with the final program, the monetized benefits presented in this analysis are an underestimate of the program's total benefits. More detailed information about the benefits analysis conducted for the final rule, including the present value of program benefits, is included in Section VIII and RIA Chapter 8.

We compare total monetized health benefits to total costs associated with the final rule in Section IX. Table I-6 shows that annual benefits of the final rule will be larger than the annual costs in 2045, with annual net benefits of $6.9 and $29 billion assuming a 3 percent discount rate, and net benefits of $5.8 and $25 billion assuming a 7 percent discount rate. The benefits of the final rule also outweigh the costs when expressed in present value terms and as equalized annual values (see Section IX for these values).

3. Summary of Economic Impacts

Section X examines the potential impacts of the final rule on heavy-duty vehicles (sales, mode shift, fleet turnover) and employment in the heavy-duty industry. The final rule may impact vehicle sales due to both changes in purchase price and longer emission warranty mileage requirements. The final rule may impact vehicle sales by increasing purchases of new vehicles before the final standards come into effect, in anticipation of higher prices after the standards (“pre-buy”). The final rule may also reduce sales after the final standards are in place (“low-buy”). In this final rule, we outline an approach to quantify potential impacts on vehicle sales due to new emission standards. Our illustrative analysis for this final rule, discussed in RIA Chapter 10.1, suggest pre- and low-buy for Class 8 trucks may range from zero to approximately 2 percent increase in sales over a period of up to 8 months before the 2027 standards begin (pre-buy), and a decrease in sales from zero to approximately 3 percent over a period of up to 12 months after the 2027 standards begin (low-buy). We expect little mode shift due to the final rule because of the large difference in cost of moving goods via trucks versus other modes of transport ( e.g., planes or barges).

Employment impacts of the final rule depend on the effects of the rule on sales, the share of labor in the costs of the rule, and changes in labor intensity due to the rule. We quantify the effects of costs on employment, and we discuss the effects due to sales and labor intensity qualitatively. In response to comments, we have added a discussion in Chapter 10 of the RIA describing a method that could be used to quantitatively estimate a demand effect on employment, as well as an illustrative application of that method. The partial quantification of employment impacts due to increases in the costs of vehicles and parts, holding labor intensity constant, shows an increase in employment by 1,000 to 5,300 job-years in 2027. See Section X for further detail on limitations and assumptions of this analysis.

D. EPA Statutory Authority for This Action

This section briefly summarizes the statutory authority for the final rule. Title II of the Clean Air Act provides for comprehensive regulation of mobile sources, authorizing EPA to regulate emissions of air pollutants from all mobile source categories. Specific Title II authorities for this final rule include: CAA sections 202, 203, 206, 207, 208, 213, 216, and 301 (42 U.S.C. 7521, 7522, 7525, 7541, 7542, 7547, 7550, and 7601). We discuss some key aspects of these sections in relation to this final action immediately below (see also Section XIII of this preamble), as well as in each of the relevant sections later in this preamble. As noted in Section I.B.2.v, the final rule includes confidentiality determinations for much of the information collected by EPA for certification and compliance under Title II; see Section XI.A. for discussion of relevant statutory authority for these final rule provisions.

Statutory authority for the final NO_X, PM, HC, and CO emission standards in this action comes from CAA section 202(a), which states that “the Administrator shall by regulation prescribe (and from time to time revise) . . . standards applicable to the emission of any air pollutant from any class or classes of new . . . motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” Standards under CAA section 202(a) take effect after such period as the Administrator finds necessary to permit the development and application of the requisite technology, giving appropriate consideration to the cost of compliance within such period.”

Section 202(a)(3) further addresses EPA authority to establish standards for emissions of NO_X, PM, HC, and CO from heavy-duty engines and vehicles. Section 202(a)(3)(A) requires that such standards “reflect the greatest degree of emission reduction achievable through the application of technology which the Administrator determines will be available for the model year to which such standards apply, giving appropriate consideration to cost, energy, and safety factors associated with the application of such technology.” Section 202(a)(3)(B) allows EPA to take into account air quality information in revising such standards. Section 202(a)(3)(C) provides that standards shall apply for a period of no less than three model years beginning no earlier than the model year commencing four years after promulgation. CAA section 202(a)(3)(A) is a technology-forcing provision and reflects Congress' intent that standards be based on projections of future advances in pollution control capability, considering costs and other statutory factors. CAA section 202(a)(3) neither requires that EPA consider all the statutory factors equally nor mandates a specific method of cost-analysis; rather EPA has discretion in determining the appropriate consideration to give such factors.

CAA section 202(d) directs EPA to prescribe regulations under which the useful life of vehicles and engines are determined and establishes minimum values of 10 years or 100,000 miles, whichever occurs first, unless EPA determines that a period of greater duration or mileage is appropriate. EPA may apply adjustment factors to assure compliance with requirements in use throughout useful life (CAA section 206(a)). CAA section 207(a) requires manufacturers to provide emissions-related warranty, which EPA last updated in its regulations for heavy-duty engines in 1983 (see 40 CFR 86.085-2).

EPA is promulgating the final emission standards pursuant to its authority under CAA section 202(a), including 202(a)(3)(A). Section II and Chapter 4 of the RIA describe EPA's analysis of information regarding heavy-duty engines' contribution to air pollution and how that pollution adversely impacts public health and welfare. Sections III and IV discuss our feasibility analysis of the emission standards and useful life periods in the final rule, with more detail in Chapter 3 of the RIA. Our analysis shows that the final emission standards and useful life periods are feasible and will result in the greatest emission reductions achievable for the model years to which they will apply, pursuant to CAA section 202(a)(3), giving appropriate consideration to costs, lead time, and other factors. Our analysis of the final standards includes providing manufacturers with sufficient time to ensure that emission control components are durable enough for the longer useful life periods in the final program. In setting the final emission standards, EPA appropriately assessed the statutory factors specified in CAA section 202(a)(3)(A), including giving appropriate consideration to the cost associated with the application of technology EPA determined will be available for the model year the final standards apply ( i.e., cost of compliance for the manufacturer associated with the application of such technology). EPA's assessment of the relevant statutory factors in CAA section 202(a)(3)(A) justify the final emission standards. We also evaluated additional factors, including factors to comply with E.O. 12866; our assessment of these factors lend further support to the final rule.

As proposed, we are finalizing new emission standards along with new and revised test procedures for both laboratory-based duty-cycles and off-cycle testing. Manufacturers demonstrate compliance over specified duty-cycle test procedures during pre-production testing, as well as confirmatory testing during production, which is conducted by EPA or the manufacturer. Test data and other information submitted by the manufacturer as part of their certification application are the basis on which EPA issues certificates of conformity pursuant to CAA section 206. Under CAA section 203, sales of new vehicles are prohibited unless the vehicle is covered by a certificate of conformity. Compliance with engine emission standards is required throughout the regulatory useful life of the engine, not only at certification but throughout the regulatory useful life in-use in the real word. In-use engines can be tested for compliance with duty-cycle and off-cycle standards, with testing over corresponding specific duty-cycle test procedures and off-cycle test procedures, either on the road or in the laboratory (see Section III for more discussion on for testing at various stages in the life of an engine).

Also as proposed, we are finalizing lengthened regulatory useful life and emission warranty periods to better reflect the mileages and time periods over which heavy-duty engines are driven today. These and other provisions in the final rule are further discussed in the preamble sections that follow. The proposed rule (87 FR 17414, March 28, 2022) includes additional information relevant to the development of this rule, including: History of Emissions Standards for Heavy-duty Engines and Vehicles; Petitions to EPA for Additional NO_X control; the California Heavy-Duty Highway Low NO_X Program Development; and the Advance Notice of Proposed Rulemaking.

II. Need for Additional Emissions Control

This final rule will reduce emissions from heavy-duty engines that contribute to ambient levels of ozone, PM, NO_X and CO, which are all pollutants for which EPA has established health-based NAAQS. These pollutants are linked to premature death, respiratory illness (including childhood asthma), cardiovascular problems, and other adverse health impacts. Many groups are at greater risk than healthy people from these pollutants, including people with heart or lung disease, outdoor workers, older adults and children. These pollutants also reduce visibility and negatively impact ecosystems. This final rule will also reduce emissions of air toxics from heavy-duty engines. A more detailed discussion of the health and environmental effects associated with the pollutants affected by this rule is included in Sections II.B and II.C and Chapter 4 of the RIA.

Populations who live, work, or go to school near high-traffic roadways experience higher rates of numerous adverse health effects, compared to populations far away from major roads. We note that there is substantial evidence that people who live or attend school near major roadways are more likely to be people of color, Hispanic ethnicity, and/or low socioeconomic status.

Across the United States, NO_X emissions from heavy-duty engines are important contributors to concentrations of ozone and PM_2.5 and their resulting threat to public health. The emissions modeling done for the final rule (see Chapter 5 of the RIA) indicates that without these standards, heavy-duty engines will continue to be one of the largest contributors to mobile source NO_X emissions nationwide in the future, representing 32 percent of the mobile source NO_X in calendar year 2045. Furthermore, it is estimated that heavy-duty engines would represent 90 percent of the onroad NO_X inventory in calendar year 2045. The emission reductions that will occur from the final rule are projected to reduce air pollution that is (and is projected to continue to be) at levels that endanger public health and welfare. For the reasons discussed in this Section II, EPA concludes that new standards are warranted to address the emissions of these pollutants and their contribution to national air pollution. We note that in the summer of 2016 more than 20 organizations, including state and local air agencies from across the country, petitioned EPA to develop more stringent NO_X emission standards for on-road heavy-duty engines. Among the reasons stated by the petitioners for such an EPA rulemaking was the need for NO_X emission reductions to reduce adverse health and welfare impacts and to help areas attain the NAAQS. EPA responded to the petitions on December 20, 2016, noting that an opportunity exists to develop a new national NO_X reduction strategy for heavy-duty highway engines. We subsequently initiated this rulemaking and issued an Advanced Notice of Proposed Rulemaking in January 2020. This final rule culminates the rulemaking proceeding and is responsive to those petitions.

Many state and local agencies across the country commented on the NPRM and have asked the EPA to reduce NO_X emissions, specifically from heavy-duty engines, because such reductions will be a critical part of many areas' strategies to attain and maintain the ozone and PM NAAQS. These state and local agencies anticipate challenges in attaining the NAAQS, maintaining the NAAQS in the future, and/or preventing nonattainment. Some nonattainment areas have already been “bumped up” to higher classifications because of challenges in attaining the NAAQS; others say they are struggling to avoid nonattainment. Others note that the ozone and PM NAAQS are being reconsidered so they could be made more stringent in the future. Many state and local agencies commented on the NPRM that heavy-duty vehicles are one of their largest sources of NO_X emissions. They commented that without action to reduce emissions from heavy-duty vehicles, they will have to adopt other potentially more burdensome and costly measures to reduce emissions from other sources under their state or local authority, such as local businesses. More information on the projected emission reductions and air quality impacts that will result from this rule is provided in Sections VI and VII.

In their comments on the NPRM, many nonprofit groups, citizen groups, individuals, and state, local, and Tribal organizations emphasized the role that emissions from trucks have in harming communities and that communities living near truck routes are disproportionately people of color and those with lower incomes. They supported additional NO_X reductions from heavy-duty vehicles to address concerns about environmental justice and ensuring that all communities benefit from improvements in air quality. In addition, many groups and commenters noted the link between emissions from heavy duty trucks and harmful health effects, in particular asthma in children. Commenters also supported additional NO_X reductions from heavy-duty vehicles to address concerns about regional haze, and damage to terrestrial and aquatic ecosystems. They mentioned the impacts of NO_X emissions on numerous locations, such as the Chesapeake Bay, Long Island Sound, the Rocky Mountains, Sierra Nevada Mountains, Appalachian Mountains, Southwestern Desert ecosystems, and other areas. For further detail regarding these comments and EPA's responses, see Section 2 of the Response to Comments document for this rulemaking.

A. Background on Pollutants Impacted by This Proposal

1. Ozone

Ground-level ozone pollution forms in areas with high concentrations of ambient nitrogen oxides (NO_X) and volatile organic compounds (VOCs) when solar radiation is strong. Major U.S. sources of NO_X are highway and nonroad motor vehicles, engines, power plants and other industrial sources, with natural sources, such as soil, vegetation, and lightning, serving as smaller sources. Vegetation is the dominant source of VOCs in the United States. Volatile consumer and commercial products, such as propellants and solvents, highway and nonroad vehicles, engines, fires, and industrial sources also contribute to the atmospheric burden of VOCs at ground-level.

The processes underlying ozone formation, transport, and accumulation are complex. Ground-level ozone is produced and destroyed by an interwoven network of free radical reactions involving the hydroxyl radical (OH), NO, NO₂, and complex reaction intermediates derived from VOCs. Many of these reactions are sensitive to temperature and available sunlight. High ozone events most often occur when ambient temperatures and sunlight intensities remain high for several days under stagnant conditions. Ozone and its precursors can also be transported hundreds of miles downwind, which can lead to elevated ozone levels in areas with otherwise low VOC or NO_X emissions. As an air mass moves and is exposed to changing ambient concentrations of NO_X and VOCs, the ozone photochemical regime (relative sensitivity of ozone formation to NO_X and VOC emissions) can change.

When ambient VOC concentrations are high, comparatively small amounts of NO_X catalyze rapid ozone formation. Without available NO_X, ground-level ozone production is severely limited, and VOC reductions would have little impact on ozone concentrations. Photochemistry under these conditions is said to be “NO_X -limited.” When NO_X levels are sufficiently high, faster NO₂ oxidation consumes more radicals, dampening ozone production. Under these “VOC-limited” conditions (also referred to as “NO_X -saturated” conditions), VOC reductions are effective in reducing ozone, and NO_X can react directly with ozone, resulting in suppressed ozone concentrations near NO_X emission sources. Under these NO_X -saturated conditions, NO_X reductions can actually increase local ozone under certain circumstances, but overall ozone production (considering downwind formation) decreases. Even in VOC-limited areas, NO_X reductions are not expected to increase ozone levels if the NO_X reductions are sufficiently large—large enough to become NO_X -limited.

The primary NAAQS for ozone, established in 2015 and retained in 2020, is an 8-hour standard with a level of 0.07 ppm. EPA announced that it will reconsider the decision to retain the ozone NAAQS. The EPA is also implementing the previous 8-hour ozone primary standard, set in 2008, at a level of 0.075 ppm. As of August 31, 2022, there were 34 ozone nonattainment areas for the 2008 ozone NAAQS, composed of 141 full or partial counties, with a population of more than 90 million, and 49 ozone nonattainment areas for the 2015 ozone NAAQS, composed of 212 full or partial counties, with a population of more than 125 million. In total, there are currently, as of August 31, 2022, 57 ozone nonattainment areas with a population of more than 130 million people.

States with ozone nonattainment areas are required to take action to bring those areas into attainment. The attainment date assigned to an ozone nonattainment area is based on the area's classification. The attainment dates for areas designated nonattainment for the 2008 8-hour ozone NAAQS are in the 2015 to 2032 timeframe, depending on the severity of the problem in each area. Attainment dates for areas designated nonattainment for the 2015 ozone NAAQS are in the 2021 to 2038 timeframe, again depending on the severity of the problem in each area. The final NO_X standards will take effect starting in MY 2027 and will assist areas with attaining the NAAQS and may relieve areas with already stringent local regulations from some of the burden associated with adopting additional local controls. The rule will also provide assistance to counties with ambient concentrations near the level of the NAAQS who are working to ensure long-term attainment or maintenance of the NAAQS.

2. Particulate Matter

Particulate matter (PM) is a complex mixture of solid particles and liquid droplets distributed among numerous atmospheric gases which interact with solid and liquid phases. Particles in the atmosphere range in size from less than 0.01 to more than 10 micrometers (μm) in diameter. Atmospheric particles can be grouped into several classes according to their aerodynamic diameter and physical sizes. Generally, the three broad classes of particles include ultrafine particles (UFPs, generally considered as particles with a diameter less than or equal to 0.1 μm [typically based on physical size, thermal diffusivity or electrical mobility]), “fine” particles (PM_2.5; particles with a nominal mean aerodynamic diameter less than or equal to 2.5 μm), and “thoracic” particles (PM₁₀; particles with a nominal mean aerodynamic diameter less than or equal to 10 μm). Particles that fall within the size range between PM_2.5 and PM₁₀, are referred to as “thoracic coarse particles” (PM_10−2.5, particles with a nominal mean aerodynamic diameter greater than 2.5 μm and less than or equal to 10 μm). EPA currently has NAAQS for PM_2.5 and PM₁₀ .

Most particles are found in the lower troposphere, where they can have residence times ranging from a few hours to weeks. Particles are removed from the atmosphere by wet deposition, such as when they are carried by rain or snow, or by dry deposition, when particles settle out of suspension due to gravity. Atmospheric lifetimes are generally longest for PM_2.5 , which often remains in the atmosphere for days to weeks before being removed by wet or dry deposition. In contrast, atmospheric lifetimes for UFP and PM_10−2.5 are shorter. Within hours, UFP can undergo coagulation and condensation that lead to formation of larger particles, or can be removed from the atmosphere by evaporation, deposition, or reactions with other atmospheric components. PM_10−2.5 are also generally removed from the atmosphere within hours, through wet or dry deposition.

Particulate matter consists of both primary and secondary particles. Primary particles are emitted directly from sources, such as combustion-related activities ( e.g., industrial activities, motor vehicle operation, biomass burning), while secondary particles are formed through atmospheric chemical reactions of gaseous precursors ( e.g., sulfur oxides (SO_X), NO_X, and VOCs).

There are two primary NAAQS for PM_2.5 : An annual standard (12.0 micrograms per cubic meter (μg/m )) and a 24-hour standard (35 μg/m ), and there are two secondary NAAQS for PM_2.5 : An annual standard (15.0 μg/m ) and a 24-hour standard (35 μg/m ). The initial PM_2.5 standards were set in 1997 and revisions to the standards were finalized in 2006 and in December 2012 and then retained in 2020. On June 10, 2021, EPA announced that it will reconsider the decision to retain the PM NAAQS.

There are many areas of the country that are currently in nonattainment for the annual and 24-hour primary PM_2.5 NAAQS. As of August 31, 2022, more than 19 million people lived in the 4 areas that are designated as nonattainment for the 1997 PM_2.5 NAAQS. Also, as of August 31, 2022, more than 31 million people lived in the 14 areas that are designated as nonattainment for the 2006 PM_2.5 NAAQS and more than 20 million people lived in the 5 areas designated as nonattainment for the 2012 PM_2.5 NAAQS. In total, there are currently 15 PM_2.5 nonattainment areas with a population of more than 32 million people. The final NO_X standards will take effect in MY 2027 and will assist areas with attaining the NAAQS and may relieve areas with already stringent local regulations from some of the burden associated with adopting additional local controls. The rule will also assist counties with ambient concentrations near the level of the NAAQS who are working to ensure long-term attainment or maintenance of the PM_2.5 NAAQS.

3. Nitrogen Oxides

Oxides of nitrogen (NO_X) refers to nitric oxide (NO) and nitrogen dioxide (NO₂). Most NO₂ is formed in the air through the oxidation of NO emitted when fuel is burned at a high temperature. NO₂ is a criteria pollutant, regulated for its adverse effects on public health and the environment, and highway vehicles are an important contributor to NO₂ emissions. NO_X, along with VOCs, are the two major precursors of ozone and NO_X is also a major contributor to secondary PM_2.5 formation. There are two primary NAAQS for NO₂ : An annual standard (53 ppb) and a 1-hour standard (100 ppb). In 2010, EPA established requirements for monitoring NO₂ near roadways expected to have the highest concentrations within large cities. Monitoring within this near-roadway network began in 2014, with additional sites deployed in the following years. At present, there are no nonattainment areas for NO₂.

4. Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes. Nationally, particularly in urban areas, the majority of CO emissions to ambient air come from mobile sources. There are two primary NAAQS for CO: An 8-hour standard (9 ppm) and a 1-hour standard (35 ppm). There are currently no CO nonattainment areas; as of September 27, 2010, all CO nonattainment areas have been redesignated to attainment. The past designations were based on the existing community-wide monitoring network. EPA made an addition to the ambient air monitoring requirements for CO during the 2011 NAAQS review. Those new requirements called for CO monitors to be operated near roads in Core Based Statistical Areas (CBSAs) of 1 million or more persons, in addition to the existing community-based network (76 FR 54294, August 31, 2011).

5. Diesel Exhaust

Diesel exhaust is a complex mixture composed of particulate matter, carbon dioxide, oxygen, nitrogen, water vapor, carbon monoxide, nitrogen compounds, sulfur compounds and numerous low-molecular-weight hydrocarbons. A number of these gaseous hydrocarbon components are individually known to be toxic, including aldehydes, benzene and 1,3-butadiene. The diesel particulate matter present in diesel exhaust consists mostly of fine particles (<2.5 μm), of which a significant fraction is ultrafine particles (<0.1 μm). These particles have a large surface area which makes them an excellent medium for adsorbing organics and their small size makes them highly respirable. Many of the organic compounds present in the gases and on the particles, such as polycyclic organic matter, are individually known to have mutagenic and carcinogenic properties.

Diesel exhaust varies significantly in chemical composition and particle sizes between different engine types (heavy-duty, light-duty), engine operating conditions (idle, acceleration, deceleration), and fuel formulations (high/low sulfur fuel). Also, there are emissions differences between on-road and nonroad engines because the nonroad engines are generally of older technology. After being emitted in the engine exhaust, diesel exhaust undergoes dilution as well as chemical and physical changes in the atmosphere. The lifetime of the components present in diesel exhaust ranges from seconds to days.

Because diesel particulate matter (DPM) is part of overall ambient PM, varies considerably in composition, and lacks distinct chemical markers that enable it to be easily distinguished from overall primary PM, we do not have direct measurements of DPM in the ambient air. DPM concentrations are estimated using ambient air quality modeling based on DPM emission inventories. DPM emission inventories are computed as the exhaust PM emissions from mobile sources combusting diesel or residual oil fuel. DPM concentrations were estimated as part of the 2018 national Air Toxics Screening Assessment (AirToxScreen). Areas with high concentrations are clustered in the Northeast and Great Lake States, with a smaller number of higher concentration locations in Western states. The highest impacts occur in major urban cores, and are also distributed throughout the rest of the United States near high truck traffic, coasts with marine diesel activity, construction sites, and rail facilities. Approximately half of the average ambient DPM concentration in the United States can be attributed to heavy-duty diesel engines, with the remainder attributable to nonroad engines.

6. Air Toxics

The most recent available data indicate that millions of Americans live in areas where air toxics pose potential health concerns. The levels of air toxics to which people are exposed vary depending on where people live and work and the kinds of activities in which they engage, as discussed in detail in EPA's 2007 Mobile Source Air Toxics Rule. According to EPA's Air Toxics Screening Assessment (AirToxScreen) for 2018, mobile sources were responsible for 40 percent of outdoor anthropogenic toxic emissions and were the largest contributor to national average cancer and noncancer risk from directly emitted pollutants. Mobile sources are also significant contributors to precursor emissions which react to form air toxics. Formaldehyde is the largest contributor to cancer risk of all 71 pollutants quantitatively assessed in the 2018 AirToxScreen. Mobile sources were responsible for 26 percent of primary anthropogenic emissions of this pollutant in 2018 and are significant contributors to formaldehyde precursor emissions. Benzene is also a large contributor to cancer risk, and mobile sources account for about 60 percent of average exposure to ambient concentrations.

B. Health Effects Associated With Exposure to Pollutants Impacted by This Rule

Heavy-duty engines emit pollutants that contribute to ambient concentrations of ozone, PM, NO₂, CO, and air toxics. This section of the preamble discusses the health effects associated with exposure to these pollutants.

Additionally, because children have increased vulnerability and susceptibility for adverse health effects related to air pollution exposures, EPA's findings regarding adverse effects for children related to exposure to pollutants that are impacted by this rule are noted in this section. The increased vulnerability and susceptibility of children to air pollution exposures may arise because infants and children generally breathe more relative to their size than adults do, and consequently may be exposed to relatively higher amounts of air pollution. Children also tend to breathe through their mouths more than adults and their nasal passages are less effective at removing pollutants, which leads to greater lung deposition of some pollutants, such as PM. Furthermore, air pollutants may pose health risks specific to children because children's bodies are still developing. For example, during periods of rapid growth such as fetal development, infancy, and puberty, their developing systems and organs may be more easily harmed. EPA's America's Children and the Environment is a tool which presents national trends on air pollutants and other contaminants and environmental health of children.

Information on environmental effects associated with exposure to these pollutants is included in Section II.C, and information on environmental justice is included in Section VII.H. Information on emission reductions and air quality impacts from this rule are included in Section VI and VII.

1. Ozone

This section provides a summary of the health effects associated with exposure to ambient concentrations of ozone. The information in this section is based on the information and conclusions in the April 2020 Integrated Science Assessment for Ozone (Ozone ISA). The Ozone ISA concludes that human exposures to ambient concentrations of ozone are associated with a number of adverse health effects and characterizes the weight of evidence for these health effects. The following discussion highlights the Ozone ISA's conclusions pertaining to health effects associated with both short-term and long-term periods of exposure to ozone.

For short-term exposure to ozone, the Ozone ISA concludes that respiratory effects, including lung function decrements, pulmonary inflammation, exacerbation of asthma, respiratory-related hospital admissions, and mortality, are causally associated with ozone exposure. It also concludes that metabolic effects, including metabolic syndrome ( i.e., changes in insulin or glucose levels, cholesterol levels, obesity, and blood pressure) and complications due to diabetes are likely to be causally associated with short-term exposure to ozone. The evidence is also suggestive of a causal relationship between short-term exposure to ozone and cardiovascular effects, central nervous system effects, and total mortality.

For long-term exposure to ozone, the Ozone ISA concludes that respiratory effects, including new onset asthma, pulmonary inflammation, and injury, are likely to be causally related with ozone exposure. The Ozone ISA characterizes the evidence as suggestive of a causal relationship for associations between long-term ozone exposure and cardiovascular effects, metabolic effects, reproductive and developmental effects, central nervous system effects, and total mortality. The evidence is inadequate to infer a causal relationship between chronic ozone exposure and increased risk of cancer.

Finally, interindividual variation in human responses to ozone exposure can result in some groups being at increased risk for detrimental effects in response to exposure. In addition, some groups are at increased risk of exposure due to their activities, such as outdoor workers and children. The Ozone ISA identified several groups that are at increased risk for ozone-related health effects. These groups are people with asthma, children and older adults, individuals with reduced intake of certain nutrients ( i.e., Vitamins C and E), outdoor workers, and individuals having certain genetic variants related to oxidative metabolism or inflammation. Ozone exposure during childhood can have lasting effects through adulthood. Such effects include altered function of the respiratory and immune systems. Children absorb higher doses (normalized to lung surface area) of ambient ozone, compared to adults, due to their increased time spent outdoors, higher ventilation rates relative to body size, and a tendency to breathe a greater fraction of air through the mouth. Children also have a higher asthma prevalence compared to adults. Recent epidemiologic studies provide generally consistent evidence that long-term ozone exposure is associated with the development of asthma in children. Studies comparing age groups reported higher magnitude associations for short-term ozone exposure and respiratory hospital admissions and emergency room visits among children than among adults. Panel studies also provide support for experimental studies with consistent associations between short-term ozone exposure and lung function and pulmonary inflammation in healthy children. Additional children's vulnerability and susceptibility factors are listed in Section XII of this preamble.

2. Particulate Matter

Scientific evidence spanning animal toxicological, controlled human exposure, and epidemiologic studies shows that exposure to ambient PM is associated with a broad range of health effects. These health effects are discussed in detail in the Integrated Science Assessment for Particulate Matter, which was finalized in December 2019 (PM ISA). In addition, there is a more targeted evaluation of studies published since the literature cutoff date of the 2019 p.m. ISA in the Supplement to the Integrated Science Assessment for PM (Supplement). The PM ISA characterizes the causal nature of relationships between PM exposure and broad health categories ( e.g., cardiovascular effects, respiratory effects, etc.) using a weight-of-evidence approach. Within this characterization, the PM ISA summarizes the health effects evidence for short-term ( i.e., hours up to one month) and long-term ( i.e., one month to years) exposures to PM_2.5, PM_10−2.5, and ultrafine particles, and concludes that exposures to ambient PM_2.5 are associated with a number of adverse health effects. The following discussion highlights the PM ISA's conclusions, and summarizes additional information from the Supplement where appropriate, pertaining to the health effects evidence for both short- and long-term PM exposures. Further discussion of PM-related health effects can also be found in the 2022 Policy Assessment for the review of the PM NAAQS.

EPA has concluded that recent evidence in combination with evidence evaluated in the 2009 p.m. ISA supports a “causal relationship” between both long- and short-term exposures to PM_2.5 and premature mortality and cardiovascular effects and a “likely to be causal relationship” between long- and short-term PM_2.5 exposures and respiratory effects. Additionally, recent experimental and epidemiologic studies provide evidence supporting a “likely to be causal relationship” between long-term PM_2.5 exposure and nervous system effects, and long-term PM_2.5 exposure and cancer. Because of remaining uncertainties and limitations in the evidence base, EPA determined a “suggestive of, but not sufficient to infer, a causal relationship” for long-term PM_2.5 exposure and reproductive and developmental effects ( i.e., male/female reproduction and fertility; pregnancy and birth outcomes), long- and short-term exposures and metabolic effects, and short-term exposure and nervous system effects.

As discussed extensively in the 2019 p.m. ISA and the Supplement, recent studies continue to support a “causal relationship” between short- and long-term PM_2.5 exposures and mortality. For short-term PM_2.5 exposure, multi-city studies, in combination with single- and multi-city studies evaluated in the 2009 p.m. ISA, provide evidence of consistent, positive associations across studies conducted in different geographic locations, populations with different demographic characteristics, and studies using different exposure assignment techniques. Additionally, the consistent and coherent evidence across scientific disciplines for cardiovascular morbidity, particularly ischemic events and heart failure, and to a lesser degree for respiratory morbidity, including exacerbations of chronic obstructive pulmonary disease (COPD) and asthma, provide biological plausibility for cause-specific mortality and ultimately total mortality. Recent epidemiologic studies evaluated in the Supplement, including studies that employed alternative methods for confounder control, provide additional support to the evidence base that contributed to the 2019 p.m. ISA conclusion for short-term PM_2.5 exposure and mortality.

The 2019 p.m. ISA concluded a “causal relationship” between long-term PM_2.5 exposure and mortality. In addition to reanalyses and extensions of the American Cancer Society (ACS) and Harvard Six Cities (HSC) cohorts, multiple new cohort studies conducted in the United States and Canada consisting of people employed in a specific job ( e.g., teacher, nurse), and that apply different exposure assignment techniques, provide evidence of positive associations between long-term PM_2.5 exposure and mortality. Biological plausibility for mortality due to long-term PM_2.5 exposure is provided by the coherence of effects across scientific disciplines for cardiovascular morbidity, particularly for coronary heart disease, stroke, and atherosclerosis, and for respiratory morbidity, particularly for the development of COPD. Additionally, recent studies provide evidence indicating that as long-term PM_2.5 concentrations decrease there is an increase in life expectancy. Recent cohort studies evaluated in the Supplement, as well as epidemiologic studies that conducted accountability analyses or employed alternative methods for confounder controls, support and extend the evidence base that contributed to the 2019 p.m. ISA conclusion for long-term PM_2.5 exposure and mortality.

A large body of studies examining both short- and long-term PM_2.5 exposure and cardiovascular effects builds on the evidence base evaluated in the 2009 p.m. ISA. The strongest evidence for cardiovascular effects in response to short-term PM_2.5 exposures is for ischemic heart disease and heart failure. The evidence for short-term PM_2.5 exposure and cardiovascular effects is coherent across scientific disciplines and supports a continuum of effects ranging from subtle changes in indicators of cardiovascular health to serious clinical events, such as increased emergency department visits and hospital admissions due to cardiovascular disease and cardiovascular mortality. For long-term PM_2.5 exposure, there is strong and consistent epidemiologic evidence of a relationship with cardiovascular mortality. This evidence is supported by epidemiologic and animal toxicological studies demonstrating a range of cardiovascular effects including coronary heart disease, stroke, impaired heart function, and subclinical markers ( e.g., coronary artery calcification, atherosclerotic plaque progression), which collectively provide coherence and biological plausibility. Recent epidemiologic studies evaluated in the Supplement, as well as studies that conducted accountability analyses or employed alternative methods for confounder control, support and extend the evidence base that contributed to the 2019 p.m. ISA conclusion for both short- and long-term PM_2.5 exposure and cardiovascular effects.

Studies evaluated in the 2019 p.m. ISA continue to provide evidence of a “likely to be causal relationship” between both short- and long-term PM_2.5 exposure and respiratory effects. Epidemiologic studies provide consistent evidence of a relationship between short-term PM_2.5 exposure and asthma exacerbation in children and COPD exacerbation in adults, as indicated by increases in emergency department visits and hospital admissions, which is supported by animal toxicological studies indicating worsening allergic airways disease and subclinical effects related to COPD. Epidemiologic studies also provide evidence of a relationship between short-term PM_2.5 exposure and respiratory mortality. However, there is inconsistent evidence of respiratory effects, specifically lung function declines and pulmonary inflammation, in controlled human exposure studies. With respect to long term PM_2.5 exposure, epidemiologic studies conducted in the United States and abroad provide evidence of a relationship with respiratory effects, including consistent changes in lung function and lung function growth rate, increased asthma incidence, asthma prevalence, and wheeze in children; acceleration of lung function decline in adults; and respiratory mortality. The epidemiologic evidence is supported by animal toxicological studies, which provide coherence and biological plausibility for a range of effects including impaired lung development, decrements in lung function growth, and asthma development.

Since the 2009 p.m. ISA, a growing body of scientific evidence examined the relationship between long-term PM_2.5 exposure and nervous system effects, resulting for the first time in a causality determination for this health effects category of a “likely to be causal relationship.” The strongest evidence for effects on the nervous system come from epidemiologic studies that consistently report cognitive decrements and reductions in brain volume in adults. The effects observed in epidemiologic studies in adults are supported by animal toxicological studies demonstrating effects on the brain of adult animals including inflammation, morphologic changes, and neurodegeneration of specific regions of the brain. There is more limited evidence for neurodevelopmental effects in children, with some studies reporting positive associations with autism spectrum disorder and others providing limited evidence of an association with cognitive function. While there is some evidence from animal toxicological studies indicating effects on the brain ( i.e., inflammatory and morphological changes) to support a biologically plausible pathway for neurodevelopmental effects, epidemiologic studies are limited due to their lack of control for potential confounding by copollutants, the small number of studies conducted, and uncertainty regarding critical exposure windows.

Building off the decades of research demonstrating mutagenicity, DNA damage, and other endpoints related to genotoxicity due to whole PM exposures, recent experimental and epidemiologic studies focusing specifically on PM_2.5 provide evidence of a relationship between long-term PM_2.5 exposure and cancer. Epidemiologic studies examining long-term PM_2.5 exposure and lung cancer incidence and mortality provide evidence of generally positive associations in cohort studies spanning different populations, locations, and exposure assignment techniques. Additionally, there is evidence of positive associations with lung cancer incidence and mortality in analyses limited to never smokers. In addition, experimental and epidemiologic studies of genotoxicity, epigenetic effects, carcinogenic potential, and that PM_2.5 exhibits several characteristics of carcinogens provide biological plausibility for cancer development. This collective body of evidence contributed to the conclusion of a “likely to be causal relationship.”

For the additional health effects categories evaluated for PM_2.5 in the 2019 p.m. ISA, experimental and epidemiologic studies provide limited and/or inconsistent evidence of a relationship with PM_2.5 exposure. As a result, the 2019 p.m. ISA concluded that the evidence is “suggestive of, but not sufficient to infer a causal relationship” for short-term PM_2.5 exposure and metabolic effects and nervous system effects, and long-term PM_2.5 exposures and metabolic effects as well as reproductive and developmental effects.

In addition to evaluating the health effects attributed to short- and long-term exposure to PM_2.5, the 2019 p.m. ISA also conducted an extensive evaluation as to whether specific components or sources of PM_2.5 are more strongly related with health effects than PM_2.5 mass. An evaluation of those studies resulted in the 2019 p.m. ISA concluding that “many PM_2.5 components and sources are associated with many health effects, and the evidence does not indicate that any one source or component is consistently more strongly related to health effects than PM_2.5 mass.”

For both PM_10-2.5 and UFPs, for all health effects categories evaluated, the 2019 p.m. ISA concluded that the evidence was “suggestive of, but not sufficient to infer, a causal relationship” or “inadequate to determine the presence or absence of a causal relationship.” For PM_10-2.5, although a Federal Reference Method (FRM) was instituted in 2011 to measure PM_10-2.5 concentrations nationally, the causality determinations reflect that the same uncertainty identified in the 2009 p.m. ISA persists with respect to the method used to estimate PM_10-2.5 concentrations in epidemiologic studies. Specifically, across epidemiologic studies, different approaches are used to estimate PM_10-2.5 concentrations ( e.g., direct measurement of PM_10-2.5, difference between PM₁₀ and PM_2.5 concentrations), and it remains unclear how well correlated PM_10-2.5 concentrations are both spatially and temporally across the different methods used.

For UFPs, which have often been defined as particles <0.1 µm, the uncertainty in the evidence for the health effect categories evaluated across experimental and epidemiologic studies reflects the inconsistency in the exposure metric used ( i.e., particle number concentration, surface area concentration, mass concentration) as well as the size fractions examined. In epidemiologic studies the size fraction examined can vary depending on the monitor used and exposure metric, with some studies examining number count over the entire particle size range, while experimental studies that use a particle concentrator often examine particles up to 0.3 µm. Additionally, due to the lack of a monitoring network, there is limited information on the spatial and temporal variability of UFPs within the United States, as well as population exposures to UFPs, which adds uncertainty to epidemiologic study results.

The 2019 p.m. ISA cites extensive evidence indicating that “both the general population as well as specific populations and life stages are at risk for PM_2.5 -related health effects.” For example, in support of its “causal” and “likely to be causal” determinations, the ISA cites substantial evidence for (1) PM-related mortality and cardiovascular effects in older adults; (2) PM-related cardiovascular effects in people with pre-existing cardiovascular disease; (3) PM-related respiratory effects in people with pre-existing respiratory disease, particularly asthma exacerbations in children; and (4) PM-related impairments in lung function growth and asthma development in children. The ISA additionally notes that stratified analyses ( i.e., analyses that directly compare PM-related health effects across groups) provide strong evidence for racial and ethnic differences in PM_2.5 exposures and in the risk of PM_2.5 -related health effects, specifically within Hispanic and non-Hispanic Black populations, with some evidence of increased risk for populations of low socioeconomic status. Recent studies evaluated in the Supplement support the conclusion of the 2019 p.m. ISA with respect to disparities in both PM_2.5 exposure and health risk by race and ethnicity and provide additional support for disparities for populations of lower socioeconomic status. Additionally, evidence spanning epidemiologic studies that conducted stratified analyses, experimental studies focusing on animal models of disease or individuals with pre-existing disease, dosimetry studies, as well as studies focusing on differential exposure suggest that populations with pre-existing cardiovascular or respiratory disease, populations that are overweight or obese, populations that have particular genetic variants, and current/former smokers could be at increased risk for adverse PM_2.5 -related health effects. The 2022 Policy Assessment for the review of the PM NAAQS also highlights that factors that may contribute to increased risk of PM_2.5 -related health effects include lifestage (children and older adults), pre-existing diseases (cardiovascular disease and respiratory disease), race/ethnicity, and socioeconomic status.

3. Nitrogen Oxides

The most recent review of the health effects of oxides of nitrogen completed by EPA can be found in the 2016 Integrated Science Assessment for Oxides of Nitrogen—Health Criteria (ISA for Oxides of Nitrogen). The primary source of NO₂ is motor vehicle emissions, and ambient NO₂ concentrations tend to be highly correlated with other traffic-related pollutants. Thus, a key issue in characterizing the causality of NO₂ -health effect relationships consists of evaluating the extent to which studies supported an effect of NO₂ that is independent of other traffic-related pollutants. EPA concluded that the findings for asthma exacerbation integrated from epidemiologic and controlled human exposure studies provided evidence that is sufficient to infer a causal relationship between respiratory effects and short-term NO₂ exposure. The strongest evidence supporting an independent effect of NO₂ exposure comes from controlled human exposure studies demonstrating increased airway responsiveness in individuals with asthma following ambient-relevant NO₂ exposures. The coherence of this evidence with epidemiologic findings for asthma hospital admissions and emergency department visits as well as lung function decrements and increased pulmonary inflammation in children with asthma describe a plausible pathway by which NO₂ exposure can cause an asthma exacerbation. The 2016 ISA for Oxides of Nitrogen also concluded that there is likely to be a causal relationship between long-term NO₂ exposure and respiratory effects. This conclusion is based on new epidemiologic evidence for associations of NO₂ with asthma development in children combined with biological plausibility from experimental studies.

In evaluating a broader range of health effects, the 2016 ISA for Oxides of Nitrogen concluded that evidence is “suggestive of, but not sufficient to infer, a causal relationship” between short-term NO₂ exposure and cardiovascular effects and mortality and between long-term NO₂ exposure and cardiovascular effects and diabetes, birth outcomes, and cancer. In addition, the scientific evidence is inadequate (insufficient consistency of epidemiologic and toxicological evidence) to infer a causal relationship for long-term NO₂ exposure with fertility, reproduction, and pregnancy, as well as with postnatal development. A key uncertainty in understanding the relationship between these non-respiratory health effects and short- or long-term exposure to NO₂ is copollutant confounding, particularly by other roadway pollutants. The available evidence for non-respiratory health effects does not adequately address whether NO₂ has an independent effect or whether it primarily represents effects related to other or a mixture of traffic-related pollutants.

The 2016 ISA for Oxides of Nitrogen concluded that people with asthma, children, and older adults are at increased risk for NO₂ -related health effects. In these groups and lifestages, NO₂ is consistently related to larger effects on outcomes related to asthma exacerbation, for which there is confidence in the relationship with NO₂ exposure.

4. Carbon Monoxide

Information on the health effects of CO can be found in the January 2010 Integrated Science Assessment for Carbon Monoxide (CO ISA). The CO ISA presents conclusions regarding the presence of causal relationships between CO exposure and categories of adverse health effects. This section provides a summary of the health effects associated with exposure to ambient concentrations of CO, along with the CO ISA conclusions.

Controlled human exposure studies of subjects with coronary artery disease show a decrease in the time to onset of exercise-induced angina (chest pain) and electrocardiogram changes following CO exposure. In addition, epidemiologic studies observed associations between short-term CO exposure and cardiovascular morbidity, particularly increased emergency room visits and hospital admissions for coronary heart disease (including ischemic heart disease, myocardial infarction, and angina). Some epidemiologic evidence is also available for increased hospital admissions and emergency room visits for congestive heart failure and cardiovascular disease as a whole. The CO ISA concludes that a causal relationship is likely to exist between short-term exposures to CO and cardiovascular morbidity. It also concludes that available data are inadequate to conclude that a causal relationship exists between long-term exposures to CO and cardiovascular morbidity.

Animal studies show various neurological effects with in-utero CO exposure. Controlled human exposure studies report central nervous system and behavioral effects following low-level CO exposures, although the findings have not been consistent across all studies. The CO ISA concludes that the evidence is suggestive of a causal relationship with both short- and long-term exposure to CO and central nervous system effects.

A number of studies cited in the CO ISA have evaluated the role of CO exposure in birth outcomes such as preterm birth or cardiac birth defects. There is limited epidemiologic evidence of a CO-induced effect on preterm births and birth defects, with weak evidence for a decrease in birth weight. Animal toxicological studies have found perinatal CO exposure to affect birth weight, as well as other developmental outcomes. The CO ISA concludes that the evidence is suggestive of a causal relationship between long-term exposures to CO and developmental effects and birth outcomes.

Epidemiologic studies provide evidence of associations between short-term CO concentrations and respiratory morbidity such as changes in pulmonary function, respiratory symptoms, and hospital admissions. A limited number of epidemiologic studies considered copollutants such as ozone, SO₂, and PM in two-pollutant models and found that CO risk estimates were generally robust, although this limited evidence makes it difficult to disentangle effects attributed to CO itself from those of the larger complex air pollution mixture. Controlled human exposure studies have not extensively evaluated the effect of CO on respiratory morbidity. Animal studies at levels of 50-100 ppm CO show preliminary evidence of altered pulmonary vascular remodeling and oxidative injury. The CO ISA concludes that the evidence is suggestive of a causal relationship between short-term CO exposure and respiratory morbidity, and inadequate to conclude that a causal relationship exists between long-term exposure and respiratory morbidity.

Finally, the CO ISA concludes that the epidemiologic evidence is suggestive of a causal relationship between short-term concentrations of CO and mortality. Epidemiologic evidence suggests an association exists between short-term exposure to CO and mortality, but limited evidence is available to evaluate cause-specific mortality outcomes associated with CO exposure. In addition, the attenuation of CO risk estimates that was often observed in copollutant models contributes to the uncertainty as to whether CO is acting alone or as an indicator for other combustion-related pollutants. The CO ISA also concludes that there is not likely to be a causal relationship between relevant long-term exposures to CO and mortality.

5. Diesel Exhaust

In EPA's 2002 Diesel Health Assessment Document (Diesel HAD), exposure to diesel exhaust was classified as likely to be carcinogenic to humans by inhalation from environmental exposures, in accordance with the revised draft 1996/1999 EPA cancer guidelines. A number of other agencies (National Institute for Occupational Safety and Health, the International Agency for Research on Cancer, the World Health Organization, California EPA, and the U.S. Department of Health and Human Services) made similar hazard classifications prior to 2002. EPA also concluded in the 2002 Diesel HAD that it was not possible to calculate a cancer unit risk for diesel exhaust due to limitations in the exposure data for the occupational groups or the absence of a dose-response relationship.

In the absence of a cancer unit risk, the Diesel HAD sought to provide additional insight into the significance of the diesel exhaust cancer hazard by estimating possible ranges of risk that might be present in the population. An exploratory analysis was used to characterize a range of possible lung cancer risk. The outcome was that environmental risks of cancer from long-term diesel exhaust exposures could plausibly range from as low as 10⁻⁵ to as high as 10⁻³ . Because of uncertainties, the analysis acknowledged that the risks could be lower than 10⁻⁵, and a zero risk from diesel exhaust exposure could not be ruled out.

Noncancer health effects of acute and chronic exposure to diesel exhaust emissions are also of concern to EPA. EPA derived a diesel exhaust reference concentration (RfC) from consideration of four well-conducted chronic rat inhalation studies showing adverse pulmonary effects. The RfC is 5 µg/m³ for diesel exhaust measured as diesel particulate matter. This RfC does not consider allergenic effects such as those associated with asthma or immunologic or the potential for cardiac effects. There was emerging evidence in 2002, discussed in the Diesel HAD, that exposure to diesel exhaust can exacerbate these effects, but the exposure-response data were lacking at that time to derive an RfC based on these then-emerging considerations. The Diesel HAD states, “With [diesel particulate matter] being a ubiquitous component of ambient PM, there is an uncertainty about the adequacy of the existing [diesel exhaust] noncancer database to identify all the pertinent [diesel exhaust]-caused noncancer health hazards.” The Diesel HAD also notes “that acute exposure to [diesel exhaust] has been associated with irritation of the eye, nose, and throat, respiratory symptoms (cough and phlegm), and neurophysiological symptoms such as headache, lightheadedness, nausea, vomiting, and numbness or tingling of the extremities.” The Diesel HAD notes that the cancer and noncancer hazard conclusions applied to the general use of diesel engines then on the market and as cleaner engines replace a substantial number of existing ones, the applicability of the conclusions would need to be reevaluated.

It is important to note that the Diesel HAD also briefly summarizes health effects associated with ambient PM and discusses EPA's then-annual PM_2.5 NAAQS of 15 µg/m³ . There is a large and extensive body of human data showing a wide spectrum of adverse health effects associated with exposure to ambient PM, of which diesel exhaust is an important component. The PM_2.5 NAAQS is designed to provide protection from the noncancer health effects and premature mortality attributed to exposure to PM_2.5. The contribution of diesel PM to total ambient PM varies in different regions of the country and also, within a region, from one area to another. The contribution can be high in near-roadway environments, for example, or in other locations where diesel engine use is concentrated.

Since 2002, several new studies have been published which continue to report increased lung cancer risk associated with occupational exposure to diesel exhaust from older engines. Of particular note since 2011 are three new epidemiology studies that have examined lung cancer in occupational populations, for example, truck drivers, underground nonmetal miners, and other diesel motor-related occupations. These studies reported increased risk of lung cancer with exposure to diesel exhaust with evidence of positive exposure-response relationships to varying degrees. These newer studies (along with others that have appeared in the scientific literature) add to the evidence EPA evaluated in the 2002 Diesel HAD and further reinforce the concern that diesel exhaust exposure likely poses a lung cancer hazard. The findings from these newer studies do not necessarily apply to newer technology diesel engines ( i.e., heavy-duty highway engines from 2007 and later model years) since the newer engines have large reductions in the emission constituents compared to older technology diesel engines.

In light of the growing body of scientific literature evaluating the health effects of exposure to diesel exhaust, in June 2012 the World Health Organization's International Agency for Research on Cancer (IARC), a recognized international authority on the carcinogenic potential of chemicals and other agents, evaluated the full range of cancer-related health effects data for diesel engine exhaust. IARC concluded that diesel exhaust should be regarded as “carcinogenic to humans.” This designation was an update from its 1988 evaluation that considered the evidence to be indicative of a “probable human carcinogen.”

6. Air Toxics

Heavy-duty engine emissions contribute to ambient levels of air toxics that are known or suspected human or animal carcinogens, or that have noncancer health effects. These compounds include, but are not limited to, benzene, formaldehyde, acetaldehyde, and naphthalene. These compounds were identified as national or regional cancer risk drivers or contributors in the 2018 AirToxScreen Assessment and have significant inventory contributions from mobile sources. Chapter 4 of the RIA includes additional information on the health effects associated with exposure to each of these pollutants.

7. Exposure and Health Effects Associated With Traffic

Locations in close proximity to major roadways generally have elevated concentrations of many air pollutants emitted from motor vehicles. Hundreds of studies have been published in peer-reviewed journals, concluding that concentrations of CO, CO₂, NO, NO₂, benzene, aldehydes, PM, black carbon, and many other compounds are elevated in ambient air within approximately 300-600 meters (about 1,000-2,000 feet) of major roadways. The highest concentrations of most pollutants emitted directly by motor vehicles are found at locations within 50 meters (about 165 feet) of the edge of a roadway's traffic lanes.

A large-scale review of air quality measurements in the vicinity of major roadways between 1978 and 2008 concluded that the pollutants with the steepest concentration gradients in vicinities of roadways were CO, UFPs, metals, elemental carbon (EC), NO, NO_X , and several VOCs. These pollutants showed a large reduction in concentrations within 100 meters downwind of the roadway. Pollutants that showed more gradual reductions with distance from roadways included benzene, NO₂, PM_2.5, and PM₁₀ . In reviewing the literature, Karner et al., (2010) reported that results varied based on the method of statistical analysis used to determine the gradient in pollutant concentration. More recent studies continue to show significant concentration gradients of traffic-related air pollution around major roads.^{127 128 129 130} There is evidence that EPA's regulations for vehicles have lowered the near-road concentrations and gradients. Starting in 2010, EPA required through the NAAQS process that air quality monitors be placed near high-traffic roadways for determining concentrations of CO, NO₂, and PM_2.5 (in addition to those existing monitors located in neighborhoods and other locations farther away from pollution sources). The monitoring data for NO₂ indicate that in urban areas, monitors near roadways often report the highest concentrations of NO₂ . More recent studies of traffic-related air pollutants continue to report sharp gradients around roadways, particularly within several hundred meters.

For pollutants with relatively high background concentrations relative to near-road concentrations, detecting concentration gradients can be difficult. For example, many carbonyls have high background concentrations as a result of photochemical breakdown of precursors from many different organic compounds. However, several studies have measured carbonyls in multiple weather conditions and found higher concentrations of many carbonyls downwind of roadways. These findings suggest a substantial roadway source of these carbonyls.

In the past 30 years, many studies have been published with results reporting that populations who live, work, or go to school near high-traffic roadways experience higher rates of numerous adverse health effects, compared to populations far away from major roads. In addition, numerous studies have found adverse health effects associated with spending time in traffic, such as commuting or walking along high-traffic roadways, including studies among children. The health outcomes with the strongest evidence linking them with traffic-associated air pollutants are respiratory effects, particularly in asthmatic children, and cardiovascular effects. Commenters on the NPRM stressed the importance of consideration of the impacts of traffic-related air pollution, especially NO_X, on children's health.

Numerous reviews of this body of health literature have been published. In a 2022 final report, an expert panel of the Health Effects Institute (HEI) employed a systematic review focusing on selected health endpoints related to exposure to traffic-related air pollution. The HEI panel concluded that there was a high level of confidence in evidence between long-term exposure to traffic-related air pollution and health effects in adults, including all-cause, circulatory, and ischemic heart disease mortality. The panel also found that there is a moderate-to-high level of confidence in evidence of associations with asthma onset and acute respiratory infections in children and lung cancer and asthma onset in adults. This report follows on an earlier expert review published by HEI in 2010, where it found strongest evidence for asthma-related traffic impacts. Other literature reviews have been published with conclusions generally similar to the HEI panels'. Additionally, in 2014, researchers from the U.S. Centers for Disease Control and Prevention (CDC) published a systematic review and meta-analysis of studies evaluating the risk of childhood leukemia associated with traffic exposure and reported positive associations between “postnatal” proximity to traffic and leukemia risks, but no such association for “prenatal” exposures. The U.S. Department of Health and Human Services' National Toxicology Program (NTP) published a monograph including a systematic review of traffic-related air pollution and its impacts on hypertensive disorders of pregnancy. The NTP concluded that exposure to traffic-related air pollution is “presumed to be a hazard to pregnant women” for developing hypertensive disorders of pregnancy.

Health outcomes with few publications suggest the possibility of other effects still lacking sufficient evidence to draw definitive conclusions. Among these outcomes with a small number of positive studies are neurological impacts ( e.g., autism and reduced cognitive function) and reproductive outcomes ( e.g., preterm birth, low birth weight).

In addition to health outcomes, particularly cardiopulmonary effects, conclusions of numerous studies suggest mechanisms by which traffic-related air pollution affects health. For example, numerous studies indicate that near-roadway exposures may increase systemic inflammation, affecting organ systems, including blood vessels and lungs. Additionally, long-term exposures in near-road environments have been associated with inflammation-associated conditions, such as atherosclerosis and asthma.

Several studies suggest that some factors may increase susceptibility to the effects of traffic-associated air pollution. Several studies have found stronger adverse health associations in children experiencing chronic social stress, such as in violent neighborhoods or in homes with low incomes or high family stress.

The risks associated with residence, workplace, or schools near major roads are of potentially high public health significance due to the large population in such locations. The 2013 U.S. Census Bureau's American Housing Survey (AHS) was the last AHS that included whether housing units were within 300 feet of an “airport, railroad, or highway with four or more lanes.” The 2013 survey reports that 17.3 million housing units, or 13 percent of all housing units in the United States, were in such areas. Assuming that populations and housing units are in the same locations, this corresponds to a population of more than 41 million U.S. residents in close proximity to high-traffic roadways or other transportation sources. According to the Central Intelligence Agency's World Factbook, based on data collected between 2012-2014, the United States had 6,586,610 km of roadways, 293,564 km of railways, and 13,513 airports. As such, highways represent the overwhelming majority of transportation facilities described by this factor in the AHS.

EPA also conducted a study to estimate the number of people living near truck freight routes in the United States. Based on a population analysis using the U.S. Department of Transportation's (USDOT) Freight Analysis Framework 4 (FAF4) and population data from the 2010 decennial census, an estimated 72 million people live within 200 meters of these freight routes. In addition, relative to the rest of the population, people of color and those with lower incomes are more likely to live near FAF4 truck routes. They are also more likely to live in metropolitan areas. The EPA's Exposure Factor Handbook also indicates that, on average, Americans spend more than an hour traveling each day, bringing nearly all residents into a high-exposure microenvironment for part of the day.

As described in Section VII.H.1, we estimate that about 10 million students attend schools within 200 meters of major roads. Research into the impact of traffic-related air pollution on school performance is tentative. A review of this literature found some evidence that children exposed to higher levels of traffic-related air pollution show poorer academic performance than those exposed to lower levels of traffic-related air pollution. However, this evidence was judged to be weak due to limitations in the assessment methods.

While near-roadway studies focus on residents near roads or others spending considerable time near major roads, the duration of commuting results in another important contributor to overall exposure to traffic-related air pollution. Studies of health that address time spent in transit have found evidence of elevated risk of cardiac impacts. Studies have also found that school bus emissions can increase student exposures to diesel-related air pollutants, and that programs that reduce school bus emissions may improve health and reduce school absenteeism.

C. Environmental Effects Associated With Exposure to Pollutants Impacted by This Rule

This section discusses the environmental effects associated with pollutants affected by this rule, specifically PM, ozone, NO_X and air toxics.

1. Visibility

Visibility can be defined as the degree to which the atmosphere is transparent to visible light. Visibility impairment is caused by light scattering and absorption by suspended particles and gases. It is dominated by contributions from suspended particles except under pristine conditions. Visibility is important because it has direct significance to people's enjoyment of daily activities in all parts of the country. Individuals value good visibility for the well-being it provides them directly, where they live and work, and in places where they enjoy recreational opportunities. Visibility is also highly valued in significant natural areas, such as national parks and wilderness areas, and special emphasis is given to protecting visibility in these areas. For more information on visibility see the final 2019 p.m. ISA.

EPA is working to address visibility impairment. Reductions in air pollution from implementation of various programs associated with the Clean Air Act Amendments of 1990 provisions have resulted in substantial improvements in visibility and will continue to do so in the future. Nationally, because trends in haze are closely associated with trends in particulate sulfate and nitrate due to the relationship between their concentration and light extinction, visibility trends have improved as emissions of SO₂ and NO_X have decreased over time due to air pollution regulations such as the Acid Rain Program. However between 1990 and 2018, in the western part of the country, changes in total light extinction were smaller, and the contribution of particulate organic matter to atmospheric light extinction was increasing due to increasing wildfire emissions.

In the Clean Air Act Amendments of 1977, Congress recognized visibility's value to society by establishing a national goal to protect national parks and wilderness areas from visibility impairment caused by manmade pollution. In 1999, EPA finalized the regional haze program to protect the visibility in Mandatory Class I Federal areas. There are 156 national parks, forests and wilderness areas categorized as Mandatory Class I Federal areas. These areas are defined in CAA section 162 as those national parks exceeding 6,000 acres, wilderness areas, and memorial parks exceeding 5,000 acres, and all international parks which were in existence on August 7, 1977.

EPA has also concluded that PM_2.5 causes adverse effects on visibility in other areas that are not targeted by the Regional Haze Rule, such as urban areas, depending on PM_2.5 concentrations and other factors such as dry chemical composition and relative humidity ( i.e., an indicator of the water composition of the particles). The secondary (welfare-based) PM NAAQS provide protection against visibility effects. In recent PM NAAQS reviews, EPA evaluated a target level of protection for visibility impairment that is expected to be met through attainment of the existing secondary PM standards.

2. Plant and Ecosystem Effects of Ozone

The welfare effects of ozone include effects on ecosystems, which can be observed across a variety of scales, i.e., subcellular, cellular, leaf, whole plant, population and ecosystem. When ozone effects that begin at small spatial scales, such as the leaf of an individual plant, occur at sufficient magnitudes (or to a sufficient degree), they can result in effects being propagated along a continuum to higher and higher levels of biological organization. For example, effects at the individual plant level, such as altered rates of leaf gas exchange, growth and reproduction, can, when widespread, result in broad changes in ecosystems, such as productivity, carbon storage, water cycling, nutrient cycling, and community composition.

Ozone can produce both acute and chronic injury in sensitive plant species depending on the concentration level and the duration of the exposure. In those sensitive species, effects from repeated exposure to ozone throughout the growing season of the plant can tend to accumulate, so even relatively low concentrations experienced for a longer duration have the potential to create chronic stress on vegetation. Ozone damage to sensitive plant species includes impaired photosynthesis and visible injury to leaves. The impairment of photosynthesis, the process by which the plant makes carbohydrates (its source of energy and food), can lead to reduced crop yields, timber production, and plant productivity and growth. Impaired photosynthesis can also lead to a reduction in root growth and carbohydrate storage below ground, resulting in other, more subtle plant and ecosystems impacts. These latter impacts include increased susceptibility of plants to insect attack, disease, harsh weather, interspecies competition, and overall decreased plant vigor. The adverse effects of ozone on areas with sensitive species could potentially lead to species shifts and loss from the affected ecosystems, resulting in a loss or reduction in associated ecosystem goods and services. Additionally, visible ozone injury to leaves can result in a loss of aesthetic value in areas of special scenic significance like national parks and wilderness areas and reduced use of sensitive ornamentals in landscaping. In addition to ozone effects on vegetation, newer evidence suggests that ozone affects interactions between plants and insects by altering chemical signals ( e.g., floral scents) that plants use to communicate to other community members, such as attraction of pollinators.

The Ozone ISA presents more detailed information on how ozone affects vegetation and ecosystems. The Ozone ISA reports causal and likely causal relationships between ozone exposure and a number of welfare effects and characterizes the weight of evidence for different effects associated with ozone. The Ozone ISA concludes that visible foliar injury effects on vegetation, reduced vegetation growth, reduced plant reproduction, reduced productivity in terrestrial ecosystems, reduced yield and quality of agricultural crops, alteration of below-ground biogeochemical cycles, and altered terrestrial community composition are causally associated with exposure to ozone. It also concludes that increased tree mortality, altered herbivore growth and reproduction, altered plant-insect signaling, reduced carbon sequestration in terrestrial ecosystems, and alteration of terrestrial ecosystem water cycling are likely to be causally associated with exposure to ozone.

3. Atmospheric Deposition

The Integrated Science Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter—Ecological Criteria documents the ecological effects of the deposition of these criteria air pollutants. It is clear from the body of evidence that NO_X, oxides of sulfur (SO_X), and PM contribute to total nitrogen (N) and sulfur (S) deposition. In turn, N and S deposition cause either nutrient enrichment or acidification depending on the sensitivity of the landscape or the species in question. Both enrichment and acidification are characterized by an alteration of the biogeochemistry and the physiology of organisms, resulting in harmful declines in biodiversity in terrestrial, freshwater, wetland, and estuarine ecosystems in the United States. Decreases in biodiversity mean that some species become relatively less abundant and may be locally extirpated. In addition to the loss of unique living species, the decline in total biodiversity can be harmful because biodiversity is an important determinant of the stability of ecosystems and their ability to provide socially valuable ecosystem services.

Terrestrial, wetland, freshwater, and estuarine ecosystems in the United States are affected by N enrichment/eutrophication caused by N deposition. These effects have been consistently documented across the United States for hundreds of species. In aquatic systems increased N can alter species assemblages and cause eutrophication. In terrestrial systems N loading can lead to loss of nitrogen-sensitive lichen species, decreased biodiversity of grasslands, meadows and other sensitive habitats, and increased potential for invasive species. For a broader explanation of the topics treated here, refer to the description in Chapter 4 of the RIA.

The sensitivity of terrestrial and aquatic ecosystems to acidification from N and S deposition is predominantly governed by geology. Prolonged exposure to excess nitrogen and sulfur deposition in sensitive areas acidifies lakes, rivers, and soils. Increased acidity in surface waters creates inhospitable conditions for biota and affects the abundance and biodiversity of fishes, zooplankton, and macroinvertebrates and ecosystem function. Over time, acidifying deposition also removes essential nutrients from forest soils, depleting the capacity of soils to neutralize future acid loadings and negatively affecting forest sustainability. Major effects in forests include a decline in sensitive tree species, such as red spruce (Picea rubens) and sugar maple (Acer saccharum).

Building materials including metals, stones, cements, and paints undergo natural weathering processes from exposure to environmental elements ( e.g., wind, moisture, temperature fluctuations, sunlight, etc.). Pollution can worsen and accelerate these effects. Deposition of PM is associated with both physical damage (materials damage effects) and impaired aesthetic qualities (soiling effects). Wet and dry deposition of PM can physically affect materials, adding to the effects of natural weathering processes, by potentially promoting or accelerating the corrosion of metals, by degrading paints, and by deteriorating building materials such as stone, concrete, and marble. The effects of PM are exacerbated by the presence of acidic gases and can be additive or synergistic due to the complex mixture of pollutants in the air and surface characteristics of the material. Acidic deposition has been shown to have an effect on materials including zinc/galvanized steel and other metal, carbonate stone (such as monuments and building facings), and surface coatings (paints). The effects on historic buildings and outdoor works of art are of particular concern because of the uniqueness and irreplaceability of many of these objects. In addition to aesthetic and functional effects on metals, stone, and glass, altered energy efficiency of photovoltaic panels by PM deposition is also becoming an important consideration for impacts of air pollutants on materials.

4. Environmental Effects of Air Toxics

Emissions from producing, transporting, and combusting fuel contribute to ambient levels of pollutants that contribute to adverse effects on vegetation. VOCs, some of which are considered air toxics, have long been suspected to play a role in vegetation damage. In laboratory experiments, a wide range of tolerance to VOCs has been observed. Decreases in harvested seed pod weight have been reported for the more sensitive plants, and some studies have reported effects on seed germination, flowering, and fruit ripening. Effects of individual VOCs or their role in conjunction with other stressors ( e.g., acidification, drought, temperature extremes) have not been well studied. In a recent study of a mixture of VOCs including ethanol and toluene on herbaceous plants, significant effects on seed production, leaf water content, and photosynthetic efficiency were reported for some plant species.

Research suggests an adverse impact of vehicle exhaust on plants, which has in some cases been attributed to aromatic compounds and in other cases to NO_X . The impacts of VOCs on plant reproduction may have long-term implications for biodiversity and survival of native species near major roadways. Most of the studies of the impacts of VOCs on vegetation have focused on short-term exposure and few studies have focused on long-term effects of VOCs on vegetation and the potential for metabolites of these compounds to affect herbivores or insects.

III. Test Procedures and Standards

In applying heavy-duty criteria pollutant emission standards, EPA divides engines primarily into two types: Compression ignition (CI) (primarily diesel-fueled engines) and spark-ignition (SI) (primarily gasoline-fueled engines). The CI standards and requirements also apply to the largest natural gas engines. Battery-electric and fuel-cell vehicles are also subject to criteria pollutant standards and requirements. Criteria pollutant exhaust emission standards apply for four criteria pollutants: Oxides of nitrogen (NO_X ), particulate matter (PM), hydrocarbons (HC), and carbon monoxide (CO). In this Section III we describe new emission standards that will apply for these pollutants starting in MY 2027. We also describe new and updated test procedures we are finalizing in this rule.

Section III.A provides an overview of provisions that broadly apply for this final rule. Section III.B and Section III.D include the new laboratory-based standards and final updates to test procedures for heavy-duty compression-ignition and spark-ignition engines, respectively. Section III.C introduces the final off-cycle standards and test procedures that apply for compression-ignition engines and extend beyond the laboratory to on-the-road, real-world conditions. Section III.E describes the new refueling standards we are finalizing for certain heavy-duty spark-ignition engines. Each of these sections describe the final new standards and their basis, as well as describe the new test procedures and any updates to current test procedures, and describe our rationale for the final program, including feasibility demonstrations, available data, and comments received.

A. Overview

1. Migration and Clarifications of Regulatory Text

As noted in Section I of this preamble, we are migrating our criteria pollutant regulations for model year 2027 and later heavy-duty highway engines from their current location in 40 CFR Part 86, subpart A, to 40 CFR Part 1036. Consistent with this migration, the compliance provisions discussed in this preamble refer to the regulations in their new location in part 1036. In general, this migration is not intended to change the compliance program specified in part 86, except as specifically finalized in this rulemaking. EPA submitted a memorandum to the docket describing how we proposed to migrate certification and compliance provisions into 40 CFR part 1036.

i. Compression- and Spark-Ignition Engines Regulatory Text

For many years, the regulations of 40 CFR part 86 have referred to “diesel heavy-duty engines” and “Otto-cycle heavy-duty engines”; however, as we migrate the heavy-duty provisions of 40 CFR part 86, subpart A, to 40 CFR part 1036 in this rule, we proposed to refer to these engines as “compression-ignition” (CI) and “spark-ignition” (SI), respectively, which are more comprehensive terms and consistent with existing language in 40 CFR part 1037 for heavy-duty motor vehicle regulations. We also proposed to update the terminology for the primary intended service classes in 40 CFR 1036.140 to replace Heavy heavy-duty engine with Heavy HDE, Medium heavy-duty engine with Medium HDE, Light heavy-duty engine with Light HDE, and Spark-ignition heavy-duty engine with Spark-ignition HDE. We received no adverse comment and are finalizing these terminology changes, as proposed. This final rule revises 40 CFR parts 1036 and 1037 to reflect this updated terminology. Throughout this preamble, reference to diesel and Otto-cycle engines and the previous service class nomenclature is generally limited to discussions relating to current test procedures and specific terminology used in 40 CFR part 86. Heavy-duty engines not meeting the definition of compression-ignition or spark-ignition are deemed to be compression-ignition engines for purposes of part 1036, per 40 CFR 1036.1(c) and are subject to standards in 40 CFR 1036.104.

ii. Heavy-Duty Hybrid Regulatory Text

Similar to our updates to more comprehensive and consistent terminology for CI and SI engines, as part of this rule we are also finalizing three main updates and clarifications to regulatory language for hybrid engines and hybrid powertrains. First, as proposed, we are finalizing an updated definition of “engine configuration” in 40 CFR 1036.801; the updated definition clarifies that an engine configuration includes hybrid components if it is certified as a hybrid engine or hybrid powertrain. Second, we are finalizing, as proposed, a clarification in 40 CFR 1036.101(b) that regulatory references in part 1036 to engines generally apply to hybrid engines and hybrid powertrains. Third, we are finalizing as proposed that manufacturers may optionally test the hybrid engine and powertrain together, rather than testing the engine alone. The option to test hybrid engine and powertrain together allows manufacturers to demonstrate emission performance of the hybrid technology that are not apparent when testing the engine alone. If the emissions results of testing the hybrid engine and powertrain together show NO_X emissions lower than the final standards, then EPA anticipates that manufacturers may choose to participate in the NO_X ABT program in the final rule (see preamble Section IV.G for details on the final ABT program).

We requested comment on our proposed clarification in 40 CFR 1036.101(b) that manufacturers may optionally test the hybrid engine and powertrain together, rather than testing the engine alone, and specifically, whether EPA should require all hybrid engines and powertrains to be certified together, rather than making it optional. For additional details on our proposed updates and clarifications to regulatory language for hybrid engines and hybrid powertrains, as well as our specific requests for comment on these changes, see the proposed rule preamble (87 FR 17457, March 28, 2022).

Several commenters support the proposal to allow manufacturers to certify hybrid powertrains with a powertrain test procedure, but urge EPA to continue to allow manufacturers to certify hybrid systems using engine dynamometer testing procedures. These commenters stated that the powertrain dynamometer test procedures produce emission results that are more representative of hybrid engine or powertrain on-road operation than engine-only testing, however, commenters also stated the proposed test cycles are not reflective of real-world applications where hybrid technology works well and urged EPA to finalize different duty-cycles. In contrast, one commenter pointed to data collected from light-duty hybrid electric vehicles in Europe that the commenter stated shows hybrid-electric vehicles (HEVs) emit at higher levels than demonstrated in current certification test procedures; based on those data the commenter stated that EPA should not allow HEVs to generate NO_X emissions credits. Separately, some commenters also stated that requiring powertrain testing for hybrid engines or hybrid powertrains certification would add regulatory costs or other logistical challenges.

After considering these comments, EPA has determined that powertrain testing for hybrid systems should remain an option in this final rule. This option allows manufacturers to demonstrate emission performance of the hybrid technology, without requiring added test burden or logistical constraints. We are therefore finalizing as proposed the allowance for manufacturers to test the hybrid engine and powertrain together. If testing the hybrid engine and hybrid powertrain together results in NO_X emissions that are below the final standards, then manufacturers can choose to certify to a FEL below the standard, and then generate NO_X emissions credits as provided under the final ABT program (see Section IV.G). We disagree with one commenter who asserted that manufacturers should not be allowed to generate NO_X emissions credits from HEVs based on data showing higher emissions from HEVs operating in the real-world compared to certification test data in Europe. Rather, we expect the powertrain test procedures we are finalizing will accurately reflect NO_X emissions from HEVs due to the specifications we are including in the final test procedures, which differ from the certification test procedures to which the commenter referred. See preamble Section III.B.2.v for more details on the powertrain test procedures that we are finalizing.

Similarly, we disagree with those commenters urging EPA to finalize different duty-cycle tests to reflect hybrid real-world operations. While the duty-cycles suggested by commenters would represent some hybrid operations, they would not represent the duty-cycles of other hybrid vehicle types. See Section 3 of the Response to Comments document for additional details on our responses to comments on different duty-cycles for hybrid vehicles, and responses to other comments on hybrid engines and hybrid powertrains.

In addition to our three main proposed updates and clarifications to regulatory language for hybrid engines and hybrid powertrain, we also proposed that manufacturers would certify a hybrid engine or hybrid powertrain to criteria pollutant standards by declaring a primary intended service class of the engine configuration using the proposed, updated 40 CFR 1036.140. Our proposal included certifying to the same useful life requirements of the primary intended service class, which would provide truck owners and operators with similar assurance of durability regardless of the powertrain configuration they choose. Finally, we proposed an update to 40 CFR 1036.230(e) such that engine configurations certified as a hybrid engine or hybrid powertrain may not be included in an engine family with conventional engines, which is consistent with the current provisions. We received no adverse comment and are finalizing as proposed these updates to 40 CFR 1036.140 and 1036.230(e).

iii. Heavy-Duty Zero Emissions Vehicles Regulatory Text

As part of this final rule we are also updating and consolidating regulatory language for battery-electric vehicles and fuel cell electric vehicles (BEVs and FCEVs), collectively referred to as zero emissions vehicles (ZEVs). For ZEVs, we are finalizing as proposed a consolidation and update to our regulations as part of a migration of heavy-duty vehicle regulations from 40 CFR part 86 to 40 CFR part 1037. In the HD GHG Phase 1 rulemaking, EPA revised the heavy-duty vehicle and engine regulations to make them consistent with our regulatory approach to electric vehicles (EVs) under the light-duty vehicle program. Specifically, we applied standards for all regulated criteria pollutants and GHGs to all heavy-duty vehicle types, including EVs. Starting in MY 2016, criteria pollutant standards and requirements applicable to heavy-duty vehicles at or below 14,000 pounds gross vehicle weight rating (GVWR) in 40 CFR part 86, subpart S, applied to heavy-duty EVs above 14,000 pounds GVWR through the use of good engineering judgment (see current 40 CFR 86.016-1(d)(4)). Under the current 40 CFR 86.016-1(d)(4), heavy-duty vehicles powered solely by electricity are deemed to have zero emissions of regulated pollutants; this provision also provides that heavy-duty EVs may not generate NO_X or PM emission credits.

As proposed, this final rule consolidates certification requirements for ZEVs over 14,000 pounds GVWR in 40 CFR part 1037 such that manufacturers of ZEVs over 14,000 pounds GVWR will certify to meeting the emission standards and requirements of 40 CFR part 1037. There are no criterial pollutant emission standards in 40 CFR part 1037, so we state in a new 40 CFR 1037.102, with revisions from the proposed rule, that heavy-duty vehicles without propulsion engines are subject to the same criteria pollutant emission standards that apply for engines under 40 CFR part 86, subpart A, and 40 CFR part 1036. We further specify in the final 40 CFR 1037.102 that ZEVs are deemed to have zero tailpipe emissions of criteria pollutants. As discussed in Section IV.G, we are choosing not to finalize our proposal to allow manufacturers to generate NO_X emission credits from ZEVs if the vehicle met certain proposed requirements. We are accordingly carrying forward in the final 40 CFR 1037.102 a provisions stating that manufacturers may not generate emission credits from ZEVs. We are choosing not to finalize the proposed durability requirements for ZEVs, but we may choose in a future action to reexamine this issue. We are finalizing as proposed to continue to not allow heavy-duty ZEVs to generate PM emission credits since we are finalizing as proposed not to allow any manufacturer to generate PM emission credits for use in MY 2027 and later under the final ABT program presented in Section IV.G.

The provisions in existing and final 40 CFR 1037.5 defer to 40 CFR 86.1801-12 to clarify how certification requirements apply for heavy-duty vehicles at or below 14,000 pounds GVWR. Emission standards and certification requirements in 40 CFR part 86, subpart S, generally apply for complete heavy-duty vehicles at or below 14,000 pounds GVWR. We proposed to also apply emission standards and certification requirements under 40 CFR part 86, subpart S, for all incomplete vehicles at or below 14,000 pounds GVWR. We decided not to adopt this requirement and are instead continuing to allow manufacturers to choose whether to certify incomplete vehicles at or below 14,000 pounds GVWR to the emission standards and certification requirements in either 40 CFR part 86, subpart S, or 40 CFR part 1037.

2. Numeric Standards and Test Procedures for Compression-Ignition and Spark-Ignition Engines

As summarized in preamble Section I.B and detailed in this preamble Section III, we are finalizing numeric NO_X standards and useful life periods that are largely consistent with the most stringent proposed option for MY 2027. The specific standards are summarized in Section III.B, Section 0, Section III.D, and Section III.E. As required by CAA section 202(a)(3), EPA is finalizing new NO_X, PM, HC, and CO emission standards for heavy-duty engines that reflect the greatest degree of emission reduction achievable through the application of technology that we have determined would be available for MY 2027, and in doing so have given appropriate consideration to additional factors, namely lead time, cost, energy, and safety. For all heavy-duty engine classes, the final numeric NO_X standards for medium- and high-load engine operations match the most stringent standards proposed for MY 2027; for low-load operations we are finalizing the most stringent standard proposed for any model year (see III.B.2.iii for discussion). For smaller heavy-duty engine service classes ( i.e., light and medium heavy-duty engines CI and SI heavy-duty engines), the numeric standards are combined with the longest useful life periods we proposed. For the largest heavy-duty engines ( i.e., heavy heavy-duty engines), the final numeric standards are combined with the longest useful life mileage that we proposed for MY 2027. The final useful life periods for the largest heavy-duty engines are 50 percent longer than today's useful life periods, which will play an important role in ensuring continued emissions control while the engines operate on the road. The final numeric emissions standards and useful life periods for all heavy-duty engines are based on further consideration of data included in the proposal from our engine demonstration programs that show the final emissions standards are feasible at the final useful life periods applicable to these each heavy-duty engine service class. Our assessment of the data available at the time of proposal is further supported by our evaluation of additional information and public comments stating that the proposed standards are feasible. Our technical assessments are primarily based on results from testing several diesel engine and aftertreatment systems at Southwest Research Institute and at EPA's National Vehicle and Fuel Emissions Laboratory (NVFEL), as well as heavy-duty gasoline engine testing conducted at NVFEL; we also considered heavy-duty engine certification data submitted to EPA by manufacturers, ANPR and NPRM comments, and other data submitted by industry stakeholders or studies conducted by EPA, as more specifically identified in the sections that follow.

After further consideration of the data included in the proposal, as well as information submitted by commenters and additional data we collected since the time of proposal, we are finalizing two updates from our proposed testing requirements in order to ensure the greatest emissions reductions technically achievable are met throughout the final useful life periods; these updates are tailored to the larger engine classes (medium and heavy heavy-duty engines). First, we are finalizing a requirement for manufacturers to demonstrate before heavy heavy-duty engines are in-use that the emissions control technology is durable through a period of time longer than the final useful life mileage. For these largest engines with the longest useful life mileages, the extended laboratory durability demonstration will better ensure the final standards will be met throughout the regulatory useful life under real-world operations where conditions are more variable. Second, we are finalizing an interim in-use compliance allowance that applies when EPA evaluates whether heavy or medium heavy-duty engines are meeting the final standards after these engines are in use in the real-world. When combined with the final useful life values, we believe the interim in-use compliance allowance will address concerns raised in comments from manufacturers that the more stringent proposed MY 2027 standards would not be feasible to meet over the very long useful life periods of heavy heavy-duty engines, or under the challenging duty-cycles of medium heavy-duty engines. This interim, in-use compliance allowance is generally consistent with our past practice (for example, see 66 FR 5114, January 18, 2001); also consistent with past practice, the compliance allowance is included as an interim provision that we may reassess in the future through rulemaking based on the performance of emissions controls over the final useful life periods for medium and heavy heavy-duty engines. To set standards that result in the greatest emission reductions achievable for medium and heavy heavy-duty engines, we considered additional data that we and others collected since the time of the proposal; these data show the significant technical challenge of maintaining very low NO_X emissions throughout very long useful life periods for heavy heavy-duty engines, and greater amounts of certain aging mechanisms over the long useful life periods of medium heavy-duty engines. In addition to these data, in setting the standards we gave appropriate consideration to costs associated with the application of technology to achieve the greatest emissions reductions in MY 2027 ( i.e., cost of compliance for manufacturers associated with the standards ) and other statutory factors, including energy and safety. We determined that for heavy heavy-duty engines the combination of: (1) The most stringent MY 2027 standards proposed, (2) longer useful life periods compared to today's useful life periods, (3) targeted, interim compliance allowance approach to in-use compliance testing, and (4) the extended durability demonstration for emissions control technologies is appropriate, feasible, and consistent with our authority under the CAA to set technology-forcing criteria pollutant standards for heavy-duty engines for their useful life. Similarly, for medium heavy-duty engines we determined that the combination of the first three elements ( i.e., most stringent MY 2027 standards proposed, increase in useful life periods, and interim compliance allowance for in-use testing) is appropriate, feasible, and consistent with our CAA authority to set technology-forcing criteria pollutant standards for heavy-duty engines for their useful life.

In addition to the final standards for the defined duty cycle and off-cycle test procedures, the final standards include several other provisions for controlling emissions from specific operations in CI or SI engines. First, we are finalizing, as proposed, to allow CI engine manufacturers to voluntarily certify to idle standards using a new idle test procedure that is based on an existing California Air Resources Board (CARB) procedure.

We are also finalizing two options for manufacturers to control engine crankcase emissions. Specifically, manufacturers will be required to either: (1) As proposed, close the crankcase, or (2) measure and account for crankcase emissions using an updated version of the current requirements for an open crankcase. We believe that either will ensure that the total emissions are accounted for during certification testing and throughout the engine operation during useful life. See Section III.B for more discussion on both the final idle and crankcase provisions.

For heavy-duty SI, we are finalizing as proposed a new refueling emission standard for incomplete vehicles above 14,000 lb GVWR starting in MY 2027. The final refueling standard is based on the current refueling standard that applies to complete heavy-duty gasoline-fueled vehicles. Consistent with the current evaporative emission standards that apply for these same vehicles, we are finalizing a requirement that manufacturers can use an engineering analysis to demonstrate that they meet our final refueling standard. We are also adopting an optional alternative phase-in compliance pathway that manufacturers can opt into in lieu of being subject to this implementation date for all incomplete heavy-duty vehicles above 14,000 pounds GVWR (see Section III.E for details).

Consistent with our proposal, we are also finalizing several provisions to reduce emissions from a broader range of engine operating conditions. First, we are finalizing new standards for our existing test procedures to reduce emissions under medium- and high-load operations ( e.g., when trucks are traveling on the highway). Second, we are finalizing new standards and a corresponding new test procedure to measure emissions during low-load operations ( i.e., the low-load cycle, LLC). Third, we are finalizing new standards and updates to an existing test procedure to measure emissions over the broader range of operations that occur when heavy-duty engines are operating on the road ( i.e., off-cycle).

The new, more stringent numeric standards for the existing laboratory-based test procedures that measure emissions during medium- and high-load operations will ensure significant emissions reductions from heavy-duty engines. Without this final rule, these medium- and high-load operations are projected to contribute the most to heavy-duty NO_X emissions in 2045.

We are finalizing as proposed a new LLC test procedure, which will ensure demonstration of emission control under sustained low-load operations. After further consideration of data included in the proposal, as well as additional information from the comments summarized in this section, we are finalizing the most stringent numeric standard for the LLC that we proposed for any model year. As discussed in our proposal, data from our CI engine demonstration program showed that the lowest numeric NO_X standard proposed would be feasible for the LLC throughout a useful life period similar to the useful life we are finalizing for the largest heavy-duty engines. After further consideration of this data, and additional support from data collected since the time of proposal, we are finalizing the most stringent standard proposed for any model year.

We are finalizing new numeric standards and revisions to the proposed off-cycle test procedure. We proposed updates to the current off-cycle test procedure that included binning emissions measurements based on the type of operation the engine is performing when the measurement data is being collected. Specifically, we proposed that emissions data would be grouped into three bins, based on if the engine was operating in idle (Bin 1), low-load (Bin 2), or medium-to-high load (Bin 3) operation. Given the different operational profiles of each of the three bins, we proposed a separate standard for each bin. Based on further consideration of data included in the proposal, as well as additional support from our consideration of data provided by commenters, we are finalizing off-cycle standards for two bins, rather than three bins; correspondingly, we are finalizing a two-bin approach for grouping emissions data collected during off-cycle test procedures. Our evaluation of available information shows that two bins better represent the differences in engine operations that influence emissions ( e.g., exhaust temperature, catalyst efficiency) and ensure sufficient data is collected in each bin to allow for an accurate analysis of the data to determine if emissions comply with the standard for each bin. Preamble Section III.C further discusses the final off-cycle standards.

3. Implementation of the Final Program

As discussed in this section, we have evaluated the final standards in terms of technological feasibility, lead time, and stability, and given appropriate consideration to cost, energy, and safety, consistent with the requirements in CAA section 202(a)(3). The final standards are based on data from our CI and SI engine feasibility demonstration programs that was included in the proposal, and further supported by information submitted by commenters and additional data we collected since the time of proposal. Our evaluation of available data shows that the final standards and useful life periods are feasible and will result in the greatest emission reductions achievable for MY 2027, pursuant to CAA section 202(a)(3), giving appropriate consideration to cost, lead time, and other factors. We note that CAA section 202(a)(3) neither requires that EPA consider all the statutory factors equally nor mandates a specific method of cost analysis; rather EPA has discretion in determining the appropriate consideration to give such factors. As discussed in the Chapter 3 of the RIA, the final standards are achievable without increasing the overall fuel consumption and CO₂ emissions of the engine (1) for each of the duty cycles (SET, FTP, and LLC), and (2) for the fuel mapping test procedures defined in 40 CFR 1036.535 and 1036.540. Finally, the final standards will have no negative impact on safety, based on the existing use of these technologies in light-duty and heavy-duty engines on the road today (see section 3 of the Response to Comments document for additional discussion on our assessment that the final standards will have no negative impact on safety). This includes the safety of closed crankcase systems, which we received comment on. As discussed in Section 3 of the RTC, one commenter stated that requiring closed crankcases could increase the chance of engine run away caused by combustion of engine oil that could enter the intake from the closed-crankcase system. We disagree with the commenter since closed crankcase systems are used on engines today with no adverse effect on safety; however, we are providing flexibility for manufactures to meet the final standards regarding crankcase emissions (see preamble Section III.B.2.vi for details).

While we have referenced a technology pathway for complying with our standards (Chapter 3 of the RIA) that is consistent with CAA section 202(a)(3), there are other technology pathways that manufacturers may choose in order to comply with the performance-based final standards. We did not rely on alternative technology pathways in our assessment of the feasibility of the final standards, however, manufacturers may choose from any number of technology pathways to comply with the final standards ( e.g., alternative fuels, including biodiesel, renewable diesel, renewable natural gas, renewable propane, or hydrogen in combination with relevant emissions aftertreatment technologies, and electrification, including plug-in hybrid electric vehicles, battery-electric or fuel cell electric vehicles). As noted in Section I, we are finalizing a program that will begin in MY 2027, which is the earliest year that standards can begin to apply under CAA section 202(a)(3)(C). The final NO_X standards are a single-step program that reflect the greatest emission reductions achievable starting in MY 2027, giving appropriate consideration to costs and other factors. In this final rule, we are focused on achieving the greatest emission reductions achievable in the MY 2027 timeframe, and have applied our judgment in determining the appropriate standards for MY 2027 under this authority for a national program. As the heavy-duty industry continues to transition to zero-emission technologies, EPA could consider additional criteria pollutant standards for model years beyond 2027 in future rules.

In the event that manufacturers start production of some engine families sooner than four years from our final rule, we are finalizing a provision for manufacturers to split the 2027 model year, with an option for manufacturers to comply with the final MY 2027 standards for all engines produced for that engine family in MY 2027. Specifically, we are finalizing as proposed that a MY 2027 engine family that starts production within four years of the final rule could comply with the final MY 2027 standards for all engines produced for that engine family in MY2027, or could split the engine family by production date in MY 2027 such that engines in the family produced prior to four years after the date that the final rule is promulgated would continue to be subject to the existing standards. The split model year provision for MY 2027 provides assurance that all manufacturers, regardless of when they start production of their engine families, will have four years of lead time to the MY 2027 standards under this final rule, while also maximizing emission reductions, which is consistent with our CAA authority. This final rule is promulgated upon the date of signature, upon which date EPA also provided this signed final rule to manufacturers and other stakeholders by email and posted it on EPA's public website.

4. Severability

This final rule includes new and revised requirements for numerous provisions under various aspects of the highway heavy-duty emission control program, including numeric standards, test procedures, regulatory useful life, emission-related warranty, and other requirements. Further, as explained in Sections I and XI, it modernizes and amends numerous other CFR parts for other standard-setting parts for various specific reasons. Therefore, this final rule is a multifaceted rule that addresses many separate things for independent reasons, as detailed in each respective section of this preamble. We intended each portion of this rule to be severable from each other, though we took the approach of including all the parts in one rulemaking rather than promulgating multiple rules to modernize each part of the program.

For example, the following portions of this rulemaking are mutually severable from each other, as numbered: (1) The emission standards in section III; (2) warranty in Section IV.B.1; (3) OBD requirements in Section IV.C; (4) inducements requirements in Section IV.D; (5) ABT program in Section IV.G; (6) the migration and clarification of regulatory text in Section III.A; and (7) other regulatory amendments discussed in Section XI. Each emission standard in Section III is also severable from each other emission standard, including for each duty-cycle, off-cycle, and refueling standard; each pollutant; and each primary intended service class. For example, the NO_X standard for the FTP duty-cycle for Heavy HDE is severable from all other emission standards. Each of the migration and clarification regulatory amendments in Section III.A is also severable from all the other regulatory amendments in that Section, and each of the regulatory amendments in Section XI is also severable from all the other regulatory amendments in that Section. If any of the above portions is set aside by a reviewing court, then we intend the remainder of this action to remain effective, and the remaining portions will be able to function absent any of the identified portions that have been set aside. Moreover, this list is not intended to be exhaustive, and should not be viewed as an intention by EPA to consider other parts of the rule not explicitly listed here as not severable from other parts of the rule.

B. Summary of Compression-Ignition Exhaust Emission Standards and Duty Cycle Test Procedures

EPA is finalizing new NO_X, PM, HC, and CO emission standards for heavy-duty compression-ignition engines that will be certified under 40 CFR part 1036. We are finalizing new emission standards for our existing laboratory test cycles ( i.e., SET and FTP) and finalizing new NO_X , PM, HC and CO emission standards based on a new LLC, as described in this section. The standards for NO_X, PM, and HC are in units of milligrams/horsepower-hour instead of the grams/horsepower-hour used for existing standards because using units of milligrams better reflects the precision of the new standards, rather than adding multiple zeros after the decimal place. Making this change will require updates to how manufacturers report data to the EPA in the certification application, but it does not require changes to the test procedures that define how to determine emission values.

The final duty cycle emission standards in 40 CFR 1037.104 apply starting in model year 2027. This final rule includes new standards over the SET and FTP duty cycles currently used for certification, as well as new standards over a new LLC duty cycle to ensure manufacturers of compression-ignition engines are designing their engines to address emissions in during lower load operation that is not covered by the SET and FTP. The new standards are shown in Table III-1.

This Section III.B describes the duty cycle emission standards and test procedures we are finalizing for compression-ignition engines. We describe compression-ignition engine technology packages that demonstrate the feasibility of achieving these standards in Section III.B.3.ii. The proposed rule provided an extensive discussion of the rationale and information supporting the proposed duty cycle standards (87 FR 17460, March 28, 2022). Chapters 1, 2, and 3 of the RIA include additional information related to the range of technologies to control criteria emissions, background on applicable test procedures, and the full feasibility analysis for compression-ignition engines. See also section 3 of the Response to Comments for a detailed discussion of the comments and how they have informed this final rule.

As part of this rulemaking, we are finalizing an increase in the useful life for each engine class as described in Section IV.A. The emission standards outlined in this section will apply for the longer useful life periods and manufacturers will be responsible for demonstrating that their engines will meet these standards as part of the revisions to durability requirements described in Section IV.F. In Section IV.G, we discuss the updates to the ABT program, including updates to account for the three laboratory cycles (SET, FTP, and LLC) with unique standards.

1. Background on Existing Duty Cycle Test Procedures and Standards

We begin by providing background information on the existing duty cycle test procedures and standards as relevant to this final rule, including the SET and FTP standards and test procedures, powertrain and hybrid powertrain test procedures, test procedure adjustments to account for production and measurement variability, and crankcase emissions. Current criteria pollutant standards must be met by compression-ignition engines over both the SET and FTP duty cycles. The FTP duty cycles, which date back to the 1970s, are composites of a cold-start and a hot-start transient duty cycle designed to represent urban driving. There are separate FTP duty cycles for both SI and CI engines. The cold-start emissions are weighted by one-seventh and the hot-start emissions are weighted by six-sevenths. The SET is a more recent duty cycle for diesel engines that is a continuous cycle with ramped transitions between the thirteen steady-state modes. The SET does not include engine starting and is intended to represent fully warmed-up operating modes not emphasized in the FTP, such as more sustained high speeds and loads.

Emission standards for criteria pollutants are currently set to the same numeric value for SET and FTP test cycles, as shown in Table III-2. Manufacturers of compression-ignition engines have the option under the existing regulations to participate in our ABT program for NO_X and PM, as discussed in the background of Section IV.G. These pollutants are subject to FEL caps under the existing regulations of 0.50 g/hp-hr for NO_X and 0.02 g/hp-hr for PM.

EPA developed powertrain and hybrid powertrain test procedures for the HD GHG Phase 2 Heavy-Duty Greenhouse Gas rulemaking (81 FR 73478, October 25, 2016) with updates in the HD Technical Amendments final rule (86 FR 34321, June 29, 2021). The powertrain and hybrid powertrain tests allow manufacturers to directly measure the effectiveness of the engine, the transmission, the axle and the integration of these components as an input to the Greenhouse gas Emission Model (GEM) for compliance with the greenhouse gas standards. As part of the technical amendments, EPA updated the powertrain test procedure to allow use of test cycles beyond the current GEM vehicle drive cycles, to include the SET and FTP engine-based test cycles and to facilitate hybrid powertrain testing (40 CFR 1036.510, 1036.512, and 1037.550).

These heavy-duty diesel-cycle engine standards are applicable for a useful life period based on the primary intended service class of the engine. For certification, manufacturers must demonstrate that their engines will meet these standards throughout the useful life by performing a durability test and applying a deterioration factor (DF) to their certification value. Additionally, manufacturers must adjust emission rates for engines with exhaust aftertreatment to account for infrequent regeneration events accordingly. To account for variability in these measurements, as well as production variability, manufacturers typically add margin between the DF plus infrequent regeneration adjustment factor (IRAF) adjusted test result and the FEL. A summary of the margins manufacturers have added for MY 2019 and newer engines is summarized in Chapter 3.1.2 of the RIA.

Current regulations restrict the discharge of crankcase emissions directly into the ambient air. Blowby gases from gasoline engine crankcases have been controlled for many years by sealing the crankcase and routing the gases into the intake air through a positive crankcase ventilation (PCV) valve. However, in the past there have been concerns about applying a similar technology for diesel engines. For example, high PM emissions venting into the intake system could foul turbocharger compressors. As a result of this concern, diesel-fueled and other compression-ignition engines equipped with turbochargers (or other equipment) were not required to have sealed crankcases (see 40 CFR 86.007-11(c)). For these engines, manufacturers are allowed to vent the crankcase emissions to ambient air as long as they are measured and added to the exhaust emissions during all emission testing to ensure compliance with the emission standards. Because all new highway heavy-duty diesel engines on the market today are equipped with turbochargers, they are not required to have closed crankcases under the current regulations. Chapter 1.1.4 of the RIA describes EPA's recent test program to evaluate the emissions from open crankcase systems on two modern heavy-duty diesel engines. Results suggest HC and CO emitted from the crankcase can be a notable fraction of overall tailpipe emissions. By closing the crankcase, those emissions would be rerouted to the engine or aftertreatment system to ensure emission control.

2. Test Procedures and Standards

As described in Section III.B.3.ii, we have determined that the technology packages evaluated for this final action can achieve the new duty-cycle standards. We are finalizing a single set of standards that take effect starting in MY 2027, including not only new numerical standards for new and existing duty-cycles but also other new numerical standards for revised off-cycles test procedures and compliance provisions, longer useful life periods, and other requirements.

The final standards were derived to achieve the maximum feasible emissions reductions from heavy-duty diesel engines for MY 2027, considering lead time, stability, cost, energy, and safety. To accomplish this, we evaluated what operation made up the greatest part of the inventory, as discussed in Section VI.B, and what technologies can be used to reduce emissions in these areas. As discussed in Section I, we project that emissions from operation at low power, medium-to-high power, and mileages beyond the current regulatory useful life of the engine will account for the majority of heavy-duty highway emissions in 2045. To achieve reductions in these three areas, we identified options for cycle-specific standards to ensure that the maximum achievable reductions are seen across the operating range of the engine. As described in Section IV, we are finalizing an increase in the regulatory useful life periods for each heavy-duty engine class to ensure these new standards are met for a greater portion of the engine's operational life. Also as described in Section IV, we are separately lengthening the warranty periods for each heavy-duty engine class, which is expected to help to maintain the benefits of the emission controls for a greater portion of the engine's operational life.

To achieve the goal of reducing emissions across the operating range of the engine, we are finalizing standards for three duty cycles (SET, FTP, and LLC). In finalizing these standards, we assessed the performance of the best available aftertreatment systems under various operating conditions. For example, we observed that these systems are more effective at reducing NO_X emissions at the higher exhaust temperatures that occur at high engine power than they are at reducing NO_X emissions at low exhaust temperatures that occur at low engine power. To achieve the maximum NO_X reductions from the engine at maximum power, the aftertreatment system was designed to ensure that the downstream selective catalytic reduction (SCR) catalyst was properly sized, diesel exhaust fluid (DEF) was fully mixed with the exhaust gas ahead of the SCR catalyst and the diesel oxidation catalyst (DOC) was designed to provide a molar ratio of NO to NO₂ of near one. The final standards for the FTP and LLC are 80 to 90 percent, or more, lower as compared to current standards, which will contribute to reductions in emissions under low power operation and under cold-start conditions. The standards are achievable by utilizing cylinder deactivation (CDA), dual-SCR aftertreatment configuration, closed crankcase, and heated diesel exhaust fluid (DEF) dosing. To reduce emissions under medium to high power, the final standards for the SET are greater than 80 percent lower as compared to current standards. The SET standards are achievable by utilizing improvements to the SCR formulation, SCR catalyst sizing, and improved mixing of DEF with the exhaust. Further information about these technologies can be found in Chapters 1 and 3 of the RIA.

The final PM standards are set at a level that requires heavy-duty engines to maintain the emissions performance of current diesel engines. The final standards for HC and CO are set at levels that are equivalent to the maximum emissions reductions achievable by spark-ignition engines over the FTP, with the general intent of making the final standards fuel neutral. Compared to current standards, the final standards for the SET and FTP duty cycles are 50 percent lower for PM, 57 percent lower for HC, and 61 percent lower for CO. Each of these standards are discussed in more detail in the following sections.

For Heavy HDE, we are finalizing NO_X standards to a useful life of 650,000 miles with a durability demonstration out to 750,000 miles, as discussed later in Section III.B.2. We recognize the greater demonstration burden of a useful life of 650,000 miles for these engines, and after careful analysis are updating our DF demonstration provisions to include two options for an accelerated aging demonstration. However, we also are taking into account that extending a durability demonstration, given that it is conducted in the controlled laboratory environment, will better ensure the final standards will be met throughout the longer final regulatory useful life mileage of 650,000 miles when these engines are operating in the real-world where conditions are more variable. We are thus requiring the durability demonstration to show that the emission control system hardware is designed to comply with the NO_X standards out to 750,000 miles. As discussed further in Section III.B, the aging demonstration out to 750,000 miles in a controlled laboratory environment ensures that manufacturers are designing Heavy HDE to meet the final standards out to the regulatory useful life of 650,000 miles once the engine is in the real-world, while reducing the risk of greater real world uncertainties impacting emissions at the longest useful life mileages in the proposed rule. This approach both sets standards that result in the maximum emission reductions achievable in MY 2027 while addressing the technical issues raised by manufacturers regarding various uncertainties in variability and the degradation of system performance over time due to contamination of the aftertreatment from, for example, fuel contamination (the latter of which is out of the manufacturer's control).

As discussed in Section III.B.3, we have assessed the feasibility of the standards for compression-ignition engines by testing a Heavy HDE equipped with cylinder CDA technology, closed crankcase, and dual-SCR aftertreatment configuration with heated DEF dosing. The demonstration work consisted of two phases. The first phase of the demonstration was led by CARB and is referred to as CARB Stage 3. In this demonstration the aftertreatment was chemically- and hydrothermally-aged to the equivalent of 435,000 miles. During this aging the emissions performance of the engine was assessed after the aftertreatment was degreened , at the equivalent of 145,000 miles, 290,000 miles and 435,000 miles. The second phase of the demonstration was led by EPA and is referred to as the EPA Stage 3 engine. In this phase, improvements were made to the aftertreatment by replacing the zone-coated catalyzed soot filter with a separate DOC and diesel particulate filter (DPF) that were chemically- and hydrothermally-aged to the equivalent of 800,000 miles and improving the mixing of the DEF with exhaust prior to the downstream SCR catalyst. The EPA Stage 3 engine was tested at an age equivalent to 435,000, 600,000, and 800,000 miles. We also tested two additional aftertreatment systems, referred to as “System A” and “System B,” which are each also a dual-SCR aftertreatment configuration with heated DEF dosing. However, they each have unique catalyst washcoat formulation and the “System A” aftertreatment has greater SCR catalyst volume. The details of these aftertreatment systems, along with the test results, can be found in RIA Chapter 3.

i. FTP

We are finalizing new emission standards for testing over the FTP duty cycle, as shown in Table III-3. These brake-specific FTP standards apply across the Heavy HDE, Medium HDE, and Light HDE primary intended service classes over the useful life periods shown in Table III-4. The numeric levels of the NO_X FTP standards at the time of certification are consistent with the most stringent proposed for MY 2027; as summarized in Section III.A.2 and detailed in this Section III.B we are also finalizing an interim, in-use compliance allowance for Medium and Heavy HDEs. The numeric level of the PM and CO FTP standards are the same as proposed, and the numeric level of the HC FTP standard is consistent with the proposed Option 1 standard starting in MY 2027. These standards have been shown to be feasible for compression-ignition engines based on testing of the CARB Stage 3 and EPA Stage 3 engine with a chemically- and hydrothermally-aged aftertreatment system. The EPA Stage 3 engine, was aged to and tested at the equivalent of 800,000 miles. EPA's System A demonstration engine, was aged to and tested at the equivalent of 650,000 miles. The System B demonstration engine was not aged and was only tested after it was degreened. A summary of the data used for EPA's feasibility analysis can be found in Section III.B.3. See Section III.B.3 for details on how we addressed compliance margin when setting the standards, including discussion of the interim in-use testing allowance for Medium and Heavy HDE for determining the interim in-use testing standards for these primary intended service classes.

As further discussed in Section III.B.3, taking into account measurement variability of the PM measurement test procedure and the low numeric level of the new PM standards, we believe PM emissions from current diesel engines are at the lowest feasible level for standards starting in MY 2027. As summarized in Section III.B.3.ii.b, manufacturers are submitting certification data to the agency for current production engines well below the existing PM standards over the FTP duty cycle. Setting the new PM FTP standards lower than the existing FTP PM standards, at 5 mg/hp-hr (0.005 g/hp-hr), ensures that future engines will maintain the low level of PM emissions of the current engines and not increase PM emissions. We received comment stating that a 5 mg/hp-hr standard did not provide enough margin for some engine designs and that a 7.5 mg/hp-hr would be a more appropriate standard to maintain current PM emissions levels while providing enough margin to account for the measurement variability of the PM measurement test procedure. The reason submitted in comment to justify the 7.5 mg/hp-hr standard was that data from the Stage 3 testing at Southwest Research Institute (SwRI) shows that in some conditions PM values exceed the 5 mg/hp-hr emission standard. EPA took a further look at this data and determined that the higher PM emission data points occur immediately following DPF ash cleaning, and that the PM level returns to a level well below the 5 mg/hp-hr standards shortly after return to service once a soot cake layer reestablishes itself in the DPF. EPA concluded from this assessment that these very short-term elevations in PM that occur after required maintenance of the DPF should not be the basis for the stringency of the PM standards and that the standards are feasible.

As noted earlier in this section, we are finalizing HC and CO FTP standards based on the feasibility demonstration for SI engines. As summarized in Section III.B.3.ii.b, manufacturers are submitting data to the agency that show emissions performance for current production CI engines that are well below the current standards. Keeping FTP standards at the same value for all fuels is consistent with the agency's approach to previous criteria pollutant standards. See Section III.D for more information on how the numeric values of the HC and CO standards were determined.

In the NPRM, we did not propose any changes to the weighting factors for the FTP cycle for heavy-duty engines. The current FTP weighting of cold-start and hot-start emissions was promulgated in 1980 (45 FR 4136, January 21, 1980). It reflects the overall ratio of cold and hot operation for heavy-duty engines generally and does not distinguish by engine size or intended use. We received comment to change the weighting factors to reduce the effect of the cold start portion of the FTP on the composite FTP emission results or to add 300 seconds of idle before the first acceleration in the cold start FTP to reduce the emissions impact of the cold start on the first acceleration. Duty-cycles are an approximation of the expected real-world operation of the engine and no duty cycle captures all aspects of the real-world operation. Changing the cold/hot weighting factors would not fully capture all aspects of what really occurs in-use, and there is precedent in experience and historical approach with the current 1/7 cold and 6/7 hot weighting factors. Adding 300 seconds of idle to the beginning of the FTP would simply reduce the stringency of the standard by reducing the impact of cold start emissions, as the 300 seconds of idle would allow the aftertreatment to light off prior to the first major acceleration in the FTP. Although the case can be made that many vehicles idle for some amount of time after start up, any attempt to add idle time before the first acceleration is simply an approximation and this “one size fits all” approach doesn't afford an improvement over the current FTP duty-cycle, nor does it allow determination of cold start emissions where the vehicle is underway shortly after start up. After considering these comments we are also not including any changes to the weighting factors for the FTP duty-cycle in this final rule.

For Heavy HDE, we are finalizing test procedures for the determination of deterioration factors in 40 CFR 1036.245 that require these engines to be aged to an equivalent of 750,000 miles, which is 15 percent longer than the regulatory useful life of those engines. As explained earlier in this section, we are finalizing this requirement for Heavy HDE to ensure the final NO_X standard will be met through the lengthy regulatory useful life of 650,000 miles. See preamble Section IV.A for details on how we set the regulatory useful life for Heavy HDE.

ii. SET

We are finalizing new emissions standards for testing over the SET duty-cycle as shown in Table III-3. These brake-specific SET standards apply across the Heavy HDE, Medium HDE, and Light HDE primary intended service classes, as well as the SI HDE primary intended service class as discussed in Section III.D, over the same useful life periods shown in Table III-4. The numeric levels of the NO_X SET standards at the time of certification are consistent with the most stringent standard proposed for MY 2027. The numeric level of the CO SET standard is consistent with the most stringent standard proposed for MY 2027 for all CI engine classes. The numeric level of the PM SET standard is the same as proposed, and the numeric level of the HC SET standard is consistent with the proposed Option 1 standard starting in MY 2027. Consistent with our current standards, we are finalizing the same numeric values for the standards over the SET and FTP duty cycles for the CI engine classes. As with the FTP cycle, the standards have been shown to be feasible for compression-ignition engines based on testing of the CARB Stage 3 and EPA Stage 3 engines with a chemically- and hydrothermally-aged aftertreatment system. The EPA Stage 3 engine was aged to and tested at the equivalent of 800,000 miles. EPA's Team A demonstration engine was aged to and tested at the equivalent of 650,000 miles. See Section III.B.3 for details on how we addressed compliance margin when setting the standards, including discussion of the interim in-use testing allowance for Medium and Heavy HDEs for determining the interim in-use testing standards for these primary intended service classes. A summary of the data used for EPA's feasibility analysis can be found in Section III.B.3.

As with the PM standards for the FTP (see Section III.B.2.i), and as further discussed in Section III.B.3, taking into account measurement variability of the PM measurement test procedure and the low numeric level of the new PM standards, we believe PM emissions from current diesel engines are at the lowest feasible level for standards starting in MY 2027. Thus, the PM standard for the SET duty-cycle is intended to ensure that there is not an increase in PM emissions from future engines. We are finalizing new PM SET standards of 5 mg/hp-hr for the same reasons outlined for the FTP in Section III.B.2.i. Also similar to the FTP (see Section III.B.2.i), we are finalizing HC and CO SET standards based on the feasibility demonstration for SI engines (see Section III.D).

We have also observed an industry trend toward engine down-speeding—that is, designing engines to do more of their work at lower engine speeds where frictional losses are lower. To better reflect this trend in our duty cycle testing, in the HD GHG Phase 2 final rule we promulgated new SET weighting factors for measuring CO₂ emissions (81 FR 73550, October 25, 2016). Since we believe these new weighting factors better reflect in-use operation of current and future heavy-duty engines, we are finalizing application of these new weighting factors to criteria pollutant measurement, as show in Table III-5, for NO_X and other criteria pollutants as well. To assess the impact of the new test cycle on criteria pollutant emissions, we analyzed data from the EPA Stage 3 engine that was tested on both versions of the SET. The data summarized in Section III.B.3.ii.a show that the NO_X emissions from the EPA Stage 3 engine at an equivalent of 435,000 miles are slightly lower using the SET weighting factors in 40 CFR 1036.510 versus the current SET procedure in 40 CFR 86.1362. The lower emissions using the SET cycle weighting factors in 40 CFR 1036.510 are reflected in the stringency of the final SET standards.

iii. LLC

EPA is finalizing the addition of new standards for testing over the new low-load duty-cycle, that will require CI engine manufacturers to demonstrate that the emission control system maintains functionality during low-load operation where the catalyst temperatures have historically been found to be below the catalyst's operational temperature (see Chapter 2.2.2 of the RIA). We believe the addition of this LLC will complement the expanded operational coverage of our new off-cycle testing requirements (see Section III.C).

During “Stage 2” of the CARB Low NO_X Demonstration program, SwRI and NREL developed several candidate cycles with average power and duration characteristics intended to test current diesel engine emission controls under three low-load operating conditions: Transition from high- to low-load, sustained low-load, and transition from low- to high-load. In September 2019, CARB selected the 92-minute “LLC Candidate #7” as the low load cycle they adopted for their Low NO_X Demonstration program and subsequent Omnibus regulation.

We are adopting CARB's Omnibus LLC as a new duty-cycle, the LLC. This cycle is described in Chapter 2 of the RIA for this rulemaking and the test procedures are specified in 40 CFR 1036.514. The LLC includes applying the accessory loads defined in the HD GHG Phase 2 rule, that were based on data submitted to EPA as part of the development of the HD GHG Phase 2. These accessory loads are 1.5, 2.5 and 3.5 kW for Light HDE, Medium HDE, and Heavy HDE engines, respectively. As detailed further in section 3 of the Response to Comments, we received comments that EPA should revise the accessory loads. One commenter provided specific recommendations for engines installed in tractors but in all cases commenters didn't provide data to support their comments; after consideration of these comments and further consideration of the basis of the proposal, we are finalizing the accessory loads for the LLC as proposed. To allow vehicle level technologies to be recognized on this cycle, we are including a powertrain test procedure option for the LLC. More information on the powertrain test procedure can be found in Section III.B.2.v. IRAF determination for the LLC follows the test procedures defined in 40 CFR 1036.580, which are the same test procedures used for the SET and FTP. The IRAF test procedures that apply to the SET and FTP in 40 CFR 1065.680 are appropriate for the LLC as the procedures in 40 CFR 1065.680 were developed to work with any engine-based duty-cycle. We are finalizing as proposed that, while the IRAF procedures in 40 CFR 1036.580 and 1065.680 require that manufacturers determine an IRAF for the SET, FTP, and LLC duty cycles, manufacturers may omit the adjustment factor for a given duty cycle if they determine that infrequent regeneration does not occur over the types of engine operation contained in the duty cycle as described in 40 CFR 1036.580(c).

The final emission standards for the LLC are presented in Table III-6, over the useful life periods shown in Table III-4. The numeric levels of the NO_X LLC standards at the time of certification are the most stringent proposed for any model year. The numeric level of the PM and CO LLC standards are the same as proposed, and the numeric level of the HC LLC standard is consistent with the proposed Option 1 standard starting in MY 2027. As with the FTP cycle, these standards have been shown to be feasible for compression-ignition engines based on testing of the EPA Stage 3 demonstration engine with chemically- and hydrothermally-aged aftertreatment system, and for the LLC the data shows that the standards are feasible for all engine service classes with available margins between the data and the standards. The summary of this data along with how we addressed compliance margin can be found in Section III.B.3, including discussion of the interim in-use compliance allowance for Medium and Heavy HDEs for determining the interim in-use standards for these primary intended service classes.

We are finalizing an LLC PM standard of 5 mg/hp-hr for the same reasons outlined for the FTP in Section III.B.2.i. We are finalizing HC and CO standards based on data from the CARB and EPA Stage 3 engine discussed in Section III.B.3. We are finalizing the same numeric standard for CO on the LLC as we have for the SET and FTP cycles because the demonstration data from the EPA Stage 3 engine shows that CO emissions on the LLC are similar to CO emissions from the SET and FTP. We are finalizing HC standards that are different than the standards of the SET and FTP cycles, to reflect our assessment of the performance of the EPA Stage 3 engine on the LLC. The data discussed in Section III.B.3 of this preamble shows that the PM, HC, and CO standards are feasible for both current and future new engines.

iv. Idle

CARB currently has an optional idle test procedure and accompanying standard of 30 g/hr of NO_X for diesel engines to be “Clean Idle Certified.”. In the CARB Omnibus rule, the CARB lowered the optional NO_X standard to 10 g/hr for MY 2024 to MY 2026 engines and 5 g/hr for MY 2027 and beyond. In the NPRM, we proposed optional NO_X idle standards with a corresponding idle test procedure, with potentially different numeric levels of the NO_X idle standards for MY 2023, MY 2024 to MY 2026 engines, and for MY 2027 and beyond, that would allow compression ignition engine manufacturers to voluntarily choose to certify ( i.e., it would be optional for a manufacturer to include the idle standard in an EPA certification but once included the idle standard would become mandatory and full compliance would be required). We proposed to require that the brake-specific HC, CO, and PM emissions during the Clean Idle test may not exceed measured emission rates from the idle mode in the SET or the idle segments of the FTP, in addition to meeting the applicable idle NO_X standard. We requested comment on whether EPA should make the idle standards mandatory instead of voluntary for MY 2027 and beyond, as well as whether EPA should set clean idle standards for HC, CO, and PM emissions (in g/hr) rather than capping the idle emissions for those pollutants based on the measured emission levels during the idle mode in the SET or the idle segments of the FTP. We also requested comment on the need for EPA to define a label that would be put on the vehicles that are certified to the optional idle standard.

We received comments on the EPA's proposal to adopt California's Clean Idle NO_X standard as a voluntary emission standard for Federal certification. All commenters provided general support for EPA's proposal to set idle standards for heavy duty engines, with some qualifications. Some commentors supported making idle standards mandatory, while others commented that the idle standards should be optional. With regard to the level of the idle standard, there was support from many commenters that the standards should be set at the Proposed Option 1 levels or lower, while several manufactures stated that 10 g/hr for certification and 15 g/hr in-use would be the lowest feasible standards for NO_X . One manufacturer commented that EPA must set standards that do not increase CO₂ emissions. EPA has considered these comments, along with the available data including the data from the EPA Stage 3 engine, and we are finalizing optional idle standards in 40 CFR 1036.104(b) and a new idle test procedure in 40 CFR 1036.525. The standards are based on CARB's test procedure with revisions to not require the measurement of PM, HC and CO, to allow compression-ignition engine manufacturers to voluntarily certify to an idle NO_X standard of 30.0 g/hr for MY 2024 to MY 2026, which is consistent with proposed Option 1 for MY 2023. For MY 2027 and beyond, the final NO_X idle standard is 10.0 g/hr, which is the same as proposed Option 2 for those MYs. Manufacturers certifying to the optional idle standard must comply with the standard and related requirements as if they were mandatory.

We received comments stating that the proposed PM, HC, and CO standards are unworkable since the standards are set at the level the engine emits at during idle over the engine SET and FTP duty cycles and that variability in the emissions between the different tests could cause the engine to fail the idle PM, HC, and CO standards. EPA recognized this issue in the proposal and requested comment on if EPA should instead set PM, HC, and CO standards that are fixed and not based on the emissions from the engine during the SET and FTP. EPA has considered these comments and we are not finalizing the proposed requirement to measure brake-specific HC, CO, and PM emissions during the Clean Idle test for comparison to emission rates from the idle modes in the SET or the idle segments of the FTP. The measurement of these additional pollutants would create unnecessary test burden for the manufacturers at this time, especially with respect to measuring PM during idle segments of the SET or FTP as it would require running duplicate tests or adding a PM sampler. Further, setting the PM, HC and CO standards right at the idle emissions level of the engine on the SET and FTP could cause false failures due to test-to-test variability from either the SET or FTP, or the Clean Idle test itself. Idle operation is included as part of off-cycle testing and the SET, FTP, and LLC duty cycles; standards for off-cycle and duty-cycle testing ensure that emissions of HC, CO, and PM are well controlled as aftertreatment temperatures are not as critical to controlling these pollutants over extended idle periods as they are for NO_X. We are therefore not requiring the measurement of these other pollutants to meet EPA voluntary clean idle standards.

We are finalizing a provision in new 40 CFR 1036.136 requiring engine manufacturers that certify to the Federal Clean Idle NO_X standard to create stickers to identify their engines as meeting the Federal Clean Idle NO_X standard. The regulatory provisions require that the stickers meet the same basic requirements that apply for stickers showing that engines meet CARB's Clean Idle NO_X standard. For example, stickers must be durable and readable throughout each vehicle's operating life, and the preferred placement for Clean Idle stickers is on the driver's side of the hood. Engine manufacturers must provide exactly the right number of these stickers to vehicle manufacturers so they can apply the stickers to vehicles with the engines that the engine manufacturer has certified to meet the Federal Clean Idle NO_X standard. If engine manufacturers install engines in their own vehicles, they must apply the stickers themselves to the appropriate vehicles. Engine manufacturers must keep the following records for at least five years: (1) Written documentation of the vehicle manufacturer's request for a certain number of stickers, and (2) tracking information for stickers the engine manufacturer sends and the date they sent them. 40 CFR 1036.136 also clarifies that the provisions in 40 CFR 1068.101 apply for the Clean Idle sticker in the same way that those provisions apply for emission control information labels. For example, manufacturing, selling, and applying false labels are all prohibited actions subject to civil penalties.

v. Powertrain

EPA recently finalized a separate rulemaking that included an option for manufacturers to certify a hybrid powertrain to the SET and FTP greenhouse gas engine standards by using a powertrain test procedure (86 FR 34321, June 29, 2021). In this rulemaking, we are similarly finalizing as proposed that manufacturers may certify hybrid powertrains to criteria pollutant emissions standards by using the powertrain test procedure. In this section we describe how manufacturers would apply the powertrain test procedure to certify hybrid powertrains.

a. Development of Powertrain Test Procedures

Powertrain testing allows manufacturers to demonstrate emission benefits that cannot be captured by testing an engine alone on a dynamometer. For hybrid engines and powertrains, powertrain testing captures when the engine operates less or at lower power levels due to the use of the hybrid powertrain function. However, powertrain testing requires the translation of an engine test procedure to a powertrain test procedure. Chapter 2 of the RIA describes how we translated the SET, FTP, and LLC engine test cycles to the powertrain test cycles. The two primary goals of this process were to make sure that the powertrain version of each test cycle was equivalent to each respective engine test cycle in terms of positive power demand versus time and that the powertrain test cycle had appropriate levels of negative power demand. To achieve this goal, over 40 engine torque curves were used to create the powertrain test cycles.

b. Testing Hybrid Engines and Hybrid Powertrains

As noted in the introduction of this Section III, we are finalizing clarifications in 40 CFR 1036.101 that manufacturers may optionally test the hybrid engine and hybrid powertrain to demonstrate compliance. We are finalizing as proposed with one clarification that the powertrain test procedures specified in 40 CFR 1036.510 and 1036.512, which were previously developed for demonstrating compliance with GHG emission standards on the SET and FTP test cycles, are applicable for demonstrating compliance with criteria pollutant standards on the SET and FTP test cycles. The clarification in 40 CFR 1036.510 provides direction that the idle points in the SET should be run as neutral or parked idle. In addition, for GHG emission standards we are finalizing updates to 40 CFR 1036.510 and 1036.512 to further clarify how to carry out the test procedure for plug-in hybrids. We have done additional work for this rulemaking to translate the LLC to a powertrain test procedure, and we are finalizing that manufacturers can similarly certify hybrid engines and hybrid powertrains to criteria pollutant emission standards on the LLC using the test procedures defined in 40 CFR 1036.514.

We are allowing manufacturers to use the powertrain test procedures to certify hybrid engine and powertrain configurations to all MY 2023 and later criteria pollutant engine standards. Manufacturers can choose to use either the SET duty-cycle in 40 CFR 86.1362 or the SET in 40 CFR 1036.510 in model years prior to 2027, and may use only the SET in 40 CFR 1036.510 for model year 2027 and beyond.

We are allowing the use of these procedures starting in MY 2023 for plug-in hybrids and, consistent with the requirements for light-duty plug-in hybrids, we are finalizing that the applicable criteria pollutant standards must be met under the worst-case conditions, which is achieved by testing and evaluating emission under both charge-depleting and charge-sustaining operation. This is to ensure that under all drive cycles the powertrain meets the criteria pollutant standards and is not based on an assumed amount of zero emissions range. We received comment stating that the charge-depleting and charge-sustaining operation should be weighted together for criteria pollutants as well as GHG pollutants, but consistent with the light-duty test procedure we want to ensure that criteria pollutant emissions are controlled under all conditions, which would include under conditions where the vehicle is not charged and is only operated in charge sustaining-operation.

We are finalizing changes to the test procedures defined in 40 CFR 1036.510 and 1036.512 to clarify how to weight together the charge-depleting and charge-sustaining greenhouse gas emissions for determining the greenhouse gas emissions of plug-in hybrids for the SET and FTP duty cycles. This weighting is done using an application specific utility factor curve that is approved by EPA. We are also finalizing a provision to not apply the cold and hot weighting factors for the determination of the FTP composite emission result for greenhouse gas pollutants because the charge-depleting and sustaining test procedures finalized in 40 CFR 1036.512 include both cold and hot start emissions by running repeat FTP cycles back-to-back. By running back-to-back FTPs, the finalized test procedure captures both cold and hot emissions and their relative contribution to daily greenhouse gas emissions per unit work, removing the need for weighting the cold and hot emissions.

We are finalizing the application of the powertrain test procedure only for hybrid powertrains, to avoid having two different testing pathways (engine only and powertrain) for non-hybrid engines for the same standards. That said, we recognize there may be other technologies where the emissions performance is not reflected on the engine test procedures, so in such cases manufacturers may seek approval from EPA to use the powertrain test procedure for non-hybrid engines and powertrains consistent with 40 CFR 1065.10(c)(1).

Finally, for all pollutants, we requested comment on if we should remove 40 CFR 1037.551 or limit the use of it to only selective enforcement audits (SEAs). 40 CFR 1037.551 was added as part of the HD GHG Phase 2 rulemaking to provide flexibility for an SEA or a confirmatory test, by allowing just the engine of the powertrain to be tested. Allowing just the engine to be tested over the engine speed and torque cycle that was recorded during the powertrain test enables the testing to be conducted in more widely available engine dynamometer test cells, but this flexibility could increase the variability of the test results. We didn't receive any comments on this topic and, for the reason just stated, we are limiting the use of 40 CFR 1037.551 to SEA testing.

vi. Crankcase Emissions

During combustion, gases can leak past the piston rings sealing the cylinder and into the crankcase. These gases are called blowby gases and generally include unburned fuel and other combustion products. Blowby gases that escape from the crankcase are considered crankcase emissions (see 40 CFR 86.402-78). Current regulations restrict the discharge of crankcase emissions directly into the ambient air. Blowby gases from gasoline engine crankcases have been controlled for many years by sealing the crankcase and routing the gases into the intake air through a PCV valve. However, in the past there have been concerns about applying a similar technology for diesel engines. For example, high PM emissions venting into the intake system could foul turbocharger compressors. As a result of this concern, diesel-fueled and other compression-ignition engines equipped with turbochargers (or other equipment) were not required to have sealed crankcases (see 40 CFR 86.007-11(c)). For these engines, manufacturers were allowed to vent the crankcase emissions to ambient air as long as they are measured and added to the exhaust emissions during all emission testing to ensure compliance with the emission standards.

Because all new highway heavy-duty diesel engines on the market today are equipped with turbochargers, they are not required to have closed crankcases under the current regulations. We estimate approximately one-third of current highway heavy-duty diesel engines have closed crankcases, indicating that some heavy-duty engine manufacturers have developed systems for controlling crankcase emissions that do not negatively impact the turbocharger. EPA proposed provisions in 40 CFR 1036.115(a) to require a closed crankcase ventilation system for all highway compression-ignition engines to prevent crankcase emissions from being emitted directly to the atmosphere starting for MY 2027 engines. Comments were received regarding concerns closing the crankcase that included coking, degraded performance and turbo efficiencies leading to increased CO₂ emissions, secondary damage to components, and increased engine-out PM (see section 3 of the Response to Comments document for further details). After considering these comments, we are finalizing a requirement for manufacturers to use one of two options for controlling crankcase emissions, either: (1) As proposed, closing the crankcase, or (2) an updated version of the current requirements for an open crankcase that includes additional requirements for measuring and accounting for crankcase emissions. We believe that either approach is appropriate, so long as the total emissions are accounted for during certification and in-use testing through useful life (including full accounting for crankcase emission deterioration).

a. Closed Crankcase Option

As EPA explained at proposal, the environmental advantages to closing the crankcase are twofold. While the exception in the current regulations for certain compression-ignition engines requires manufacturers to quantify their engines' crankcase emissions during certification, they report non-methane hydrocarbons in lieu of total hydrocarbons. As a result, methane emissions from the crankcase are not quantified. Methane emissions from diesel-fueled engines are generally low; however, they are a concern for compression-ignition-certified natural gas-fueled heavy-duty engines because the blowby gases from these engines have a higher potential to include significant methane emissions. We note that in the HD GHG Phase 2 rule we set methane standards which required natural gas engines to close the crankcase in order to comply with the methane standard. EPA proposed to require that all natural gas-fueled engines have closed crankcases in the HD GHG Phase 2 rulemaking, but opted to wait to finalize any updates to regulations in a future rulemaking, where we could then propose to apply these requirements to natural gas-fueled engines and to the diesel fueled engines that many of the natural gas-fueled engines are based off of (81 FR 73571, October 25, 2016).

In addition to our concern of unquantified methane emissions, we believe another benefit to closed crankcases would be reduced engine wear due to improved engine component durability. We know that the performance of piston seals reduces as the engine ages, which would allow more blowby gases and could increase crankcase emissions. While crankcase emissions are currently included in the durability tests that estimate an engine's deterioration at useful life, those tests were not designed to capture the deterioration of the crankcase. These unquantified age impacts continue throughout the operational life of the engine. Closing crankcases could be a means to ensure those emissions are addressed long-term to the same extent as other exhaust emissions.

After considering all of the manufacturer concerns, we still believe, noting that one-third of current highway heavy-duty diesel engines have closed crankcases, that improvements in the design of engine hardware would allow manufacturers to close the crankcase, with the potential for increased maintenance intervals on some components. For these reasons, EPA is finalizing provisions in 40 CFR 1036.115(a) to require a closed crankcase ventilation system as one of two options for all highway compression-ignition engines to control crankcase emissions for MY 2027 and later engines.

b. Open Crankcase Option

Given consideration of the concerns from commenters regarding engine hardware durability associated with closing the crankcase, we have decided to finalize an option that allows the crankcase to remain open. This option requires manufacturers of compression ignition engines that choose to leave the crankcase open to account for any increase in the contribution of crankcase emissions (due to reduction in performance of piston seals, etc.) to the total emissions from the engine throughout the engine's useful life. Manufacturers that choose to perform engine dynamometer-based testing out to useful life will provide a deterioration factor that includes deteriorated crankcase emissions because the engine components will be aged out to the engine's useful life. Manufacturers that choose to use the accelerated aging option in 40 CFR 1036.245(b), where the majority of the emission control system aging is done, must use good engineering judgment to determine the impact of engine deterioration on crankcase emissions and adjust the tailpipe emissions at useful life to reflect this deterioration. For example, manufacturers may determine deteriorated crankcase emissions from the assessment of field-aged engines.

Manufacturers who choose this option must also account for crankcase criteria pollutant emissions during any manufacturer run in-use testing to determine the overall compliance of the engine as described in 40 CFR 1036.415(d)(2). The crankcase emissions must be measured separately from the tailpipe emissions or be routed into the exhaust system, downstream from the last catalyst in the aftertreatment system, to ensure that there is proper mixing of the two streams prior to the sample point. In lieu of these two options, manufacturers may use the contribution of crankcase emissions over the FTP duty-cycle at useful life from the deterioration factor determination testing in 40 CFR 1036.245, as described in 40 CFR 1036.115(a) and add them to the binned emission results determined in 40 CFR 1036.530.

Chapter 1.1.4 of the RIA describes EPA's recent test program to evaluate the emissions from open crankcase systems on two modern heavy-duty diesel engines. Results suggest HC and CO emitted from the crankcase can be a notable fraction of overall tailpipe emissions. By closing the crankcase, those emissions would be rerouted to the engine or aftertreatment system to ensure control of the crankcase emissions. If a manufacturer chooses the option to keep the crankcase open, overall emission control will still be achieved, but the manufacturer will have to design and optimize the emission control system for lower tailpipe emissions to offset the emissions from the crankcase as the total emissions are accounted for both in-use and at useful life.

3. Feasibility of the Diesel (Compression-Ignition) Engine Standards

i. Summary of Technologies Considered

Our finalized standards for compression-ignition engines are based on the performance of technology packages described in Chapters 1 and 3 of the RIA for this rulemaking. Specifically, we are evaluating the performance of next-generation catalyst formulations in a dual SCR catalyst configuration with a smaller SCR catalyst as the first substrate in the aftertreatment system for improved low-temperature performance, and a larger SCR catalyst downstream of the diesel particulate filter to improve NO_X conversion efficiency during high power operation and to allow for passive regeneration of the particulate filter. Additionally, the technology package includes CDA that reduces the number of active cylinders, resulting in increased exhaust temperatures for improved catalyst performance under light-load conditions and can be used to reduce fuel consumption and CO₂ emissions. The technology package also includes the use of a heated DEF injector for the upfront SCR catalyst; the heated DEF injector allows DEF injection at temperatures as low as approximately 140°C. The heated DEF injector also improves the mixing of DEF and exhaust gas within a shorter distance than with unheated DEF injectors, which enables the aftertreatment system to be packaged in a smaller space. Finally, the technology package includes hardware needed to close the crankcase of diesel engines.

ii. Summary of Feasibility Analysis

a. Projected Technology Package Effectiveness and Cost

Based upon data from EPA's and CARB's Stage 3 Heavy-duty Low NO_X Research Programs (see Chapter 3.1.1.1 and Chapter 3.1.3.1 of the RIA), an 80 percent reduction in the Heavy HDE NO_X standard as compared to the current NO_X standard is technologically feasible when using CDA or other valvetrain-related air control strategies in combination with dual SCR systems, and closed crankcase. As noted in the proposal, EPA continued to evaluate aftertreatment system durability via accelerated aging of advanced emissions control systems as part of EPA's diesel engine demonstration program that is described in Chapter 3 of the RIA. In assessing the technical feasibility of each of our final standards, we have taken into consideration the emissions of the EPA Stage 3 engine and other available data, the additional emissions from infrequent regenerations, the final longer useful life, test procedure variability, emissions performance of other child engines in an engine family, production and engine variability, fuel and DEF quality, sulfur, soot and ash levels on the aftertreatment, aftertreatment aging due to severe-service operation, aftertreatment packaging and lead time for manufacturers.

Manufacturers are required to design engines that meet the duty cycle and off-cycle standards throughout the engines' useful life. In recognition that emissions performance will degrade over time, manufacturers generally design their engines to perform significantly better than the standards when first sold to ensure that the emissions are below the standard throughout useful life even as the emissions controls deteriorate. As discussed in this section and in Chapter 3 of the RIA and shown in Table III-12 and Table III-13, some manufactures have submitted certification data with zero emissions (with rounding), which results in a margin at 100 percent of the FEL, while other manufacturers have margin that is less than 25 percent of the FEL.

To assess the feasibility of the final MY 2027 standards for Light, Medium, and Heavy HDE at the corresponding final useful lives, EPA took into consideration and evaluated the data from the EPA Stage 3 engine as well as other available data and comments received on the proposed standards. See section 3 of the Response to Comment document for further information on the comments received and EPA's detailed response.

As discussed in Section III.B.2, the EPA Stage 3 engine includes improvements beyond the CARB Stage 3 engine, namely replacing the zone-coated catalyzed soot filter with a separate DOC and DPF and improving the mixing of the DEF with exhaust for the downstream SCR catalyst. These improvements lowered the emissions on the SET, FTP, and LLC below what was measured with the CARB Stage 3 engine. The emissions for the EPA Stage 3 engine on the SET, FTP, and LLC aged to an equivalent of 435,000, 600,000 and 800,000 miles are shown in Table III-7, Table III-8, and Table III-9. To account for the IRAF for both particulate matter and sulfur on the aftertreatment system, we assessed and determined it was appropriate to rely on an analysis by SwRI that is summarized in Chapter 3 of the RIA. In this analysis SwRI determined that IRAF NO_X emissions were at 2 mg/hp-hr for both the SET and FTP cycles and 5 mg/hp-hr for the LLC. To account for the crankcase emissions, we assessed and determined it was appropriate to rely on an analysis by SwRI that is summarized in Chapter 3 of the RIA. In this analysis, SwRI determined that the NO_X emissions from the crankcase were at 6 mg/hp-hr for the LLC, FTP, and SET cycles.

To determine whether or how to account for the effects of test procedure variability, emissions performance of other ratings in an engine family, production and engine variability, fuel and DEF quality, sulfur, soot and ash levels on the aftertreatment, aftertreatment aging due to severe-service operation, and aftertreatment packaging—and given the low level of the standards under consideration—EPA further assessed two potential approaches after taking into consideration comments received. The first approach considered was assigning standard deviation and offsets to each of these effects and then combining them using a mathematical method similar to what one commenter presented in their comments to the NPRM. The second approach considered was defining the margin as a percentage of the standards, similar to assertions by two commenters. We considered both of these approaches, the comments and supporting information submitted, historical approaches by EPA to compliance margin in previous heavy-duty criteria pollutant standards rules, and the data collected from the EPA Stage 3 engine and other available data, to determine the numeric level of each standard over the corresponding useful life that is technically feasible.

For the first approach, we determined that a minimum of 15 mg/hp-hr of margin between an emission standard and the NO_X emissions of the EPA Stage 3 engine for each of the duty cycles was appropriate. For the second approach, we first assessed the average emissions rates from the EPA Stage 3 engine at the respective aged miles. For Light HDEs, we looked at the data at the equivalent of 435,000 miles. For the Medium and Heavy HDEs standards the interpolated emissions performance at 650,000 miles was determined from the tests at the equivalent of 600,000 and 800,000 miles, which is shown in Table III-10. Second, the average emissions values were then adjusted to account for the IRAF and crankcase emissions from the EPA Stage 3 engine. Third, we divided the adjusted emissions values by 0.55 to calculate an emission standard that would provide 45 percent margin to the standard. We determined it would be appropriate to apply a 45 percent margin in this case after evaluating the margin in engines that meet the current standards as outlined in RIA chapter 3 and in CARB's comment to the NPRM and considering the level of the standards in this final rule. Our determination is based on our analysis that the certification data from engines meeting today's standards shows that more than 80 percent of engine families are certified with less than 45 percent compliance margin. For Light HDEs, we took the resulting values from the third step of our approach and rounded them. EPA then also checked that each of these values for each of the duty cycles (resulting from the second approach) provided a minimum of 15 mg/hp-hr of margin between those values and the NO_X emissions of the EPA Stage 3 engine (consistent with the first approach). For Light HDEs, we determined those resulting values were appropriate final numeric emission standards (as specified in Preamble Section III.B.2). The last step of checking that the Light HDE standards provide a minimum of 15 mg/hp-hr of NO_X margin was to ensure that the margin determined from the percent of the standard (the second approach to margin) also provided the margin that we determined under the first approach to margin. For Light HDEs, given the level of the final standards and the length of the final useful life mileages, we determined that this approach to margin was appropriate for both certification and in-use testing of engines.

Given the very long useful life mileages for Heavy HDE and greater amounts of certain aging mechanisms over the long useful life periods of Medium HDE, we determined that a different application of considering these two approaches to margin was appropriate. The in-use standards of Medium and Heavy HDEs were determined using the second approach for determining margin. The certification standards where then determined by subtracting the margin from the first approach (15 mg/hp-hr) from the in-use standards.

Separating the standards from the level that applies for in-use testing was appropriate because we recognize that laboratory aging of the engine doesn't fully capture all the sources of deterioration of the aftertreatment that can occur once the engine enters the real-world and those uncertainties would be most difficult for these engine classes at the level of the final standards and the final useful life mileages. Some of these effects are SCR sulfation, fuel quality, DEF quality, sensor variability, and field aging from severe duty cycles. Thus, the last step in determining the standards for Medium and Heavy HDE was to subtract the 15 mg/hp-hr from the rounded value that provided 45 percent margin to the Stage 3 data. We determined each of the resulting final duty cycle NO_X standards for Medium and Heavy HDE that must be demonstrated at the time of certification out to 350,000 and 750,000 miles, respectively, are feasible with enough margin to account for test procedure variability. We determined this by comparing the EPA Stage 3 emissions results at 800,000 miles (Table III-9) after adjusting for IRAF and crankcase emissions to each of the NO_X standards in Section III.B.2. The EPA Stage 3 NO_X emissions results at 800,000 miles adjusted for IRAF and crankcase emissions are 26 mg/hp-hr for the SET, 33 mg/hp-hr for the FTP, and 33 mg/hp-hr for the LLC. For any in-use testing of Medium and Heavy HDEs, a 15 mg/hp-hr compliance allowance is added to the applicable standard, in consideration of the other sources of variability and deterioration of the aftertreatment that can occur once the engine enters the real world.

As explained in the proposal, our technology cost analysis included an increased SCR catalyst volume from what was used on the EPA and CARB Stage 3 engines. By increasing the SCR catalyst volume, the NO_X reduction performance of the aftertreatment system should deteriorate slower than what was demonstrated with the EPA Stage 3 engine. The increase in total SCR catalyst volume relative to the EPA and CARB Stage 3 SCR was approximately 23.8 percent. We believe this further supports our conclusion that the final standards are achievable in MY 2027, including for the final useful life of 650,000 miles for Heavy HDEs. In addition to NO_X, the final HC and CO standards are feasible for CI engines on all three cycles. This is shown in Table III-10, where the demonstrated HC and CO emission results are below the final standards discussed in Section III.B.2. The final standard for PM of 5 mg/hp-hr for the SET, FTP, and LLC continue to be feasible with the additional technology and control strategies needed to meet the final NO_X standards, as seen by the PM emissions results in Table III-10. As discussed in Section III.B.2, taking into account measurement variability of the PM measurement test procedure, we believe PM emissions from current diesel engines are at the lowest feasible level for standards starting in MY 2027.

In addition to evaluating the feasibility of the new criteria pollutant standards, we also evaluated how CO₂ was impacted on the CARB Stage 3 engine (which is the same engine that was used for EPA's Stage 3 engine with modifications to the aftertreatment system and engine calibration to lower NO_X emissions). We did this by evaluating how CO₂ emissions changed from the base engine over the SET, FTP, and LLC, as well as the fuel mapping test procedures defined in 40 CFR 1036.535 and 1036.540. For all three cycles the CARB Stage 3 engine emitted CO₂ with no measurable difference compared to the base 2017 Cummins X15 engine. Specifically, we compared the CARB Stage 3 engine including the 0-hour (degreened) aftertreatment with the 2017 Cummins X15 engine including degreened aftertreatment and found the percent reduction in CO₂ was 0 percent for the SET, 1 percent for the FTP, and 1 percent for the LLC.

We note that while the data from the EPA Stage 3 engine (the same engine as the CARB Stage 3 engine but after SwRI made changes to the thermal management strategies) at the equivalent age of 435,000 miles showed an increase in CO₂ emissions for the SET, FTP, and LLC of 0.6, 0.7 and 1.3 percent respectively, which resulted in the CO₂ emissions for the EPA Stage 3 engine being higher than the 2017 Cummins X15 engine, this is not directly comparable because the baseline 2017 Cummins X15 aftertreatment had not been aged to an equivalent of 435,000 miles. As discussed in Chapter 3 of the RIA, aging the EPA Stage 3 engine included exposing the aftertreatment to ash, that increased the back pressure on the engine, which contributed to the increase in CO₂ emissions from the EPA Stage 3 engine. We would expect the same increase in backpressure and in CO₂ emissions from the 2017 Cummins X15 engine if the aftertreatment of the 2017 Cummins X15 engine was aged to an equivalent of 435,000 miles.

To evaluate how the technology on the CARB Stage 3 engine compares to the 2017 Cummins X15 engine with respect to the HD GHG Phase 2 vehicle CO₂ standards, both engines were tested on the fuel mapping test procedures defined in 40 CFR 1036.535 and 1036.540. These test procedures define how to collect the fuel consumption data from the engine for use in GEM. For these tests the CARB Stage 3 engine was tested with the development aged aftertreatment. The fuel maps from these tests were run in GEM and the results from this analysis showed that the EPA and CARB Stage 3 engine emitted CO₂ at the same rate as the 2017 Cummins X15 engine. The details of this analysis are described in Chapter 3.1 of the RIA.

The technologies included in the EPA Stage 3 engine were selected to both demonstrate the lowest criteria pollutant emissions and have a negligible effect on GHG emissions. Manufactures may choose to use other technologies to meet the final standards, but manufacturers will still also need to comply with the GHG standards that apply under HD GHG Phase 2. We have, therefore, not projected an increase in GHG emissions resulting from compliance with the final standards.

Table III-11 summarizes the incremental direct manufacturing costs for the final standards, from the baseline costs shown in Table III-15. These values include aftertreatment system, closed crankcase, and CDA costs. As discussed in Chapter 7 of the RIA, the direct manufacturing costs include the technology costs plus some costs to improve the durability of the technology through regulatory useful life. The details of this analysis can be found in Chapters 3 and 7 of the RIA.²⁷⁶ The cost of the final standards and useful life periods are further accounted for in the indirect costs as discussed in Chapter 7 of the RIA.²⁷⁷

b. Baseline Emissions and Cost

The basis for our baseline technology assessment is the data provided by manufacturers in the heavy-duty in-use testing program. This data encompasses in-use operation from nearly 300 Light HDE, Medium HDE, and Heavy HDE vehicles. Chapter 5 of the RIA describes how the data was used to update the MOVES model emissions rates for HD diesel engines. Chapter 3 of the RIA summarizes the in-use emissions performance of these engines.

We also evaluated the certification data submitted to the agency. The data includes test results adjusted for IRAF and FEL that includes adjustments for deterioration and margin. The certification data, summarized in Table III-12 and Table III-13, shows that manufacturers vary in their approach to how much margin is built into the FEL. Some manufactures have submitted certification data with zero emissions (with rounding), which results in a margin at 100 percent of the FEL, while other manufacturers have margin that is less than 25 percent of the FEL.

In addition to analyzing the on-cycle certification data submitted by manufacturers, we tested three modern HD diesel engines on an engine dynamometer and analyzed the data. These engines were a 2018 Cummins B6.7, 2018 Detroit DD15 and 2018 Navistar A26. These engines were tested on cycles that range in power demand from the creep mode of the Heavy Heavy-Duty Diesel Truck (HHDDT) schedule to the HD SET cycle defined in 40 CFR 1036.510. Table III-14 summarizes the range of results from these engines on the SET, FTP, and LLC. As described in Chapter 3 of the RIA, the emissions of current production heavy-duty engines vary from engine to engine but the largest difference in NO _X between engines is seen on the LLC.

Table III-15 summarizes the baseline sales-weighted total aftertreatment cost of Light HDEs, Medium HDEs, Heavy HDEs and urban bus engines. The details of this analysis can be found in Chapters 3 and 7 of the RIA.

C. Summary of Compression-Ignition Off-Cycle Standards and Off-Cycle Test Procedures

In this Section 0, we describe the final off-cycle standards and test procedures that will apply for model year 2027 and later heavy-duty compression-ignition engines. The final off-cycle standards and test procedures cover the range of operation included in the duty cycle test procedures and operation that is outside of the duty cycle test procedures for each regulated pollutant (NO_X, HC, CO, and PM). As described in Section III.C.1, our current not-to-exceed (NTE) test procedures were not designed to capture and control low-load operation. In contrast to the current NTE approach that evaluates engine operation within the NTE zone and excludes operation out of the NTE zone, we are finalizing a moving average window (MAW) approach that divides engine operation into two categories (or “bins”) based on the time-weighted average engine power of each MAW of engine data. See Section III.C.2 for a discussion of the derivation of the final off-cycle standards for each bin. For bin 1, the NO_X emission standard is 10.0 g/hr. The final off-cycle standards for bin 2 are shown in Table III-16.

The proposed rule provided an extensive discussion of the rationale and information supporting the proposed off-cycle standards (87 FR 17472, March 28, 2022). Chapters 2 and 3 of the RIA include additional information including background on applicable test procedures and the full feasibility analysis for compression-ignition engines. See also section 11.3 of the Response to Comments for a detailed discussion of the comments and how they have informed this final rule.

1. Existing NTE Standards and Need for Changes to Off-Cycle Test Procedures

Heavy-duty CI engines are currently subject to Not-To-Exceed (NTE) standards that are not limited to specific test cycles, which means they can be evaluated not only in the laboratory but also in-use. NTE standards and test procedures are generally referred to as “off-cycle” standards and test procedures. These off-cycle emission standards are 1.5 (1.25 for CO) times the laboratory certification standard for NO_X, HC, PM and CO and can be found in 40 CFR 86.007-11. NTE standards have been successful in broadening the types of operation for which manufacturers design their emission controls to remain effective, including steady cruise operation. However, there remains a significant proportion of vehicle operation not covered by NTE standards.

Compliance with an NTE standard is based on emission test data (whether collected in a laboratory or in use) analyzed pursuant to 40 CFR 86.1370 to identify NTE events, which are intervals of at least 30 seconds when engine speeds and loads remain in the NTE control area or “NTE zone”. The NTE zone excludes engine operation that falls below certain torque, power, and speed values. The NTE procedure also excludes engine operation that occurs in certain ambient conditions ( i.e., high altitudes, high intake manifold humidity), or when aftertreatment temperatures are below 250 °C. Collected data is considered a valid NTE event if it occurs within the NTE zone, lasts at least 30 seconds, and does not occur during any of the exclusion conditions (ambient conditions or aftertreatment temperature).

The purpose of the NTE test procedure is to measure emissions during engine operation conditions that could reasonably be expected to occur during normal vehicle use; however, only data in a valid NTE event is then compared to the NTE emission standard. Our analysis of existing heavy-duty in-use vehicle test data indicates that less than ten percent of a typical time-based dataset are part of valid NTE events, and hence subject to the NTE standards; the remaining test data are excluded from consideration. We also found that emissions are high during many of the excluded periods of operation, such as when the aftertreatment temperature drops below the 250 °C exclusion criterion. Our review of in-use data indicates that extended time at low load and idle operation results in low aftertreatment temperatures, which in turn lead to diesel engine SCR-based emission control systems not functioning over a significant fraction of real-world operation. Test data collected as part of EPA's manufacturer-run in-use testing program indicate that low-load operation could account for greater than 50 percent of the NO_X emissions from a vehicle over a given workday.

For example, 96 percent of tests in response to 2014, 2015, and 2016 EPA in-use testing orders passed with NO_X emissions for valid NTE events well below the 0.3 g/hp-hr NO_X NTE standard. When we used the same data to calculate NO_X emissions over all operation measured, not limited to valid NTE events, the NO_X emissions were more than double those within the valid NTE events (0.5 g/hp-hr). The results were even higher when we analyzed the data to consider only NO_X emissions that occur during low load events.

EPA and others have compared the performance of US-certified engines and those certified to European Union emission standards and concluded that the European engines' NO_X emissions are lower in low-load conditions, but comparable to US-certified engines subject to MY 2010 standards under city and highway operation. This suggests that manufacturers are responding to the European certification standards by designing their emission controls to perform well under low-load operations, as well as highway operations.

The European Union “Euro VI” emission standards for heavy-duty engines require manufacturers to check for “in-service conformity” by operating their engines over a mix of urban, rural, and motorway driving on prescribed routes using portable emission measurement system (PEMS) equipment to measure emissions. Compliance is determined using a work-based windows approach where emissions data are evaluated over segments or “windows.” A window consists of consecutive 1 Hz data points that are summed until the engine performs an amount of work equivalent to the European transient engine test cycle (World Harmonized Transient Cycle).

EPA is finalizing new off-cycle test procedures similar to the European Euro VI in-service conformity program, with key distinctions that build upon the Euro VI approach, as discussed in the following section. This new approach will require manufacturers to account for a relatively larger proportion of engine operation and thereby further ensure that real-world emissions meet the off-cycle standards.

2. Off-Cycle Standards and Test Procedures

We are replacing the NTE test procedures and standards (for NO_X, PM, HC and CO) for model year 2027 and later engines. Under the final new off-cycle standards and test procedures, engine operation and emissions test data must be assessed in test intervals that consist of 300-second moving average windows (MAWs) of continuous engine operation. Our evaluation accounts for our current understanding that shorter windows are more sensitive to measurement variability and longer windows make it difficult to distinguish between duty cycles. In contrast to the current NTE approach that divides engine operation into two categories (in the NTE zone and out of the NTE zone), this approach will divide engine operation into two categories (or “bins”) based on the time-weighted average engine power of each MAW of engine data, with some limited exclusions from the two bins, as described in more detail in the following discussion.

In the NPRM, we requested comment on the proposed off-cycle standards and test procedures, including the 300 second length of the window. We first note that commenters broadly agree that the current NTE methodology should be revised, and that a MAW structure is preferable for off-cycle standards. Some commenters were concerned that individual seconds of data would be “smeared,” with the same 1-Hz data appearing in both bins as the 300 second windows are placed in the appropriate bin. We are finalizing the window length that we proposed, as the 300 second length provides an adequate averaging time to smooth any anomalous emission events and we anticipate that the final bin structure described in Section III.C.2.i. should also help address these concerns. See Response to Comments Section 11.1 through 11.3 for further details on these comments and EPA's response to these comments.

Although this program has similarities to the European Euro VI approach, we are not limiting our off- cycle standards and test procedures to operation on prescribed routes. Our current NTE program is not limited to prescribed routes, and we would consider it an unnecessary step backward to change that aspect of the procedure.

In Section IV.G, we discuss the final rule updates to the ABT program to account for these new off-cycle standards.

i. Moving Average Window Operation Bins

The final bin structure includes two bins of operation that represent two different domains of emission performance. Bin 1 represents extended idle operation and other very low load operation where engine exhaust temperatures may drop below the optimal temperature for aftertreatment function. Bin 2 represents higher power operation including much of the operation currently covered by the NTE. Operation in bin 2 naturally involves higher exhaust temperatures and catalyst efficiencies. Because this approach divides 300 second windows into bins based on time-averaged engine power of the window, any of the bins could include some idle or high-power operation. Like the duty cycle standards, we believe more than a single standard is needed to apply to the entire range of operation that heavy-duty engines experience. A numerical standard that is technologically feasible under worst case conditions such as idle would necessarily be much higher than the levels that are achievable when the aftertreatment is functioning optimally. Section III.C.2.iii includes the final numeric off-cycle standards.

Given the challenges of measuring engine power directly in-use, we are using the CO₂ emission rate (grams per second) as a surrogate for engine power in defining the bins for an engine. We are further normalizing CO₂ emission rates relative to the nominal maximum CO₂ rate of the engine. So, if an engine with a maximum CO₂ emission rate of 50 g/sec was found to be emitting CO₂ at a rate of 10 g/sec, its normalized CO₂ emission rate would be 20 percent. The maximum CO₂ rate is defined as the engine's rated maximum power multiplied by the engine's CO₂ family certification level (FCL) for the FTP certification cycle.

In the proposal, we requested comment on whether the maximum CO₂ mass emission rate should instead be determined from the steady-state fuel mapping procedure in 40 CFR 1036.535 or the torque mapping procedure defined in 40 CFR 1065.510. After considering comments, EPA is finalizing the use of the CO₂ emission rate as a surrogate for engine power with the proposed approach to determining the maximum CO₂ mass emission rate. We have two main reasons for finalizing the determination of maximum CO₂ mass emission rate as proposed. First, the FTP FCL and maximum engine power are already reported to the EPA, so no new requirements are needed under the finalized approach. Second, our assessment of the finalized approach has shown that this approach for the determination of maximum CO₂ mass emission rate matches well with the other options we requested comment on. EPA believes that using the CO₂ emission rate will automatically account for additional fuel usage not directly used for driveshaft torque and minimizes concerns about the accuracy and data alignment in the use of broadcast torque. EPA acknowledges that there is some small variation in efficiency, and thus CO₂ emissions rates, among engines. However, the test procedure accounts for improvements to the engine efficiency by using the FTP FCL to convert CO₂ specific NO_X to work specific NO_X . This is because the FTP FCL captures the efficiency of the engine over a wide range of operation, from cold start, idle and steady-state higher power operation. Furthermore, the FTP FCL can also capture the CO₂ improvements from hybrid technology when the powertrain test option described in preamble Section III.B.2.v is utilized.

The bins are defined as follows:

• Bin 1: 300 second windows with normalized average CO₂ rate ≤6 percent.

• Bin 2: 300 second windows with normalized average CO₂ rate >6 percent.

The bin cut point of six percent is near the average power of the low-load cycle. In the NPRM, we proposed a three-bin structure and requested comment on the proposed number of bins and the value of the cut point(s). After considering comments, EPA agrees with commenters to the extent the commenters recommend combining the proposed bins 2 and 3 into a single “non-idle” bin 2. Results from the EPA Stage 3 real world testing indicate that emissions in bins 2 and 3 (expressed as emissions/normalized CO₂) are substantially similar, minimizing the advantage of separating these modes of operation. See Response to Comments Section 11.1 for further details on these comments and EPA's response to these comments.

To ensure that there is adequate data in each of the bins to compare to the off-cycle standards, the final requirements specify that there must be a minimum of 2,400 moving average windows in bin 1 and 10,000 moving average windows in bin 2. In the NPRM, we proposed a minimum of 2,400 windows for all bins and requested comment on the appropriate minimum number of windows required to sufficiently reduce variability in the results while not requiring an unnecessary number of shift days to be tested to meet the requirement. EPA received comments both supporting the proposed 2,400 window minimum and supporting an increase to 10,000 windows total for the non-idle bins (now a single bin 2 in this final rule). After considering comments, we believe requiring a minimum of 10,000 windows in final bin 2 to define a valid test is appropriate. Analysis of data from the EPA Stage 3 off-cycle test data has shown that emissions are stable after 6,000 windows of data at moderate temperatures but NO_X emissions under low ambient temperatures need closer to 10,000 windows to be stable. EPA believes the larger number of required windows will better characterize the emissions performance of the engine.

If during the first shift day any of the bins do not include at least the minimum number of windows, then the engine will need to be tested for additional day(s) until the minimum requirement is met. Additionally, the engine can be idled at the end of the shift day to meet the minimum window count requirement for the idle bin. This is to ensure that even for duty cycles that do not include significant idle operation the minimum window count requirement for the idle bin can be met without testing additional days.

We received comments on the timing and duration of the optional end-of-day idle. After considering comments, the final requirements specify that the ability to add idle time is restricted to the end of the shift day, and manufacturers may extend this end-of-day idle period to be as long as they choose. Additional idle in the middle of the shift day is contrary to the intent of real-world testing, and the end of the shift day is the only realistic time to add windows. Since idle times of varying lengths are encountered in real-world operation, we do not think that requiring a specific length of idle time would necessarily make the resulting data set more representative.

As described further in section III.C.2.ii, after consideration of comment, EPA is including requirements in 40 CFR 1036.420 that specify that during the end-of-day idle period, when testing vehicles with automated engine shutdown features, manufacturers will be required to override the automated shutdown feature where possible. This will ensure that the test data will contain at least 2,400 windows in the idle bin, which otherwise would be unobtainable. For automated shutdown features that cannot be overridden, the manufacturer may populate the bin with zero emission values for idle until exactly 2,400 windows are achieved.

ii. Off-Cycle Test Procedures

The final off-cycle test procedures include measuring off-cycle emissions using the existing test procedures that specify measurement equipment and the process of measuring emissions during testing in 40 CFR part 1065. Part 1036, subpart E contains the process for recruiting test vehicles, how to test over the shift day, how to evaluate the data, what constitutes a valid test, and how to determine if an engine family passes. Measurements may use either the general laboratory test procedures or the field-testing procedures in 40 CFR part 1065, subpart J. However, we are finalizing special calculations for bin 2 in 40 CFR 1036.530 that will supersede the brake-specific emission calculations in 40 CFR part 1065. The test procedures require second-by-second measurement of the following parameters:

• Molar concentration of CO₂ (ppm)

• Molar concentration of NO_X (ppm)

• Molar concentration of HC (ppm)
• Molar concentration of CO (ppm)

• Concentration of PM (g/m³ )

• Exhaust flow rate (m³ /s)

Mass emissions of CO₂ and each regulated pollutant are separately determined for each 300-second window and are binned based on the normalized CO₂ rate for each window.

Additionally, EPA agrees with commenters that the maximum allowable engine coolant temperature at the start of the day should be raised to 40 degrees Celsius and we are finalizing this change in 40 CFR 1036.530. In the NPRM, we proposed 30 °C which is 86 °F. It is possible that ambient temperatures in some regions of the United States won't drop below this overnight. We are therefore finalizing 40 °C which is 104 °F as this should ensure that high overnight ambient temperatures do not prevent a manufacturer from testing a vehicle.

The standards described in Section III.C.2.iii are expressed in units of g/hr for bin 1 and mg/hp-hr for bin 2. However, unlike most of our exhaust standards, the hp-hr values for the off-cycle standards do not refer to actual brake work. Rather, they refer to nominal equivalent work calculated proportional to the CO₂ emission rate. Thus, in 40 CFR 1036.530 the NO_X emissions (“e”) in g/hp-hr are calculated as:

The final requirements include a limited number of exclusions (six total) in 40 CFR 1036.530(c)(3) that exclude some data from being subject to the off-cycle standards. The first exclusion in 40 CFR 1036.530(c)(3)(i) is for data collected during periodic PEMS zero and span drift checks or calibrations, where the emission analyzers and/or flow meter are not available to measure emissions during that time and these checks/calibrations are needed to ensure the robustness of the data.

The second exclusion in 40 CFR 1036.530(c)(3)(ii) is for data collected anytime the engine is off during the course of the shift day, with modifications from proposal that (1) this exclusion does not include engine off due to automated stop-start, and (2) specific requirements for vehicles with stop-start technology. In the NPRM, we proposed excluding data for vehicles with stop-start technology when the engine was off and requested comment on the appropriateness of this exclusion. We received comment suggesting provisions for vehicles equipped with automated stop-start technology. After considering comments, EPA has included in the final rule requirements applicable when testing vehicles with automatic engine shutdown (AES) and/or stop-start technology. Under the final requirements, the manufacturer shall disable AES and/or stop-start if it is not tamper resistant as described in 40 CFR 1036.415(g), 1036.420(c), and 1036.530(c)(3). If stop-start is tamper resistant, the 1-Hz emission rate for all GHG and criteria pollutants shall be set to zero when AES and/or stop-start is active and the engine is off, and these data are included in the normal windowing process ( i.e., the engine-off data are not treated as exclusions). If at the end of the shift day there are not 2,400 windows in bin 1 for a vehicle with AES and/or stop-start technology, the manufacturer must populate the bin with additional windows with the emission rate for each GHG and criteria pollutant set to zero to achieve exactly 2,400 idle bin windows. This process accounts for manufacturers who implement a start/stop mode that cannot be overridden and applies the windowing and binning process in a way that is similar to the process applied to a conventionally idling vehicle.

The third exclusion in 40 CFR 1036.530(c)(3)(iii) is for data collected during infrequent regeneration events. The data collected for the test order may not collect enough operation to properly weight the emissions rates during an infrequent regeneration event with emissions that occur without an infrequent regeneration event.

The fourth exclusion in 40 CFR 1036.530(c)(3)(iv) is for data collected when ambient temperatures are below 5 °C (this aspect includes some modifications from proposal), or when ambient temperatures are above the altitude-based value determined using Equation 40 CFR 1036.530-1. The colder temperatures can significantly inhibit the engine's ability to maintain aftertreatment temperature above the minimum operating temperature of the SCR catalyst while the higher temperature conditions at altitude can limit the mass airflow through the engine, which can adversely affect the engine's ability to reduce engine out NO_X through the use of exhaust gas recirculation (EGR). In addition to affecting EGR, the air-fuel ratio of the engine can decrease under high load, which can increase exhaust temperatures above the conditions where the SCR catalyst is most efficient at reducing NO_X. However, we also do not want to select temperature limits that overly exclude operation, such as setting a cold temperature limit so high that it excludes important initial cold start operation from all tests, or a number of return to service events. These are important operational regimes, and the MAW protocol is intended to capture emissions over the entire operation of the vehicle. The final rule strikes an appropriate balance between these considerations.

In the NPRM, we proposed excluding data when ambient temperatures were below −7 °C and requested comment on the appropriateness of this exclusion. Several comments disagreed with the proposed low temperature exclusion level and recommended a higher temperature of 20 °C as well as additional exemptions for coolant and oil temperatures, and recommended low temperature exclusion temperatures that ranged from 20 to 70 °C. After considering comments, we adjusted the final ambient temperature exclusion to 5 °C. We have additionally incorporated a temperature-based adjustment to the final numerical NO_X standards, as described in Section III.C.iii. However, we have not incorporated exclusions based on coolant and oil temperatures. These changes are supported by data recently generated from testing at SwRI with the EPA Stage 3 engine at low temperatures over the CARB Southern Route Cycle and Low Load Cycle. This testing consisted of operation of the engine over the duty-cycle with the test cell ambient temperature set at 5 °C with air flow moving over the aftertreatment system to simulate the airflow over the aftertreatment during over the road operation. The results indicated that there were cold ambient air temperature effects on aftertreatment temperature that reduced NO_X reduction efficiency, which supports that the temperature should be increased. With these changes, our analysis, as described in section III.C, shows that the off-cycle standards are achievable for MY 2027 and later engines down to 5 °C, taking into account the temperature-based adjustment to the final numerical standards. We have concerns about whether the off-cycle standards could be met below 5 °C after taking a closer look at all data regarding real world effects and based on this we are exempting data from operation below 5 °C from being subject to the standards.

The fifth exclusion in 40 CFR 1036.530(c)(3)(v) is for data collected where the altitude is greater than 5,500 feet above sea level for the same reasons as for the high temperatures at altitude exclusion.

The sixth exclusion in 40 CFR 1036.530(c)(3)(vi) is for data collected when any approved Auxiliary Emission Control Device (AECD) for emergency vehicles are active because the engines are allowed to exceed the emission standards while these AECDs are active.

To reduce the influence of environmental conditions on the accuracy and precision of the PEMS for off-cycle in-use testing, we are adding additional changes to those proposed in requirements in 40 CFR 1065.910(b). These requirements are to minimize the influence of temperature, electromagnetic frequency, shock, and vibration on the emissions measurement. If the design of the PEMS or the installation of the PEMS does not minimize the influence of these environmental conditions, the final requirements specify that the PEMS must be installed in an environmental chamber during the off-cycle test to minimize these effects.

iii. Off-Cycle Standards

For NO_X, we are finalizing separate standards for distinct modes of operation. To ensure that the duty-cycle NO_X standards and the off-cycle NO_X standards are set at the same relative stringency level, the bin 1 standard is proportional to the Voluntary Idle standard discussed in Section III.B.2.iv, and the bin 2 standard is proportional to a weighted combination of the LLC standard discussed in Section III.B.2.iii and the SET standard discussed in Section III.B.2.ii. For bin 1, the NO_X emission standard for all CI primary intended service classes is 10.0 g/hr starting in model year 2027. For PM, HC and CO we are not setting standards for bin 1 because the emissions from these pollutants are very small under idle conditions and idle operation is extensively covered by the SET, FTP, and LLC duty cycles discussed in Section III.B.2. The combined NO_X bin 2 standard is weighted at 25 percent of the LLC standard and 75 percent of the SET standard, reflecting the nominal flow difference between the two cycles. For HC, the bin 2 standard is also set at values proportional to a 25 percent/75 percent weighted combination of the LLC standard and the SET standard. For PM and CO, the SET, FTP, and LLC standards are the same numeric value, so bin 2 is proportional to that numeric standard. The numerical values of the off-cycle standards for bin 2 are shown in Table III-17.

The final numerical off-cycle bin 1 NO_X standard reflect a conformity factor of 1.0 times the Clean Idle standard discussed in Section III.B.2.iv. The final numerical off-cycle bin 2 standards for all pollutants reflect a conformity factor of 1.5 times the duty-cycle standards set for the LLC and SET cycles discussed in Section III.B.2.ii and Section III.B.2.iii. Additionally, as discussed in Section III.B.2, the in-use NO_X off-cycle standard for Medium and Heavy HDE reflects an additional 15 mg/hp-hr NO_X allowance above the bin 2 standard. Similar to the duty cycle standards, the off-cycle standards were set at a level that resulted in at least 40 percent compliance margin for the EPA Stage 3 engine. We requested and received comments on the appropriate scaling factors or other approaches to setting off-cycle standards. After consideration of the comments, we believe the final numerical standards are feasible and appropriate for certification and in-use testing. We note that the final standards are similar, but not identical to, the options proposed in the NPRM. As with the duty cycle standards discussed in Preamble Section III.B, the data from the EPA Stage 3 engine supported the most stringent numeric standards we proposed under low-load operation and the most stringent numeric standards we proposed for MY 2027 under high load operation. More discussion of the feasibility of these standards can be found in the following discussion and in Section III.C.3 and Response to Comments Section 11.3.1.

In the proposal, we requested comment on the in-use test conditions over which engines should be required to comply with the standard, asking commentors to take into consideration any tradeoffs that broader or narrower conditions might have on the stringency of the standard we set. After considering comments on low ambient air temperature and the available data from the low-temperature Stage 3 testing at SwRI described in section III.C.2.ii, we are also incorporating an adjustment to the numerical off-cycle bin 1 and bin 2 standards for NO_X as a function of ambient air temperature below 25 °C. The results demonstrated higher NO_X emissions at low temperatures, indicating that standards should be numerically higher to account for real-world temperature effects on the aftertreatment system. To determine the magnitude of this adjustment, we calculated the increase in the Stage 3 engine NO_X emissions over the CARB Southern Route Cycle at low temperature over the NO_X emissions at 25 °C. These values were linearly extrapolated to determine the projected increase at 5 °C versus 25 °C. Table III-18 presents the numerical value of each off-cycle bin 1 and bin 2 NO_X standard at both 25 °C and 5 °C.

Under the final requirements in 40 CFR 1036.104, the ambient temperature adjustment is applied based on the average 1-Hz ambient air temperature during the shift day for all data not excluded under 40 CFR 1036.530(c), calculated as the time-averaged temperature of all included data points. If this average temperature is 25 °C or above, no adjustment to the standard is made. If the average temperature is below 25 °C, the applicable NO_X standard is calculated using the equations in Table 3 to paragraph (a)(3) of 40 CFR 1036.104 Table III-18 for the appropriate service class and bin.

3. Feasibility of the Diesel (Compression-Ignition) Off-Cycle Standards

i. Technologies

As a starting point for our determination of the appropriate numeric levels of the off-cycle emission standards, we considered whether manufacturers could meet the duty-cycle standard corresponding to the type of engine operation included in a given bin, as follows:

• Bin 1 operation is generally similar to operation at idle and the lower speed portions of the LLC.
• Bin 2 operation is generally similar to operation over the LLC, the FTP and much of the SET.

An important question is whether the off-cycle standards would require technology beyond what we are projecting would be necessary to meet the duty-cycle standards. As described in this section, we do not expect the off-cycle standards to require different technologies.

This is not to say that we expect manufacturers to be able to meet these standards with no additional work. Rather, we project that the off-cycle standards can be met primarily through additional effort to calibrate the duty-cycle technologies to function properly over the broader range of in-use conditions. We also recognize that manufacturers can choose to include additional technology, if it provided a less expensive or otherwise preferred option.

When we evaluated the technologies discussed in Section III.B.3.i with emissions controls that were designed to cover a broad range of operation, it was clear that we should set the off-cycle standards to higher numerical values than the duty-cycle standards to take into account the broader operations covered by the off-cycle test procedures. Section III.C.3.ii explains how the technology and controls performed when testing with the off-cycle test procedures over a broad range of operation. The data presented in Section III.C.3.ii shows that even though there are similarities in the operation between the duty cycles (SET, FTP, and LLC) and the off-cycle bins 1 and 2, the broader range of operation covered by the off-cycle test procedure results in a broader range of emissions performance, which justifies setting the numeric off-cycle standards higher than the corresponding duty cycle standards for equivalent stringency. In addition to this, the off-cycle test procedures and standards cover a broader range of ambient temperature and pressure, which can also increase the emissions from the engine as discussed in Section III.C.2.ii.

ii. Summary of Feasibility Analysis

To identify appropriate numerical levels for the off-cycle standards, we evaluated the performance of the EPA Stage 3 engine in the laboratory on five different cycles that were created from field data of HD engines that cover a range of off-cycle operation. These cycles are the CARB Southern Route Cycle, Grocery Delivery Truck Cycle, Drayage Truck Cycle, Euro-VI ISC Cycle (EU ISC) and the Advanced Collaborative Emissions Study (ACES) cycle. The CARB Southern Route Cycle is predominantly highway operation with elevation changes resulting in extended motoring sections followed by high power operation. The Grocery Delivery Truck Cycle represents goods delivery from regional warehouses to downtown and suburban supermarkets and extended engine-off events characteristic of unloading events at supermarkets. Drayage Truck Cycle includes near dock and local operation of drayage trucks, with extended idle and creep operation. Euro-VI ISC Cycle is modeled after Euro VI ISC route requirements with a mix of 30 percent urban, 25 percent rural and 45 percent highway operation. ACES Cycle is a 5-mode cycle developed as part of ACES program. Chapter 3 of the RIA includes figures that show the engine speed, engine torque and vehicle speed of the cycles.

The engine was initially calibrated to minimize NO_X emissions for the dynamometer duty cycles (SET, FTP, and LLC). It was then further calibrated to achieve more optimal performance over off-cycle operation. The test results shown in Table III-19 provide a reasonable basis for evaluating the feasibility of controlling off-cycle emissions to a useful life of 435,000 miles and 800,000 miles. Additionally, the engine tested did not include the SCR catalyst volume that is included in our cost analysis and that we determined should enable lower bin 2 NO_X emissions, further supporting that the final standards are feasible. Additionally, the 800,000 mile aged aftertreatment was tested over the CARB Southern Route Cycle with an ambient temperature between 2 °C and 9 °C (6.8 °C average), the average of which is slightly above the 5 °C minimum ambient temperature that the final requirements specify as the level below which test data are excluded. The summary of the results is in Chapter 3 of the RIA. For Light HDE standards, we looked at the data at the equivalent of 435,000 miles. For the Medium and Heavy HDE standards we looked at the data at the equivalent of 800,000 miles.

a. Bin 1 Evaluation

Bin 1 includes the idle operation and some of the lower speed operation that occurs during the FTP and LLC. However, it also includes other types of low-load operation observed with in-use vehicles, such as operation involving longer idle times than occur in the LLC. To ensure that the bin 1 standard is feasible, we set the idle bin standard at the level projected to be achievable engine-out with exhaust temperatures below the aftertreatment light-off temperature. As can be seen from the results in Table III-19, the EPA Stage 3 engine performed well below the bin 1 NO_X standards. The summary of the results is located in Chapter 3 of the RIA.

For bin 1 we are finalizing NO_X standard at a level above what we have demonstrated because there are conditions in the real world that may prevent the emissions control technology from being as effective as demonstrated with the EPA Stage 3 engine. For example, under extended idle operation the EGR rate may need to be reduced to maintain engine durability. Under extended idle operation with cold ambient temperatures, the aftertreatment system can lose NO_X reduction efficiency which can also increase NO_X emissions. Taking this under consideration, as well as other factors, we believe that the final bin 1 NO_X standard in Table III-17 is the lowest achievable standard in MY 2027.

b. Bin 2 Evaluations

As can be seen see from the results in Table III-19, the NO_X emissions from the Stage 3 engine in bin 2 were below the final off-cycle standards for each of the off-cycle duty-cycles. The HC and CO emissions measured for each of these off-cycle duty cycles were well below the final off-cycle standards for bin 2. PM emissions were not measured during the off-cycle tests, but based on the effectiveness of DPFs over all engine operation as seen with the SET, FTP, and LLC, our assessment is that the final PM standards in Bin 2 are feasible. The summary of the results is located in Chapter 3 of the RIA.

For bin 2, all the 25 °C off-cycle duty cycles at a full useful life of 800,000 miles had emission results below the NO_X certification standard of 58 mg/hp-hr shown in Table III-19. Additionally, the CARB Southern Route Cycle run at ambient temperatures under 10 °C had emission results below the Heavy HDE NO_X in-use off-cycle standard of 106 mg/hp-hr which is the standard at 10 °C as determined from Equation 40 CFR 1036.104-2. While this cycle was run at temperatures above the minimum ambient temperature exclusion limit of 5 °C that we are finalizing, we expect actual HDIUT testing to be less severe than the demonstration. Nonetheless, since the results of the low ambient temperature testing demonstrated higher NO_X emissions at low temperatures, as shown in Table III-19, we have finalized standards that are numerically higher at lower temperatures to account for real-world temperature effects on the aftertreatment system.

In the NPRM, we requested comment on the numerical values of the off-cycle standards, as well as the overall structure of the off-cycle program. We received comments recommending both lower and higher numerical standards than were proposed. After considering comments, we believe the off-cycle standards that we are finalizing are appropriate and feasible values. See Response to Comments Section 11.3.1 for further details on these comments and EPA's response to these comments.

4. Compliance and Flexibilities for Off-Cycle Standards

Given the similarities of the off-cycle standards and test procedures to the current NTE requirements that we are replacing starting in MY 2027, we evaluated the appropriateness of applying the current NTE compliance provisions to the off-cycle standards we are finalizing and determined which final compliance requirements and flexibilities are applicable to the new final off-cycle standards, as discussed immediately below.

i. Relation of Off-Cycle Standards To Defeat Devices

CAA section 203 prohibits bypassing or rendering inoperative a certified engine's emission controls. When the engine is designed or modified to do this, the engine is said to have a defeat device. With today's engines, the greatest risks with respect to defeat devices involve manipulation of the engine's electronic controls. EPA refers to an element of design that manipulates emission controls as an Auxiliary Emission Control Device (AECD). Unless explicitly permitted by EPA, AECDs that reduce the effectiveness of emission control systems under conditions which may reasonably be expected to be encountered in normal vehicle operation and use are prohibited as defeat devices under current 40 CFR 86.004-2.

For certification, EPA requires manufacturers to identify and describe all AECDs. For any AECD that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use, manufacturers must provide a detailed justification. We are migrating the definition of defeat device from 40 CFR 86.004-2 to 40 CFR 1036.115(h) and clarifying that an AECD is not a defeat device if such conditions are substantially included in the applicable procedure for duty-cycle testing as described in 40 CFR 1036, subpart F. Such AECDs are not treated as defeat devices because the manufacturer shows that their engines are able to meet standards during duty-cycle testing while the AECD is active. The AECD might reduce the effectiveness of emission controls, but not so much that the engine fails to meet the standards that apply.

We do not extend this same treatment to off-cycle testing, for two related reasons. First, we can have no assurance that the AECD is adequately exercised during any off-cycle operation to support the conclusion that the engine will consistently meet emission standards over all off-cycle operation. Second, off-cycle testing may involve operation over an infinite combination of engine speeds and loads, so excluding AECDs from consideration as defeat devices during off-cycle testing would make it practically impossible to conclude that an engine has a defeat device.

If an engine meets duty-cycle standards and the engine has no defeat devices, we should be able to expect engines to achieve a comparable level of emission control for engine operation that is different than what is represented by the certification duty cycles. The off-cycle standards and measurement procedures allow for a modest increase in emissions for operation that is different than the duty cycle, but manufacturers may not change emission controls to increase emissions to the off-cycle standard if those controls were needed to meet the duty-cycle standards. The finalized off-cycle standards are set at a level that is feasible under all operating conditions, so we expect that under much of the engine operation the emissions are well below the final off-cycle standards.

ii. Heavy-Duty In-Use Testing Program

Under the current manufacturer-run heavy-duty in-use testing (HDIUT) program, EPA annually selects engine families to evaluate whether engines are meeting current emissions standards. Once we submit a test order to the manufacturer to initiate testing, it must contact customers to recruit vehicles that use an engine from the selected engine family. The manufacturer generally selects five unique vehicles that have a good maintenance history, no malfunction indicators on, and are within the engine's regulatory useful life for the requested engine family. The tests require use of portable emissions measurement systems (PEMS) that meet the requirements of 40 CFR part 1065, subpart J. Manufacturers collect data from the selected vehicles over the course of a day while they are used for their normal work and operated by a regular driver, and then submit the data to EPA. Compliance is currently evaluated with respect to the NTE standards.

With some modifications from proposal, we are continuing the HDIUT program, with compliance with respect to the new off-cycle standards and test procedures added to the program beginning with MY 2027 engines. As proposed, we are not carrying forward the Phase 2 HDIUT requirements in 40 CFR 86.1915 once the NTE phases out after MY 2026. Under the current NTE based off-cycle test program, if a manufacturer is required to test ten engines under Phase 1 testing and less than eight fully comply with the vehicle pass criteria in 40 CFR 86.1912, we could require the manufacturer to initiate Phase 2 HDIUT testing which would require manufacturers to test an additional 10 engines. After consideration of comments, we are generally finalizing our overall long term HDIUT program's engine testing steps and pass/fail criteria as proposed; however, EPA believes that an interim approach in the initial two years of the program is appropriate, as manufacturers transition to the final standards, test procedures, and requirements, while still providing overall compliance assurance during that transition. More specifically, we are finalizing that compliance with the off-cycle standards would be determined by testing a maximum of fifteen engines for MYs 2027 and MY 2028 under the interim provisions, and ten engines for MYs 2029 and later. As noted in the proposal, the testing of a maximum of ten engines was the original limit under Phase 1 HDIUT testing in 40 CFR 86.1915. Similar to the current Phase 1 HDIUT requirements in 40 CFR 86.1912, the finalized 40 CFR 1036.425 and finalized interim provision in 40 CFR 1036.150(z) require initially testing five engines. Various outcomes are possible based on the observed number of vehicle passes or failures from manufacturer-run in-use testing, as well as other supplemental information. Under the interim provisions for MYs 2027 and 2028, if four of the first test vehicles meet the off-cycle standards, testing stops, and no other action is required of the manufacturer for that diesel engine family. For MYs 2029 and later, if five of the first test vehicles meet the off-cycle standards, testing stops, and no other action is required of the manufacturer for that diesel engine family. For MYs 2027 and 2028, if two of those engines do not comply fully with the off-cycle bin standards, the manufacturer would then test five additional engines for a total of ten. For MYs 2029 and later, if one of those engines does not comply fully with the off-cycle bin standards, the manufacturer would then test a sixth engine. For MYs 2027 and 2028, if eight of the ten engines tested pass, testing stops, and no other action is required of the manufacturer for that diesel engine family under the program for that model year. For MYs 2029 and later, if five of the six engines tested pass, testing stops, and no other action is required of the manufacturer for that diesel engine family under the program for that model year. For MYs 2027 and 2028, if three or more of the first ten engines tested do not pass, the manufacturer may test up to five additional engines until a maximum of fifteen engines have been tested. For MYs 2029 and later, when two or more of the first six engines tested do not pass, the manufacturer must test four additional engines until a total of ten engines have been tested. If the arithmetic mean of the emissions from the ten, or up to fifteen under the interim provisions, engine tests determined in § 1036.530(g), or § 1036.150(z) under the interim provisions, is at or below the off-cycle standard for each pollutant, the engine family passes and no other action is required of the manufacturer for that diesel engine family. If the arithmetic mean of the emissions from the ten, or up to fifteen under the interim provisions, engines for either of the two bins for any of the pollutants is above the respective off-cycle bin standard, the engine family fails and the manufacturer must join EPA in follow-up discussions to determine whether any further testing, investigations, data submissions, or other actions may be warranted. Under the final requirements, the manufacturer may accept a fail result for the engine family and discontinue testing at any point in the sequence of testing the specified number of engines.

We received comment on the elimination of Phase 2 testing. See Response to Comment Section 11.5.1 for further information on these comments and EPA's response to these comments. As noted in the preceding paragraphs, we are finalizing elimination of Phase 2 testing. However, we also are clarifying what happens when an engine family fails under the final program. In such a case, three outcomes are possible. First, we may ultimately decide not to take further action if no nonconformity is indicated after a thorough evaluation of the causes or conditions that caused vehicles in the engine family to fail the off-cycle standards, and a review of any other supplemental information obtained separately by EPA or submitted by the manufacturer shows that no significant nonconformity exists. Testing would then stop, and no other action would be required of the manufacturer for that diesel engine family under the program for that year. Second, we may seek some form of remedial action from the manufacturer based on our evaluation of the test results and review of other supplemental information. Third, and finally, in situations where a significant nonconformity is observed during testing, we may order a recall action for the diesel engine family in question if the manufacturer does not voluntarily initiate an acceptable remedial action.

In the NPRM, we proposed allowing manufacturers to test a minimum of 2 engines using PEMS, in response to a test order program, provided they measure, and report in-use data collected from the engine's on-board NO_X measurement system. EPA received comments expressing concerns on the feasibility of this alternate in-use testing option. Given meaningful uncertainties in whether technological advancement of measurement capabilities of these sensors will occur by MY 2027, at this time, EPA is not including the proposed option in 40 CFR 1036.405(g) and not finalizing this alternative test program option in this action. The final in-use option for manufacturers to show compliance with the off-cycle standard will require the use of currently available PEMS to measure criteria pollutant emissions, with the sampling and measurement of emission concentrations in a manner similar to the current NTE in-use test program as described in 40 CFR part 1036, subpart E, and Section III.C of this preamble. See Response to Comment Section 11.5.3 for further information on these comments and EPA's response to these comments.

In the NPRM, we proposed to not carry forward the provision in 40 CFR 86.1908(a)(6) that considers an engine misfueled if operated on a biodiesel fuel blend that is either not listed as allowed or otherwise indicated to be an unacceptable fuel in the vehicle's owner or operator manual. We also proposed in 40 CFR 1036.415(c)(1) to allow vehicles to be tested for compliance with the new off-cycle standards on any commercially available biodiesel fuel blend that meets the specifications for ASTM D975 or ASTM D7467.

We received comments on these proposed requirements. After considering the comments, we have altered provisions in the final rule from what was proposed. EPA agrees with the commenters' recommendation to restrict in-use off-cycle standards testing on vehicles that have been fueled with biodiesel to those that are either expressly allowed in the vehicle's owner or operator manual or not otherwise indicated as an unacceptable fuel in the vehicle's owner or operator manual or in the engine manufacturer's published fuel recommendations. EPA believes, as explained in section IV.H of this preamble, that data show biodiesel is compliant with ASTM D975, D7467 and D6751, that the occurrence of metal contamination in the fuel pool is extremely low, and that the metal content of biodiesel is low. However, EPA understands that manufacturers have little control over the quality of fuel that their engines will encounter over years of in-use operation. To address uncertainties, EPA is modifying the proposed approach to in-use off-cycle standards testing and will allow manufacturers to continue to exempt engines from in-use off-cycle standards testing if the engine is being operated on biofuel that exceeds the manufacturers maximum allowable biodiesel percentage usable in their engines, as specified in the engine owner's manual. See 40 CFR 1036.415(c)(1).

EPA requested comment on a process for a manufacturer to receive EPA approval to exempt test results from in-use off-cycle standards testing from being considered for potential recall if an engine manufacturer can show that the vehicle was historically fueled with biodiesel blends whose B100 blendstock did not meet the ASTM D6751-20a limit for Na, K, Ca, and/or Mg metal (metals which are a byproduct of biodiesel production) or contaminated petroleum based fuels ( i.e. if the manufacturer can show that the vehicle was misfueled), and the manufacturer can show that misfueling lead to degradation of the emission control system performance. 40 CFR 1068.505 describes how recall requirements apply for engines that have been properly maintained and used. Given the risk of metal contamination from biofuels and in some rare cases petroleum derived fuels, EPA will be willing to engage with any information manufacturers can share to demonstrate that the fueling history caused an engine to be noncompliant based on improper maintenance or use. It is envisioned that this engagement would include submission by the manufacturer of a comparison of the degraded emission control system to a representative compliant system of similar miles with respect to content of the contaminant, including an analysis of the level of the poisoning agents on the catalysts in the engine's aftertreatment system. This process addresses concerns expressed by a commentor who stated that it would be difficult if not impossible for a manufacturer to provide “proof of source” of the fuel contamination that led to the degradation in catalyst performance. This clarifies that the manufacturer must only determine the amount of poisoning agent present versus a baseline aftertreatment system.

In the NPRM, we requested comment on the need to measure PM emissions during in-use off-cycle testing of engines that comply with MY 2027 or later standards if they are equipped with a DPF. PEMS measurement is more complicated and time-consuming for PM measurements than for gaseous pollutants such as NO_X and eliminating it for some or all of in-use off-cycle standards testing would provide significant cost savings. We received comments both in support of and in opposition to continuing to require measurement of PM during in-use off-cycle standards testing. After considering these comments, EPA believes that historic test results from the manufacturer run in-use test program indicate that there is not a PM compliance problem for properly maintained engines. Additionally, we believe that removing the requirement for in-use off-cycle PM standards testing will not lead manufacturers to stop using wall flow DPF technology to meet the PM standards. Therefore, EPA is not including the proposed requirement for manufacturers to measure PM in the final 40 CFR 1036.415(d)(1) but is modifying that requirement from proposal to include a final provision in this paragraph that EPA may request PM measurement and that manufacturers must provide that measurement if EPA requests it. Generally, EPA expects that test orders issued by EPA under 40 CFR 1036.405 will not include a requirement to measure PM.

Furthermore, EPA received comments on the subject of the need to measure NMHC emissions during in-use off-cycle testing of engines that comply with MY 2027 or later standards. After considering comments, EPA believes that historic test results from the manufacturer run in-use test program indicate that there is not an NMHC compliance problem for properly maintained engines. EPA is not including the proposed requirement for manufacturers to measure NMHC in the final 40 CFR 1036.415(d)(1) but is modifying that requirement from proposal to include a provision in this paragraph that EPA may request NMHC measurement and that manufacturers must provide that measurement if EPA requests it. Generally, EPA expects that test orders issued by EPA under 40 CFR 1036.405 will not include a requirement to measure NMHC. See Response to Comment Section 11.5.5 for further information on these comments and EPA's response to comments on the subject of in-use off-cycle standards PM and NMHC testing.

iii. PEMS Accuracy Margin

EPA worked with engine manufacturers on a joint test program to establish measurement allowance values to account for the measurement uncertainty associated with in-use testing in the 2007-time frame for gaseous emissions and the 2010-time frame for PM emissions to support NTE in-use testing. PEMS measurement allowance values in 40 CFR 86.1912 are 0.01 g/hp-hr for HC, 0.25 g/hp-hr for CO, 0.15 g/hp-hr for NO_X, and 0.006 g/hp-hr for PM. We are maintaining the same values for HC, CO, and PM in this rulemaking. For NO_X we are finalizing an off-cycle NO_X accuracy margin (formerly known as measurement allowance) that is 5 percent of the off-cycle standard for a given bin. This final accuracy margin is supported by PEMS accuracy margin work at SwRI. The SwRI PEMS accuracy margin testing was done on the Stage 3 engine, which was tested over five field cycles with three different commercially available PEMS. EPA's conclusion after assessing the results of that study, was that accuracy margins set at 0.4 g/hr for bin 1 and 5 mg/hp-hr for bin 2 were appropriate.

The accuracy margins we are finalizing differ from the 10 percent of the standard margin proposed in the NPRM, which was based on an earlier study by JRC. This SwRI PEMS accuracy margin study was on-going at the time the NPRM was published, and the results were only available post-NPRM publication. However, the NPRM did note that we would consider the results of the SwRI PEMS study when they became available, and that the final off-cycle bin NO_X standards could be higher or lower than what we proposed. EPA requested and received comments on the value of the PEMS accuracy margin for NO_X; some commenters encouraged EPA to account for the SwRI PEMS accuracy work that was carried out on the Stage 3 engine. We initially planned to consider the results of this work and this was further supported through recommendations by some commentors; thus, we believe that incorporating the results of the latest study to determine an off-cycle NO_X accuracy margin is appropriate. The SwRI PEMS study is further discussed in RIA Chapter 2. The study consisted of testing the Stage 3 engine with three commercially available PEMS units over 19 different tests. These tests were 6 to 9 hours long, covering a wide range of field operation. In addition, the Stage 3 engine was tested in three different configurations to cover the range of emissions levels expected from an engine both meeting and failing the final standards. We believe, based on this robust data set that was evaluating using the finalized test procedures, the SwRI study provides a more accurate assessment of PEMS measurement uncertainty from field testing of heavy-duty engines than what was determined from the JRC study that we relied on in the proposal for the proposed 10 percent margin. See Response to Comment Section 11.6 for further information on these comments and EPA's response to these comments.

It should be noted that our off-cycle test procedures already include a linear zero and span drift correction over at least the shift day, and we are finalizing requirements for at least hourly zero drift checks over the course of the shift day on purified air. We believe that the addition of these checks and the additional improvements we implemented helped facilitate a measurement error that is lower than the analytically derived JRC value of 10 percent.

We are updating 40 CFR 1065.935 to require hourly zeroing of the PEMS analyzers using purified air for all analyzers. We are also updating the drift limits for NO_X analyzers to improve data quality. Specifically, for NO_X analyzers, we are requiring an hourly or more frequent zero verification limit of 2.5 ppm, a zero-drift limit over the entire shift day of 10 ppm, and a span drift limit between the beginning and end of the shift day or more frequent span verification(s) of ±4 percent of the measured span value. In the NPRM, we requested comment on the test procedure updates in 40 CFR 1065.935 and any changes that would reduce the PEMS measurement uncertainty. We received no comments on this topic other than a few minor edits and are finalizing these updates with minor edits for clarification.

iv. Demonstrating Off-Cycle Standards for Certification

Consistent with current certification requirements in 40 CFR 86.007-21(p)(1), we are finalizing a new paragraph in 40 CFR 1036.205(p) that requires manufacturers to provide a statement in their application for certification that their engine complies with the off-cycle standards, along with testing or other information to support that conclusion. We are finalizing this provision as proposed.

D. Summary of Spark-Ignition HDE Exhaust Emission Standards and Test Procedures

This section summarizes the exhaust emission standards, test procedures, and other requirements and flexibilities we are finalizing for certain spark-ignition (SI) heavy-duty engines. The exhaust emission provisions in this section apply for SI engines installed in vehicles above 14,000 lb GVWR and incomplete vehicles at or below 14,000 lb GVWR, but do not include engines voluntarily certified to or installed in vehicles subject to 40 CFR part 86, subpart S.

As described in this Section III.D, Spark-ignition HDE certification will continue to be based on emission performance in lab-based engine dynamometer testing, which will include a new SET duty cycle to address high load operation. High load temperature protection and idle emission control requirements are also added to supplement our current FTP and new SET duty cycles. We are also lengthening the useful life and emissions-related warranty periods for all heavy-duty engines, including Spark-ignition HDE, as detailed in Sections IV.A and IV.B.1 of this preamble.

The final exhaust emission standards in 40 CFR 1037.104 apply starting in MY 2027. This final rule includes new standards over the FTP duty cycle currently used for certification, as well as new standards over the SET duty cycle to ensure manufacturers of Spark-ignition HDE are designing their engines to address emissions in during operation that is not covered by the FTP. The new standards are shown in Table III-20.

Our proposal included two options of fuel-neutral standards that applied the same numerical standards across all primary intended service classes. The proposed NO_X and PM standards for the SET and FTP duty cycles were based on the emission performance of technologies evaluated in our HD CI engine technology demonstration program. We based the proposed SET and FTP standards for HC and CO on HD SI engine performance.

Three organizations specifically expressed support for adopting the standards of proposed Option 1 for Spark-ignition HDE. The final standards are based largely on the emission levels of proposed Option 1, with some revisions to account for a single-step program, starting in MY 2027. Some organizations commented that the proposed SI standards were challenging enough to need the flexibility of ABT for HC and CO. Consistent with the proposal for this rule, we are finalizing an ABT program for NO_X credits only and are discontinuing the current options for manufacturers to generate HC and PM credits. We did not request comment on and are not finalizing an option for manufacturers to generate credits for CO. See Section IV.G of this preamble and section 12 of the Response to Comments document for more information on the final ABT program.

We are remaining generally consistent with a fuel neutral approach in the final SET and FTP standards, with the exception of CO for Spark-ignition HDE over the new SET duty cycle. We expand on our rationale for this deviation from fuel neutrality in Section III.D.1 where we also describe our rationale for the final program, including a summary of the feasibility demonstration, available data, and comments received.

After considering comments, we are revising three other proposed provisions for Spark-ignition HDE as described in Section . Two new requirements in 40 CFR 1036.115(j) focus on ensuring catalyst efficiency at low loads and proper thermal management at high loads. We are finalizing, with additional clarification, a new OBD flexibility for “sister vehicles”. We did not propose and are not finalizing separate off-cycle standards, manufacturer-run in-use testing requirements, or a low-load duty cycle for Spark-ignition HDE at this time.

The proposed rule provided an extensive discussion of the rationale and information supporting the proposed standards (87 FR 17479, March 28, 2022). The RIA includes additional information related to the range of technologies to control criteria emissions, background on applicable test procedures, and the full feasibility analysis for Spark-ignition HDE. See also section 3 of the Response to Comments for a detailed discussion of the comments and how they have informed this final rule.

1. Basis of the Final Exhaust Emission Standards and Test Procedures

EPA conducted a program with SwRI to better understand the emissions performance limitations of current heavy-duty SI engines as well as investigate the feasibility of advanced three-way catalyst aftertreatment and technologies and strategies to meet our proposed exhaust emission standards. Our demonstration included the use of advanced catalyst technologies artificially aged to the equivalent of 250,000 miles and engine downspeeding. Our feasibility analyses for the exhaust emission standards are based on the SwRI demonstration program. Feasibility of the FTP standards is further supported by compliance data submitted by manufacturers for the 2019 model year. We also support the feasibility of the SET standards using engine fuel mapping data from a test program performed by the agency as part of the HD GHG Phase 2 rulemaking. See Chapter 3.2 of the RIA for more details related to the SwRI demonstration program and the two supporting datasets.

Results from our SI HDE technology demonstration program (see Table III-21 and Table III-22) show that the NO_X standards based on our CI engine feasibility analysis are also feasible for SI HDEs over the SET and FTP duty cycles. The NO_X standard was achieved in this test program by implementing an advanced catalyst with minor catalyst system design changes, and NO_X levels were further improved with engine down-speeding. The emission control strategies that we evaluated did not specifically target PM emissions, but we note that PM emissions remained low in our demonstration. We project SI HDE manufacturers will maintain near-zero PM levels with limited effort. The following sections discuss the feasibility of the HC and CO standards over each of the duty cycles and the basis for our final numeric standards' levels.

i. Federal Test Procedure and Standards for Spark-Ignition HDE

After considering comments, we are finalizing FTP standards that differ from our proposed options for Spark-ignition HDE. We are finalizing standards of 35 mg/hp-hr NO_X, 5 mg/hp-hr PM, 60 mg/hp-hr HC, and 6.0 g/hp-hr CO over the FTP duty cycle in a single step for MY 2027 and later engines. The NO_X and HC standards match the MY 2027 step of proposed Option 1; the PM and CO standards match the MY 2031 step of Option 1. All of these standards were demonstrated to be technologically feasible in EPA's SI engine test program.

As shown in Table III-21, use of advanced catalysts provided NO_X emission levels over the FTP duty cycle well below today's standards and below the certification levels of some of the best performing engines certified in recent years. Engine down-speeding further decreased CO emissions while maintaining NO_X, NMHC, and PM control. Engine down-speeding also resulted in a small improvement in fuel consumption over the FTP duty cycle, with fuel consumption being reduced from 0.46 to 0.45 lb/hp-hr. See Chapter 3.2.3 of the RIA for an expanded description of the test program and results.

All SI HDEs currently on the market use a three-way catalyst (TWC) to simultaneously control NO_X , HC, and CO emissions. We project most manufacturers will continue to use TWC technology and will also adopt advanced catalyst washcoat technologies and refine their existing catalyst thermal protection (fuel enrichment) strategies to prevent damage to engine and catalyst components over the longer useful life period we have finalized. We expect manufacturers, who design and have full access to the engine controls, could achieve similar emission performance as we demonstrated by adopting other, more targeted approaches, including a combination of calibration changes, optimized catalyst location, and fuel control strategies that EPA was unable to evaluate in our demonstration program due to limited access to proprietary engine controls.

In the proposal we described how the FTP duty cycle did not sufficiently incentivize SI HDE manufacturers to address fuel enrichment and the associated CO emissions that are common under higher load operations in the real-world. In response to our proposed rule, one manufacturer shared technical information with us regarding an SI engine architecture under development that is expected to reduce or eliminate enrichment and the associated CO emissions. The company indicated that the low CO emissions may come at the expense of HC emission reduction in certain operation represented by the FTP duty cycle, and reiterated their request for an 80 mg/hp-hr HC standard, as was stated in their written comments. We are not finalizing an HC standard of 80 mg/hp-hr as requested in comment. For the FTP duty cycle, the EPA test program achieved HC levels more than half of the requested level without compromising NO_X or CO emission control (see Table III-21), which clearly demonstrates feasibility.

While we demonstrated emission levels below the final standards of 60 mg HC/hp-hr and 35 mg NO_X /hp-hr over the FTP duty cycle in our SI HDE testing program, we expect manufacturers to apply a compliance margin to their certification test results to account for uncertainties, such as production variation. Additionally, we believe manufacturers would have required additional lead time to implement the demonstrated emission levels broadly across all heavy-duty SI engine platforms for the final useful life periods. Since we are finalizing a single-step program starting in MY 2027, as discussed in Section III.A.3 of this preamble, we continue to consider 60 mg HC/hp-hr and 35 mg NO_X /hp-hr the appropriate level of the standards for that model year, as proposed in the MY 2027 step of proposed Option 1.

ii. Supplemental Emission Test and Standards for Spark-Ignition HDE

The existing SET duty cycle, currently only applicable to CI engines, is a ramped modal cycle covering 13 steady-state torque and engine speed points that is intended to exercise the engine over sustained higher load and higher speed operation. Historically, in light of the limited range of applications and sales volumes of SI heavy-duty engines, especially compared to CI engines, we believed the FTP duty cycle was sufficient to represent the high-load and high-speed operation of SI engine-powered heavy-duty vehicles. As the market for SI engines increases for use in larger vehicle classes, these engines are more likely to operate under extended high-load conditions. To address these market shifts, we proposed to apply the SET duty cycle and new SET standards to Spark-ignition HDE, starting in model year 2027. This new cycle would ensure that emission controls are properly functioning in the high load and speed conditions covered by the SET.

We are finalizing the addition of the SET duty cycle for the Spark-ignition HDE primary intended service class, as proposed. We requested comment on revisions we should consider for the CI-based SET procedure to adapt it for SI engines. We received no comments on changes to the procedure itself and the SET standards for Spark-ignition HDE are based on the same SET procedure as we are finalizing for heavy-duty CI engines. After considering comments, we are finalizing SET standards that differ from our proposed options for Spark-ignition HDE.

The EPA HD SI technology demonstration program evaluated emission performance over the SET duty cycle. As shown in Table III-22, the NO_X and NMHC emissions over the SET duty cycle were substantially lower than the emissions from the FTP duty cycle (see Table III-21). Lower levels of NMHC were demonstrated, but at the expense of increased CO emissions in those higher load operating conditions. Engine down-speeding improved CO emissions significantly, while NO_X , NMHC, and PM remained low. The considerably lower NO_X and HC in our SET duty cycle demonstration results leave enough room for manufacturers to calibrate the tradeoff in TWC emission control of NO_X, HC, and CO to continue to fine-tune CO. See Chapter 3.2 of the RIA for an expanded description of the test program and results.

Similar to our discussion related to the FTP standards, we expect manufacturers, who design and have full access to the engine controls, could achieve emission levels comparable to or lower than our feasibility demonstration over the SET duty cycle by adopting other approaches, including a combination of calibration changes, optimized catalyst location, and fuel control strategies that EPA was unable to evaluate due to limited access to proprietary engine controls. In fact, we are aware of advanced engine architectures that can reduce or eliminate enrichment, and the associated CO emissions, by maintaining closed loop operation.

We proposed Spark-ignition HDE standards for HC and CO emissions on the SET cycle that were numerically equivalent to the respective proposed FTP standards. Our intent was to ensure that SI engine manufacturers utilize emission control hardware and calibration strategies to control emissions during high load operation to levels similar to the FTP duty cycle. We retain this approach for HC, but, after considering comments, the final CO standard is revised from that proposed. One commenter indicated that manufacturers would need CO credits to achieve the proposed standards. Another commenter suggested that EPA underestimated the modifications manufacturers would need to make to fully transition away from the fuel enrichment strategies they currently use to protect their engines. The same commenter requested that EPA delay the SET to start in model year 2031 or temporarily exclude the highest load points over the test to provide additional lead time for manufacturers.

We are not finalizing an option for manufacturers to generate CO credits. We believe a delayed implementation of SET, as requested, would further delay manufacturers' motivation to focus on high load operation to reduce enrichment and the associated emissions reductions that would result. Additionally, our objective for adding new standards over the SET duty cycle is to capture the prolonged, high-load operation not currently represented in the FTP duty cycle, and the commenter's recommendation to exclude the points of highest load would be counter to that objective.

We agree with commenters that the new SET duty cycle and standards will be a challenge for heavy-duty SI manufacturers but maintain that setting a feasible technology-forcing CO standard is consistent with our authority under the CAA. After further considering the comments and assessing CO data from the EPA heavy-duty SI test program, the final new CO standard we are adopting is less stringent than proposed to provide manufacturers additional margin for ensuring compliance with that pollutant's standard over the new test procedure for Spark-ignition HDE. Given this final standard, we determined that neither ABT or more lead time are appropriate or required. The Spark-ignition HDE standard for CO emissions on the SET duty-cycle established in this final rule is numerically equivalent to the current FTP standard of 14.4 g/hp-hr.

2. Other Provisions for Spark-Ignition HDE

This Section III.D.2 describes other provisions we proposed and are finalizing with revisions from proposal in this rule. The following three provisions address information manufacturers will share with EPA as part of their certification and we are adding clarification where needed after considering comments. See also section 3 of the Response to Comments for a detailed discussion of the comments summarized in this section and how they have informed the updates we are finalizing for these three provisions.

Idle Control for Spark-Ignition HDE

We proposed to add a new paragraph at 40 CFR 1036.115(j)(1) to require manufacturers to show how they maintain a catalyst bed temperature of 350 °C in their application for certification or get approval for an alternative strategy that maintains low emissions during idle. As described in Chapter 3.2 of the RIA, prolonged idling events may allow the catalyst to cool and reduce its efficiency, resulting in emission increases until the catalyst temperatures increase. Our recent HD SI test program showed idle events that extend beyond four minutes allow the catalyst to cool below the light-off temperature of 350 °C. The current heavy-duty SET and FTP duty cycles do not include sufficiently long idle periods to represent these real-world conditions where the exhaust system cools below the catalyst's light-off temperature.

We continue to believe that a 350 °C lower bound for catalysts will sufficiently ensure emission control is maintained during idle without additional manufacturer testing. We are finalizing the 350 °C target and the option for manufacturers to request approval for a different strategy, as proposed. We are revising the final requirement from our proposal to also allow manufacturers to request approval of a temperature lower than 350 °C, after considering comments that requested that we replace the 350 °C temperature with the more generic “light-off temperature” to account for catalysts with other formulations or locations relative to the engine.

i. Thermal Protection Temperature Modeling Validation

The existing regulations require manufacturers to report any catalyst protection strategy that reduces the effectiveness of emission controls as an AECD in their application for certification. The engine controls used to implement these strategies often rely on a modeling algorithm to predict high exhaust temperatures and to disable the catalyst, which can change the emission control strategy and directly impact real world emissions. The accuracy of these models used by manufacturers is critical in both ensuring the durability of the emission control equipment and preventing excessive emissions that could result from unnecessary or premature activation of thermal protection strategies.

To ensure that a manufacturer's model accurately estimates the temperatures at which thermal protection modes are engaged, we proposed a validation process during certification in a new paragraph 40 CFR 1036.115(j)(2) to demonstrate the model performance.

Several commenters opposed the proposed requirement that manufacturers demonstrate a 5 °C accuracy between modelled and actual exhaust and emission component temperatures and expressed concern with the ability to prove correlation at this level and lack of details on the procedure for measuring the temperatures. Our final, revised approach still ensures EPA has the information needed to appropriately assess a manufacturer's AECD strategy, without a specific accuracy requirement.

Our final 40 CFR 1036.115(j)(2) clarifies that the new validation process is a requirement in addition to the requirements for any SI engine applications for certification that include an AECD for thermal protection. Instead of the proposed 5 °C accuracy requirement, a manufacturer will describe why they rely on any AECDs, instead of other engine designs, for thermal protection of catalyst or other emission-related components. They will also describe the accuracy of any modeled or measured temperatures used to activate the AECD. Instead of requiring manufacturers to submit second-by-second data upfront in the application for certification to demonstrate a specific accuracy requirement is met, the final requirement gives EPA discretion to request the information at certification. We note that our final revised requirements apply the same validation process for modeled and measured temperatures that activate an AECD and that this requirement would not apply if manufacturers certify their engines without an AECD for enrichment as thermal protection.

ii. OBD Flexibilities

In recognition that there can be some significant overlap in the technologies and emission control systems adopted for products in the chassis-certified and engine-certified markets, we proposed an OBD flexibility to limit the data requirements for engine-certified products that use the same engines and generally share similar emission controls ( i.e., are “sister vehicles”) with chassis-certified products. Specifically, in a new 40 CFR 1036.110(a)(2), we proposed to allow vehicle manufacturers the option to request approval to certify the OBD of their SI, engine-certified products using data from similar chassis-certified Class 2b and Class 3 vehicles that meet the provisions of 40 CFR 86.1806-17.

Two organizations commented in support of the proposed OBD flexibility and with one suggesting some revisions to the proposed regulatory language. The commenter suggested that the expression `share essential design characteristics' was too vague, and requested EPA provide more specific information on what EPA will use to make their determination. We disagree that more specific information is needed. We are relying on the manufacturers to identify the design characteristics and justify their request as part of the certification process. We are adjusting the final regulatory text to clarify how the vehicles above and below 14,000 lbs GVWR must use the same engine and share similar emission controls, but are otherwise finalizing this OBD flexibility as proposed.

E. Summary of Spark-Ignition HDV Refueling Emission Standards and Test Procedures

All sizes of complete and incomplete heavy-duty vehicles have been subject to evaporative emission standards for many years. Similarly, all sizes of complete heavy-duty vehicles are subject to refueling standards. We most recently applied the refueling standards to complete heavy-duty vehicles above 14,000 pounds GVWR starting with model year 2022 (81 FR 74048, Oct. 25, 2016).

We proposed to amend 40 CFR 1037.103 to apply the same refueling standard of 0.20 grams hydrocarbon per gallon of dispensed fuel to incomplete heavy-duty vehicles above 14,000 pounds GVWR starting with model year 2027 over a useful life of 150,000 miles or 15 years (whichever comes first). We further proposed to apply the same testing and certification procedures that currently apply for complete heavy-duty vehicles. We are adopting this standard and testing and certification procedures as proposed, with some changes to the proposed rule as noted in this section. As noted in 40 CFR 1037.103(a)(2), the standards apply for vehicles that run on gasoline, other volatile liquid fuels, and gaseous fuels.

The proposed rule provided an extensive discussion of the history of evaporative and refueling standards for heavy-duty vehicles, along with rationale and information supporting the proposed standards (87 FR 17489, March 28, 2022). The RIA includes additional information related to control technology, feasibility, and test procedures. See also section 3 of the Response to Comments for a detailed discussion of the comments and the changes we made to the proposed rule.

Some commenters advocated for applying the refueling standards also to incomplete heavy-duty vehicles at or below 14,000 pounds GVWR. Specifically, some manufacturers commented that they would need a phase-in schedule that allowed more lead time beyond the proposed MY 2027 start of the refueling standards for incomplete vehicles above 14,000 pounds GVWR, and that EPA should consider a longer phase-in that also included refueling standards for incomplete vehicles at or below 14,000 pounds GVWR. In EPA's judgment, the design challenge for meeting the new refueling standards will mainly involve larger evaporative canisters, resizing purge valves, and recalibrating for higher flow of vapors from the evaporative canister into the engine's intake. Four years of lead time is adequate for designing, certifying, and implementing these design solutions. We are therefore finalizing the proposed start of refueling standards in MY 2027 for all incomplete heavy-duty vehicles above 14,000 pounds GVWR.

At the same time, as manufacturers suggested, expanding the scope of certification over a longer time frame may be advantageous for implementing design changes across their product line in addition to the environmental gain from applying refueling controls to a greater number of vehicles. We did not propose refueling standards for vehicles at or below 14,000 pounds GVWR and we therefore do not adopt such standards in this final rule. However, the manufacturers' suggestion to consider a package of changes to both expand the scope of the standards and increase the lead time for meeting standards has led us to adopt an optional alternative phase-in. Under the alternative phase-in compliance pathway, instead of certifying all vehicles above 14,000 pounds GVWR to the refueling standard in MY 2027, manufacturers can opt into the alternate phase-in that applies for all incomplete heavy-duty vehicles, regardless of GVWR. The alternative phase-in starts at 40 percent of production in MYs 2026 and 2027, followed by 80 percent of production in MYs 2028 and 2029, ramping up to 100 percent of production in MY 2030. Phase-in calculations are based on projected nationwide production volume of all incomplete heavy-duty vehicles subject to refueling emission standards under 40 CFR 86.1813-17. Specifying the phase-in schedule in two-year increments allows manufacturers greater flexibility for integrating emission controls across their product line.

Manufacturers may choose either schedule of standards; however, they must satisfy at least one of the two. That is, if manufacturers do not certify all their incomplete heavy-duty vehicles above 14,000 pounds GVWR to the refueling standards in MY 2027, the alternate phase-in schedule described in 40 CFR 86.1813-17(b) becomes mandatory to avoid noncompliance. Conversely, if manufacturers do not meet the alternative phase-in requirement for MY 2026, they must certify all their incomplete heavy-duty vehicles above 14,000 pounds GVWR to the refueling standard in MY 2027 to avoid noncompliance. See the final 40 CFR 86.1813-17(b) for the detailed specifications for the alternative phase-in schedule.

We received several comments suggesting that we adjust various aspects of the testing and certification procedures for heavy-duty vehicles meeting the evaporative and refueling standards. Consideration of these comments led us to include some changes from proposal for the final rule. First, we are revising 40 CFR 1037.103 to add a reference to the provisions from 40 CFR part 86, subpart S, that are related to the refueling standards. This is intended to make clear that the overall certification protocol from 40 CFR part 86, subpart S, applies for heavy-duty vehicles above 14,000 pounds GVWR (see also existing 40 CFR 1037.201(h)). This applies, for example, for durability procedures, useful life, and information requirements for certifying vehicles. Along those lines, we are adding provisions to 40 CFR 86.1821-01 to clarify how manufacturers need to separately certify vehicles above 14,000 pounds GVWR by dividing them into different families even if they have the same design characteristics as smaller vehicles. This is consistent with the way we have been certifying vehicles to evaporative and refueling standards.

Second, we are modifying the test procedures for vehicles with fuel tank capacity above 50 gallons. These vehicles have very large quantities of vapor generation and correspondingly large evaporative and refueling canisters. The evaporative test procedures call for manufacturers to design their vehicles to purge a canister over about 11 miles of driving (a single FTP duty cycle) before the diurnal test, which requires the vehicle to control the vapors generated over two simulated hot summer days of parking. We share manufacturers' concern that the operating characteristics of these engines and vehicles do not support achieving that level of emission control. We are therefore revising the two-day diurnal test procedure at 40 CFR 86.137-94(b)(24) and the Bleed Emission Test Procedure at 40 CFR 86.1813-17(a)(2)(iii) to include a second FTP duty cycle with an additional 11 miles of driving before starting the diurnal measurement procedure.

Third, manufacturers pointed out that the existing test procedures don't adequately describe how to perform a refueling emission measurement with vehicles that have two fuel tanks with separate filler necks. We are amending the final rule to include a provision to direct manufacturers to use good engineering judgment for testing vehicles in a dual-tank configuration. It should be straightforward to do the testing with successive refills for the two tanks and combining the measured values into a single result. Rather than specifying detailed adjustments to the procedure, allowing manufacturers the discretion to perform that testing and computation consistent with good engineering judgment will be enough to ensure a proper outcome.

Table III-23 summarizes the cost estimations for the different technological approaches to controlling refueling emissions that EPA evaluated. See Chapter 3.2.3.2 of the RIA for the details. In calculating the overall cost, we used $25 (2019 dollars), the average of both approaches, to represent the cost for manufacturers to adopt the additional canister capacity and hardware to meet our new refueling emission standards for incomplete vehicles above 14,000 lb GVWR. See also Section V of this preamble for a summary of our overall program cost and Chapter 7 of the RIA for more details on our overall program cost.

Incomplete vehicles above 14,000 lb GVWR with dual fuel tanks may require some unique accommodations to adopt onboard refueling vapor recovery (ORVR) systems. A chassis configuration with dual fuel tanks would need separate canisters and separate filler pipes and seals for each fuel tank. Depending on the design, a dual fuel tank chassis configuration may require a separate purge valve for each fuel tank. We assume manufacturers will install one additional purge valve for dual fuel tank applications that also incorporate independent canisters for the second fuel tank/canister configuration, and that manufacturers adopting a mechanical seal in their filler pipe will install an anti-spitback valve for each filler pipe. See Chapter 1.2.4.5 of the RIA for a summary of the design considerations for these fuel tank configurations. We did not include an estimate of the impact of dual fuel tank vehicles in our cost analysis of the new refueling emission standards, as the population of these vehicles is very low and we expect minimal increase in the total average costs.

IV. Compliance Provisions and Flexibilities

EPA certification is a fundamental requirement of the Clean Air Act for manufacturers of heavy-duty highway engines. EPA has employed significant discretion over the past several decades in designing and updating many aspects of our heavy-duty engine and vehicle certification and compliance programs. In the following sections, we discuss several revised provisions that we believe will increase the effectiveness of our regulations.

As noted in Section I, we are migrating our criteria pollutant regulations for model years 2027 and later heavy-duty highway engines from their current location in 40 CFR part 86, subpart A, to 40 CFR part 1036. Consistent with this migration, the compliance provisions discussed in this section refer to the final regulations in their new location in part 1036. In general, this migration is not intended to change the compliance program specified in part 86, except as specifically finalized in this rulemaking. See Section III.A.1.

A. Regulatory Useful Life

Useful life represents the period over which emission standards apply for certified engines, and, practically, any difference between the regulatory useful life and the generally longer operational life of in-use engines represents miles and years of operation without an assurance that emission standards will continue to be met. In addition to promulgating new emission standards and promulgating new and updating existing test procedures described in Section III, we are updating regulatory useful life periods to further assure emission performance of heavy-duty highway engines. In this section, we present the updated regulatory useful life periods we are finalizing in this rule. In Section IV.A.1, we present our revised useful life periods that will apply for the new exhaust emission standards for criteria pollutants, OBD, and requirements related to crankcase emissions. In Section IV.A.2, we present the useful life periods that will apply for the new refueling emission standards for certain Spark-ignition HDE. As described in Section G.10 of this preamble, we are not finalizing the proposed allowance for manufacturers to generate NO_X emissions credits from heavy-duty zero emissions vehicles (ZEVs) or the associated useful life requirements.

1. Regulatory Useful Life Periods by Primary Intended Service Class

In this final rule, we are increasing the regulatory useful life mileage values for new heavy-duty engines to better reflect real-world usage, extend the emissions durability requirement for heavy-duty engines, and improve long-term emission performance. In this Section IV.1, we describe the regulatory useful life periods we are finalizing for the four primary intended service classes for heavy-duty highway engines. Our longer useful life periods vary by engine class to reflect the different lengths of their estimated operational lives. As described in the proposal for this rule, we continue to consider operational life to be the average mileage at rebuild for CI engines and the average mileage at replacement for SI engines.

In determining the appropriate longer useful life values to set in the final rule, we retain our proposed objective to set useful life periods that cover a significant portion of the engine's operational life. However, as explained in the proposal, we also maintain that the emission standards presented in Section III must be considered together with their associated useful life periods. After further consideration of the basis for the proposal, comments received, supporting data available since the proposal, and the numeric level of the final standards, we are selecting final useful life values within the range of options proposed that cover a significant portion of the engine's operational life and take into account the combined effect of useful life and the final numeric standards on the overall stringency and emissions reductions of the program. As described in the final RIA, we concluded two engine test programs for this rule that demonstrated technologies that are capable of meeting lower emission levels at much longer mileages than current useful life periods. We evaluated a heavy-duty diesel engine to a catalyst-aged equivalent of 800,000 miles for the compression-ignition demonstration program, and a heavy-duty gasoline engine to a catalyst-aged equivalent of 250,000 miles for the spark-ignition demonstration program. As described in Section III of this preamble, the results of those demonstration programs informed the appropriate standard levels for the useful life periods we are finalizing for each engine class. Our final useful life values were also informed by comments, including additional information on uncertainties and potential corresponding costs. We summarize key comments in Section IV.1.ii, and provide complete responses to useful life comments in section 3.8 of the Response to Comments document.

Our final useful life periods for Spark-ignition HDE, Light HDE, Medium HDE, and Heavy HDE classes are presented in Table IV-1 and specified in a new 40 CFR 1036.104(e). The final useful life values that apply for Spark-ignition HDE, Light HDE, and Medium HDE starting in MY 2027 match the most stringent option we proposed, that is, MY 2031 step of proposed Option 1. The final useful life values for Heavy HDE, which has a distinctly longer operational life than the smaller engine classes, match the longest useful life mileage we proposed for model year 2027 ( i.e., the Heavy HDE mileage of proposed Option 2). We are also increasing the years-based useful life from the current 10 years to values that vary by engine class and match the proposed value in the respective proposed option. After considering comments, we are also adding hours-based useful life values to all primary intended service classes based on a 20 mile per hour speed threshold and the corresponding final mileage values.

For hybrid engines and powertrains, we are finalizing the proposal that manufacturers certifying hybrid engines and powertrains would declare the primary intended service class of their engine family using 40 CFR 1036.140. Once a primary intended service class is declared, the engine configuration would be subject to the corresponding emission standards and useful life values from 40 CFR 1036.104.

i. Summary of the Useful Life Proposal

For CI engines, the proposed Option 1 useful life periods included two steps in MYs 2027 and 2031 that aligned with the final useful life periods of CARB's HD Omnibus regulation, and the proposed MY 2031 periods covered close to 80 percent of the expected operational life of CI engines based on mileage at out-of-frame rebuild. The useful life mileages of proposed Option 2, which was a single-step option starting in MY 2027, generally corresponded to the average mileages at which CI engines undergo the first in-frame rebuild. The rebuild data indicated that CI engines can last well beyond the in-frame rebuild mileages. We noted in the proposal that it was unlikely that we would finalize a single step program with useful life mileages shorter than proposed Option 2; instead, we signaled that we would likely adjust the numeric value of the standards to address any feasibility concerns.

For Spark-ignition HDE, the useful life mileage in proposed Option 1 was about 90 percent of the operational life of SI engines based on mileage at replacement. The useful life of proposed Option 2 aligned with the current SI engine useful life mileage that applies for GHG standards. In the proposal, we noted that proposed Option 2 also represented the lowest useful life mileage we would consider finalizing for Spark-ignition HDE.

In proposed Option 1, we increased the years-based useful life values for all engine classes to account for engines that accumulate fewer miles annually. We also proposed to update the hours-based useful life criteria for the Heavy HDE class to account for engines that operated frequently, but accumulated relatively few miles due to lower vehicle speeds. We calculated the proposed hours values by applying the same 20 mile per hour conversion factor to the proposed mileages as was applied when calculating the useful life hours that currently apply for Heavy HDE. The proposed hours specification was limited to the Heavy HDE class to be consistent with current regulations, but we requested comment on adding hours-based useful life values to apply for the other service classes.

ii. Basis for the Final Useful Life Periods

In this Section IV.1.ii, we provide the rationale for our final useful life periods, including summaries and responses to certain comments that informed our final program. The complete set of useful life comments and our responses are in section 3.8 of the Response to Comments document. As explained in the NPRM, CAA section 202(d) provides that the minimum useful life for heavy-duty vehicles and engines is a period of 10 years or 100,000 miles, whichever occurs first, and further authorizes EPA to adopt longer useful life periods that we determine to be appropriate.

Many commenters expressed general support for our proposal to lengthen useful life periods in this rulemaking. Several commenters expressed specific support for the useful life periods of proposed Option 1 or proposed Option 2. Other commenters recommended EPA revise the proposal to either lengthen or shorten the useful life periods to values outside of the range of our proposed options.

We are lengthening the current useful life mileages to capture the greatest amount of the operational life for each engine class that we have determined is appropriate at this time. We disagree with commenters recommending that we finalize useful life periods below the mileages of proposed Option 2. As noted in our proposal, proposed Option 2 represented the lower bound of useful life mileages we would consider finalizing for all engine classes. Furthermore, as described in Section III of this preamble and Chapter 3 of the RIA for this final rule, both of EPA's engine test programs successfully demonstrated that CI and SI engine technologies can achieve low emission levels at mileages (800,000 miles and 250,000 miles, respectively) well beyond Option 2. Even after taking into consideration uncertainties of the impacts of variability and real world operation on emission levels at the longest mileages, the test programs' data supports that mileages at least as long as Option 2 are appropriate, and the final standards are feasible at those mileages. We also disagree with commenters suggesting we finalize mileages longer than proposed Option 1. We did not propose and for the reasons just explained about impacts on emission level at the longest mileages do not believe it is appropriate at this time to require useful life periods beyond proposed Option 1.

Organizations submitting adverse comments on useful life focused mostly on the useful life mileages proposed for the Heavy HDE service class. Technology suppliers and engine manufacturers expressed concern with the lack of data from engines at mileages well beyond the current useful life. Suppliers commented that it could be costly and challenging to design components without more information on component durability, failure modes, and use patterns at high mileages. Engine manufacturers claimed that some uncertainties relating to real world use would limit the feasibility of the proposed Option 1 useful life periods, including: The range of applications in which these engines are used, variable operator behavior (including 2nd and 3rd owners), and the use of new technology that is currently unproven in the field. In Sections III and IV.F of this preamble, we describe other areas where useful life plays a role and manufacturers expressed concern over uncertainties, including certification, DF testing, engine rating differences, lab-to-lab variability, production variability, and in-use engine variability. Due to these combined uncertainties, manufacturers stated that they expect to be conservative in their design and maintenance strategies, and some may opt to schedule aftertreatment replacement as a means to ensure compliance with new NO_X emission standards, particularly for proposed Option 1 numeric standards and useful life values. Comments did not indicate a concern that manufacturers may schedule aftertreatment replacement for the smaller engine classes at the proposed Option 1 useful life periods.

We agree that there are uncertainties associated with implementing new technology to meet new emission standards, and recognize that the uncertainties are highest for Heavy HDE that are expected to have the longest operational life and useful life periods. We acknowledge that higher useful life mileage is one factor that may contribute to a risk that manufacturers would schedule aftertreatment replacement to ensure compliance for the heaviest engine class. Specific to Heavy HDE, the final useful life mileage of 650,000 miles matches the longest useful life mileage we proposed for model year 2027 and we expect manufacturers have experience with their engines at this mileage through their extended warranty offerings, thus reducing uncertainties of real world operation compared to the longest useful life mileage we proposed ( i.e., 800,000 miles). For Heavy HDE, the final numeric emission standards and useful life periods matching proposed Option 2, combined with other test procedure revisions to provide clarity and address variability, will require less conservative compliance strategies than proposed Option 1 and will not require manufacturers to plan for the replacement of the entire catalyst system. See Section III for further discussion on the basis and feasibility of the final emission standards.

Many commenters supported proposed Option 1, including useful life periods out to 800,000 miles for the Heavy HDE class. Several commenters pointed to EPA's engine testing results on an engine aged to the equivalent of 800,000 miles as adequately demonstrating feasibility of an 800,000-mile useful life for Heavy HDE. We agree that CI engines are capable of meeting low emission levels at very high mileages in a controlled laboratory environment, but manufacturer liability for maintaining certified emission levels over the regulatory useful life period is not restricted to laboratory tests. Manufacturers expressed specific concern about the uncertainties outside the controlled laboratory environment after an engine enters commerce. In Sections III and IV.F of this preamble we summarize comments relating to how useful life factors into certification, DF testing, and in-use testing. In Section III.B, we describe a certification requirement we are finalizing for manufacturers to demonstrate the emission controls on Heavy HDE are durable through the equivalent of 750,000 miles; this durability demonstration will extend beyond the 650,000 mile useful life period for these engines. We expect this extended laboratory-based demonstration, in a controlled environment, will translate to greater assurance that an engine will maintain its certified emission levels in real world operation where conditions are more variable throughout the regulatory useful life. This greater assurance would be achieved while minimizing the compliance uncertainties identified by manufacturers in comments for the highest proposed useful life mileages.

We believe manufacturers can adequately ensure the durability of their smaller engines over useful life periods that match proposed Option 1 both for meeting emission standards in the laboratory at certification and in the laboratory and applicable in-use testing after operation in the real world. The final durability demonstration requirements for Spark-ignition HDE, Light HDE, and Medium HDE match the final useful life periods for those smaller engines classes.

As shown in Table IV-1, we are also finalizing useful life periods in years and hours for all primary intended service classes. We are updating the years values from the current 10 years to 15 years for Spark-ignition HDE and Light HDE, 12 years for Medium HDE, and 11 years for Heavy HDE. The final years values match the years values we proposed and vary by engine class corresponding to the proposed mileage option we are finalizing. We are also adding hours as a useful life criteria for all engine classes. We received no adverse comments for hours-based useful life periods and are finalizing hours values by applying a 20-mph conversion factor, as proposed, to calculate hours values from the final mileage values.

We have finalized a combination of emissions standards and useful life values that our analysis and supporting data demonstrate are feasible for all heavy-duty engine classes. We are lengthening the existing useful life mileages to capture the greatest amount of the operational life for each engine class that we have determined is appropriate at this time, while considering the impact of useful life length on the stringency of the standards and other requirements of this final rule. Preamble Section III describes how our analysis and the EPA engine test programs demonstrated feasibility of the standards at these useful life values, including data on emission levels at the equivalent useful life mileages.

2. Useful Life for Incomplete Vehicle Refueling Emission Standards

As described in Section III.E., we are finalizing a refueling emission standard for incomplete vehicles above 14,000 lb GVWR. Manufacturers would meet the refueling emission standard by installing onboard refueling vapor recovery (ORVR) systems on these incomplete vehicles. Since ORVR systems are based on the same carbon canister technology that manufacturers currently use to control evaporative emissions on these incomplete vehicles, we proposed to align the useful life periods for the two systems. In 40 CFR 1037.103(f), we are finalizing a useful life of 15 years or 150,000 miles, whichever comes first, for refueling standards for incomplete vehicles above 14,000 lb GVWR, as proposed.

Evaporative emission control systems are currently part of the fuel system of incomplete vehicles, and manufacturers are meeting applicable standards and useful life requirements for evaporative systems today. ORVR is a mature technology that has been installed on complete vehicles for many years, and incomplete vehicle manufacturers have experience with ORVR systems through their complete vehicle applications. Considering the manufacturers' experience with evaporative emission standards for incomplete vehicles, and their familiarity with ORVR systems, we continue to believe it would be feasible for manufacturers to apply the same evaporative emission standard useful life periods to refueling standards. We received no adverse comments relating to the proposed 15 years/150,000 miles useful life for refueling standards, and several manufacturers commented in support of our proposed periods.

B. Ensuring Long-Term In-Use Emissions Performance

In the proposal, we introduced several ideas for an enhanced, comprehensive strategy to ensure in-use emissions performance over more of an engine's operational life. In this section, we discuss the final provisions to lengthen emission-related warranty periods, update maintenance requirements, and improve serviceability in this rule. Taken together, these updates are intended to increase the likelihood that engine emission controls will be maintained properly through more of the service life of heavy-duty engines and vehicles, including beyond useful life.

1. Emission-Related Warranty

The emission-related warranty period is the period over which CAA section 207 requires an engine manufacturer to warrant to a purchaser that the engine is designed, built, and equipped so as to conform with applicable regulations under CAA section 202 and is free from defects in materials or workmanship which would cause the engine not to conform with applicable regulations for the warranty period. If an emission-related component fails during the regulatory emission warranty period, the manufacturer is required to pay for the cost of repair or replacement. A manufacturer's general emissions warranty responsibilities are currently set out in 40 CFR 1068.115. Note that while an emission warranty provides protection to the owner against emission-related repair costs during the warranty period, the owner is responsible for properly maintaining the engine (40 CFR 1068.110(e)), and the manufacturer may deny warranty claims for failures that have been caused by the owner's or operator's improper maintenance or use (40 CFR 1068.115(a)).

In this section, we present the updated emission-related warranty periods we are finalizing for heavy-duty highway engines and vehicles included in this rule. As described in Section G.10 of this preamble, we are not finalizing the proposed allowance for manufacturers to generate NO_X emissions credits from heavy-duty zero emissions vehicles (ZEVs) or the associated warranty requirements.

i. Final Warranty Periods by Primary Intended Service Class

We are updating and significantly strengthening our emission-related warranty periods for model year 2027 and later heavy-duty engines. We are finalizing most of the emission-related warranty provisions of 40 CFR 1036.120 as proposed. Following our approach for useful life, we are revising the proposed warranty periods for each primary intended service class to reflect the difference in average operational life of each class and after considering additional information provided by commenters. See section 4 of the Response to Comments document for our detailed responses, including descriptions of revisions to the proposed regulatory text in response to commenter requests for clarification.

EPA's current emissions-related warranty periods for heavy-duty engines range from 22 percent to 54 percent of the current regulatory useful life; the warranty periods have not changed since 1983 even as the useful life periods were lengthened. The revised warranty periods are expected to result in better engine maintenance and less tampering, which would help to maintain the benefits of the emission controls. In addition, longer regulatory warranty periods may lead engine manufacturers to simplify repair processes and make them more aware of system defects that need to be tracked and reported to EPA.

Our final emission-related warranty periods for heavy-duty engines are presented in Table IV-2 and specified in a new 40 CFR 1036.120. The final warranty mileages that apply starting in MY 2027 for Spark-ignition HDE, Light HDE, and Medium HDE match the longest warranty mileages proposed ( i.e., MY 2031 step of proposed Option 1) for these primary intended service classes. For Heavy HDE, the final warranty mileage matches the longest warranty mileage proposed for MY 2027 ( i.e., MY 2027 step of proposed Option 1). We are also increasing the years-based warranty from the current 5 years to 10 years for all engine classes. After considering comments, we are also adding hours-based warranty values to all primary intended service classes based on a 20 mile per hour speed threshold and the corresponding final mileage values. Consistent with current warranty provisions, the warranty period would be whichever warranty value ( i.e., mileage, hours, or years) occurs first. We summarize key comments in Section IV.B.1.i.a, and provide complete responses to warranty comments in section 4 of the Response to Comments document.

We note that we are finalizing as proposed that when a manufacturer's certified configuration includes hybrid system components ( e.g., batteries, electric motors, and inverters), those components are considered emission-related components, which would be covered under the warranty requirements in new 40 CFR 1036.120. Similar to the approach for useful life in Section IV.A, a manufacturer certifying a hybrid engine or hybrid powertrain would declare a primary intended service class for the engine family and apply the corresponding warranty periods in 40 CFR 1036.120 when certifying the engine configuration. This approach to clarify that hybrid components are part of the broader engine configuration provides vehicle owners and operators with consistent warranty coverage based on the intended vehicle application.

We estimated the emissions impacts of the final warranty periods in our inventory analysis, which is summarized in Section VI and discussed in detail in Chapter 5 of our RIA. In Section V, we estimate costs associated with the final warranty periods, including indirect costs for manufacturers and operating costs for owners and operators.

a. Summary of the Emission-Related Warranty Proposal

In the proposal, we included several justifications for lengthened warranty periods that continue to apply for the final provisions. First, we expected longer emission-related warranty periods would lead owners to continue maintain their engines and vehicles over a longer period of time and ensure longer-term benefits of emission controls. Since emission-related repairs would be covered by manufacturers for a longer period of time, an owner would be more likely to have systems repaired and less likely to tamper to avoid the cost of a repair.

Second, emission-related repair processes may get more attention from manufacturers if they are responsible for repairs over a longer period of time. The current, relatively short warranty periods provide little incentive for manufacturers to evaluate the complexity of their repair processes, since the owner pays for the repairs after the warranty period ends. As manufacturers try to remain competitive, longer emission warranty periods may lead manufacturers to simplify repair processes and provide better training to technicians in an effort to reduce their warranty repair costs. Simplifying repair processes could include modifying emission control components in terms of how systems are serviced and how components are replaced ( e.g., modular sub-assemblies that could be replaced individually, resulting in a quicker, less expensive repair). Improved technician training may also reduce warranty repair costs by improving identification and diagnosing component failures more quickly and accurately, thus reducing downtime for owners and avoiding repeated failures, misdiagnoses of failures, and higher costs from repeat repair events at service facilities.

Finally, longer regulatory emission warranty periods would increase the period over which the engine manufacturer would be made aware of emission-related defects. Manufacturers are currently required to track and report defects to the Agency under the defect reporting provisions of 40 CFR part 1068. Under 40 CFR 1068.501(b), manufacturers investigate possible defects whenever a warranty claim is submitted for a component. Therefore, manufacturers can easily monitor defect information from dealers and repair shops who are performing those warranty repair services, but after the warranty period ends, the manufacturer would not necessarily know about these events, since repair facilities are less likely to be in contact with the manufacturers and they are less likely to use OEM parts. A longer warranty period would allow manufacturers to have access to better defect information over a period of time more consistent with engine useful life.

In the proposal, we also highlighted that a longer warranty period would encourage owners of vehicles powered by SI engines (as for CI engines) to follow manufacturer-prescribed maintenance procedures for a longer period of time, as failure to do so would void the warranty. We noted that the impact of a longer emissions warranty period may be slightly different for SI engines from a tampering perspective. Spark-ignition engine systems rely on mature technologies, including evaporative emission systems and three-way catalyst-based emission controls, that have been consistently reliable for light-duty and heavy-duty vehicle owners. SI engine owners may not currently be motivated to tamper with their catalyst systems to avoid repairs, but they may purchase defeat devices intended to disable emission controls to boost the performance of their engines. We expected SI engine owners may be less inclined to install such defeat devices during a longer warranty period.

We proposed two options that generally represented the range of revised emission warranty periods we considered adopting in the final rule. Proposed Option 1 included warranty periods that aligned with the MY 2027 and MY 2031 periods of the CARB HD Omnibus program and were close to 80 percent of useful life. At the time of the proposal, we assumed most manufacturers would continue to certify 50-state compliant engines in MY 2027 and later, and it would simplify the certification process if there would be consistency between CARB and Federal requirements. The warranty periods of proposed Option 2 were proposed to apply in a single step beginning in model year 2027 and to match CARB's Step 1 warranty periods for engines sold in California. The proposed Option 2 mileages covered 40 to 55 percent of the proposed Option 1 MY 2031 useful life mileages and represented an appropriate lower end of the range of the revised regulatory emission warranty periods we considered.

While we noted that a majority of engines would reach the warranty mileage in a reasonable amount of time, some applications may have very low annual mileage due to infrequent use or low speed operation and may not reach the warranty mileage for many years. To ensure manufacturers are not indefinitely responsible for components covered under emissions warranty in these situations, we proposed to revise the years-based warranty periods and proposed hours-based warranty periods for all engine classes in proposed Option 1.

For the years-based period, which would likely be reached first by engines with lower annual mileage due to infrequent use, we proposed to increase the current period from 5 years to 7 years for MY 2027 through 2030, and to 10 years starting with MY 2031. We also proposed to add an hours-based warranty period to cover engines that operate at low speed and/or are frequently in idle mode. In contrast to infrequent use, low speed and frequent idle operation can strain emission control components. We proposed an hours-based warranty period to allow manufacturers to factor gradually-accumulated work into their warranty obligations.

b. Basis for the Final Emission-Related Warranty Periods

As detailed in section 4 of the Response to Comments document for this rule, commenter support for lengthening emission-related warranty periods varied. Many commenters expressed general support for our proposal to lengthen warranty periods in this rulemaking. Several commenters expressed specific support for the warranty periods of proposed Option 1 or proposed Option 2. Other commenters recommended EPA revise the proposal to either lengthen or shorten the warranty periods to values outside of the range of our proposed options.

Our final warranty periods continue to be influenced by the potential beneficial outcomes of lengthening emission-related warranty periods that we discussed in the proposal. Specifically, we continue to believe lengthened warranty periods will effectively assure owners properly maintain and repair their emission controls over a longer period, reduce the likelihood of tampering, provide additional information on failure modes, and create a greater incentive for manufacturers to simplify repair processes to reduce costs. Several commenters agreed with our list of potential outcomes, with some noting that any associated emissions benefits would be accelerated by pulling ahead the warranty periods of the MY 2031 step of proposed Option 1 to begin in MY 2027.

Organizations submitting adverse comments on lengthening warranty periods focused mostly the warranty mileages proposed for the Heavy HDE service class. Technology suppliers and engine manufacturers expressed concern with the lack of data from engines at high mileages, including uncertainties related to frequency and cause of failures, varying vehicle applications, and operational changes as the engine ages. We considered commenters' concerns regarding how uncertainties for the highest mileages of proposed Option 1 could cause manufacturers to respond by conservatively estimating their warranty cost. We continue to expect, as noted in the proposal, that manufacturers are likely to recoup the costs of warranty by increasing the purchase price of their products. We agree with comments indicating that increases in purchase price can increase the risk of pre-buy or low-buy, especially for the heaviest engine class, Heavy HDE.

As described in this section, the final warranty periods are within the range of periods over which we expect manufacturers have access to failure data, which should limit the need for manufacturers to conservatively estimate warranty costs. We summarize our updated cost and economic impact analyses, which reflect the final warranty periods, in Sections V and X of this preamble, respectively. For more information, see our complete assessments of costs in Chapter 7 and economic impacts in Chapter 10 of the Regulatory Impact Analysis for this final rule.

We retain our proposed objectives to lengthen warranty periods to cover a larger portion of the operational lives and to be more consistent with the final useful life periods. Similar to our approach for the useful life mileages in this final rule (see Section IV.A of this preamble), we believe it is appropriate to pull ahead the longest proposed MY 2031 warranty periods to apply in MY 2027 for the smaller engine classes. For Spark-ignition HDE, Light HDE, and Medium HDE, the final warranty mileages are 160,000 miles, 210,000 miles, and 280,000 miles, respectively, which cover about 80 percent of the corresponding final useful life mileages. In response to commenters concerned with data limitations, we expect any component failure and wear data available from engines in the largest engine class would be applicable to the smaller engine classes. As such, manufacturers and suppliers have access to failure and wear data at the mileages we are finalizing for the smaller engine classes through their current R&D and in-use programs evaluating components for larger engines that currently have a 435,000 mile useful life.

We are not applying the same pull-ahead approach for the Heavy HDE warranty mileage. We do not believe it is appropriate at this time to finalize a 600,000-mile warranty for the Heavy HDE class that would uniquely cover greater than 90 percent of the 650,000- mile final useful life, especially considering the comments pointing to uncertainties, lack of data, and potential high costs specific to Heavy HDE. We are also not applying the approach of adopting the warranty mileage of proposed Option 2, as was done for Heavy HDE useful life, as we do not believe the proposed Option 2 warranty of 350,000 miles would provide emission control assurance over a sufficient portion of the useful life. Instead, we are finalizing a warranty mileage that matches the longest mileage proposed for MY 2027 (450,000 miles), covering a percentage of the final useful life that is more consistent with the warranty periods of the smaller engine classes. The final warranty mileage for Heavy HDE is only 15,000 miles longer than the current useful life for this engine class. As noted for the warranties of the smaller engine classes, we expect manufacturers and suppliers have access to failure data nearing 450,000 miles through their R&D programs evaluating Heavy HDE over their current useful life. We expect manufacturers also have experience with their engines at this mileage through their extended warranty offerings; thus, they already possess real world operational data in addition to their internal evaluations.

Several organizations commented on the proposed years or hours criteria for warranty. One supplier noted that analyses focused on tractors and their relatively high mileages may not accurately predict the use of vocational vehicles that are more limited by hours of operation. The same supplier suggested EPA should further differentiate warranties by vehicles classes and vocations. Another organization cautioned against warranty periods that are one-size-fits-all. Two organizations supported applying an hours-based warranty period for all engine classes to cover lower-speed applications and the 20-mph conversion factor that we proposed.

We agree that vocational vehicles have distinct use patterns; however, we did not propose and are not finalizing warranty periods at the vehicle level to distinguish between vehicle types in this rule. We are finalizing three warranty thresholds for each heavy-duty engine class: A mileage threshold that is likely to reached first by vehicles driving many miles annually, a years threshold that is likely to be reached first by vehicles that drive infrequently or seasonally, and an hours threshold that is likely to be reached first by vehicles that drive frequently at lower speeds or with significant idling. We believe adding an hours threshold in the final rule to the mileage- and years-based warranty periods for all engine classes will lead to more equitable warranty obligations across the range of possible vehicle applications for which a heavy-duty engine may be used.

ii. Warranty for Incomplete Vehicle Refueling Emission Controls

As noted in Section III.E, we are finalizing refueling emission standards for Spark-ignition HDE that are certified as incomplete vehicles above 14,000 lb GVWR. Our refueling standards are equivalent to the refueling standards that are in effect for light- and heavy-duty complete Spark-ignition HDVs. We project manufacturers would meet the new refueling standards by adapting the existing onboard refueling vapor recovery (ORVR) systems from systems designed for complete vehicles. The new ORVR systems will likely supplement existing evaporative emission control systems installed on these vehicles.

We are finalizing warranty periods for the ORVR systems of incomplete vehicles above 14,000 lb GVWR that align with the current warranty periods for the evaporative systems on those vehicles. Specifically, warranty periods for refueling emission controls would be 5 years or 50,000 miles on incomplete Light HDV, and 5 years or 100,000 miles on incomplete Medium HDV and Heavy HDV, as proposed. See our final updates to 40 CFR 1037.120. Our approach to apply the existing warranty periods for evaporative emission control systems to the ORVR systems is similar to our approach to the final regulatory useful life periods associated with our final refueling standards discussed in Section IV.A. We received no adverse comments on our proposed warranty periods for refueling emission controls.

2. Maintenance

In this section, we describe the migrated and updated maintenance provisions we are finalizing for heavy-duty highway engines. Section IV.F of this preamble summarizes the current durability demonstration requirements and our final updates.

Our final maintenance provisions, in a new section 40 CFR 1036.125, combine and amend the existing criteria pollutant maintenance provisions from 40 CFR 86.004-25 and 86.010-38. Similar to other part 1036 sections we are adding in this rule, the structure of the new 40 CFR 1036.125 is consistent with the maintenance sections in the standard-setting parts of other sectors ( e.g., nonroad compression-ignition engines in 40 CFR 1039.125). In 40 CFR 1036.205(i), we are codifying the current manufacturer practice of including maintenance instructions in their application for certification such that approval of those instructions would be part of a manufacturer's certification process. We are also finalizing a new paragraph 40 CFR 1036.125(h) outlining several owner's manual requirements, including migrated and updated provisions from 40 CFR 86.010-38(a).

This section summarizes the final provisions that clarify the types of maintenance, update the options for demonstrating critical emission-related maintenance will occur and the minimum scheduled maintenance intervals for certain components, and specify the requirements for maintenance instructions. The proposed rule provided an extensive discussion of the rationale and information supporting the proposed maintenance provisions (87 FR 17520, March 28, 2022). See also section 6 of the Response to Comments for a detailed discussion of the comments and how they may have informed changes we are making to the proposal in this final rule.

i. Types of Maintenance

The new 40 CFR 1036.125 clarifies that maintenance includes any inspection, adjustment, cleaning, repair, or replacement of components and, consistent with 40 CFR 86.004-25(a)(2), broadly classifies maintenance as emission-related or non-emission-related and scheduled or unscheduled. As proposed, we are finalizing five types of maintenance that manufacturers may choose to schedule: Critical emission-related maintenance, recommended additional maintenance, special maintenance, noncritical emission-related maintenance, and non-emission-related maintenance. As we explained in the proposal, identifying and defining these maintenance categories in final 40 CFR 1036.125 distinguishes between the types of maintenance manufacturers may choose to recommend to owners in maintenance instructions, identifies the requirements that apply to maintenance performed during certification durability demonstrations, and clarifies the relationship between the different types of maintenance, emissions warranty requirements, and in-use testing requirements. The final provisions thus also specify the conditions for scheduling each of these five maintenance categories.

We summarize several revisions to the proposed critical emission-related maintenance provisions in Section 0 with additional details in section 6 of the Response to Comments document. As proposed, the four other types of maintenance will require varying levels of EPA approval. In 40 CFR 1036.125(b), we propose to define recommended additional maintenance as maintenance that manufacturers recommend owners perform for critical emission-related components in addition to what is approved for those components under 40 CFR 1036.125(a). We are finalizing this provision as proposed except for a clarification in wording to connect additional recommended maintenance and critical emission-related maintenance more clearly. Under the final provisions, a manufacturer may recommend that owners replace a critical emission-related component at a shorter interval than the manufacturer received approval to schedule for critical emission-related maintenance; however, the manufacturer will have to clearly distinguish their recommended intervals from the critical emission-related scheduled maintenance in their maintenance instructions. As described in this Section III.B.2 and the proposal, recommended additional maintenance is not performed in the durability demonstration and cannot be used to deny a warranty claim, so manufacturers will not be limited by the minimum maintenance intervals or need the same approval from EPA by demonstrating the maintenance would occur.

In 40 CFR 1036.125(c), we proposed that special maintenance would be more frequent maintenance approved at shorter intervals to address special situations, such as atypical engine operation. We received one comment requesting we clarify special maintenance in proposed 40 CFR 1036.125(c) and we are finalizing this provision as proposed except that we are including an example of biodiesel use in the final paragraph (c). Under the final provisions, manufacturers will clearly state that the maintenance is associated with a special situation in the maintenance instructions provided to EPA and owners.

In 40 CFR 1036.125(d), as proposed, we are finalizing that noncritical emission-related maintenance includes inspections and maintenance that is performed on emission-related components but is considered “noncritical” because emission control will be unaffected (consistent with existing 40 CFR 86.010-38(d)). Under this final provision, manufacturers may recommend noncritical emission-related inspections and maintenance in their maintenance instructions if they clearly state that it is not required to maintain the emissions warranty.

In 40 CFR 1036.125(e), we are updating the paragraph heading from nonemission-related maintenance to maintenance that is not emission-related to be consistent with other sectors. The final provision, as proposed, describes the maintenance as unrelated to emission controls ( e.g., oil changes) and states that manufacturers' maintenance instructions can include any amount of maintenance unrelated to emission controls that is needed for proper functioning of the engine.

Critical Emission-Related Components

Consistent with the existing and proposed maintenance provisions, the final provisions continue to distinguish certain components as critical emission-related components. The proposal did not migrate the specific list of components defined as “critical emission-related components” from 40 CFR 86.004-25(b)(6)(i); instead, we proposed and are finalizing that manufacturers identify their specific critical components by obtaining EPA's approval for critical emission-related maintenance using 40 CFR 1036.125(a). Separately, we also proposed a new definition for critical emission-related components in 40 CFR 1068.30 and are finalizing with revision. The final definition is consistent with paragraph 40 CFR 86.004-25(b)(6)(i)(I) and the current paragraph IV of 40 CFR part 1068, appendix A, as proposed. We are removing the proposed reference to 40 CFR 1068, appendix A, in the final definition, since appendix A specifies emission-related components more generally. To avoid having similar text in two locations, we are also replacing the current text of paragraph IV of 40 CFR 1068, appendix A, with a reference to the new part 1068 definition of critical emission-related components.

ii. Critical Emission-Related Maintenance

A primary focus of the final maintenance provisions is critical emission-related maintenance. Critical emission-related maintenance includes any adjustment, cleaning, repair, or replacement of emission-related components that manufacturers identify as having a critical role in the emission control of their engines. The final 40 CFR 1036.125(a), consistent with current maintenance provisions in 40 CFR part 86 and the proposal, will continue to allow manufacturers to seek advance approval from EPA for new emission-related maintenance they wish to include in maintenance instructions and perform during durability demonstration. The final 40 CFR 1036.125(a) retains the same proposed structure that includes a maintenance demonstration and minimum maintenance intervals, and a pathway for new technology that may be applied in engines after model year 2020.

We are finalizing with revision the maintenance demonstration proposed in 40 CFR 1036.125(a)(1). The final provision includes the five proposed options for manufacturers to demonstrate the maintenance is reasonably likely to be performed in-use, with several clarifying edits detailed in the Response to Comments document . As further discussed in Section IV.D, we are finalizing the separate statement in 40 CFR 1036.125(a)(1) that points to the final inducement provisions, noting that we will accept DEF replenishment as reasonably likely to occur if an engine meets the specifications in proposed 40 CFR 1036.111; we are not setting a minimum maintenance interval for DEF replenishment. Also, as noted in the proposal and reiterated here, the first maintenance demonstration option, described in 40 CFR 1036.125(a)(1)(i), is intended to cover emission control technologies that have an inherent performance degradation that coincides with emission increases, such as back pressure resulting from a clogged DPF.

Consistent with the current and proposed maintenance provisions, we are specifying minimum maintenance intervals for certain emission-related components, such that manufacturers may not schedule more frequent maintenance than we allow. In 40 CFR 1036.125(a)(2), we are updating the list of components with minimum maintenance intervals to more accurately reflect components in use today and extending the replacement intervals such that they reflect replacement intervals currently scheduled for those components. See the NPRM preamble for a discussion of our justification for terminology changes we are applying in the final rule, and the list of components that we are not migrating from 40 CFR part 86 because they are obsolete or covered by other parts.

Consistent with current maintenance provisions, we proposed to disallow replacement of catalyst beds and particulate filter elements within the regulatory useful life of the engine. We are removing reference to catalyst beds and particular filter elements in the introductory text of paragraph (a)(2) and instead are adding them, with updated terminology, as a separate line in the list of components in Table 1 of 40 CFR 1036.125(a)(2) with minimum maintenance intervals matching the final useful life values of this rule. Including catalyst substrates and particulate filter substrates directly in the table of minimum maintenance intervals more clearly connects the intervals to the useful life values. In response to manufacturer comments requesting clarification, we are also adding a reference to 40 CFR 1036.125(g) in paragraph (a)(2) to clarify that manufacturers are not restricted from scheduling maintenance more frequent than the minimum intervals, including replacement of catalyst substrates and particulate filter substrates, if they pay for it.

We are finalizing as proposed the addition of minimum intervals for replacing hybrid system components in engine configurations certified as hybrid engines or hybrid powertrains, which would include the rechargeable energy storage system (RESS). Our final minimum intervals for hybrid system components equal the current useful life for the primary intended service classes of the engines that these electric power systems are intended to supplement or replace.

Table IV-3 summarizes the minimum replacement intervals we are finalizing in a new table in 40 CFR 1036.125(a)(2). As explained in the proposal, we believe it is appropriate to account for replacement intervals that manufacturers have already identified and demonstrated will occur for these components and the final replacement intervals generally match the shortest mileage interval ( i.e., most frequent maintenance) of the published values, with some adjustments after considering comments. Commenters noted that some sensors are not integrated with a listed system and requested EPA retain a discrete set of minimum intervals for sensors, actuators, and related ECMs. We agree and are specifying minimum intervals that match the current intervals for sensors, actuators, and related control modules that are not integrated into other systems. We are retaining the proposed text to indicate that intervals specified for a given system would apply for all to actuators, sensors, tubing, valves, and wiring associated with that component associated with that system. We are also revising the minimum intervals for ignition wires from the proposed 100,000 miles to 50,000 miles to match the current intervals and adding an interval for ignition coils at the same 50,000 miles after considering comments. See section 6 of the Response to Comments document for other comments we considered when developing the final maintenance provisions.

We proposed to retain the maintenance intervals specified in 40 CFR 86.004-25 for adjusting or cleaning components as part of critical emission-related maintenance. We are finalizing the proposed maintenance intervals for adjusting and cleaning with one correction. Commenters noted that the proposal omitted an initial minimum interval for adjusting or cleaning EGR system components. Consistent with 40 CFR 86.004-25(b), we are correcting the proposed intervals for several components (catalyst system components, EGR system components (other than filters or coolers), particulate filtration system components, and turbochargers) from 150,000 miles or 4,500 hours to include an initial interval of 100,000 miles or 3,000 hours, with subsequent intervals of 150,000 miles or 4,500 hours. We did not reproduce the new Table 2 from 40 CFR 1036.125(a)(2) showing the minimum intervals for adjusting or cleaning components in this preamble.

We received no adverse comments on the proposed approach to calculate the corresponding hours values for each minimum maintenance interval. Consistent with our current maintenance provisions and the proposal, we are finalizing minimum hours values based on the final mileage and a 33 miles per hour vehicle speed ( e.g., 150,000 miles would equate to 4,500 hours). Consistent with the current maintenance intervals specified in part 86 and the proposal, we are not including year-based minimum intervals; OEMs can use good engineering judgment if they choose to include a scheduled maintenance interval based on years in their owner's manuals.

For new technology, not used on engines before model year 2020, we are providing a process for manufacturers to seek approval for new scheduled maintenance, consistent with the current maintenance provisions. We received no adverse comment on the proposal to migrate 40 CFR 86.094-25(b)(7)(ii), which specifies a process for approval of new critical emission-related maintenance associated with new technology, and 40 CFR 86.094-25(b)(7)(iii), which allows manufacturers to ask for a hearing if they object to our decision. We are finalizing a new 40 CFR 1036.125(a)(3), as proposed.

iii. Source of Parts and Repairs

Consistent with CAA section 207 and our existing regulations for heavy duty vehicles under part 1037, we proposed a new paragraph 40 CFR 1036.125(f) to clarify that manufacturers' written instructions for proper maintenance and use, discussed further in Section IV.B.2.vi, generally cannot limit the source of parts and service owners use for maintenance unless the component or service is provided without charge under the purchase agreement, with two specified exceptions. We are moving, with revisions, the content of the proposed paragraph (f) to 40 CFR 1036.125(h)(2). See section 6 of the Response to Comments. Consistent with the proposal, we are finalizing that manufacturers cannot specify a particular brand, trade, or corporate name for components or service and cannot deny a warranty claim due to “improper maintenance” based on owners choosing not to use a franchised dealer or service facility or a specific brand of part unless the component or service is provided without charge under the purchase agreement. Consistent with current maintenance provisions and CAA section 207(c)(3)(B), a second exception is that manufacturers can specify a particular service facility and brand of parts only if the manufacturer convinces EPA during the approval process that the engine will only work properly with the identified service or component. We are not finalizing at this time the proposed 40 CFR 1036.125(f) requirement regarding specific statements on the first page of written maintenance instructions; after consideration of comments, we agree with commenters that the final regulatory text accomplishes the intent of our proposal without the additional proposed first sentence.

iv. Payment for Scheduled Maintenance

We proposed 40 CFR 1036.125(g) to allow manufacturers to schedule maintenance not otherwise allowed by 40 CFR 1036.125(a)(2) if they pay for it. The proposed paragraph (g) also included four criteria to identify components for which we would require manufacturers to pay for any scheduled maintenance within the regulatory useful life. The four criteria, which are based on current provisions that apply for nonroad compression-ignition engines, would require manufacturers to pay for components that were not in general use on similar engines before 1980, whose primary purpose is to reduce emissions, where the cost of the scheduled maintenance is more than 2 percent of the price of the engine, and where failure to perform the scheduled maintenance would not significantly degrade engine performance. We continue to believe that components meeting the four criteria are less likely to be maintained without the incentive of manufacturers paying for it and we are finalizing 40 CFR 1036.125(g) as proposed.

As noted in Section IV.B.2.ii, manufacturers cannot schedule replacement of catalyst substrates or particulate filter substrates within the regulatory useful life of the engine unless they pay for it. As explained in the proposed rule, in addition to catalyst substrates and particulate filter substrates, we expect that replacement of EGR valves, EGR coolers, and RESS of certain hybrid systems also meet the 40 CFR 1036.125(g) criteria and manufacturers will only be able to schedule replacement of these components if the manufacturer pays for it.

In the proposal, we requested comment on restricting the replacement of turbochargers irrespective of the four criteria of proposed 40 CFR 1036.125(g). One commenter suggested that EPA should follow the CARB approach that requires manufacturers to pay for scheduled maintenance of turbochargers within the regulatory useful life. The comment indicated the cost of repairs and “significant impact” of a failed turbocharger on emissions justify requiring that manufacturers pay for replacement. We disagree and are not finalizing a separate requirement for turbochargers. Turbochargers are not added to engines specifically to control emissions and we expect the performance degredation associated with a failing turbocharger is likely to motivate owners to fix the problem. We continue to believe the four criteria in 40 CFR 1036.125(g) are an appropriate means of distinguishing components for which manufacturers should pay in order to ensure the components are maintained.

v. Maintenance Instructions

As proposed, our final 40 CFR 1036.125 preserves the requirement that the manufacturer provide written instructions for properly maintaining and using the engine and emission control system, consistent with CAA section 207(c)(3)(A). The new 40 CFR 1036.125(h) describes the information that we are requiring manufacturers to include in an owner's manual, consistent with CAA sections 202 and 207. The new 40 CFR 1036.125(h)(1) generally migrates the existing maintenance instruction provisions specified in 40 CFR 86.010-38(a). As described in Section IV.B.2.iii, final 40 CFR 1036.125(h)(2) includes revised content from proposed 40 CFR 1036.125(f). The final paragraph (h)(2) is also revised from the proposed regulatory text to clarify that EPA did not intend the proposed paragraph as a requirement for owners to maintain records in order to make a warranty claim. While 40 CFR 1036.120(d) allows manufacturers to deny warranty claims for improper maintenance and use, owners have expressed concern that it is unclear what recordkeeping is needed to document proper maintenance and use, and both the proposed and final 40 CFR 1036.125(h)(2) are intended to ensure manufacturers are communicating their expectations to owners.

Consistent with the current 40 CFR 86.010-38(a)(2), our final 40 CFR 1036.125(h)(2) also requires manufacturers to describe in the owner's manual if manufacturers expect owners to maintain any documentation to show the engine and emission control system have been properly maintained and, if so, to specify what documentation. Manufacturers should be able to identify their expectations for documenting routine maintenance and repairs related to warranty claims. For instance, if a manufacturer requires a maintenance log as part of their process for reviewing warranty claims and determining whether the engine was properly maintained, we expect the owner's manual would provide an example log with a clear statement that warranty claims require an up-to-date maintenance record. We note that 40 CFR 1036.125 specifies minimum maintenance intervals for critical emission-related maintenance, and limits manufacturers from invalidating warranty if certain other types of allowable maintenance are not performed ( i.e., recommended additional maintenance and noncritical emission-related maintenance). Any required maintenance tasks and intervals must be consistent with the requirements and limitations in 40 CFR 1036.125. As explained at proposal, we may review a manufacturer's information describing the parameters and documentation for demonstrating proper maintenance before granting certification for an engine family.

The maintenance instructions requirements we are finalizing for the remainder of 40 CFR 1036.125(h) are covered in the serviceability discussion in Section IV.B.3 and inducements discussion in Section IV.C of this preamble. As noted in Section IV.B.3, our serviceability provisions supplement the service information provisions specified in 40 CFR 86.010-38(j).

vi. Performing Scheduled Maintenance on Test Engines

We are finalizing our proposed update to 40 CFR 1065.410(c) to clarify that inspections performed during testing include electronic monitoring of engine parameters. While we intended the proposed update to include prognostic systems, the proposed text referred only to electronic tools, and we are revising from the proposed text in the final provision to include “or internal engine systems” to clarify. Manufacturers that include prognostic systems as part of their engine packages to identify or predict malfunctioning components may use those systems during durability testing and would describe any maintenance performed as a result of those systems, consistent with 40 CFR 1065.410(d), in their application for certification. We note that, to apply these electronic monitoring systems in testing, the inspection tool ( e.g., prognostic system) must be readable without specialized equipment so it is available to all customers or accessible at dealerships and other service outlets consistent with CAA sections 202(m) and 206.

3. Serviceability

This Section IV.B.3 describes the provisions we are finalizing to improve serviceability, reduce mal-maintenance, and ensure owners are able to maintain emission control performance throughout the entire in-use life of heavy-duty engines. See section IV.B.2 of this preamble for a discussion of manufacturers' obligations to provide maintenance instructions to operators. Also see the preamble of the proposed rule for further discussion of why EPA proposed these serviceability and maintenance information provisions. The final serviceability and maintenance information provisions were informed by comments, and we summarize key comments in this section. We provide complete responses to the serviceability-related comments in section 5 of the Response to Comments.

i. Background

Without proper maintenance, the emission controls on heavy-duty engines may not function as intended, which can result in increased emissions. Mal-maintenance, which includes delayed or improper repairs and delayed or unperformed maintenance, can be intentional ( e.g., deferring repairs due to costs) or unintentional ( e.g., not being able to diagnose the actual problem and make the proper repair).

In the NPRM, EPA discussed stakeholder concerns with the reliability of MY 2010 and later heavy-duty engines, and significant frustration expressed by owners concerning their experiences with emission control systems on such engines. EPA explained that stakeholders have communicated to EPA that, although significant improvements have been made to emission control systems since they were first introduced into the market, reliability and serviceability continue to cause them concern. EPA received comments on the NPRM further highlighting problems from fleets, owners, and operators. Commenters noted issues with a range of emission-related components, including: Sensors (DPF and SCR-related), DEF dosers, hoses, filters, EGR valves, EGR coolers and EGR actuators, SCR catalysts, DOC, turbos, wiring, decomposition tubes, cylinder heads, and DPFs. Specifically, for example, comments included described experiences with aftertreatment wiring harness failures, DEF nozzles plugging or over-injecting, NO_X sensor failures, defective DEF pumps and level sensors, systems being less reliable in rain and cold weather, more frequent required cleaning of DPFs than anticipated, and problems related to DEF build-up. See section 5 of the Response to Comment for further information and the detailed comments.

In addition to existing labeling, diagnostic, and service information requirements, EPA proposed to require important maintenance information be made available in the owner's manual as a way to improve factors that may contribute to mal-maintenance. The proposed serviceability provisions were expected to result in better service experiences for independent repair technicians, specialized repair technicians, owners who repair their own equipment, and possibly vehicle inspection and maintenance technicians. Furthermore, the proposed provisions were intended to improve owner experiences operating and maintaining heavy-duty engines and provide greater assurance of long-term in-use emission reductions by reducing the likelihood of occurrences of tampering.

Given the importance and complexity of emission control systems and the impact to drivers for failing to maintain such systems ( e.g., inducements), EPA believes it is critical to include additional information about emission control systems in the owner's manual. We proposed to require manufacturers to provide more information concerning the emission control system in the owner's manual to include descriptions of how the emissions systems operate, troubleshooting information, and diagrams. EPA has imposed similar requirements in the past, such as when EPA required vacuum hose diagrams be included on the emission label to improve serviceability and help inspection and maintenance facilities identify concerns with that system.

ii. Final Maintenance Information Requirements for Improved Serviceability

EPA received both supportive and adverse comments from a number of stakeholders on the serviceability proposals (see section 5 of the Response to Comments). For example, comments from service providers and manufacturers largely objected to the proposed serviceability requirements, while owners and operators supported the proposed requirements. EPA is finalizing requirements for improved serviceability so that owners and operators can more easily understand advanced emission control system operation and identify issues in such systems as they arise during operation. To the extent EPA can ensure this information is harmonized among manufacturers, we believe this will improve the experiences of owners, operators, parts counter specialists, and repair technicians, and reduce frustration that could otherwise create an incentive to tamper.

CAA section 207(c)(3)(A) requires manufacturers to provide instructions for the proper maintenance and use of a vehicle or engine by the ultimate purchaser and requires such instructions to correspond to EPA regulations. The final rule includes maintenance provisions migrated and updated from 40 CFR part 86, subpart A, to a new 40 CFR 1036.125, that specify the maintenance instructions manufacturers must provide in an owner's manual to ensure that owners can properly maintain their vehicles (see Section IV.B.2). Additionally, as a part of the new 40 CFR 1036.125(h), we are finalizing specific maintenance information manufacturers must provide in the owner's manual to improve serviceability:

• EPA is finalizing with revision the proposed requirement for manufacturers to provide a description of how the owner can use the OBD system to troubleshoot problems and access emission-related diagnostic information and codes stored in onboard monitoring systems. The revision replaces the proposed requirement that the owner's manual include general information on how to read and understand OBD codes with a more specific set of required information. The final requirement specifies that, at a minimum, manufacturers provide a description of how to use the OBD system to troubleshoot and access information and codes, including (1) identification of the OBD communication protocol used, (2) location and type of OBD connector, (3) a brief description of what OBD is (including type of information stored, what a malfunction indicator light (MIL) is, explanation that some MILs may self-extinguish), and (4) a note that certain engine and emission data is publicly available using any scan tool, as required by EPA. As we describe further in section IV.C.1.iii, we are not taking final action on the proposed health monitors. Therefore, we are also not requiring manufacturers to provide information about the role of the health monitor to help owners service their engines before components fail in the description of the OBD system.
• EPA is finalizing as proposed, with a few clarifications in wording, a requirement for manufacturers to identify critical emission systems and components, describe how they work, and provide a general description of how the emission control systems operate.
• EPA is finalizing as proposed the requirement for manufacturers to include one or more diagrams of the engine and its emission-related components, with two exceptions: (1) We are not finalizing the proposed requirements to include the identity, location, and arrangement of wiring in the diagram, and we are not requiring information related to the expected pressures at the particulate filter and exhaust temperatures throughout the aftertreatment system. The final requirement specifies the following information is required, as proposed:

○ The flow path for intake air and exhaust gas.

○ The flow path of evaporative and refueling emissions for spark-ignition engines, and DEF for compression-ignition engines, as applicable.

○ The flow path of engine coolant if it is part of the emission control system described in the application for certification.

○ The identity, location, and arrangement of relevant emission sensors, DEF heater and other DEF delivery components, and other critical emission-related components.

○ Terminology to identify components must be consistent with codes the manufacturer uses for the OBD system.

• EPA is revising the proposed requirement relating to exploded-view drawings and basic assembly requirements in the owner's manual. The final provision replaces a general reference to aftertreatment devices with a specific list of components that should be included in one or more diagrams in the owner's manual, including: EGR Valve, EGR actuator, EGR cooler, all emission sensors ( e.g., NO_X, soot sensors, etc.), temperature and pressure sensors (EGR, DPF, DOC, and SCR-related, including DEF-related temperature and pressure sensors), fuel (DPF-related) and DEF dosing units and components ( e.g., pumps, filters, metering units, nozzles, valves, injectors), DEF quality sensors, DPF filter, DOC, SCR catalyst, aftertreatment-related control modules, any other DEF delivery-related components ( e.g., lines and freeze protection components), and aftertreatment-related wiring harnesses if replaceable separately. The revision also notes that the information could be provided in multiple diagrams. We are also revising the proposed requirement to include part numbers for all components in the drawings and instead are only requiring part numbers for sensors and filters related to SCR or DPF systems. We are not finalizing at this time the broader requirement that this information include enough detail to allow a mechanic to replace any of these components. Finally, once published for a given model year, manufacturers will not be required to revise their owner's manual with updated part numbers if a part is updated in that model year. We recognize that manufacturers are able to use outdated part numbers to find updated parts.

• EPA is finalizing as proposed the requirement for manufacturers to provide a statement instructing owners or service technicians where and how to find emission recall and technical repair information available without charge from the National Highway Traffic Safety Administration.

• EPA is finalizing with some modifications from the proposal the requirement for manufacturers to include a troubleshooting guide to address SCR inducement-related and DPF regeneration-related warning signals. For the SCR system this requirement includes:

○ The inducement derate schedule (including indication that DEF quantity-related inducements will be triggered prior to the DEF tank being completely empty).

○ The meaning of any trouble lights that indicate specific problems ( e.g., DEF level).

○ A description of the three types of SCR-related derates (DEF quantity, DEF quality and tampering) and a notice that further information on the cause of ( e.g., trouble codes) is available using the OBD system.

• For the DPF system the troubleshooting guide requirement includes:

○ Information on the occurrence of DPF-related derates.

○ EPA is finalizing in 40 CFR 1036.110(c) that certain information must be displayed on-demand for operators. Specifically, EPA is finalizing the requirement that for SCR-related inducements, information such as the derate and associated fault code must be displayed on-demand for operators (see section IV.D.3 for further information). EPA is also finalizing requirements that the number of DPF regenerations, DEF consumption rate, and the type of derate ( e.g., DPF- or SCR-related) and associated fault code for other types of emission-related derates be displayed on-demand for operators (see section IV.C.1.iii for further information).

EPA proposed that manufacturers include a Quick Response (QR) code on the emission label that would direct repair technicians, owners, and inspection and maintenance facilities to a website providing critical emission systems information at no cost. We are not taking final action at this time on the proposed requirement to include QR codes on the emission control information label. After considering manufacturers' comments, we intend to engage in further outreach and analysis before adopting electronic labeling requirements, such as QR codes. In this rule, we are instead finalizing that the owner's manual must include a URL directing owners to a web location for the manufacturer's service information required in 40 CFR 86.010-38(j). We recognize the potential for electronic labels with QR codes or similar technology to provide useful information for operators, inspectors, and others. Manufacturers from multiple industry sectors are actively pursuing alternative electronic labeling. In the absence of new requirements for electronic labeling, manufacturers must continue to meet requirements for applying physical labels to their engines. Manufacturers may include on the vehicle or engine any QR codes or other electronic labeling information that goes beyond what is required for the physical emission control information label. EPA is also not taking final action at this time on the proposed requirement to include a basic wiring diagram for aftertreatment-related components in the owner's manual. Finally, EPA is not taking final action at this time on requirements related to DPF cleaning; instead, EPA intends to continue to follow the work CARB has undertaken in this area and may consider taking action in a future rule.

iii. Other Emission Controls Education Options

In addition to our proposed provisions to provide more easily accessible service information for operators, we sought comment on whether educational programs and voluntary incentives could lead to better maintenance and real-world emission benefits. We received comments in response to the NPRM supportive of improving such educational opportunities to promote an understanding of how advanced emission control technologies function and the importance of emissions controls as they relate to the broader economy and the environment (see section 5.4 of the Response to Comment for further details). EPA is not finalizing any requirements related to this request for comment at this time but will look for future opportunities to improve the availability of information on emission control systems.

C. Onboard Diagnostics

As used here, the terms “onboard diagnostics” and “OBD” refer to systems of electronic controllers and sensors required by regulation to detect malfunctions of engines and emission controls. EPA's OBD regulations for heavy-duty engines are contained in 40 CFR 86.010-18, which were initially promulgated on February 24, 2009 (74 FR 8310). Those requirements were harmonized with CARB's OBD program then in place. Consistent with our authority under CAA section 202(m), EPA is finalizing an update to our OBD regulations in 40 CFR 1036.110 to align with existing CARB OBD requirements as appropriate, better address newer diagnostic methods and available technologies, and to streamline provisions.

1. Incorporation of California OBD Regulations by Reference

CARB OBD regulations for heavy-duty engines are codified in title 13, California Code of Regulations, sections 1968.2, 1968.5, 1971.1, and 1971.5. EPA is finalizing our proposal to incorporate by reference in 40 CFR 1036.810 the OBD requirements CARB adopted October 3, 2019. In response to the NPRM, EPA received a number of comments supportive of EPA's adoption of the revised CARB OBD program, including the 2019 rule amendments. As discussed in this section and reflected in final 40 CFR 1036.110(b), our final rule will harmonize with the majority of CARB's existing OBD regulations, as appropriate and consistent with the CAA, and make these final requirements mandatory beginning in MY 2027 and optional in earlier model years. These new requirements better address newer diagnostic methods and available technologies and have the additional benefit of being familiar to industry. For example, the new tracking requirements contained in CARB's updated OBD program, known as the Real Emissions Assessment Logging (“REAL”) program, track real-world emissions systems performance of heavy-duty engines. The REAL tracking requirements include the collection of onboard data using existing OBD sensors and other vehicle performance parameters, which will better allow the assessment of real world, in-use emission performance.

EPA's final OBD requirements are closely aligned with CARB's existing requirements with a few exceptions, as further described in Section IV.C.1.i. We are finalizing exclusions to certain provisions that are not appropriate for a Federal program and including additional elements to improve on the usefulness of OBD systems for operators.

i. CARB OBD Provisions Revised or Not Included in the Finalized Federal Program

CARB's 2019 OBD program includes some provisions that may not be appropriate for the Federal regulations. In a new 40 CFR 1036.110(b), we are finalizing the following clarifications and changes to the 2019 CARB regulations that we are otherwise incorporating by reference:

1. Modifying the threshold requirements contained in the 2019 CARB OBD standards we are adopting (as discussed in Section IV.C.1.ii),

2. Providing flexibilities to delay compliance up to three model years for small manufacturers who have not previously certified an engine in California,

3. Allowing good engineering judgment to correlate the CARB OBD standards with EPA OBD standards,

4. Clarifying that engines must comply with OBD requirements throughout EPA's useful life as specified in 40 CFR 1036.104, which may differ from CARB's required useful life for some model years,

5. Clarifying that the purpose and applicability statements in 13 CCR 1971.1(a) and (b) do not apply,

6. Not requiring the manufacturer self-testing and reporting requirements in 13 CCR 1971.1(l)(4) “Verification of In-Use Compliance” and 1971.5(c) “Manufacturer Self-Testing” (note, in the proposal we inadvertently cited incorrect CARB provisions for the intended referenced requirements),

7. Retaining our existing deficiency policy (which we are also migrating into 40 CFR 1036.110(d)), adjusting our deficiency timing language to match CARB's, and specifying that the deficiency provisions in 13 CCR 1971.1(k) do not apply,

8. Requiring additional freeze frame data requirements (as further explained in Section IV.C.1.iii),

9. Requiring additional data stream parameters for compression- and spark-ignition engines (as further explained in Section IV.C.1.iii), and

10. Providing flexibilities to reduce redundant demonstration testing requirements for engines certified to CARB OBD requirements.

With regard to the second through the fifth items, EPA is finalizing these requirements as proposed for the reasons stated in the proposal. For the sixth item, EPA is finalizing this requirement for the reasons stated in the proposal and as proposed with the exception of a correction to the CARB reference we cited.

EPA received supportive comment from manufacturers on our proposal to migrate our existing deficiency requirements, and adverse comment from manufacturers and CARB requesting that EPA harmonize with CARB's retroactive deficiency provisions. CARB's deficiency requirements are described in 13 CCR 1971.1(k) and include descriptions of requirements such as how deficiencies are granted, fines charged for deficiencies, allowable timelines, and the application of retroactive deficiencies. We are finalizing as proposed to migrate our existing approach to deficiency provisions in 40 CFR 86.010-18(n) into 40 CFR 1036.110(d). See section 7.1 of the Response to Comments for further details on comments received and EPA's responses.

EPA also received comment concerned with EPA's regulatory language describing the allowable timeframe for deficiencies. Commenters said EPA's proposed deficiency timeline is shorter than CARB's and that EPA should harmonize with CARB and provide manufacturers with 3 years to make hardware-related changes. EPA is finalizing a change to 40 CFR 1036.110(d)(3) to ensure our language is consistent with CARB's deficiency timeline in 13 CCR 1971.1(k)(4).

EPA received supportive and adverse comment on the proposal to require additional freeze frame data requirements, including that the reference in our regulations was overly broad and possibly in error. EPA is finalizing these requirements with revisions to those proposed in 40 CFR 1036.110(b)(8) to be more targeted. It is critical for there to be sufficient emissions-related parameters captured in freeze frame data to enable proper repairs.

EPA received supportive and adverse comment on the proposal to require additional data stream parameter requirements, including comment that our regulations needed to be more specific. EPA is finalizing these requirements with revisions to those proposed in 40 CFR 1036.110(b)(9) to properly capture the additional elements we intended to add to the freeze frame and to ensure these additional parameters are interpreted properly as an expansion of the existing data stream requirements in 13 CCR 1971.1(h)(4.2). Access to important emissions-related data parameters is critical for prompt and proper repairs.

EPA is finalizing flexibilities to reduce redundant demonstration testing requirements for engines certified to CARB OBD requirements, see section IV.C.1.iv. of this preamble for further discussion on what we are finalizing.

It is important to emphasize that by not incorporating certain existing CARB OBD requirements ( e.g., the “Manufacturer Self-Testing” requirements) into our regulations, we are not waiving our authority to require such testing on a case-by-case basis. CAA section 208 gives EPA broad authority to require manufacturers to perform testing not specified in the regulations in such circumstances. Thus, should we determine in the future that such testing is needed, we would retain the authority to require it pursuant to CAA section 208.

ii. OBD Threshold Requirements

a. Malfunction Criteria Thresholds

Existing OBD requirements specify how OBD systems must monitor certain components and indicate a malfunction prior to when emissions would exceed emission standards by a certain amount, known as an emission threshold. Emission thresholds for these components under the existing requirements in the 2019 CARB OBD update that we are incorporating by reference are generally either an additive or multiplicative value above the applicable exhaust emission standard. EPA proposed to modify the threshold requirements in the 2019 CARB OBD update to be consistent with the provisions finalized by CARB in their Omnibus rule in December of 2021 and not tighten threshold requirements while finalizing lower emission standards. This meant, for example, that for monitors required to detect a malfunction before NO_X emissions exceed 1.75 times the applicable existing NO_X standard, the manufacturer would continue to use the same numeric threshold ( e.g., 0.35 g/bhp-hr NO_X) for the new emission standards finalized in this rule.

EPA received comments from manufacturers and operators in support of finalizing the threshold provisions as proposed, and a comment from CARB stating that three engine families have recently been certified to lower FELs indicating EPA should finalize lower thresholds. We note that CARB stated that two of these engine families were certified with deficiencies, and thus these engines did not fully meet all specific OBD requirements (see section 7.1 of the Response to Comment for further detail about these comments and EPA's responses). EPA is finalizing with minor revision future numerical values for OBD NO_X and PM thresholds that align with the numerical value that results under today's NO_X and PM emissions requirements.

We are finalizing as proposed a NO_X threshold of 0.40 g/hp-hr and a PM threshold of 0.03 g/hp-hr for compression-ignition engines for operation on the FTP and SET duty cycles. We are finalizing as proposed a PM threshold of 0.015 g/hp-hr for spark-ignition engines for operation on the FTP and SET duty cycles. For spark-ignition engines, we proposed NO_X thresholds of 0.30 and 0.35 g/hp-hr for monitors detecting a malfunction before NO_X emissions exceed 1.5 and 1.75 times the applicable standard, respectively. We are finalizing these numeric threshold values without reference to what percent exceedance is relevant and instead are clarifying that the 0.35g/hp-hr standard applies for catalyst monitors and that 0.30g/hp-hr applies for all other monitors, to ensure the proper numeric thresholds can be applied to engines certified under 13 CCR 1968.2 and 1971.1.. EPA intends to continue to evaluate the capability of HD OBD monitors to accommodate lower thresholds to correspond to the lower emission levels for the final emission standards and may consider updating threshold requirements in the future as more in-use data becomes available.

We also inadvertently omitted from the proposed 40 CFR 1036.110(b) the specific threshold criteria for SI and CI engine HC and CO emissions that coincided with our overall expressed intent to harmonize with the threshold requirements included in CARB's Omnibus rule and not tighten OBD emission thresholds. Consistent with this intent, we are finalizing a provision in 40 CFR 1036.110(b)(5) that instructs manufacturers to use numeric values that correspond to existing HC and CO standards (0.14 g/hp-hr for HC, 15.5 g/hp-hr for CO from compression-ignition engines, and 14.4 g/hp/hr for spark-ignition engines) to determine the required thresholds. Applying this methodology will result in calculations that produce thresholds equivalent to existing thresholds. Including this clarification avoids unintentionally lowering such thresholds.

b. Test-Out Criteria

CARB OBD requirements include “test-out” provisions in 13 CCR 1968.2 and 1971.1 which allow manufacturers to be exempt from monitoring certain components if failure of these components meets specified criteria. EPA is adopting these test-out provisions through the incorporation by reference of CARB's updated 2019 OBD requirements. Similar to the revisions we proposed and are finalizing for malfunction criteria, EPA's assessment is that for compression ignition engines test-out criteria should also not be tightened at this time. However, we inadvertently omitted from the proposed 40 CFR 1036.110(b) the specific adjustments to test-out criteria for compression-ignition engines included in CARB's Omnibus rule that are necessary to result in such criteria not being tightened. Consistent with our overall expressed intent to (1) not tighten OBD requirements, and (2) modify the 2019 CARB requirements we are adopting by harmonizing with the numeric values included in CARB's Omnibus rule, we are finalizing a revision from the proposal to include test-out criteria calculation instructions into our regulations.

Specifically, we are finalizing a provision that manufacturers seeking to use the test-out criteria to exempt engines from certain monitoring in the incorporated by reference 2019 CARB regulations 13 CCR 1968.2 and 1971.1 must calculate the criteria based on specified values provided in 40 CFR 1036.110(b)(5). For example, 13 CCR 1971.1(e)(3.2.6) specifies that one of the requirements for an EGR catalyst to be exempt from monitoring is if no malfunction of the EGR catalyst can cause emissions to increase by 15 percent or more of the applicable standard as measured from the appropriate test cycle. The requirement we are finalizing in 40 CFR 1036.110(b)(5) instructs manufacturers to use specific values for that “applicable standard” to calculate the required test-out criteria. For example, for the EGR catalyst test-out provision, this would result in a NO_X test-out criterion of 0.03 g/hp-hr (0.2 g/hp-hr • 0.15). Including this provision is consistent with the intent of our proposal and avoids unintentionally lowering such test-out criteria that would render such test-out criteria generally inconsistent with the other provisions we are finalizing in 40 CFR 1036.110(b)(5), and enables manufacturers to continue using these provisions.

c. Applicable Thresholds for Engines Certified to 40 CFR Part 1036 Used in Heavy-Duty Vehicles Less Than 14,000 Pounds GVWR

We are finalizing as proposed that engines installed in vehicles at or below 14,000 lbs GVWR are subject to OBD requirements under the light-duty program in 40 CFR 86.1806-17. Commenters pointed out that the proposed rule did not specify alternative thresholds for engines certified to 40 CFR part 1036 on an engine dynamometer that are subject to OBD requirements under 40 CFR 86.1806-17. Without such a provision, manufacturers would be subject to the existing thresholds in 40 CFR 86.1806-17 that are based on standards set for light-duty chassis-certified vehicles. Consistent with our statements in the NPRM that our proposal intended to harmonize with the threshold requirements included in CARB's Omnibus policy and not lower emission threshold levels in our proposed OBD regulations, we are clarifying in 40 CFR 86.1806-17(b)(9) that the thresholds we are finalizing in 40 CFR 1036.110(b)(5) apply equally for engines certified under 40 CFR part 1036 that are used in vehicles at or below 14,000 lbs GVWR.

iii. Additional OBD Provisions in the Proposed Federal Program

In the NPRM, EPA proposed to include additional requirements to ensure that OBD can be used to properly diagnose and maintain emission control systems to avoid increased real-world emissions. This was also a part of our effort to update EPA's OBD program and respond to numerous concerns raised in the ANPR about the difficulty of diagnosing and maintaining proper functionality of advanced emission control technologies and the important role accessible and robust diagnostics play in this process. At this time, after consideration of comments, we are finalizing a limited set of these proposed provisions (see section 7 of the Response to Comments documents for further detail on comments and EPA's responses). Where OBD requirements between EPA and CARB may differ, EPA is finalizing as proposed provisions allowing us to accept CARB OBD approval as long as a manufacturer can demonstrate that the CARB program meets the intent of EPA OBD requirements and submits documentation as specified in 40 CFR 1036.110(b).

In this section we describe the final additional EPA certification requirements in 40 CFR 1036.110 for OBD systems, which, consistent with CAA section 202(m), are intended to provide more information and value to the operator and play an important role in ensuring expected in-use emission reductions are achieved long-term. With respect to our proposed provisions to require additional information from OBD systems be made publicly available, we received supportive comments from operators and adverse comments from manufacturers. After considering these comments, we are revising our final provision from those proposed, as summarized here and provide in more detail in section 7 of the Response to Comments document. We are not taking final action at this time on the proposed requirement to include health monitors. In addition to driver information requirements we are adopting to increase the availability of serviceability and inducement-related information (see section IV.B.3 and IV.D.3 respectively of this preamble), we are also finalizing in 40 CFR 1036.110(c) that the following information must be made available in the cab on-demand in lieu of the proposed health monitors:

• The total number of diesel particulate filter regeneration events that have taken place since installing the current particulate filter.

• Historical and current rate of DEF consumption ( e.g., gallons of DEF consumed per mile or gallons of DEF consumed per gallon of diesel fuel consumed.) This information is designed such that operators can reset it as needed to capture specific data for comparison purposes.

• For AECD conditions (outside of inducements) related to SCR or DPF systems that derate the engine ( e.g., either a speed or torque reduction), the fault code for the detected problem, a description of the fault code, and the current restriction.

For all other health monitor provisions proposed in 40 CFR 1036.110(c)(3), we are not taking final action on those proposed provisions at this time.

In addition to incorporating an improved list of publicly available data parameters by harmonizing with updated CARB OBD requirements, in 40 CFR 1036.110(b)(9) EPA is finalizing as proposed for the reasons explained further in the proposal to add signals to the list, including to specifically require that all parameters related to fault conditions that trigger vehicle inducement also be made readily available using generic scan tools. EPA expects that each of these additional requirements will be addressed even where manufacturers relied in part on a CARB OBD approval to satisfy Federal requirements in order to demonstrate under 40 CFR 1036.110(b) that the engine meets the intent of 40 CFR 1036.110. The purpose of including additional parameters is to make it easier to identify malfunctions of critical aftertreatment related components, especially where failure of such components would trigger an inducement. We are revising the proposed new parameters for HD SI engines in 40 CFR 1036.110(b)(10) after considering comments. See section 3 of the Response to Comments.

We are also finalizing a general requirement in 40 CFR 1036.110(b)(9)(vi) to make all parameters available that are used as the basis for the decision to put a vehicle into an SCR- or DPF-related derate. For example, if the failure of an open-circuit check for a DEF quality sensor leads to an engine inducement, the owner/operator would be able to identify this fault condition using a generic scan tool. We are finalizing a requirement that manufacturers make additional parameters available for all engines so equipped, including:

• For Compression Ignition engines:

○ Inlet DOC and Outlet DOC pressure and temperature

○ DPF Filter Soot Load (for all installed DPFs)

○ DPF Filter Ash Load (for all installed DPFs)

○ Engine Exhaust Gas Recirculation Differential Pressure

○ DEF quality-related signals

○ Parking Brake, Neutral Switch, Brake Switch, and Clutch Switch Status

○ Aftertreatment Dosing Quantity Commanded and Actual

○ Wastegate Control Solenoid Output

○ Wastegate Position Commanded and Actual

○ DEF Tank Temperature

○ DEF Doser Control Status

○ DEF System Pressure

○ DEF Pump Commanded Percentage

○ DEF Coolant Control Valve Control Position Commanded and Actual

○ DEF Line Heater Control Outputs

○ Speed and output shaft torque consistent with 40 CFR 1036.115(d)

• For Spark Ignition Engines:

○ Air/Fuel Enrichment Enable flags: Throttle based, Load based, Catalyst protection based

○ Percent of time not in stoichiometric operation (including per trip and since new)

One of the more useful features in the CARB OBD program for diagnosing and repairing emissions components is the requirement for “freeze frame” data to be stored by the system. To comply with this requirement, manufacturers must capture and store certain data parameters ( e.g., vehicle operating conditions such as the NO_X sensor output reading) within 10 seconds of the system detecting a malfunction. The purpose of storing this data is in part to record the likely area of malfunction. EPA is finalizing a requirement in 40 CFR 1036.110(b)(8) to require that manufacturers capture the following elements as freeze frame data: Those data parameters specified in 1971.1(h)(4.2.3)(E), 1971.1(h)(4.2.3)(F), and 1971.1(h)(4.2.3)(G). We are also specifying that these additional parameters would be added according to the specifications in 13 CCR 1971.1(h)(4.3). EPA believes this is essential information to make available to operators for proper maintenance.

iv. Demonstration Testing Requirements

Existing requirements of 40 CFR 86.010-18(l) and 13 CCR 1971.1(l) specify the number of test engines for which a manufacturer must submit monitoring system demonstration emissions data. Specifically, a manufacturer certifying one to five engine families in a given model year must provide emissions test data for a single test engine from one engine rating, a manufacturer certifying six to ten engine families in a given model year must provide emissions test data for a single test engine from two different engine ratings, and a manufacturer certifying eleven or more engine families in a given model year must provide emissions test data for a single test engine from three different engine ratings.

EPA received supportive and adverse comment on a proposed flexibility to reduce redundant demonstration testing requirements for certain engines where an OBD system designed to comply with California OBD requirements is being used in both a CARB proposed family and a proposed EPA-only family and the two families are also identical in all aspects material to expected emission characteristics. EPA issued guidance last year on this issue. We are finalizing as proposed to codify this guidance as a provision, subject to certain information submission requirements for EPA to evaluate if this provision's requirements have been met, for model years 2027 and later engines in 40 CFR 1036.110(b)(11). Manufacturers may also use the flexibility in earlier model years. More specifically, we are finalizing the provision as proposed to count two equivalent engines families as one for the purposes of determining OBD demonstration testing requirements, where equivalent means they are identical in all aspects material to emission characteristics, as such, testing is not necessary to ensure a robust OBD program. 40 CFR 1036.110(b)(11) requires manufacturers to submit additional information as needed to demonstrate that the engines meet the requirements of 40 CFR 1036.110 that are not covered by the California Executive order, as well as results from any testing performed for certifying engine families (including equivalent engine families) with the California Air Resources Board and any additional information we request as needed to evaluate whether the requirements of this provision are met.

We took comment on and are finalizing language that this flexibility will apply for cases where equivalent engine families also have different inducement strategies. We are aware that the auxiliary emission control devices (AECDs) needed to implement the engine derating associated with inducements do not affect engine calibrations in a way that would prevent OBD systems from detecting when emissions exceed specified levels. Rather, those AECDs simply limit the range of engine operation that is available to the driver. Thus, testing of different inducement strategies in these AECDs would also not be necessary to ensure a robust OBD program and we would consider such differences between engines to not be material to emission characteristics relevant to these OBD testing requirements. Any difference in impacts between the engines would be a consequence of the driver's response to the inducement itself, which could also occur even with the same inducement strategy, rather than a difference in the functioning of the OBD systems in the engines. In that way, inducements are analogous to warranty for purposes of counting engine families for OBD testing requirements. See section 8 of the Response to Comments for details on the comments received and EPA's responses.

v. Use of CARB OBD Approval for EPA OBD Certification

Existing EPA OBD regulations allow manufacturers seeking an EPA certificate of conformity to comply with the Federal OBD requirements by demonstrating to EPA how the OBD system they have designed to comply with California OBD requirements also meets the intent behind Federal OBD requirements, as long as the manufacturer complies with certain certification documentation requirements. EPA has implemented these requirements by allowing a manufacturer to submit an OBD approval letter from CARB for the equivalent engine family where a manufacturer can demonstrate that the CARB OBD program has met the intent of the EPA OBD program. In other words, EPA has interpreted these requirements to allow OBD approval from CARB to be submitted to EPA for approval. We are finalizing as proposed to migrate the language from 40 CFR 86.010-18(a)(5) to 40 CFR 1036.110(b) to allow manufacturers to continue to use a CARB OBD approval letter to demonstrate compliance with Federal OBD regulations for an equivalent engine family where manufacturers can demonstrate that the CARB OBD program has met the intent of the EPA OBD program.

To demonstrate that your engine meets the intent of EPA OBD requirements, we are finalizing as proposed that the OBD system must address all the provisions described in 40 CFR 1036.110(b) and (c) and adding clarification in 40 CFR 1036.110(b) that manufacturers must submit information demonstrating that all EPA requirements are met. In the case where a manufacturer chooses not to include information showing compliance with additional EPA OBD requirements in their CARB certification package ( e.g., not including the additional EPA data parameters in their CARB certification documentation), EPA expects manufacturers to provide separate documentation along with the CARB OBD approval letter to show they have met all EPA OBD requirements. This process also applies in potential future cases where CARB has further modified their OBD requirements such that they are different from but meet the intent of existing EPA OBD requirements. EPA expects manufacturers to submit documentation as is currently required by 40 CFR 86.010-18(m)(3), detailing how the system meets the intent of EPA OBD requirements and information on any system deficiencies. As a part of this update to EPA OBD regulations, we are clarifying as proposed in 40 CFR 1036.110(b)(11)(iii) that we can request that manufacturers send us information needed for us to evaluate how they meet the intent of our OBD program using this pathway. This would often mean sending EPA a copy of documents submitted to CARB during the certification process.

vi. Use of the SAE J1979-2 Communications Protocol

In a February 2020 workshop, CARB indicated their intent to propose allowing the use of Unified Diagnostic Services (“UDS”) through the SAE J1979-2 communications protocol for heavy-duty OBD with an optional implementation as early as MY 2023. The CARB OBD update that includes this UDS proposal has not yet been finalized, but was submitted to California's Office of Administrative Law for approval in July of 2022. CARB stated that engine manufacturers are concerned about the limited number of remaining undefined 2-byte diagnostic trouble codes (“DTC”) and the need for additional DTCs for hybrid vehicles. SAE J1979-2 provides 3-byte DTCs, significantly increasing the number of DTCs that can be defined. In addition, this change would provide additional features for data access that improve the usefulness of generic scan tools to repair vehicles.

This update has not been finalized by CARB in time for us to include it in this final rule. In consideration of manufacturers who want to certify their engine families in the future for nationwide use, and after consideration of expected environmental benefits associated with the use of this updated protocol, we are finalizing as proposed a process for reviewing and approving manufacturers' requests to comply using the alternative communications protocol.

While EPA believes our existing requirements in 40 CFR 86.010-18(a)(5) allow us to accept OBD systems using SAE J1979-2 that have been approved by CARB, there may be OEMs that want to obtain an EPA-only certificate ( i.e., does not include certification to California standards) for engines that do not have CARB OBD approval for MYs prior to MY 2027 ( i.e., prior to when the 40 CFR part 1036 OBD provisions of this final rule become mandatory). EPA is finalizing as proposed to allow the use of SAE J1979-2 for manufacturers seeking EPA OBD approval. We are adopting this as an interim provision in 40 CFR 1036.150(v) to address the immediate concern for model year 2026 and earlier engines. Once EPA's updated OBD requirements are in effect for MY 2027, we expect to be able to allow the use of SAE J1979-2 based on the final language in 40 CFR 1036.110(b); however, we do not specify an end date for the provision in 40 CFR 1036.150(v) to make sure there is a smooth transition toward using SAE J1979-2 for model years 2027 and later. This provides manufacturers the option to upgrade their OBD protocol to significantly increase the amount of OBD data available to owners and repair facilities.

CAA section 202(m)(4)(C) requires that the output of the data from the emission control diagnostic system through such connectors shall be usable without the need for any unique decoding information or device, and it is not expected that the use of SAE J1979-2 would conflict with this requirement. Further, CAA section 202(m)(5) requires manufacturers to provide promptly to any person engaged in the repairing or servicing of motor vehicles or motor vehicle engines, and the Administrator for use by any such persons, with any and all information needed to make use of the emission control diagnostics system prescribed under this subsection and such other information including instructions for making emission related diagnosis and repairs. Manufacturers that voluntarily use J1979-2 as early as MY 2022 under interim provision 40 CFR 1036.150(v) would need to provide access to systems using this alternative protocol at that time and meet all the relevant requirements in 40 CFR 86.010-18 and 1036.110. EPA did not receive adverse comment on the availability of tools that can read the new protocol from manufacturers or tool providers. CARB commented that staff anticipates tool vendors will be able to fully support the SAE J1979-2 protocol at a fair and reasonable price for the vehicle repair industry and consumers.

2. Cost Impacts

Heavy-duty engine manufacturers currently certify their engines to meet CARB's OBD regulations before obtaining EPA certification for a 50-state OBD approval. We anticipate most manufacturers will continue to certify with CARB and that they will certify to CARB's 2019 updated OBD regulations well in advance of the EPA program taking effect; therefore, we anticipate the incorporation by reference of CARB's 2019 OBD requirements will not result in any additional costs. EPA does not believe the additional OBD requirements described here will result in any significant costs, as there are no requirements for: New monitors, new data parameters, new hardware, or new testing included in this rule. However, EPA has accounted for possible additional costs that may result from the final expanded list of public OBD parameters in the “Research and Development Costs” of our cost analysis in Section V. EPA recognizes that there could be cost savings associated with reduced OBD testing requirements under final 40 CFR 1036.110(c)(11). For example, cost savings could come from the provision to not count engine families certified separately by EPA and CARB, but otherwise identical in all aspects material to expected emission characteristics, as different families when determining OBD demonstration testing (see section IV.C.1.iv of this document for further discussion on this provision). This potential reduction in demonstration testing burden could reduce costs such as labor and test cell time. However, manufacturers may choose not to certify engine families in this manner which would not translate to cost savings. Therefore, given the uncertainty in the potential for savings, we did not quantify the costs savings associated with this final provision.

D. Inducements

Manufacturers have deployed urea-based SCR systems to meet the existing heavy-duty engine emission standards. EPA anticipates that manufacturers will continue to use this technology to meet the new NO_X standards finalized in this rule. SCR is very different from other emission control technologies in that it requires operators to maintain an adequate supply of diesel exhaust fluid (DEF), which is generally a water-based solution with 32.5 percent urea. Operating an SCR-equipped engine without DEF or certain components like an SCR catalyst could cause NO_X emissions to increase to levels comparable to having no NO_X controls at all.

The proposed rule described two key aspects of how our regulations currently require manufacturers to ensure engines will operate with an adequate supply of high-quality DEF, which we proposed to update and further codify. First, manufacturers currently must demonstrate compliance with our critical emissions-related schedule maintenance requirements, including 40 CFR 86.004-25(b). EPA has approved DEF refills as part of manufacturers' scheduled maintenance. EPA's approval is conditioned on manufacturers demonstrating that operators are reasonably likely to perform such maintenance. Manufacturers have consistently made this demonstration by designing their engines to go into a disabled mode that decreases a vehicle's maximum speed if the engine detects that operators are failing to provide an adequate supply of DEF. More specifically, manufacturers have generally complied by programming engines to restrict peak vehicle speeds after detecting that such maintenance has not been performed or detecting that tampering with the SCR system may have occurred. We refer to this strategy of derating engine power and vehicle speed as an “inducement.”

Second, EPA's current regulations in 40 CFR 86.094-22(e) require that manufacturers comply with emission standards over the full adjustable range of “adjustable parameters,” and that, in determining the parameters subject to adjustment, EPA considers the likelihood that settings other than the manufacturer's recommended setting will occur in-use, including the effect of settings other than the manufacturer's recommended settings on engine performance. We have historically considered DEF level and quality as parameters that can be physically adjusted and may significantly affect emissions. EPA generally has approved manufacturers strategies consistent with guidance that described recommendations on ways manufacturers could meet adjustable parameter requirements when using SCR systems. This guidance states that manufacturers should demonstrate that operators are being made aware that DEF needs to be replaced through warnings and vehicle performance deterioration that should not create undue safety concerns but be onerous enough to discourage drivers from operating without DEF ( i.e., through inducement). See the proposed rule preamble for further background and discussion of the basis of EPA's proposed inducement regulations.

With some modification from the proposal, EPA is adopting final inducement regulations in this final rule. The regulatory provisions also include changes compared to existing inducement guidance after consideration of manufacturer designs and operator experiences with SCR over the last several years. The inducement requirements included in this final rule supersede the existing guidance and are mandatory beginning in MY 2027 and voluntary prior to that and are intended to-

• Ensure that all critical emission-related scheduled maintenance has a reasonable likelihood of being performed while also deterring tampering of the SCR system.
• Set an appropriate inducement speed derating schedule that reflects experience gained over the past decade with SCR systems.
• Recognize the diversity of the real-world fleet with derate schedules that are tailored to a vehicle's operating characteristics.
• Improve the type and amount of information operators receive from the vehicle to both understand inducement actions and to help avoid or quickly remedy a problem that is causing an inducement.
• Allow operators to perform an inducement reset by using a generic scan tool or allowing for the engine to self-heal during normal driving.
• Address operator frustration with false inducements and low inducement speed restrictions that occur quickly, in part due to concern that such frustration may potentially lead to in-use tampering of the SCR system.

This final rule includes several changes from the proposed rule after consideration of numerous comments. See section 8 of the Response to Comments for the detailed comments and EPA's response to those comments, including further discussion of the changes in the final rule compared to the proposed rule. As an overview, EPA is adopting as a maintenance requirement, as proposed, in 40 CFR 1036.125(a)(1) that manufacturers must meet the specifications in new 40 CFR 1036.111, which contains requirements for inducements related to SCR, to demonstrate that timely replenishment with high-quality DEF is reasonably likely to occur on in-use engines and that adjustable parameter requirements will be met. Specifically, EPA is finalizing as proposed to specify in 40 CFR 1036.115(f) that DEF supply and DEF quality are adjustable parameters. Regarding DEF supply, we are finalizing as proposed that the physically adjustable range includes any amount of DEF that the engine's diagnostic system does not recognize as a fault condition under new 40 CFR 1036.111. We are adopting a requirement under new 40 CFR 1036.115(i) for manufacturers to size DEF tanks corresponding to refueling events, which is consistent with the regulation we are replacing under 40 CFR 86.004-25(b)(4)(v). Under the final requirements, manufacturers can no longer use the alternative option in 40 CFR 86.004-25(b)(6)(ii)(F) to demonstrate high-quality DEF replenishment is reasonably likely to be performed in use. As described in the proposed rule, EPA plans to continue to rely on the existing guidance in CD-13-13 that describes how manufacturers of heavy-duty highway engines determine the practically adjustable range for DEF quality. We inadvertently proposed to require that manufacturers use the physically adjustable range for DEF quality as the basis for defining a fault condition for inducements under 40 CFR 1036.111. Since we intended for the existing guidance to addresses issues related to the physically adjustable range for DEF quality, we are not finalizing the proposed provision in 40 CFR 1036.115(f)(2) for DEF quality. EPA intends further consider the relationship between inducements and the practically adjustable range for DEF quality and may consider updating this guidance in the future.

EPA is adopting requirements that inducements be triggered for three types of fault conditions: (1) DEF supply is low, (2) DEF quality does not meet manufacturer specifications, or (3) tampering with the SCR system. EPA is not taking final action at this time on the proposed requirement for manufacturers to include a NO_X override to prevent false inducements. After consideration of public comments, the final inducement provisions at 40 CFR 1036.111 include updates from the proposed inducement schedules; more specifically, EPA is adopting separate inducement schedules for low-, medium-, and high-speed vehicles. EPA is also finalizing requirements for manufacturers to improve information provided to operators regarding inducements. The final rule also includes a requirement for manufacturers to design their engines to remove inducements after proper repairs are made, through self-healing or with the use of a generic scan tool to ensure that operators have performed the proper maintenance.

These requirements apply starting in MY 2027, though manufacturers may optionally comply with these 40 CFR part 1036 requirements in lieu of provisions that apply under 40 CFR part 86 early. The following sections describe the inducement requirements for the final rule in greater detail.

1. Inducement Triggers

Three types of fault conditions trigger inducements under 40 CFR 1036.111. The first triggering condition is DEF quantity. Specifically, we require that SCR-equipped engines trigger an inducement when the amount of DEF in the tank has been reduced to a level corresponding to three remaining hours of engine operation. This triggering condition ensures that operators will be compelled to perform the necessary maintenance before the DEF supply runs out, which would cause emissions to increase significantly.

The second triggering condition is DEF quality failing to meet manufacturer concentration specifications. This triggering condition ensures high quality DEF is used.

Third, EPA is requiring inducements to ensure that SCR systems are designed to be tamper-resistant. We are requiring that manufacturers design their engines to monitor for and trigger an inducement for open-circuit fault conditions for the following components: (1) DEF tank level sensor, (2) DEF pump, (3) DEF quality sensor, (4) SCR wiring harness, (5) NO_X sensors, (6) DEF dosing valve, (7) DEF tank heater, (8) DEF tank temperature sensor, and (9) aftertreatment control module (ACM). EPA is also requiring that manufacturers monitor for and trigger an inducement if the OBD system has any signal indicating that a catalyst is missing (see OBD requirements for this monitor in 13 CCR 1971.1(i)(3.1.6)). This list is the same as the list from the proposed rule, with two exceptions after consideration of comments. First, we are adding the DEF tank temperature sensor in the final rule. This additional sensor is on par with the DEF tank heater for ensuring that SCR systems are capable of monitoring for freezing conditions. Second, in consideration of comment, we are removing blocked DEF lines or dosing valves as a triggering condition because such a condition could be caused by crystallized DEF rather than any operator action and thus is not directly related to protecting against tampering with the SCR-system. We believe this standardized list of required tampering inducement triggers will be important for owners, operators, and fleets in repairing their vehicles by avoiding excessive cost and time to determine the reason for inducement.

2. Derate Schedule

We are finalizing a different set of schedules than we proposed. First, we are adding a new category for medium-speed vehicles. Second, we are adjusting the low-speed category to have a lower final speed compared to the proposal and a lower average operating speed to identify this category. Third, we increased the average operating speed that qualifies a vehicle to be in the high-speed category. We are adopting derate schedules for low-, medium- and high-speed vehicles as shown in Table IV-13. Similar to the proposal, we differentiate these three vehicle categories based on a vehicle's calculated average speed for the preceding 30 hours of non-idle operation. Low-speed vehicles are those with an average operating speed below 15 mph. Medium-speed vehicles are those with average operating speeds at or above 15 and below 25 mph. High-speed vehicles are those with average operating speeds at or above 25 mph. Excluding idle from the calculation of vehicle speed allows us to more effectively evaluate each vehicle's speed profile; in contrast, time spent at idle would not help to give an indication of a vehicle's operating characteristics for purposes of selecting the appropriate derate schedule. EPA chose these final speeds after consideration of stakeholder comments (see section 8.3 of the Response to Comments for further information on comments received) and an updated analysis of real-world vehicle speed activity data from the FleetDNA database maintained by the National Renewable Energy Laboratory (NREL). Our analyses provided us with insight into the optimum way to characterize vehicles in a way to ensure these categories received appropriate inducements that would be neither ineffective nor overly restrictive.

The derate schedule for each vehicle category is set up with progressively increasing severity to induce the owner or operator to efficiently address conditions that trigger inducements. Table IV-13 shows the derate schedules in cumulative hours. The initial inducement applies immediately when the OBD system detects any of the triggering fault conditions identified in section IV.D.1. The inducement schedule then steps down over time to result in the final inducement speed corresponding to each vehicle category. The inducement schedule includes a gradual transition (1mph every 5 minutes) at the beginning of each step of derate and prior to any repeat inducement occurring after a failed repair to avoid abrupt changes, as the step down in derate speeds in the schedules will be implemented while the vehicle is in motion. Inducements are intended to deteriorate vehicle performance to a point unacceptable for typical driving in a manner that is safe but onerous enough to discourage vehicles from being operated ( i.e., impact the ability to perform work), such that operators will be compelled to replenish the DEF tank with high-quality DEF and not tamper with the SCR system's ability to detect whether there is adequate high-quality DEF. To this end, as explained in the proposal, our analyses of vehicle operational data from NREL show that even vehicles whose operation is focused on local or intracity travel depend on frequently operating at highway speeds to complete commercial work. Vehicles in an inducement under the schedules we are finalizing would not be able to maintain commercial functions. Our analysis of the NREL data also show that even medium- and low-speed vehicles travel at speeds up to 70 mph and indicate that it is likely regular highway travel is critical for low-speed vehicles to complete their work; for example, refuse trucks need to drop off collected waste at a landfill or transfer station before returning to neighborhoods.

Motorcoach operators submitted comments describing a greater sensitivity to any speed derate because of a much greater responsibility for carrying people safely to their intended destinations over longer distances, including their role in emergency response and national defense operations. After consideration of these comments, we are allowing manufacturers to design and produce engines that will be installed in motorcoaches with an alternative derate schedule that starts with a 65 mi/hr derate when a fault condition is first detected, steps down to 50 mi/hr after 80 hours, and concludes with a final derate speed of 25 mi/hr after 180 hours of non-idle operation. EPA is defining motorcoaches in 40 CFR 1036.801 to include buses that are designed to travel long distances with row seating for at least 30 passengers. This is intended to include charter services available to the general public.

Comments on the proposed inducement policy ranged from objecting to any speed restrictions to advocating that we adopt a 5 mph final derate speed. Some commenters supported the proposed rule, and some commenters asserted that decreasing final derate speeds would provide for greater assurance that operators would perform the necessary maintenance. There was a similar range of comments regarding the time specified for escalating the speed restrictions, with some commenters agreeing with the proposed schedule, and other commenters suggesting substantially more or less time.

We made several changes from proposal after consideration of comments, including three main changes. First, as noted in the preceding paragraphs, the final rule includes a medium-speed vehicle category. This allows us to adjust the qualifying criterion for high-speed vehicles to finalize a derate schedule similar to that proposed for vehicles that are clearly operating mostly on interstate highways over long distances. Similarly, the added vehicle category allows us to adjust the qualifying criterion for low-speed vehicles and adopt an appropriately more restrictive final derate schedule for those vehicles that are operating at lower speeds in local service.

Second, we developed unique schedules for escalating the speed restrictions for medium-speed and low-speed vehicles; this change was based on the expectation that vehicles with lower average speeds spend less time operating at highway speeds characteristic of inter-city driving and will therefore not need to travel substantial distances to return home for scheduling repair.

Third, we added derate speeds that go beyond the first four stages of derating that we proposed for high-speed vehicles, essentially reducing the final inducement speeds for all vehicles to be the same as low-speed vehicles. In other words, as shown in Table IV-13, both high- and medium-speed vehicles eventually derate to the same speeds as low-speed vehicles, after additional transition time after the derate begins. For example, the final derate schedule for high-speed vehicles goes through the proposed four derate stages for high-speed vehicles. At the fifth derate stage the vehicle begins to be treated like a medium-speed vehicle, starting at the third derate stage for medium-speed vehicles and progressing through the fifth derate stage for medium-speed vehicles. At the fifth derate stage the vehicle begins to be treated like a low-speed vehicle, similarly starting at the third derate stage for low-speed vehicles. A similar step-down approach applies for medium-speed vehicles, transitioning down to the derate stages for low-speed vehicles. This progression is intended to address the concern that vehicle owners might reassign vehicles in their fleet to lower-speed service, or sell vehicles to someone who would use the vehicle for different purposes that don't depend on higher-speed operations. Our assessment is that the NREL data show that no matter what category vehicles are, they do not travel exclusively at or below 25 mph, indicating that vehicles derated to 25 mph cannot be operated commercially.

For the simplest type of maintenance, DEF refills, we fully expect that the initial stage of derated vehicle speed will be sufficient to compel vehicle operators to meet their maintenance obligations. We expect operators will add DEF routinely to avoid inducements; however, inducements begin three hours prior to the DEF tank being empty to better ensure operation with an empty DEF tank is avoided.

We expect that the derate schedules in this final rule will be fully effective in compelling operators to perform needed maintenance. This effectiveness will be comparable to the current approach under existing guidance, but will reduce operating costs to operators. We believe this measured approach will also result in lower tampering rates involving time.

3. Driver Information

In addition to the driver information requirements we are adopting to improve serviceability and OBD (see section IV.B.3 and IV.C.1.iii respectively of this preamble for more details on these provisions), we are also adopting improved driver information requirements for inducements. Specifically, we are adopting as proposed the requirement for manufacturers to increase the amount of information provided to the driver about inducements, including: (1) The condition causing the derate ( i.e., DEF quality, DEF quantity or tampering), (2) the fault code and description of the code associated with the inducement, (3) the current derate speed restriction, (4) hours until the next derate speed decrease, and (5) what the next derate speed will be. It is critical that operators have clear and ready access to information regarding inducements to reduce concerns over progressive engine derates (which can lead to motivations to tamper) as well as to allow operators to make timely informed decisions, especially since inducements are used by manufacturers to demonstrate that critical emissions-related maintenance is reasonably likely to occur in-use. We note that we are finalizing this requirement at 40 CFR 1036.110(c), in a different regulatory section than proposed; however, the substance of the requirement is the same as at proposal.

EPA is requiring that all inducement-related diagnostic data parameters be made available with generic scan tools to help operators promptly respond when the engine detects fault condition requiring repair or other maintenance (see section IV.C.1.iii. for further information).

4. Clearing an Inducement Condition

Following restorative maintenance, EPA is requiring that the engine would allow the vehicle to self-heal if it confirms that the fault condition is resolved. The engine would then remove the inducement, which would allow the vehicle to resume unrestricted engine operation. EPA is also requiring that generic scan tools be able to remove an inducement condition after a successful repair. After clearing inducement-related fault codes, all fault codes are subject to immediate reevaluation that would lead to resuming the derate schedule where it was at the time the codes were cleared if the fault persists. Therefore, there is no need to limit the number of times a scan tool can clear codes. Use of a generic scan tool to clear inducements would allow owners who repair vehicles outside of commercial facilities to complete the repair without delay ( e.g., flushing and refilling a DEF tank where contaminated DEF was discovered). However, if the same fault condition repeats within 40 hours of engine operation ( e.g., in response to a DEF quantity fault an owner adds a small but insufficient quantity of DEF), this will be considered a repeat faut. In response to a repeat fault, the system will immediately resume the derate at the same point in the derate schedule when the original fault was deactivated. This is less time than the 80 hours EPA proposed in the NPRM, but it is consistent with existing EPA guidance. After consideration of comments, we believe that the shorter interval is long enough to give a reliable confirmation that a repair has properly addressed the fault condition, and are concerned that 80 hours would risk treating an unrelated occurrence of a fault condition as if it were a continuation of the same fault.

EPA is not finalizing the proposed provision that an inducement schedule is applied and tracked independently for each fault if multiple fault conditions are detected due to the software complexity for the manufacturer in applying and tracking the occurrence of multiple derate schedules. Section 4 of the Response to Comments for further discussion of EPA's thinking to assist manufacturers regarding consideration for programming diagnostic systems to handle overlapping fault conditions.

5. Further Considerations

EPA is not taking final action at this time on the proposed NO_X override provision, which was proposed to prevent speed derates for fault conditions that are caused by component failures if the catalyst is nevertheless functioning normally. We received comments describing concerns with our proposed methodology, including the reliability of NO_X sensors and use of OBD REAL NO_X data, and concerns that reliance in this way on the NO_X sensor could result in easier tampering. We are continuing to consider these issues and comments. We may consider such a provision in an appropriate future action. Our final inducement regulations will reduce the risk of false inducements and provide increased certainty during repairs by limiting inducements to well-defined fault conditions that focus appropriately on DEF supply, DEF quality, and tampering (open-circuit faults associated with missing aftertreatment hardware).

We have also learned from the last several years that it is important to monitor in-use experiences to evaluate whether the inducement provisions are striking the intended balance of ensuring an adequate supply of high-quality DEF in a way that is allowing for safe and timely resolution, even for cases involving difficult circumstances. For example, we might hypothetically learn from in-use experiences that component malfunctions, part shortages, or other circumstances are leaving operators in a place where inducements prevent them from operating and they are unable to perform maintenance that is needed to resolve the fault condition. Conversely, we might hypothetically learn that operators are routinely driving vehicles with active derates. Information from those in-use experiences may be helpful for future assessments of whether we should pursue adjustments to the derate schedules or other inducement provisions we are adopting in this final rule.

6. In-Use Retrofits To Update Existing Inducement Algorithms

In the NPRM, we sought comment on whether it would be appropriate to allow engine manufacturers to modify earlier model year engines to align with the new regulatory specifications. We did not propose changes to existing regulations to address this concern. Specifically, we sought comment on whether and how manufacturers might use field-fix practices under EPA's field fix guidance to modify in-use engines with algorithms that incorporate some or all the inducement provisions in the final rule. We received numerous comments on the need to modify existing inducement speeds and schedules from operator groups and at least one manufacturer. We received comment on the use of field-fixes for this purpose from CARB, stating that CARB staff does not support the SCR inducement strategy proposed by EPA and does not support allowing field fixes for in-use vehicles or to amend the certification application of current model year engines for the NPRM inducement strategy. CARB staff also commented that they would support allowing field fixes for in-use vehicles or amending current certification applications only if EPA adopts an inducement strategy identical or similar to the one CARB proposed in their comments on the proposed rule. For example, CARB suggested an inducement strategy with a 5 mph inducement after 10 hours, following an engine restart.

EPA believes field fixes with updated inducement algorithms may fall within EPA's field fix guidance for engines that have EPA-only certification ( i.e., does not include certification to California standards), but has concerns about such field fixes falling within the scope of the guidance for engines also certified by CARB if CARB considers such changes to be tampering with respect to requirements that apply in California. EPA intends to also consider alternative field fix inducement approaches that manufacturers choose to develop and propose to CARB and EPA, for engines certified by both EPA and CARB, such as approaches that provide a more balanced inducement strategy than that used in current certifications while still being effective.

E. Fuel Quality

EPA has long recognized the importance of fuel quality on motor vehicle emissions and has regulated fuel quality to enable compliance with emission standards. In 1993, EPA limited diesel sulfur content to a maximum of 500 ppm and put into place a minimum cetane index of 40. Starting in 2006 with the establishment of more stringent heavy-duty highway PM, NO_X and hydrocarbon emission standards, EPA phased-in a 15-ppm maximum diesel fuel sulfur standard to enable heavy-duty diesel engine compliance with the more stringent emission standards.

EPA continues to recognize the importance of fuel quality on heavy-duty vehicle emissions and is not currently aware of any additional diesel fuel quality requirements necessary for controlling criteria pollutant emissions from these vehicles.

1. Biodiesel Fuel Quality

As discussed in Chapter 2.3.2 of the RIA, metals ( e.g., Na, K, Ca, Mg) can enter the biodiesel production stream and can adversely affect emission control system performance if not sufficiently removed during production. Our review of data collected by NREL, EPA, and CARB indicates that biodiesel is compliant with the ASTM D6751-18 limits for Na, K, Ca, and Mg. As we explained in the proposed rule, the available data does not indicate that there is widespread off specification biodiesel blend stock or biodiesel blends in the marketplace. We did not propose and are not including at this time in this final rule requirements for biodiesel blend metal content.

While occasionally there are biodiesel blends with elevated levels of these metals, they are the exception. Data in the literature indicates that Na, K, Ca, and Mg levels in these fuels are less than 100 ppb on average. Data further suggests that the low levels measured in today's fuels are not enough to adversely affect emission control system performance when the engine manufacturer properly sizes the catalyst to account for low-level exposure.

Given the low levels measured in today's fuels, however, we are aware that ASTM is currently evaluating a possible revision to the measurement method for Na, K, Ca, and Mg in D6751-20a from EN14538 to a method that has lower detection limits ( e.g., ASTM D7111-16, or a method based on the ICP-MS method used in the 2016 NREL study). We anticipate that ASTM will likely specify Na, K, Ca, and Mg limits in a future update to ASTM 7467-19 for B6 to B20 blends that is an extrapolation of the B100 limits (see RIA Chapter 2.3.2 for additional discussion of ASTM test methods, as well as available data on levels of metal in biodiesel and potential impacts on emission control systems).

2. Compliance Issues Related to Biodiesel Fuel Quality

Given the concerns we raised in the ANPR and NPRM regarding the possibility of catalyst poisoning from metals contained in biodiesel blends and specifically heavy-duty vehicles fueled on biodiesel blends, and after consideration of comments on the NPRM, EPA is finalizing a process where we will consider the possibility that an engine was not properly maintained under the provisions of 40 CFR part 1068, subpart F, if an engine manufacturer demonstrates that the vehicle was misfueled in a way that exposed the engine and its aftertreatment components to metal contaminants and that misfueling degraded the emission control system performance. This allows a manufacturer to receive EPA approval to exempt test results from being considered for potential recall. For example, a manufacturer might request EPA approval through this process for a vehicle that was historically fueled on biodiesel blends whose B100 blend stock did not meet the ASTM D6751-20a limit for Na, K, Ca, and/or Mg (metals which are a byproduct of current biodiesel production methods). This process requires the engine manufacturer to provide proof of historic misfueling with off-specification fuels; more specifically, to qualify for the test result exemption(s), a manufacturer must provide documentation that compares the degraded system to a representative compliant system of similar miles with respect to the content and amount of the contaminant. We are also finalizing a change from the proposal in the fuel requirements relevant to conducting in-use testing and to recruitment of vehicles for in-use testing. The new provision in 40 CFR 1036.415(c)(1) states that the person conducting the in-use testing may use any commercially available biodiesel fuel blend that meets the specifications for ASTM D975 or ASTM D7467 that is either expressly allowed or not otherwise indicated as an unacceptable fuel in the vehicle's owner or operator manual or in the engine manufacturer's published fuel recommendations. As specified in final 40 CFR 1036.410, if the engine manufacturer finds that the engine was fueled with fuel not meeting the specifications in 40 CFR 1036.415(c)(1), they may disqualify the vehicle from in-use testing and replace it with another one.

F. Durability Testing

In this section, we describe the final deterioration factor (DF) provisions for heavy-duty highway engines, including migration and updates from their current location in 40 CFR 86.004-26(c) and (d) and 86.004-28(c) and (d) to 40 CFR 1036.245 and 1036.246. EPA regulations require that a heavy-duty engine manufacturer's application for certification include a demonstration that the engines will meet applicable emission standards throughout their regulatory useful life. This is often called the durability demonstration. Manufacturers typically complete this demonstration by following regulatory procedures to calculate a DF. Deterioration factors are additive or multiplicative adjustments applied to the results from manufacturer testing to quantify the emissions deterioration over useful life.

Currently, a DF is determined directly by aging an engine and exhaust aftertreatment system to useful life on an engine dynamometer. This time-consuming service accumulation process requires manufacturers to commit to product configurations well ahead of their pre-production certification testing to complete the durability testing so EPA can review the test results before issuing the certificate of conformity. Some manufacturers run multiple, staggered durability tests in parallel in case a component failure occurs that may require a complete restart of the aging process.

As explained in the NPRM, EPA recognizes that durability testing over a regulatory useful life is a significant undertaking, which can involve more than a full year of continuous engine operation for Heavy HDE to test to the equivalent of the current useful life of 435,000 miles. Manufacturers have been approved, on a case-by-case basis, to age their systems to between 35 and 50 percent of the current full useful life on an engine dynamometer, and then extrapolate the test results to full useful life. This extrapolation reduces the time to complete the aging process, but data from a test program shared with EPA show that while engine out emissions for SCR-equipped engines were predictable and consistent, actual tailpipe emission levels were higher by the end of useful life when compared to emission levels extrapolated to useful life from service accumulation of 75 or lower percent useful life. In response to the new data indicating DFs generated by manufacturers using service accumulation less than useful life may not be fully representative of useful life deterioration, EPA initially worked with manufacturers and CARB to address this concern through guidance for MY 2020 and later engines.

While the current DF guidance is specific to SCR-equipped engines, in this final rule we are updating our DF provisions to apply certain aspects of the current DF guidance to all engine families starting in model year 2027. We also are finalizing as proposed that manufacturers may optionally use these provisions to determine their deterioration factors for earlier model years. As noted in the following section, as proposed, we are continuing the option for Spark-ignition HDE manufacturers to request approval of an accelerated aging DF determination, as is allowed in our current regulations (see 40 CFR 86.004-26(c)(2)), and our final provision extends this option to all primary intended service classes. We are not finalizing any changes to the existing compliance demonstration provision in 40 CFR 1037.103(c) for evaporative and refueling emission standards. As introduced in Section III.E, in this rule we are also promulgating refueling emission standards for incomplete vehicles above 14,000 lb GVWR. As proposed, we are finalizing that incomplete vehicle manufacturers certifying to the refueling emission standards for the first time have the option to use engineering analyses to demonstrate durability using the same procedures that apply for the evaporative systems on their vehicles today.

In Section IV.F.1, we are finalizing two methods for determining DFs in a new 40 CFR 1036.245 with some modifications from those proposed, including a new option to bench-age the aftertreatment system to limit the burden of generating a DF over the lengthened useful life periods in Section IV.A.3. We are also codifying two DF verification options available to manufacturers in the recent DF guidance, with some modifications from our proposed DF verification requirements. As described in Section IV.F.2, under the final 40 CFR 1036.245 and 40 CFR 1036.246, the final provisions include two options for DF verification to confirm the accuracy of the DF values submitted by manufacturers for certification, and will be required upon request from EPA. In Section IV.F.3, we introduce a test program to evaluate a rapid-aging protocol for diesel catalysts, the results of which we used to develop a rapid-aging test procedure for CI engine manufacturers to be able to use in their durability demonstration under 40 CFR 1036.245(c)(6). We are finalizing this procedure in 40 CFR part 1065, subpart L, as new sections 40 CFR 1065.1131 through 40 CFR 1065.1145.

At this time we are not finalizing any additional testing requirements for manufacturers to demonstrate durability of other key components included in a hybrid configuration ( e.g., battery durability testing). We will consider additional requirements in a future rule as we pursue other durability-related provisions for EVs, PHEVs, etc.

As described in Section XI.A.8, we are also finalizing as proposed that manufacturers of nonroad engines may use the procedures described in this section to establish deterioration factors based on bench-aged aftertreatment, along with any EPA-requested in-use verification testing.

1. Options for Determining Deterioration Factor

Accurate methods to demonstrate emission durability are key to ensuring certified emission levels represent real world emissions, and the efficiency of those methods is especially important in light of the lengthening of useful life periods in this final rule. To address these needs, we are migrating our existing regulatory option from part 86 to part 1036 and including a new option for heavy-duty highway engine manufacturers to determine DFs for certification. We note that manufacturers apply these deterioration factors to determine whether their engines meet the duty cycle standards.

Consistent with existing regulations, final 40 CFR 1036.245 allows manufacturers to continue the current practice of determining DFs based on engine dynamometer-based aging of the complete engine and aftertreatment system out to regulatory useful life. In addition, under the new DF determination option, which includes some modifications from that proposed and which are described in this section, manufacturers perform dynamometer testing of an engine and aftertreatment system to a minimum required mileage that is less than regulatory useful life. Manufacturers then bench age the aftertreatment system to regulatory useful life and combine the aftertreatment system with an engine that represents the engine family. Manufacturers run the combined engine and bench-aged aftertreatment for at least 100 hours before collecting emission data for determination of the deterioration factor. Under this option, the manufacturer can use the accelerated bench-aging of diesel aftertreatment procedure described in Section IV.F.3 that is codified in the new sections 40 CFR 1065.1131 through 40 CFR 1065.1145 or propose an equivalent bench-aging procedure and obtain prior approval from the Agency. For example, a manufacturer might propose a different, established bench-aging procedure for other engines or vehicles ( e.g., procedures that apply for light-duty vehicles under 40 CFR part 86, subpart S).

We requested comment on whether the new bench-aged aftertreatment option accurately evaluates the durability of the emission-related components in a certified configuration, including the allowance for manufacturers to define and seek approval for a less-than-useful life mileage for the dynamometer portion of the bench-aging option. This request for comment specifically included whether or not there is a need to define a minimum number of engine hours of dynamometer testing beyond what is required to stabilize the engine before bench-aging the aftertreatment, noting that EPA's bench-aging proposal focused on deterioration of emission control components. We requested comment on including a more comprehensive durability demonstration of the whole engine, such as the recent diesel test procedures from CARB's Omnibus regulation that includes dynamometer-based service accumulation of 2,100 hours or more based on engine class and other factors. We also requested comment on whether EPA should prescribe a standardized aging cycle for the dynamometer portion, as was done by CARB in the Omnibus rule with their Service Accumulation Cycles 1 and 2. We also requested cost and time data corresponding to the current DF procedures, and projections of cost and time for the proposed new DF options at the proposed new useful life mileages.

Some commentors supported the removal of the fuel-based accelerated DF determination method, noting that it has been shown to underestimate emission control system deterioration. Other commentors requested that EPA retain the option, noting that it has been historically allowed. Fuel-based accelerated aging accelerates the service accumulation using higher-load operation based on equivalent total fuel flow on a dynamometer. The engine is only operated out to around 35 percent of UL based on operating hours, however the high-load operation is intended to result in an equivalent aging out to full UL. EPA has assessed data from the EMA DF test program and determined that the data indicated that the aging mechanism of accelerating the aging at higher load differs from the actual in-use deterioration mechanism. We are not including this option in the final provisions for determining DF based on our assessment of the available data and have removed the option in final 40 CFR 1036.245.

We also received general support of the use of accelerated aging cycles to manage the total cost and duration of the DF test, in addition to some commenters stating that the CARB DF determination procedure in the CARB Omnibus regulation is superior to the accelerated aging procedure EPA proposed in 40 CFR 1036.245(b)(2). The required hours of engine dynamometer aging in the CARB Omnibus procedure (roughly out to 20 percent of UL for a HHD engine) provide limited assurance on the performance of engine components out to UL, and thus primarily provide a short-term quality assurance durability program for engine hardware. While the purpose of EPA's DF determination procedure is to determine emission performance degradation over the useful life of the engine, we acknowledge that there is value in performing some engine dynamometer aging. We are finalizing an option to use accelerated reactor bench-aging of the emission control system that is ten times a dynamometer or field test (1,000 hours of accelerated aging would be equivalent to 10,000 hours of standard aging), requiring a minimum number of testing hours on an engine dynamometer, with the allowance for the manufacturer to add additional hours of engine dynamometer-aging at their discretion. The minimum required hours are by primary intended service class and follow: 300 hours for SI, 1,250 hours for Light HDE, and 1,500 hours for Medium HDE and Heavy HDE. This option allows the DF determination to be completed within a maximum of 180 days for a Heavy HDE. We recognize that a different approach, that uses the same aging duty-cycle for all manufacturers, would provide more consistency across engine manufacturers. However, no data was provided by commentors showing that the Service Accumulation Cycles 1 and 2 in the CARB Omnibus rule are any more effective at determining deterioration than cycles developed by the manufacturer and submitted to EPA for approval. EPA is also concerned regarding the amount of idle contained in each of the CARB Omnibus rule cycles. We realize that this idle operation was included to target the degradation mechanism that plagued the SAPO-34 SCR formulations used by manufacturers in the 2010s, however the catalyst developers are aware of this issue now and have developed formulations that are free from this degradation mechanism. The two most predominant degradation mechanisms are time at high temperature and sulfur exposure, including the effects of catalyst desulfation, and as such EPA favors duty-cycles with more aggressive aftertreatment temperature profiles. We understand that catalyst manufacturers now bench test the catalyst formulations under the conditions that led to the SAPO-34 degradation to ensure that this degradation mechanism is not present in newly developed SCR formulations. After taking all of the comments received into consideration, EPA has added two specified duty-cycle options in 40 CFR 1036.245(c) for DF determination, that are identical to CARB's Service Accumulation Cycles 1 and 2. Cycle 1 consists of a combination of FTP, RMC, LLC and extended idle, while Cycle 2 consists of a combination of HDTT, 55-cruise, 65-cruise, LLC, and extended idle. In the case of the second option, the manufacturer is required to use good engineering judgment to choose the vehicle subcategory and vehicle configuration that yields the highest load factor using the GEM model. EPA is also providing an option for manufacturers to use their own duty cycles for DF determination subject to EPA approval and we expect a manufacturer to include light-load operation if it is deemed to contribute to degradation of the aftertreatment performance. We also note that we are finalizing requirements to stop, cooldown, and restart the engine during service accumulation when using the options that correspond to CARB Service Accumulation Cycles 1 and 2 for harmonization purposes, however we note that manufacturers may make a request to EPA to remove this requirement on a case-by-case basis.

We are finalizing critical emission-related maintenance as described in 40 CFR 1036.125(a)(2) and 1036.245(c) in this final rule. Under this final rule, manufacturers may make requests to EPA for approval for additional emission-related maintenance actions beyond what is listed in 40 CFR 1036.125(a)(2), as described in 40 CFR 1036.125(a)(1) and as allowed during deterioration testing under 40 CFR 1036.245(c).

2. Options for Verifying Deterioration Factors

We are finalizing, with some modifications from proposal, a new 40 CFR 1036.246 where, at EPA's request, the manufacturers would be required to verify an engine family's deterioration factor for each duty cycle up to 85 percent of useful life. Because the manufacturer must comply with emission standards out to useful life, we retain the authority to verify DF. We proposed requiring upfront verification for all engine families, but have decided to make this required only in the event that EPA requests verification. We intend to make such a request primarily when EPA becomes aware of information suggesting that there is an issue with the DF generated by the manufacturer. EPA anticipates that a DF verification request may be appropriate due to consideration of, for example: (1) Information indicating that a substantial number of in-use engines tested under subpart E of this part failed to meet emission standards, (2) information from any other test program or any other technical information indicating that engines will not meet emission standards throughout the useful life, (3) a filed defect report relating to the engine family, (4) a change in the technical specifications for any critical emission-related components, and (5) the addition of a new or modified engine configuration such that the test data from the original emission-data engine do not clearly continue to serve as worst-case testing for certification. We are finalizing as proposed that manufacturers may request use of an approved DF on future model year engines for that engine family, using the final updates to carryover engine data provisions in 40 CFR 1036.235(d), with the final provision clarifying that we may request DF verification for the production year of that new model year as specified in the new 40 CFR 1036.246. As also further discussed in the following paragraphs, we are not finalizing at this time certain DF verification provisions that we had proposed regarding timing of when EPA may request DF verification and certain provisions for the first model year after a failed result. Our revisions from proposal appropriately provide flexibility for EPA to gather information based on DF concerns. The final provisions specify that we will discuss with the manufacturer the selection criteria for vehicles with respect to the target vehicle mileage(s) and production model year(s) that we want the manufacturer to test. We are finalizing that we will not require the manufacturer to select vehicles whose mileage or age exceeds 10 years or 85 percent of useful life.

We originally included three testing options in our proposed DF verification provisions. We are finalizing two of these options and we are not including the option to verify DF by measuring NO_X emissions using the vehicle's on-board NO_X measurement system at this time. For the two options we are finalizing, manufacturers select in-use engines meeting the criteria in 40 CFR 1036.246(a), including the appropriate mileage specified by EPA corresponding to the production year of the engine family.

Under the first verification option in 40 CFR 1036.246(b)(1), manufacturers test at least two in-use engines over all duty cycles with brake-specific emission standards in 40 CFR 1036.104(a) by removing each engine from the vehicle to install it on an engine dynamometer and measure emissions. Manufacturers determine compliance with the emission standards after applying infrequent regeneration adjustment factors to their measured results, just as they did when they originally certified the engine family. We are also finalizing a requirement under this option to allow EPA to request that manufacturers perform a new determination of infrequent regeneration adjustment factors to apply to the emissions from the engine dynamometer-based testing. Consistent with the proposal, the engine family passes the DF verification if 70 percent or more of the engines tested meet the duty-cycle emission standards in 40 CFR 1036.104(a), including any associated compliance allowance, for each pollutant over all duty cycles. If a manufacturer chooses to test two engines under this option, both engines have to meet the standards. Under this option, the aftertreatment system, including all the associated wiring, sensors, and related hardware or software is installed on the test engine. We are finalizing an allowance in 40 CFR 1036.246(a) for the manufacturer to use hardware or software in testing that differs from those used for engine family and power rating with EPA approval.

Under the second verification option in 40 CFR 1036.246(b)(2), as proposed, manufacturers test at least five in-use engines, to account for the increased variability of vehicle-level measurement, while installed in the vehicle using a PEMS. Manufacturers bin and report the emissions following the in-use testing provisions in 40 CFR part 1036, subpart E. Compliance is determined by comparing emission results to the off-cycle emission standards in 40 CFR 1036.104(a) with any associated compliance allowance, mean ambient temperature adjustment, and, accuracy margin for each pollutant for each bin after adjusting for infrequent regeneration. As proposed, the engine family passes the DF verification if 70 percent or more of the engines tested meet the off-cycle standards for each pollutant for each bin. In the event that EPA requested DF verification and a DF verification fails under the PEMS option, consistent with the proposal the manufacturer can reverse a fail determination for the PEMS-based testing and verify the DF using the engine dynamometer testing option in 40 CFR 1036.246(b)(1).

EPA is not including the third option we proposed, to verify DF using the vehicle's on-board NO_X measurement system ( i.e., a NO_X sensor), in the final provisions, as we have concerns that the technology has not matured enough to make this method viable for DF verification at this time. We did not receive any comments that supported the availability of technology to enable accurate on-board NO_X measurement at a level needed to show compliance with the standard. EPA acknowledges the challenges associated with the development of a functional onboard NO_X measurement method, including data acquisition and telematic system capabilities, and may reconsider this option in the future if the technology evolves.

As noted in the preceding paragraphs, we are not taking final action at this time on the proposed 40 CFR 1036.246(h) provision that proposed a process for the first MY after a DF verification resulted in failure. Instead, we are adopting a process for DF verification failures similar to the existing process used for manufacturer run in-use testing failures under 40 CFR part 1036, subpart E, such that a failure may result in an expanded discovery process that could eventually lead to recall under our existing provisions in 40 CFR part 1068, subpart F. EPA is making this change from proposal because this approach provides consistency with and builds upon existing processes.

The final 40 CFR 1036.246(a) specifies how to select and prepare engines for testing. Manufacturers may exclude selected engines from testing if they have not been properly maintained or used and the engine tested must be in a certified configuration, including its original aftertreatment components. Manufacturers may test engines that have undergone critical emission-related maintenance as allowed in 40 CFR 1065.410(d), but may not test an engine if its critical emission-related components had any other major repair.

3. Accelerated Deterioration Factor Determination

As discussed in Section IV.F.1, we are finalizing a deterioration factor procedure where manufacturers use engine dynamometer testing for the required minimum number of hours given in Table 1 to Paragraph (c)(2) of 40 CFR 1036.245 in combination with an accelerated aftertreatment catalyst aging protocol in their demonstration of heavy-duty diesel engine aftertreatment durability through useful life. EPA has approved accelerated aging protocols for spark-ignition engine manufacturers to apply in their durability demonstrations for many years. Historically, while CI engine manufacturers have the ability to request EPA approval of an accelerated aging procedure, CI engine manufacturers have largely opted to seek EPA approval to use a service accumulation fuel based accelerated test with reduce mileage and extrapolate to determine their DF.

Other regulatory agencies have promulgated accelerated aging protocols, and we have evaluated how these or similar protocols apply to our heavy-duty highway engine compliance program. EPA has validated and is finalizing an accelerated aging procedure in 40 CFR part 1065, subpart L, as new sections 40 CFR 1065.1131 through 40 CFR 1065.1145 that CI engine manufacturers can choose to use in lieu of developing their own protocol as described in 40 CFR 1036.245. The test program that validated the diesel aftertreatment rapid-aging protocol (DARAP) was built on existing accelerated aging protocols designed for light-duty gasoline vehicles (64 FR 23906, May 4, 1999) and heavy-duty engines.

i. Diesel Aftertreatment Rapid Aging Protocol

The objective of the DARAP validation program was to artificially recreate the three primary catalytic deterioration processes observed in field-aged aftertreatment components: Thermal aging based on time at high temperature, chemical aging that accounts for poisoning due to fuel and oil contamination, and deposits. The validation program had access to three baseline engines that were field-aged to the model year 2026 and earlier useful life of 435,000 miles. Engines and their corresponding aftertreatment systems were aged using the current, engine dynamometer-based durability test procedure for comparison of the results to the accelerated aging procedure. We performed accelerated aging of the catalyst-based aftertreatment systems using two different methods with one utilizing a burner and the other using an engine as the source of aftertreatment aging conditions. The validation test plan compared emissions at the following approximate intervals: 0 percent, 25 percent, 50 percent, 75 percent, and 100 percent of the model year 2026 and earlier useful life of 435,000 miles. At proposal, we included additional details of our DARAP test program in a memo to the docket.

The DARAP validation program has completed testing of two rapidly aged aftertreatment systems, engine and burner, and two engines, a single FUL aged engine and a 300-hour aged engine. Our memo to the docket includes a summary of the validation results from this program. The results show that both accelerated aging pathways, burner and engine, produced rapidly aged aftertreatment system results that were not statistically significant when compared to the 9,800-hour dynamometer aged reference system. We are currently completing postmortem testing to evaluate the deposition of chemical poisoning on the surface of the substrates to see how this compares to the dynamometer aged reference system. The complete results from our validation program are contained in a final report in the docket.

ii. Diesel Aftertreatment Accelerated Aging Test Procedure

The final provisions include an option for manufacturers to use the method from the DARAP test program for DF determination and streamline approval under 40 CFR 1036.245(c). This accelerated aging method we are finalizing in 40 CFR part 1065, subpart L, as new sections 40 CFR 1065.1131 through 40 CFR 1065.1145 is a protocol for translating field data that represents a given application ( e.g., engine family) into an accelerated aging cycle for that given application, as well as methods for carrying out reactor or engine accelerated aging using that cycle. While this testing can be carried out on an engine as well as reactor bench, the engine option should not be confused with standard engine dynamometer aging out to useful life or the historic fuel-based engine dynamometer accelerated aging typically done out to 35 percent of useful life approach that EPA will no longer allow under this final rule. The engine option in this procedure uses the engine (1) as a source of accelerated sulfur from the combusted fuel, (2) as a source for exhaust gas, and (3) to generate heat. The catalyst poisoning agents (oil and sulfur) as well as the temperature exposure are the same between the two methods and the DARAP test program data corroborates this. This protocol is intended to be representative of field aging, includes exposure to elements of both thermal and chemical aging, and is designed to achieve an acceleration of aging that is ten times a dynamometer or field test (1,000 hours of accelerated aging would be equivalent to 10,000 hours of standard aging).

The initial step in the method requires the gathering and analysis of input field data that represent a greater than average exposure to potential field aging factors. The field aging factors consist of thermal, oil, and sulfur exposure. The thermal exposure is based on the average exhaust temperature; however, if the engine family incorporates a periodic infrequent regeneration event that involves exposure to higher temperatures than are observed during normal (non-regeneration) operation, then this temperature is used. Oil exposure is based on field and laboratory measurements to determine an average rate of oil consumption in grams per hour that reaches the exhaust. Sulfur exposure is based on the sum of fuel- and oil-related sulfur consumption rates for the engine family. The procedure provides details on how to gather data from field vehicles to support the generation and analysis of the field data.

Next, the method requires determination of key components for aging. Most diesel aftertreatment systems contain multiple catalysts, each with their own aging characteristics. This accelerated aging procedure ages the system, not component-by-component. Therefore, it is necessary to determine which catalyst components are the key components that will be used for deriving and scaling the aging cycle. This includes identification of the primary and secondary catalysts in the aftertreatment system, where the primary is the catalyst that is directly responsible for most of the NO_X reduction, such as a urea SCR catalyst in a compression-ignition aftertreatment system. The secondary is the catalyst that is intended to either alter exhaust characteristics or generate elevated temperature upstream of the primary catalyst, such as a DOC placed upstream of an SCR catalyst, with or without a DPF in between.

The next step in the process is to determine the thermal deactivation rate constant(s) for each key component. This is used for the thermal heat load calculation in the accelerated aging protocol. The calculations for thermal degradation are based on the use of an Arrhenius rate law function to model cumulative thermal degradation due to heat exposure. The process of determining the thermal deactivation rate constant begins with determining what catalyst characteristic will be tracked as the basis for measuring thermal deactivation. Generally, ammonia storage is the key aging metric for zeolite-based SCR catalysts, NO_X reduction efficiency at low temperature for vanadium-based SCR catalysts, conversion rate of NO to NO₂ for DOCs with a downstream SCR catalyst, and HC reduction efficiency (as measured using ethylene) at 200 °C for DOCs where the aftertreatment system does not contain an SCR catalyst for NO_X reduction. Thermal degradation experiments are then carried out over at least three different temperatures that accelerate thermal deactivation such that measurable changes in the aging metric can be observed at multiple time points over the course of no more than 50 hours. During these experiments it is important to void temperatures that are too high to prevent rapid catalyst failure by a mechanism that does not represent normal aging.

Generation of the accelerated aging cycle for a given application involves analysis of the field data to determine a set of aging modes that will represent that field operation. There are two methods of cycle generation in 40 CFR 1065.1139, each of which is described separately. Method 1 involves the direct application of field data and is used when the recorded data includes sufficient exhaust flow and temperature data to allow for determination of aging conditions directly from the field data set. Method 2 is meant to be used when insufficient flow and temperature data is available from the field data. In Method 2, the field data is used to weight a set of modes derived from the laboratory certification cycles for a given application. These weighted modes are then combined with laboratory recorded flow and temperatures on the certification cycles to derive aging modes. There are two different cases to consider for aging cycle generation, depending on whether or not a given aftertreatment system incorporates the use of a periodic regeneration event. For the purposes of cycle generation, a regeneration is any event where the operating temperature of some part of the aftertreatment system is raised beyond levels that are observed during normal (non-regeneration) operation. The analysis of regeneration data is considered separately from normal operating data.

The process of cycle generation begins with the determination of the number of bench aging hours. The input into this calculation is the number of real or field hours that represent the useful life for the target application. The target for the accelerated aging protocol is a 10-time acceleration of the aging process, therefore the total number of aging hours is set at service accumulation hours minus required engine dynamometer aging hours divided by 10. The hours will then be among different operating modes that will be arranged to result in repetitive temperature cycling over that period. For systems that incorporate periodic regeneration, the total duration will be split between regeneration and normal (non-regeneration) operation. The analysis of the operation data develops a reduced set of aging modes that represent normal operation using either Method 1 or Method 2. Method 1 is a direct clustering method and involves three steps: Clustering analysis, mode consolidation, and cycle building. This method is used when sufficient exhaust flow and temperature data are available directly from the field data. Method 2 is a cluster-based weighting of certification cycle modes when there is insufficient exhaust flow and temperature data from the field at the time the cycle is being developed. The initial candidate mode conditions are temperature and flow rate combinations that are the centroids from the analysis of each cluster.

The target for accelerated aging cycle operation is to run all the regenerations that would be expected over the course of useful life and the procedure provides a process for determining a representative regeneration profile that will be used during aging. Heat load calculation and cycle tuning are performed after the preliminary cycles have been developed for both normal and regeneration operation. The target cumulative deactivation is determined from the input field data, and then a similar calculation is performed for the preliminary aging cycle. If the cumulative deactivation for the preliminary cycle does not match cumulative deactivation from the field data, then the cycle is tuned over a series of steps described in 40 CFR 1065.1139 until the target is matched.

The final assembly of the candidate accelerated aging cycle involves the assembly of the target modes into a schedule of modes laid out on a time basis that can be repeated until the target number of aging hours has been reached. For cycles that incorporate periodic regeneration modes, the regeneration frequency and duration, including any regeneration extension added to reach thermal targets, will be used to determine the length of the overall cycle. The number of these cycles that is run is equal to the total number of regenerations over full useful life. The duration of each cycle is total number of accelerated aging hours divided by the total number of regenerations. For multiple components with differing regeneration schedules, this calculation is performed using the component with the fewest total number of regenerations. The regeneration events for the more frequently regenerating components should be spaced evenly throughout each cycle to achieve the appropriate regeneration frequency and duration.

The regeneration duration (including extension) is then subtracted from the base cycle duration to calculate the duration of normal (non-regeneration) operation in seconds. This time is split among the normal (non-regeneration) modes in proportion to the overall target aging time in each mode. These modes are then split and arranged to achieve the maximum thermal cycling between high and low temperatures. No mode may have a duration shorter than 900 seconds, not including transition time. Mode transitions must be at least 60 seconds long and must be no longer than 300 seconds. The transition period is considered complete when you are within 5 °C of the target temperature for the primary key component. For modes longer than 1800 seconds, you may count the transition time as time in mode. For modes shorter than 1800 seconds, under the procedure you must not count the transition time as time in mode. Modes are arranged in alternating order starting with the lowest temperature mode and proceeding to the highest temperature mode, followed by the next lowest temperature mode, and so forth.

The final cycle is expressed as a schedule of target temperature, exhaust flow rate, and NO_X. For a burner-based platform with independent control of these parameters, this cycle can be used directly. For an engine-based platform, it is necessary to develop a schedule of speed and load targets that will produce the target exhaust conditions based on the capabilities of the engine platform.

The accelerated oil consumption target is calculated at 10 times the field average oil consumption that was determined from the field data and/or laboratory measurements. Under the procedure, this oil consumption rate must be achieved on average over the aging cycle, and it must at least be performed during all non-regeneration modes. Under the procedure, the lubricating oil chosen must meet the normal in-use specifications and it cannot be altered. The oil is introduced by two pathways, a bulk pathway and a volatile pathway. The bulk pathway involves introduction of oil in a manner that represents oil passing the piston rings, and the volatile pathway involves adding small amount of lubricating oil to the fuel. Under the procedure, the oil introduced by the volatile pathway must be between 10 percent and 30 percent of the total accelerated oil consumption.

Sulfur exposure related to oil is already taken care of via acceleration of the oil consumption itself. The target cumulative fuel sulfur exposure is calculated using the field recorded average fuel rate data and total field hours assuming a 10-ppm fuel sulfur level (which was determined as the 90th percentile of available fuel survey data).

For an engine-based accelerated aging platform where the engine is used as the exhaust gas source, accelerated fuel sulfur is introduced by increasing the fuel sulfur level. The cycle average fuel rate over the final aging cycle is determined once that target modes have been converted into an engine speed and load schedule. The target aging fuel sulfur level that results in reaching the target cumulative fuel sulfur exposure is determined from the field data using the aging cycle average fuel rate and the total number of accelerated aging hours.

For a burner-based platform, accelerated fuel sulfur is introduced directly as gaseous SO₂ . Under the procedure, the SO₂ must be introduced in a manner that does not impede any burner combustion, and only in a location that represents the exhaust conditions entering the aftertreatment system. Under the procedure, the mass rate of sulfur that must be introduced on a cycle average basis to reach the target cumulative fuel sulfur exposure from the field data is determined after the final aging cycle has been generated.

The accelerated aging protocol is run on a bench aging platform that includes features necessary to successfully achieve accelerated aging of thermal and chemical aging factors. This aging bench can be built around either an engine or a burner as the core heat generating element. The requirements for both kinds of bench aging platform are described in the following paragraphs.

The engine-based accelerated aging platform is built around the use of a diesel engine for generation of heat and flow. The engine used does not need to be the same engine as the application that is being aged. Any diesel engine can be used, and the engine may be modified as needed to support meeting the aging procedure requirements. You may use the same bench aging engine for deterioration factor determination from multiple engine families. The engine must be capable of reaching the combination of temperature, flow, NO_X, and oil consumption targets required. Using an engine platform larger than the target application for a given aftertreatment system can provide more flexibility to achieve the target conditions and oil consumption rates.

To increase the range of flexibility of the bench aging engine platform, the test cell setup should include additional elements to allow more independent control of exhaust temperature and flow than would be available from the engine alone. For example, exhaust heat exchangers and/or the use of cooled and uncooled exhaust pipe can be useful to provide needed flexibility. When using heat exchangers under this procedure, you must ensure that you avoid condensation in any part of the exhaust system prior to the aftertreatment. You can also control engine parameters and the calibration on the engine to achieve additional flexibility needed to reach the target exhaust conditions.

Under this procedure, oil consumption must be increased from normal levels to reach the target of 10 times oil consumption. As noted earlier, oil must be introduced through a combination of a bulk pathway, which represents the majority of oil consumption past the piston rings, and a volatile pathway, which is achieved by adding small amounts of lube oil to the fuel. The total oil exposure via the volatile pathway must be between 10 percent and 30 percent of the total accelerated oil consumption. Under this procedure, the remainder of the oil consumption must be introduced via the bulk pathway. The volatile portion of the oil consumption should be introduced and monitored continuously via a mass flow meter or controller.

Under this procedure, the engine will need to be modified to increase oil consumption via the bulk pathway. This increase is generally achieved through a combination of engine modifications and the selection of aging speed/load combinations that will result in increased oil consumption rates. To achieve this, you may modify the engine in a fashion that will increase oil consumption in a manner such that the oil consumption is still generally representative of oil passing the piston rings into the cylinder. Inversion of the top compression rings as a method which has been used to increase oil consumption successfully for the DAAAC aging program at SwRI. A secondary method that has been used in combination with the primary method involves the modification of the oil control rings in one or more cylinders to create small notches or gaps (usually no more than two per cylinder) in the top portion of the oil control rings that contact the cylinder liner (care must be taken to avoid compromising the structural integrity of the ring itself).

Under this procedure, oil consumption for the engine-based platform must be tracked at least periodically via a drain and weigh process, to ensure that the proper amount of oil consumption has been achieved. It is recommended that the test stand include a constant volume oil system with a sufficiently large oil reservoir to avoid oil “top-offs” between oil change intervals. Under this procedure, periodic oil changes will be necessary on any engine platform, and it is recommended that the engine be run for at least 72 hours following an oil change with engine exhaust not flowing through the aftertreatment system to stabilize oil consumption behavior before resuming aging. A secondary method for tracking oil consumption is to use clean DPF weights to track ash loading, and compare this mass of ash to the amount predicted using the measured oil consumption mass and the oil ash concentration. The mass of ash found by DPF weight should fall within a range of 55 percent to 70 percent of the of mass predicted from oil consumption measurements.

The engine should also include a means of introducing supplemental fuel to the exhaust to support regeneration if regeneration events are part of the aging. This can be done either via post-injection from the engine or using in-exhaust injection. The method and location of supplemental fuel introduction should be representative of the approach used on the target application, but manufacturers may adjust this methodology as needed on the engine-based aging platform to achieve the target regeneration temperature conditions.

The burner-based aging platform is built around a fuel-fired burner as the primary heat generation mechanism. For the accelerated aging application under this procedure, the burner must utilize diesel fuel and it must produce a lean exhaust gas mixture. Under this procedure, the burner must have the ability to control temperature, exhaust flow rate, NO_X, oxygen, and water to produce a representative exhaust mixture that meets the accelerated aging cycle targets for the aftertreatment system to be aged. Under this procedure, the burner must include a means to monitor these constituents in real time, except in the case of water where the system's water metering may be verified via measurements made prior to the start of aging (such as with an FTIR) and should be checked periodically by the same method. Under this procedure, the accelerated aging cycle for burner-based aging must also include representative mode targets for oxygen and water, because these will not necessarily be met by the burner itself through combustion. As a result, for this procedure the burner will need features to allow the addition of water and the displacement of oxygen to reach representative target levels of both. During non-regeneration modes, it is recommended that the burner be operated in a manner to generate a small amount of soot to facilitate proper ash distribution in the DPF system.

The burner-based platform requires methods for oil introduction for both the bulk pathway and the volatile pathway. For the bulk pathway, manufacturers may implement a method that introduces lubricating oil in a region of the burner that does not result in complete combustion of the oil, but at the same time is hot enough to oxidize oil and oil additives in a manner similar to what occurs when oil enters the cylinder of an engine past the piston rings. Care must be taken to ensure the oil is properly atomized and mixed into the post-combustion burner gases before they have cooled to normal exhaust temperatures, to insure proper digestion and oxidation of the oil constituents. The volatile pathway oil is mixed into the burner fuel supply and combusted in the burner. As noted earlier, under this procedure total oil exposure via the volatile pathway must be between 10 percent and 30 percent of the total accelerated oil consumption. The consumption of oil in both pathways should be monitored continuously via mass flow meters or controllers. A secondary method of tracking oil consumption is to use clean DPF weights to track ash loading and compare this mass of ash to the amount predicted using the measured oil consumption mass and the oil ash concentration. The mass of ash found by DPF weight should fall within a range of 55 percent to 70 percent of the of mass predicted from oil consumption measurements. This will also ensure that injected oil mass is actually done in a representative manner so that it reaches the aftertreatment system.

Under this procedure, the burner-based platform will also need a method to introduce and mix gaseous SO₂ to achieve the accelerated sulfur targets. Under this procedure, the consumption of SO₂ must be monitored continuously via a mass flow meter or controller. SO₂ does not need to be injected during regeneration modes.

The burner-based platform should also include a means of introducing supplemental fuel to the exhaust to support regeneration if regeneration events are part of the aging. We recommend that the method and location of supplemental fuel introduction be representative of the approach used on the target application, but manufacturers may adjust this methodology as needed on the bench engine platform to achieve the target regeneration temperature conditions. For example, to simulate post-injected fuel we recommend to introduce the supplemental fuel into the post-combustion burner gases to achieve partial oxidation that will produce more light and partially oxidized hydrocarbons similar to post-injection.

There are specific requirements for the implementation, running, and validation of an accelerated aging cycle developed using the processes described in this section. Some of these requirements are common to both engine-based and burner-based platforms, but others are specific to one platform type or the other.

We recommended carrying out one or more practice aging cycles to help tune the cycle and aging platform to meet the cycle requirements. These runs can be considered part of the de-greening of test parts, or these can be conducted on a separate aftertreatment.

The final target cycle is used to calculate a cumulative target deactivation for key aftertreatment components. Manufacturers must also generate a cumulative deactivation target line describing the linear relationship between aging hours and cumulative deactivation. The temperature of all key components is monitored during the actual aging test and the actual cumulative deactivation based on actual recorded temperatures is calculated. The cumulative deactivation must be maintained to within 3 percent of the target line over the course of the aging run and if you are exceeding these limits, you must adjust the aging stand parameters to ensure that you remain within these limits. Under this procedure, you must stay within these limits for all primary key components. It should be noted that any adjustments made may require adjustment of the heat rejection through the system if you are seeing different behavior than the target cycle suggests based on the field data. If you are unable to meet this requirement for any tracked secondary system (for example for a DOC where the SCR is the primary component), you may instead track the aging metric directly and show that you are within 3 percent of the target aging metric. Note that this is more likely to occur when there is a large difference between the thermal reactivity coefficients of different components.

Calculate a target line for oil accumulation and sulfur accumulation showing a linear relationship between aging hours and the cumulative oil exposure on a mass basis. Under this procedure, you must stay within ±10 percent of this target line for oil accumulation, and within ±5 percent of this target line for sulfur accumulation. In the case of engine-based bulk oil accumulation you will only be able to track this based on periodic drain and weigh measurements. For all other chemical aging components, track exposure based on the continuous data from the mass flow meters for these chemical components. If your system includes a DPF, it is recommend that you implement the secondary tracking of oil consumption using DPF ash loading measurements as describe earlier.

For the engine-based platform, it will be necessary under this procedure to develop a schedule of engine operating modes that achieve the combined temperature, flow, and oil consumption targets. You may deviate from target NO_X levels as needed to achieve these other targets, but we recommend that you maintain a NO_X level representative of the target application or higher on a cycle average basis. Note that the need to operate at modes that can reach the target oil consumption will leverage the flexibility of the engine stand, and you may need to iterate on the accelerated oil consumption modifications to achieve a final target configuration. You may need to adjust the cycle or modify the oil consumption acceleration to stay within the ±10 percent target. In the even that you find that actual fuel consumption varies from original assumptions, you may need to adjust the doped fuel sulfur level periodically to maintain the sulfur exposure within the ±5 percent limit.

If the application uses DEF, it must be introduced to the exhaust stream in a manner that represents the target application. You may use hardware that is not identical to the production hardware but ensure that hardware produces representative performance. Similarly, you may use hardware that is not identical to production hardware for fuel introduction into the exhaust as long you ensure that the performance is representative.

Under this procedure, for the burner-based platform, you will be able to directly implement the temperature, flow, NO_X, sulfur, and oil consumption targets. You will also need to implement water and O₂ targets to reach levels representative of diesel exhaust. We recommend that you monitor and adjust oil and sulfur dosing on a continuous basis to stay within targets. You must verify the performance of the oil exposure system via the secondary tracking of oil exposure via DPF ash loading and weighing measurements. This will ensure that your oil introduction system is functioning correctly. If you use a reductant, such as DEF, for NO_X reduction, use good engineering judgement to introduce DEF in a manner that represents the target application. You may use hardware that is not identical to the production hardware but ensure that the hardware produces representative performance. Similarly, you may use hardware that is not identical to production hardware for fuel introduction into the exhaust as long you ensure that the performance is representative.

The implementation and carrying out of these procedures will enable acceleration of the deterioration factor determination testing, and generally allow the determination of the deterioration factor out to useful life, over 90 days of testing.

G. Averaging, Banking, and Trading

EPA is finalizing an averaging, banking, and trading (ABT) program for heavy-duty engines that provides manufacturers with flexibility in their product planning while encouraging the early introduction of emissions control technologies and maintaining the expected emissions reductions from the program. Several core aspects of the ABT program we are finalizing are consistent with the proposed ABT program, but the final ABT program includes several updates after consideration of public comments. In particular, EPA requested comment on and agrees with commenters that a lower family emission limit (FEL) cap than proposed is appropriate for the final rule. Further, after consideration of public comments, EPA is not finalizing at this time the proposed Early Adoption Incentives program, and in turn we are not including emissions credit multipliers in the final program. Rather, we are finalizing an updated version of the proposed transitional credit program under the ABT program. As described in preamble Section IV.G.7, the revised transitional credit program that we are finalizing provides four pathways to generate straight NO_X emissions credits ( i.e., no credit multipliers) that are valued based on the extent to which the engines generating credits comply with the requirements we are finalizing for MY 2027 and later ( e.g., credits discounted at a rate of 40 percent for engines meeting a lower numeric standard but none of the other MY 2027 and later requirements) (see section 12 of the Response to Comments document and preamble Section IV.G.7 for more details). In addition, we are finalizing a production volume allowance for MYs 2027 through 2029 that is consistent with the proposal but different in several key aspects, including that manufacturers will be required to use NO_X emissions credits to certify heavy heavy-duty engines compliant with MY 2010 requirements in MYs 2027 through 2029 (see Section IV.G.9 for details). Finally, we are not finalizing the proposed allowance for manufacturers to generate NO_X emissions credits from heavy-duty zero emissions vehicles (ZEVs) (see Section IV.G.10).

Consistent with the proposed ABT program, the final ABT program will maintain several aspects of the ABT program currently specified in 40 CFR 86.007-15, including:

• Allowing ABT of NO_X credits with no expiration of the ABT program,

• calculating NO_X credits based on a single NO_X FEL for an engine family,

• specifying FELs to the same number of decimal places as the applicable standards, and
• calculating credits based on the work and miles of the FTP cycle.

In this Section we briefly describe the proposed ABT program, the comments received on the proposed ABT program, and EPA's response to those comments. Subsequent subsections provide additional details on the restrictions we are finalizing for using emission credits in model years 2027 and later, such as averaging sets (Section IV.G.2), FEL caps (Section IV.G.4), and limited credit life (Section IV.G.4). See the proposed rule preamble (87 FR 17550, March 28, 2022) for additional discussion on the proposed ABT program and the history of ABT for heavy-duty engines.

The proposed ABT program allowed averaging, banking, and trading of NO_X credits generated against applicable heavy-duty engine NO_X standards, while discontinuing a credit program for HC and PM. We also proposed new provisions to clarify how FELs apply for additional duty cycles. The proposed program included restrictions to limit the production of new engines with higher emissions than the standards; these restrictions included FEL caps, credit life for credits generated for use in MYs 2027 and later, and the expiration of currently banked credits. These provisions were included in proposed 40 CFR part 1036, subpart H. and 40 CFR 1036.104(c). In addition, we proposed interim provisions in 40 CFR 1036.150(a)(1) describing how manufacturers could generate credits in MY 2024 through 2026 to apply in MYs 2027 and later. We requested comment on several aspects of the proposed ABT program that we are updating in the final rule, including the transitional credit program and level of the FEL cap, which restrict the use of credits in MY 2027 and later.

Many commenters provided perspectives on the proposed ABT program. The majority of commenters supported the proposed ABT program, although several suggested adjustments for EPA to consider in the final rule. In contrast, a number of commenters opposed the proposed ABT program and argued that EPA should eliminate the NO_X ABT program in the final rule. Perspectives from commenters supporting and opposing the proposed ABT program are briefly summarized in this section with additional details in section 12 of the Response to Comments document.

Commenters supporting the ABT program stated that it provides an important flexibility to manufacturers for product planning during a transition to more stringent standards. They further stated that a NO_X ABT program would allow manufacturers to continue offering a complete portfolio of products, while still providing real NO_X emissions reductions. In contrast, commenters opposing the ABT program argued EPA should eliminate the NO_X ABT program in order to maximize NO_X emissions reductions nationwide, particularly in environmental justice communities and other areas impacted by freight industry. These commenters stated that the NO_X standards are feasible without the use of credits, and that eliminating the credit flexibilities of an ABT program would be most consistent with EPA's legal obligations under the CAA.

EPA agrees with those commenters who support a well-designed ABT program as a way to help us meet our emission reduction goals at a faster pace while providing flexibilities to manufacturers to meet new, more stringent emission standards. For example, averaging, banking, and trading can result in emissions reductions by encouraging the development and use of new and improved emission control technology, which results in lower emissions. The introduction of new emission control technologies can occur either in model years prior to the introduction of new standards, or during periods when there is no change in emissions standards but manufacturers still find it useful to generate credits for their overall product planning. In either case, allowing banking and trading can result in emissions reductions earlier in time, which leads to greater public health benefits sooner than would otherwise occur; benefits realized sooner in time are generally worth more to society than those deferred to a later time. These public health benefits are further ensured through the use of restrictions on how and when credits may be used ( e.g., averaging sets, credit life), which are discussed further in this Section IV.G. For manufacturers, averaging, banking, and trading provides additional flexibility in their product planning by providing additional lead time before all of their engine families must comply with all the new requirements without the use of credits. For periods when no changes in emission standards are involved, banking can provide manufacturers additional flexibility, provide assurance against any unforeseen emissions-related problems that may arise, and in general provide a means to encourage the development and introduction of new engine technology (see 55 FR 30585, July 26, 1990, for additional discussion on potential benefits of an ABT program).

While EPA also agrees with those commenters stating that the standards in the final rule are feasible without the use of credits, as described in Section III of this preamble, given the technology-forcing nature of the final standards we disagree that providing an optional compliance pathway through the final rule's ABT program is inconsistent with requirements under CAA section 202(a)(3)(A). The final ABT program appropriately balances flexibilities for manufacturers to generate NO_X emissions credits with updated final restrictions ( e.g., credit life, averaging sets, and family emissions limit (FEL) caps) that in our judgement both ensure that available emissions control technologies are adopted and maintain the emissions reductions expected from the final standards. An ABT program is also an important foundation for targeted incentives to encourage manufacturers to adopt advanced technology before required compliance dates, which we discuss further in preamble Section IV.G.7 and Section 12 of the Response to Comments document.

One commenter opposing EPA's proposed NO_X emissions ABT program provided analyses for EPA to consider in developing the final rule. EPA has evaluated the three approaches to generating credits in the commenter's analysis: (1) Engines certified below today's standards which qualify for the proposed transitional credit program, (2) engines certified to the CARB Omnibus standards which would quality for the proposed transitional program or on average achieve a standard below Federal requirements, and (3) ZEVs. For the first category (the transitional credit program), we considered several factors when designing the final transitional credit program that are more fully described in preamble Section IV.G.7; briefly, the transitional credit program we are finalizing will discount the credits manufacturers generated from engines certified to levels below today's standards unless manufacturers can meet all of the requirements in the final MY 2027 and later standards. This includes meeting standards such as the final low load cycle (LLC), which requires demonstration of emissions control in additional engine operations ( i.e., low load) compared to today's test cycles. For the second category in the commenter's analysis (engines certified to Omnibus standards), we recognize that our proposed rule preamble may have been unclear regarding how the existing regulations in part 86 and part 1036 apply for purposes of participation in the Federal ABT program to engines that are certified to state standards that are different than the Federal standards. We proposed to migrate without substantive modification the definition of “U.S.-directed production” in 40 CFR 86.004-2 to 40 CFR part 1036.801 for criteria pollutant engine requirements, to match the existing definition for GHG engine requirements, which excludes engines certified to state emission standards that are different than the Federal standards. The relevant existing NO_X ABT credit program requirements, and the relevant program requirements we are finalizing as proposed, specify that compliance through ABT does not allow credit calculations to include engines excluded from the definition of U.S.-directed production volume. For the third category in the commenter's analysis (ZEVs), as discussed in preamble Section IV.G.10 and section 12 of the Response to Comments document, we are not finalizing the proposed allowance for manufacturers to generate NO_X credits from ZEVs. For these reasons, EPA believes the final ABT program will at a minimum maintain the emissions reductions projected from the final rule, and in fact could result in greater public health benefits by resulting in emissions reductions earlier in time than they would occur without banking or trading. Further, if manufacturers generate NO_X emissions credits that they do not subsequently use ( e.g., due to transitioning product lines to ZEVs), then the early emissions reductions from generating these credits will result in more emission reductions than our current estimates reflect. In addition, the final ABT program provides an important flexibility for manufacturers, which we expect will help to ensure a smooth transition to the new standards and avoid delayed emissions reductions due to slower fleet turnover than may occur without the flexibility of the final ABT program.

In the subsections that follow we briefly summarize and provide responses to comments on several more specific topics, including: ABT for pollutants other than NO_X (IV.G.1), Applying the ABT provisions to multiple NO_X duty-cycle standards (IV.G.2), Averaging Sets (IV.G.3), FEL caps (IV.G.4), Credit Life (IV.G.5), Existing credits (IV.G.6), Transitional Credits (IV.G.7), the proposed Early Adoption Incentives (IV.G.8), and a Production Volume Allowance under ABT (IV.G.9). The final ABT program is specified in 40 CFR part 1036, subpart H. Consistent with the proposal, we are also finalizing a new paragraph at 40 CFR 1036.104(c) to specify how the ABT provisions will apply for MY 2027 and later heavy-duty engines subject to the final criteria pollutant standards in 40 CFR 1036.104(a). The Transitional Credit program in the final rule is described in the interim provision in 40 CFR 1036.150(a)(1), which we are finalizing with revisions from the proposal.

1. ABT for Pollutants Other Than NO_X

After consideration of public comments, EPA is choosing to finalize as proposed an ABT program that will not allow averaging, banking, or trading for HC (including NO_X +NMHC) or PM for MY 2027 and later engines. This includes not allowing HC and PM emissions credits from prior model years to be used for MY 2027 and later engines. For engines certified to MY 2027 or later standards, manufacturers must demonstrate in their application for certification that they meet the final PM, HC, and CO emission standards in 40 CFR 1036.104(a) without using emission credits.

Several commenters supported EPA's proposal to discontinue ABT for HC and PM. These commenters stated that current heavy-duty engine technologies can easily meet the proposed HC and PM standards, and therefore an ABT program for these pollutants is not necessary. Some commenters urged EPA to provide ABT programs for HC and CO based on the stringency of the standards for these pollutants, particularly for Spark-ignition HDE. Another commenter did not indicate support or opposition to an HC ABT flexibility in general, but stated that EPA should not base the final HC standard on the use of HC emissions credits since doing so could lead to competitive disruptions between SI engine manufacturers. One commenter also urged EPA to consider ABT programs for regulated pollutant emissions other than NO_X, including HC, PM, CO, and N₂ O.

As discussed in preamble Section III, EPA demonstrated that the final standards for NO_X, HC, CO, and PM area feasible for all engine classes, and we set the numeric values without assuming manufacturers would require the use of credits to comply. We proposed to retain and revise the NO_X ABT program and we are updating from our proposal in this final rule as described in the following sections.

For PM, manufacturers are submitting certification data to the agency for current production engines well below the final PM standard over the FTP duty cycle; the final standard ensures that future engines will maintain the low level of PM emissions of the current engines. Manufacturers are not using PM credits to certify today and we received no new data showing manufacturers would generate or use PM credits starting in MY 2027; therefore, we are finalizing as proposed.

We disagree with commenters indicating that credits will be needed for Spark-ignition HDE to meet the final HC and CO standards. Our SI engine demonstration program data show feasibility of the final standards (see preamble Section III.D for details). Furthermore, as described in Section IV.G.3, we are retaining the current ABT provisions that restrict credit use to within averaging sets and we expect SI engine manufacturers, who have few heavy-duty engine families, will have limited ability to generate and use credits. See preamble Section III.D for a discussion of the final numeric levels of the Spark-ignition HDE standards and adjustments we made to the proposed HC and CO stringencies after further consideration.

We did not propose or request comment on expanding the heavy-duty engine ABT program to include other regulated pollutant emissions, such as N₂ O, and thus are not including additional pollutants in the final ABT program.

2. Multiple Standards and Duty Cycles for NO_X ABT

Under the current and final ABT provisions, FELs serve as the emission standards for the engine family for compliance testing purposes. We are finalizing as proposed new provisions to ensure the NO_X emission performance over the FTP is proportionally reflected in the range of cycles included in the final rule for heavy-duty engines. Specifically, manufacturers will declare a FEL to apply for the FTP standards and then they will calculate a NO_X FEL for the other applicable cycles by applying an adjustment factor based on their declared FEL_FTP . As proposed, the adjustment factor in the final rule is a ratio of the declared NO_X FEL_FTP to the FTP NO_X standard to scale the NO_X FEL of the other duty cycle or off-cycle standards. For example, if a manufacturer declares an FEL_FTP of 25 mg NO_X /hp-hr in MY 2027 for a Medium HDE, where the final NO_X standard is 35 mg/hp-hr, a ratio of 25/35 or 0.71 will be applied to calculate a FEL to replace each NO_X standard that applies for these engines in the proposed 40 CFR 1036.104(a). Specifically, for this example, a Medium HDE manufacturer would replace the full useful life standards for SET, LLC, and the three off-cycle bins with values that are 0.71 of the final standards. For an SI engine manufacturer that declares an FEL_FTP of 15 mg NO_X /hp-hr compared to the final MY 2027 standard of 35 mg/hp-hr, a ratio of 15/35 or 0.43 would be applied to the SET duty cycle standard to calculate an FEL_SET . Note that an FEL_FTP can also be higher than the NO_X standard in an ABT program if it is offset by lower-emitting engines in an engine family that generates equivalent or more credits in the averaging set (see 40 CFR 1036.710). For a FEL higher than the NO_X standard, the adjustment factor will proportionally increase the emission levels allowed when manufacturers demonstrate compliance over the other applicable cycles. Manufacturers are required to set the FEL for credit generation such that the engine family's measured emissions are at or below the respective FEL of all the duty-cycle and off-cycle standards. For instance, if a CI engine manufacturer demonstrates NO_X emissions on the FTP that is 25 percent lower than the standard but can only achieve 10 percent lower NO_X emissions for the low load cycle, the declared FEL could be no less than 10 percent below the FTP standard, to ensure the proportional FEL_LLC would be met.

In the final program, manufacturers will include test results in the certification application to demonstrate their engines meet the declared FEL values for all applicable duty cycles (see 40 CFR 1036.240(a), finalized as proposed). For off-cycle standards, we are also finalizing as proposed the requirement for manufacturers to demonstrate that all the CI engines in the engine family comply with the final off-cycle emission standards (or the corresponding FELs for the off-cycle bins) for all normal operation and use by describing in sufficient detail any relevant testing, engineering analysis, or other information (see 40 CFR 1036.205(p)). These same bin standards (or FELs) apply for the in-use testing provisions finalized in 40 CFR part 1036, subpart E, and for the PEM-based DF verification in the finalized 40 CFR 1036.246(b)(2), if applicable. In addition, as discussed in Section III, we are finalizing a compliance margin for Heavy HDE to account for additional variability that can occur in-use over the useful life of HHDEs; the same 15 mg/hp-hr in-use compliance margin for HHDEs will be added to declared FELs when verifying in-use compliance for each of the duty-cycles ( i.e., compliance with duty-cycle standards once the engine has entered commerce) (see 40 CFR 1036.104(a)). Similarly, the same in-use compliance margin will be applied when verifying in-use compliance over off-cycle standards (see preamble Section III.C for discussion).

Once FEL values are established, credits are calculated based on the FTP duty cycle. We did not propose substantive revisions to the equation that applies for calculating emission credits in 40 CFR 1036.705, but we are finalizing, as proposed, to update the variable names and descriptions to apply for both GHG and criteria pollutant calculations. In Equation IV-1, we reproduce the equation of 40 CFR 1036.705 to emphasize how the FTP duty cycle applies for NO_X credits. Credits are calculated as megagrams ( i.e., metric tons) based on the emission rate over the FTP cycle. The emission credit calculation represents the emission impact that would occur if an engine operated over the FTP cycle for its full useful life. The difference between the FTP standard and the FEL is multiplied by a conversion factor that represents the average work performed over the FTP duty cycle to get the per-engine emission rate over the cycle. This value is then multiplied by the production volume of engines in the engine family and the applicable useful life mileage. Credits are calculated at the end of the model year using actual U.S. production volumes for the engine family. The credit calculations are submitted to EPA as part of a manufacturer's ABT report (see 40 CFR 1036.730).

Where:

Std_FTP = the FTP duty cycle NO_X emission standard, in mg/hp-hr, that applies for engines not participating in the ABT program

FEL = the engine family's FEL for NO_X, in mg/hp-hr.

Work_FTP = the total integrated horsepower-hour over the FTP duty cycle.

Miles_FTP = the miles of the FTP duty cycle. For Spark-ignition HDE, use 6.3 miles. For Light HDE, Medium HDE, and Heavy HDE, use 6.5 miles.

Volume = the number of engine eligible to participate in the ABT program within the given engine family during the model year, as described in 40 CFR 1036.705(c).

UL = the useful life for the standard that applies for a given engine family, in miles.

We did not receive specific comments on the proposed approach to calculate a NO_X FEL for the other applicable cycles by applying an adjustment factor based on the declared FEL_FTP. As such, we are finalizing the approach as proposed.

3. Averaging Sets

After consideration of public comments, we are finalizing, as proposed, to allow averaging, banking, and trading only within specified “averaging sets” for heavy-duty engine emission standards. Specifically, the final rule will use engine averaging sets that correspond to the four primary intended service classes, namely:

• Spark-ignition HDE
• Light HDE
• Medium HDE
• Heavy HDE

Some commenters urged EPA to allow manufacturers to move credits between the current averaging sets ( e.g., credits generated by medium heavy-duty engines could be used by heavy heavy-duty engines), while other commenters recommended that EPA finalize the proposal to maintain restrictions that do not allow movement of credits between the current averaging sets. Those supporting movement of credits between averaging sets stated that doing so would reduce the likelihood that a manufacturer would develop two engines to address regulatory requirements when they could invest in only one engine if they were able to move credits between averaging sets; commenters also stated that restrictions on ABT decrease a manufacturer's ability to respond to changes in emissions standards. Those supporting the current restrictions that do not allow movement of credits between averaging sets stated that maintaining the averaging sets was important to avoid competitive disruptions between manufacturers.

EPA agrees that maintaining the current averaging sets is important to avoid competitive disruptions between manufacturers; this is consistent with our current and historical approach to avoid creating unfair competitive advantages or environmental risks due to credit inconsistency. As described throughout this Section IV.G, we believe that the final ABT program, including this limitation, appropriately balances providing manufacturers with flexibility in their product planning, while maintaining the expected emissions reductions from the program. As we describe further in Section IV.G.7, we provide one exception to this limitation for one of the Transitional Credit pathways for reasons special to that program.

4. FEL Caps

As proposed, the final ABT program includes Family Emissions Limit (FEL) caps; however, after further consideration, including consideration of public comments, we are choosing to finalize lower FEL caps than proposed. The FEL caps in the final rule are 65 mg/hp-hr for MY 2027 through 2030, and 50 mg/hp-hr for MY 2031 and later (see 40 CFR 1036.104(c)(2)). In this section, IV.G.4, we briefly summarize our proposed FEL caps, stakeholder comments on the proposed FEL caps, and then discuss EPA's responses to comments along with our rationale for the FEL caps in the final rule.

We proposed maximum NO_X FEL_FTP values of 150 mg/hp-hr under both proposed Option 1 (for model year 2027 through 2030), and proposed Option 2 (for model year 2027 and later). This value is consistent with the average NO_X emission levels achieved by recently certified CI engines (see Chapter 3.1.2 of the RIA). We believed a cap based on the average NO_X emission levels of recent engines would be more appropriate than a cap at the current standard of 0.2 g/hp-hr (200 mg/hp-hr), particularly when considering the potential for manufacturers to apply NO_X credits generated from electric vehicles for the first time. For MY 2031 and later under Option 1, we proposed a consistent 30 mg/hp-hr allowance for each primary intended service class added to each full useful life standard.

We requested comment on our proposed FEL caps, including our approach to base the cap for MY 2027 through 2030 under Option 1, or MY 2027 and later under Option 2, on the recent average NO_X emission levels. We also requested comment on whether the NO_X FEL_FTP cap in MY 2027 should be set at a different value, ranging from the current Federal NO_X standard of approximately 200 mg/hp-hr to the 50 mg/hp-hr standard in CARB's HD Omnibus rule starting in MY 2024. We further requested comment on the proposal to set MY 2031 NO_X FEL caps at 30 mg/hp-hr above the full useful life standards under proposed Option 1. Finally, we requested comment on whether different FEL caps should be considered if we finalize standards other than those proposed ( i.e., within the range between the standards of proposed Options 1 and 2) (See 87 FR 17550, March 28, 2022, for additional discussion on our proposed FEL caps and historical perspective on FEL caps).

Several commenters provided perspectives on the proposed FEL caps. All commenters urged EPA to finalize a lower FEL cap than proposed; there was broad agreement that the FEL cap in the final rule should be 100 mg/hp-hr or lower.

One commenter stated that a FEL cap at the level of the current standard would not meet the CAA 202(a)(3)(A) requirement to set “standards which reflect the greatest degree of emission reduction achievable through the application of technology which the Administrator determines will be available for the model year to which such standards apply”. Similarly, many commenters stated that EPA should finalize FEL caps that match the CARB Omnibus FEL caps ( i.e., 100 mg-hp-hr in 2024-2026 for all engine classes; 50 mg/hp-hr in 2027 and later for LHDEs and MHDE and 65 mg/hp-hr in 2027-2030 and 70 mg/hp-hr in 2031 and later for HHDEs). These commenters argue that aligning the FEL caps in the EPA final rule with those in the CARB Omnibus would reflect the technologies available in 2027 and later, and better align with the CAA 202(a)(3)(A) requirement for standards that reflect the greatest degree of emission reduction achievable. Commenters provide several lines of support that the CARB Omnibus FEL caps should provide the technical maximum for the EPA FEL caps. Namely, commenters stated that manufacturers will have been producing products to meet CARB Omnibus standard of 50 mg/hp-hr starting in 2024. They further state that two diesel engine families have been certified with CA for MY2022 at a FEL of 160 mg/hp-hr, which is only slightly higher than the FEL EPA proposed under option 1 for MY 2027 and would continue under the proposed FEL cap until MY2030. Finally, a commenter pointed to SwRI data showing that 50 mg/hp-hr can be achieved with what the commenter considers to be “minor changes to engine configuration.”

Commenters further argue that EPA should not base the FEL cap in the final rule on the average performance of recently certified engines since these engines were designed to comply with the current standards, which were set over 20 years ago, and do not utilize the emissions controls technologies that would be available in 2027. Commenters stated that EPA did not consider the extent to which the proposed FEL cap could adversely affect the emissions reductions expected from the rule. Commenters note that although EPA has previously set the FEL cap at the level of the previous standard, the current FEL cap was set lower than the previous standard due to the 90 percent reduction between the previous standard and the current standard. Commenters argue that EPA should similarly set the FEL cap below the current standard given the same magnitude in reduction between the current and proposed standards, and the greater level of certainty in the technologies available to meet the standards in this rule compared to previous rules.

Other commenters stated that a FEL cap of 100 mg/hp-hr, or between 50 and 100 mg/hp-hr, would help to prevent competitive disruptions. Additional details on comments received on the proposed FEL caps are available in section 12.2 of the Response to Comments document.

Our analysis and rationale for finalizing FEL caps of 65 mg/hp-hr in MY 2027 through 2030, and 50 mg/hp-hr in MY 2031and later includes several factors. First, we agree with commenters that the difference between the current (0.2 g/hp-hr) standard and the standards we are finalizing for MY 2027 and later suggests that FEL caps lower than the current standard are appropriate to ensure that available emissions control technologies are adopted. This is consistent with our past practice when issuing rules for heavy-duty onroad engines or nonroad engines in which there was a substantial ( i.e., greater than 50 percent) difference between the numeric levels of the existing and new standards (69 FR 38997, June 29, 2004; 66 FR 5111, January 18, 2001). Specifically, by finalizing FEL caps below the current standards, we are ensuring that the vast majority of new engines introduced into commerce include updated emissions control technologies compared to the emissions control technologies manufacturers use to meet the current standards.

Second, finalizing FEL caps below the current standard is consistent with comments from manufacturers stating that a FEL cap of 100 mg/hp-hr or between 50 and 100 mg/hp-hr would help to prevent competitive disruptions ( i.e., require all manufactures to make improvements in their emissions control technologies).

The specific numeric levels of the final FEL caps were also selected to balance several factors. These factors include providing sufficient assurance that low-emissions technologies will be introduced in a timely manner, which is consistent with our past practice (69 FR 38997, June 29, 2004), and providing manufacturers with flexibility in their product planning or assurance against unforeseen emissions-related problems that may arise. In the early years of the program ( i.e., MY2027 through 2030), we are finalizing a FEL cap of 65 mg/hp-hr to place more emphasis on providing manufacturers flexibility and assurance against unforeseen emissions control issues in order to ensure a smooth transition to the new standards and avoid market disruptions. A smooth transition in the early years of the program will help ensure the public health benefits of the final program by avoiding delayed emissions reductions due to slower fleet turnover than may occur without the flexibility of the final ABT. Thus, the final FEL cap in MY 2027 through 2030 can help to ensure the expected emissions reductions by providing manufacturers with flexibility to meet the final standards through the use of credits up to the FEL cap. In the later years of the program ( i.e., MY 2031 and later), we are finalizing a FEL cap of 50 mg/hp-hr to place more emphasis on ensuring continued improvements in the emissions control technologies installed on new engines.

We disagree with certain commenters stating that a certain numeric level of the FEL cap does or does not align with the CAA requirement to set “standards which reflect the greatest degree of emission reduction achievable through the application of technology which the Administrator determines will be available for the model year to which such standards apply”; rather, given the technology-forcing nature of the final standards, an optional compliance pathway, including the FEL caps and other elements of the ABT program, through the final rule is consistent with requirements under CAA section 202(a)(3)(A). Nevertheless, as described in this Section IV.G.4, we are finalizing lower FEL caps than proposed as part of a carefully balanced final ABT program that provides flexibilities for manufacturers to generate NO_X emissions credits while assuring that available emissions control technologies are adopted and the emissions reductions expected from the final program are realized.

Finally, we disagree with commenters stating a FEL cap can adversely affect the emissions reductions expected from the final rule. Inherent in the ABT program is the requirement for manufacturers producing engines above the emissions standard to also produce engines below the standard or to purchase credits from another manufacturer who has produced lower emitting engines. As such, while the FEL cap constrains the extent to which engines can emit above the level of the standard, it does not reduce the expected emissions reductions because higher emitting engines must be balanced by lower emitting engines. Without credit multipliers, an ABT program, and the associated FEL cap, may impact when emissions reductions occur due to manufacturers choosing to certify some engines to a more stringent standard and then later use credits generated from those engines, but it does not impact the absolute value of the emissions reductions. Rather, to the extent that credits are banked, there would be greater emissions reductions earlier in the program, which leads to greater public health benefits sooner than would otherwise occur; as discussed earlier in this Section IV.G, benefits realized in the near term are worth more to society than those deferred to a later time.

The FEL caps for the final rule have been set at a level to ensure sizeable emission reductions from the existing 2010 standards, while providing manufacturers with flexibility to meet the final standards. When combined with the other restrictions in the final ABT program ( e.g., credit life, averaging sets, expiration of existing credit balances), we believe the final FEL caps of 65 mg/hp-hr in MY 2027 through 2030, and 50 mg/hp-hr in MY 2031 and later avoid potential adverse effects on the emissions reductions expected from the final program.

5. Credit Life for MY 2027 and Later Credits

As proposed, we are finalizing a five-year credit life for NO_X emissions credits generated and used in MY 2027 and later, which is consistent with the existing credit life for CO₂. In this section, IV.G.5, we briefly summarize our proposed credit life, stakeholder comments on the proposed credit life, and then discuss EPA's responses to comments along with our rationale for credit life in the final rule. Section IV.G.7 discusses credit life of credits generated in MYs 2022 through 2026 for use in 2027.

We proposed to update the existing credit life provisions in 40 CFR 1036.740(d) to apply for both CO₂ and NO_X credits. The proposal updated the current unlimited credit life for NO_X credits such that NO_X emission credits generated for use in MY 2027 and later could be used for five model years after the year in which they are generated. For example, under the proposal credits generated in model year 2027 could be used to demonstrate compliance with emission standards through model year 2032. We also requested comment on our proposed five-year credit life.

Several commenters provided perspectives on the proposal to revise the credit life of NO_X emissions credits from unlimited to five years. Commenters took several different positions, including supporting the proposed five-year credit life, arguing that three years, not five, is the more appropriate credit life period, and arguing that credit life should be unlimited. Additional details and a summary of comments received on the proposed credit life are available in section 12 of the Response to Comments document.

The commenter supporting the proposed five-year credit life, rather than an unlimited credit life, states that they conducted an analysis that showed manufacturers had accrued credits from 2007-2009 MYs, which could have been used to certify engines up to the FEL cap in the Omnibus 2024-2026 program and would have delayed emissions reductions in those years. They further state that unlimited credit life would allow manufacturers to produce higher emitting engines against more stringent standards for many years ( e.g., in MY2030).

The commenter arguing that three (not five) years is an appropriate credit life to average out year-to-year variability stated that three years aligns with the CAA requirement for three years of stability between changes in standards, and it represents the pace of improvement that manufacturers include in their product planning. The commenter argues that three years would be more protective under the CAA and is the duration that EPA previously used for NO_X and PM emissions credits. Finally, the commenter states that EPA has not justified its choice of five years.

Commenters who urged EPA to finalize an unlimited credit life for NO_X emissions credits did not provide data or rationale to support their assertion.

After further consideration, including consideration of public comments, EPA is finalizing as proposed a five-year credit life for credits generated and used in MY 2027 and later. The credit life in the final rule is based on consideration of several factors. First, consistent with our proposal, we continue to believe a limited credit life, rather than an unlimited credit life suggested by some commenters, is necessary to prevent large numbers of credits accumulating early in the program from interfering with the incentive to develop and transition to other more advanced emissions control technologies later in the program. Further, as discussed in Section IV.G.7, we believe the transitional credit program in the final rule addresses key aspects of manufacturers' requests for longer credit life. Second, as explained in the proposal, we believe a five-year credit life adequately covers a transition period for manufacturers in the early years of the program, while continuing to encourage technology development in later years.

We disagree with one commenter who stated that a three-year credit life is more appropriate than a five-year credit life. Rather, we believe five years appropriately balances providing flexibility in manufacturers product planning with ensuring available emissions control technologies are adopted. Further, as discussed in Section IV.G.4, inherent in an ABT program is the requirement for manufacturers producing engines above the emissions standard to also produce engines below the standard or to purchase credits from another manufacturer who has produced lower emitting engines. As such, while the five-year credit life in the final rule constrains the time period over which manufacturers can use credits, it does not impact the overall emissions reductions from the final rule. In addition, to the extent that credits are banked for five-years, the emissions reductions from those credits occur five-years earlier, and as discussed earlier in this Section IV.G, benefits realized in the near term are worth more to society than those deferred to a later time. Finally, a five-year credit life is consistent with our approach in the existing light-duty criteria and GHG programs, as well as our heavy-duty GHG program (see 40 CFR 86.1861-17, 86.1865-12, and 1037.740(c)).

As discussed in Section IV.G.7, we are finalizing a shorter credit life for credits generated in 2022 through 2026 with engines certified to a FEL below the current MY 2010 emissions standards, while complying with all other MY 2010 requirements, since these credits are generated from engines that do not meet the MY 2027 and later requirements. We are also finalizing longer credit life values for engines meeting all, or some of the key, MY 2027 and later requirements to further incentivize emissions reductions before the new standards begin (see IV.G.7 for details).

6. Existing Credit Balances

After further consideration, including information received in public comments, the final rule will allow manufacturers to generate credits in MYs 2022 and later for use in MYs 2027 and later, as described further in the following Section IV.G.7. Consistent with the proposal, in the final program, manufacturers will not be allowed to use credits generated prior to model year 2022 when certifying to model year 2027 and later requirements.

We proposed that while emission credits generated prior to MY 2027 could continue to be used to meet the existing emission standards through MY 2026 under 40 CFR part 86, subpart A, those banked credits could not be used to meet the proposed MYs 2027 and later standards (except as specified in 1036.150(a)(3) for transitional and early credits in 1036.150(a)(1) and (2)). Our rationale included that the currently banked NO_X emissions credits are not equivalent to credits that would be generated under the new program ( e.g., credits were generated without demonstrating emissions control under all test conditions of the new program), and that EPA did not rely on the use of existing credit balances to demonstrate feasibility of the proposed standards.

Some commenters urged EPA to allow the use of existing credits, or credits generated after the release of the CTI ANPR, to be used in MYs 2027 and later. Commenters stated that EPA has not demonstrated the standards are feasible without the use of credits, and that the credits were from engines with improved emissions that provide real-world NO_X benefits, even if they are not certified to all of the test conditions of the proposed program. They further stated that not allowing the use of existing credits in 2027 and later could discourage manufacturers from proactively improving emissions performance. In contrast, other commenters support the proposal to discontinue the use of old credits ( e.g., those generated before 2010) since allowing the use of these credits would delay emissions reductions and prevent a timely transition to new standards.

EPA did not rely on the use of existing or prior to MY 2027 credit balances to demonstrate feasibility of the proposed standards (see Section III) and continues to believe that credits from older model years should not be used to meet the final MY 2027 and later standards. Credits from older model years ( i.e., MY 2009 or prior) were generated as manufacturers transitioned to the current standards, and thus would not require manufacturers to introduce new emissions control technologies to generate credits leading up to MY 2027. However, EPA agrees with some commenters that credits generated in model years leading up to MY 2027 are from engines with improved emissions controls and provide some real-world NO_X benefits, even if they are not certified to all of the test conditions of the model year 2027 and later program. Therefore, the transitional credit program we are finalizing allows manufacturers to generate credits starting in model year 2022 for use in MYs 2027 and later; however, credits generated from engines in MYs 2022-2026 that do not meet all of the MY 2027 and later requirements are discounted to account for the differences in emissions controls between those engines and engines meeting all 2027 and later requirements (see Section IV.G.7 and Section 12 of the RTC for details). For credits generated in model years prior to MY 2022, we are finalizing that such emission credits could continue to be used to meet the existing emission standards through MY 2026 under 40 CFR part 86, subpart A.

We selected model year 2022 for two reasons. First, allowing MY 2022 and later credits inherently precludes emissions credits from the oldest model years ( i.e., MY 2009 or prior). These oldest years are when the vast majority of existing credit balances were accumulated, to create flexibility in transitioning to the MY 2007-2010 standards. The oldest model year credits were not generated with current emissions control technologies and are therefore quite distinct from credits generated under the final standards. Second, regarding both the oldest MY credits and those few generated in more recent years, allowing only MY 2022 and later credits incentivizes manufacturers to maximize their development and introduction of the best available emissions control technologies ahead of when they are required to do so in MY2027. As discussed in IV.G.7, this not only provides a stepping-stone to the broader introduction of this technology soon thereafter, but also encourages the early production of cleaner vehicles, which enhances the early benefits of our program. If we were to allow manufacturers to use emissions credits from older model years then there would be no incentive to apply new emissions control technologies in the years leading up to MY 2027. Further, we recognize that some manufacturers have begun to modernize some of their emissions controls in anticipation of needing to comply with the CARB Omnibus standards that begin in 2024, or potential future Federal standards under this final rule, and agree with commenters that it's appropriate to recognize the effort to proactively improve emissions performance. Thus, allowing credits generated in MY 2022 and later both recognizes improvements in emissions controls beyond what is needed to meet the current standards, and ensures that only credits generated in the model years leading up to 2027 can be used to meet the standards finalized in this rule.

7. Transitional Credits Generated in MYs 2022 Through 2026

We are finalizing a transitional credit program that includes several pathways for manufacturers to generate transitional credits in MYs 2022 through 2026 that they can then use in MYs 2027 and later. The transitional credit pathways differ in several ways from what we proposed based on further consideration, including the consideration of public comments. In this section, IV.G.7, we briefly summarize our proposed transitional credit program, stakeholder comments on the proposed transitional credit program, and then discuss EPA's responses to comments along with our rationale for the transitional credit pathways in the final rule.

Under the proposed transitional credit program, manufacturers would generate transitional credits in model years 2024 through 2026. As proposed, manufacturers would have calculated transitional credits based on the current NO_X emissions standards and useful life periods; however, manufacturers would have been required to certify to the other model year 2027 and later requirements, including the LLC and off-cycle test procedures. We proposed the same five-year credit life for transitional credits as other credits in the proposed general ABT program (see 87 FR 17553-17554 March 28, 2022, for additional details of the proposed transitional credits).

We requested comment on our proposed approach to offer transitional NO_X emission credits that incentivize manufacturers to adopt the proposed test procedures earlier than required in MY 2027. We also requested comment on whether CI engines should be required to meet the proposed off-cycle standards to qualify for the transitional credits, and were specifically interested in comments on other approaches to calculating transitional credits before MY 2027 that would account for the differences in our current and proposed compliance programs. In addition, we requested comment on our proposed five-year credit life for transitional NO_X emission credits. Finally, we also requested comment related to our proposed Early Adoption Incentives on whether EPA should adopt an incentive that reflects the MY 2024 Omnibus requirements being a step more stringent than our current standards, but less comprehensive than the proposed MY 2027 requirements.

Several commenters provided perspectives on the proposed transitional credit program under the ABT program. Most commenters either opposed allowing manufacturers to generate NO_X emissions credits, or suggested additional requirements for generating credits that could be used in MYs 2027 and later. One commenter stated that due to lead time and resource constraints, manufacturers would not be able to participate in the proposed transitional credit program. Another commenter supported the proposed transitional credit program. One commenter also stated that incentives for compliant vehicles, not just ZEVs, purchased prior to the MY 2027 will bring tremendous health benefits to at-risk communities and the nation. Similarly, one commenter encouraged EPA to further incentivize emissions reductions prior to the start of the new standards by providing additional flexibilities to use credits in MY 2027 and later if manufacturers were able to certify prior to MY 2027 a large volume of engines ( i.e., an entire engine service class) to almost all MY2027 and later requirements.

Commenters who opposed allowing manufacturers to generate NO_X emissions credits prior to MY2027 were concerned that the difference between Federal and state ( i.e., CARB Omnibus) standards would result in “windfall of credits” that would allow a large fraction of engines to emit at the FEL cap into MY2030 and later. One commenter stated that EPA has not adequately assessed the potential erosion of emissions reductions from credits generated by engines certifying to the CARB Omnibus standards. Another commenter stated that manufacturers are already certifying to levels below the current MY2010 standards, and they believe that certifying to the new test procedures will take little effort for manufacturers. The commenter stated that there is no need to incentivize manufacturers to adopt proposed test procedures ahead of MY2027 because they will already be doing so under the Omnibus program. They argued that rather than requiring new testing, EPA should encourage new technology adoption. Commenters opposing the transitional credit program stated that EPA should eliminate the transitional credit program, or if EPA choses to finalize the transitional credit program, then EPA should adjust the final standards to account for the transitional credit program impacts, or revise the transitional credit program ( e.g., shorten credit life to three years, establish a separate bank for credits generated by engines in states adopting the Omnibus standards). Two commenters stated that EPA should require engines generating credits prior to 2027 to meet all of the requirements of 2027 and beyond; they highlighted the importance of the 2027 and later low-load cycle and off-cycle standards to ensure real-world reductions on the road, and stated that there should be consistency in the way credits are generated and the way they are used. Similarly, these commenters oppose credits for legacy engines or legacy technologies ( i.e., engines or technologies used to meet the current emissions standards).

The commenter who stated that manufacturers would be unable to generate credits under the proposed transitional credit due to lead time and resource constraints argued that manufacturers would be unable to adjust their engine development plans to meet the new LLC and off-cycle test standards in MY 2024. They further stated that in many cases deterioration factor (DF) testing has already started for MY 2024 engines. The commenter also argued that they view the ABT program as part of the emissions standards, and the proposed transitional credit program provided less than the four-year lead time that the CAA requires when setting heavy-duty criteria pollutant emissions standards. In addition, the commenter stated that the proposed transitional credit program would disincentivize manufacturers to make real-world NO_X emissions reductions ahead of when new standards are in place because they would not be able to design and validate their engines to meet the requirements to generate credits.

Finally, a commenter suggested EPA further encourage additional emissions reductions prior to the start of new standards by providing greater flexibility to use credits in MYs 2027 and later. Specifically, this commenter suggested that EPA provide a longer credit life ( e.g., ten years compared to the five years proposed for the ABT program) and also allow the movement of credits between averaging sets. The commenter stated that in order to generate credits with these additional flexibilities manufacturers would need to certify an entire engine service class ( e.g., all heavy heavy-duty engines a manufacturer produced) in a given model year to a FEL of 50 mg/hp-hr or less, and meet all other MY 2027 and later requirements. They further stated that it may not be appropriate for natural gas engines to generate credits with these additional flexibilities since natural gas engines can meet a 50 mg/hp-hr FEL today. Finally, the commenter stated that engines using these credits in MYs 2027 and later should be required to certify to a FEL of 50 mg/hp-hr or less. Additional details on comments regarding the proposed transitional credit program are included in section 12 of the Response to Comments document.

After considering comments on the proposed transitional credit program, we are choosing to finalize a revised version of the proposed transitional credit program. Similar to the proposed rule, we are finalizing an optional transitional credit program to help us meet our emission reduction goals at a faster pace, while also providing flexibilities to manufacturers to meet new, more stringent emission standards. Building on the ABT program as whole, the transitional credit program in the final rule can benefit the environment and public health in two ways. First, early introduction of new emission control technologies can accelerate the entrance of lower-emitting engines and vehicles into the heavy-duty vehicle fleet, thereby reducing NO_X emissions from the heavy-duty sector and lowering its contributions to ozone and PM formation before new standards are in place. Second, the earlier improvements in ambient air quality will result in public health benefits sooner than they would otherwise occur; these benefits are worth more to society than those deferred to a later time, and could be particularly impactful for communities already overburdened with pollution. As discussed in Section II, many state and local agencies have asked the EPA to further reduce NO_X emissions, specifically from heavy-duty engines, because such reductions will be a critical part of many areas' strategies to attain and maintain the ozone and PM_2.5 NAAQS. Several of these areas are working to attain or maintain NAAQS in timeframes leading up to and immediately following the required compliance dates of the final standards, which underscores the importance of the early introduction of lower-emitting vehicles.

The transitional credit program is voluntary and as such no manufacturer is required to participate in the transitional credit program. The transitional credit program in the final rule will provide four pathways for manufacturers to generate credits in MYs 2022 through 2026 for use in MYs 2027 and later: (1) In MY 2026, certify all engines in the manufacturer's heavy heavy-duty service class to a FEL of 50 mg/hp-hr or less and meet all other EPA requirements for MYs 2027 and later to generate undiscounted credits that have additional flexibilities for use in MYs 2027 and later (2026 Service Class Pull Ahead Credits); (2) starting in MY 2024, certify one or more engine family(ies) to a FEL below the current MY2010 emissions standards and meet all other EPA requirements for MYs 2027 and later to generate undiscounted credits based on the longer UL periods included in the 2027 and later program (Full Credits); (3) starting in MY 2024, certify one or more engine family(ies) to a FEL below the current MY2010 emissions standards and meet several of the key requirements for MYs 2027 and later, while meeting the current useful life and warranty requirements to generate undiscounted credits based on the shorter UL period (Partial Credits); (4) starting in MY 2022, certify one or more engine family(ies) to a FEL below the current MY2010 emissions standards, while complying with all other MY2010 requirements, to generate discounted credits (Discounted Credits).

All credits generated in the first pathway have an eight-year credit life and can therefore be used through MY 2034. All credits generated under the second or third pathways will expire by MY2033; all credits generated in the fourth pathway will expire by MY 2030. We further describe each pathway and our rationale for each pathway in this section (see the final interim provisions in 40 CFR 1036.150(a) for additional details). In Section IV.G.8 we discuss our decision to finalize the transitional credit pathways in lieu of the proposed Early Adoption Incentives program (section 12 of the Response to Comments document includes additional details on the comments received on the proposed Early Adoption Incentives program).

In developing the final transitional credit program and each individual pathway, we considered several factors. For instance, for the transitional credit program as a whole, one commenter stated that there should be consistency in the way the credits are generated and the way they are used; several commenters urged EPA to only provide transitional credits to engines meeting all the 2027 and later requirements. The transitional credit program acknowledges these commenters' input by only providing full credit value to engines meeting all the 2027 and later requirements [ i.e., 2026 Service Class Pull Ahead Credits and Full Credits pathways], while providing a lesser value for credits generated from engines that do not meet all of the 2027 and later requirements but still demonstrate improved emissions performance compared to the current standards.

We now turn to discussing in detail each pathway, and the factors we considered in developing each pathway. The first pathway acknowledges the significant emissions reductions that would occur if manufacturers were to certify an entire service class of heavy heavy-duty engines to a much lower numeric standard than the current standards and meet all other MY 2027 requirements prior to the start of the new standards. Specifically, compared to the emissions reductions expected from the final rule, our assessment shows significant, additional reductions in the early years of the program from certifying the entire heavy heavy-duty engine fleet to a FEL of 50 mg/hp-hr or less and meeting all other MY2027 requirements in MY 2026, one model year prior to the start of the new standards. As discussed throughout this Section IV.G, emissions reductions, and the resulting public health benefits, that are realized earlier in time are worth more to society than those deferred to a later time. Based on the potential for additional, early emissions reductions, we are finalizing the 2026 Service Class Pull Ahead Credits pathway with two additional flexibilities for manufacturers to use the credits in MYs 2027 and later. First, 2026 Service Class Pull Ahead Credits have an eight-year credit life ( i.e., expire in MY 2034), which is longer than credits generated in the other transitional credit pathways, or under the main ABT program. Second, we are allowing 2026 Service Class Pull Ahead Credits to move from a heavy heavy-duty to a medium heavy-duty averaging set; however, credits moved between averaging sets will be discounted at 10 percent. We note that a recent assessment by an independent NGO shows that allowing credits to move between service classes could reduce the overall monetized health benefits of a program similar to the one in this final rule; however, the 10 percent discount rate that we are apply would more than offset the potential for reduced emissions reductions. Moreover, as noted in this section, the early emissions reductions from this transitional credit program would provide important positive benefits, particularly in communities overburdened with pollution. Further, we are balancing these additional flexibilities with restrictions on which engines can participate in the 2026 Service Class Pull Ahead Credits pathway. Specifically, only heavy heavy-duty engines may generate 2026 Service Class Pull Ahead Credits; we expect a much lower level of investment would be required for natural gas-fueled engines, light heavy-duty engines, and SI engines to meet the 2026 Service Class Pull Ahead Credits requirements compared to the investment needed for heavy- heavy-duty engines. We expect that the combination of discounting credits moved across averaging sets and only allowing the heavy heavy-duty engine service class to participate in the 2026 Service Class Pull Ahead Credits pathway will appropriately balance the potential for meaningful emissions reductions in the early years of the program with the potential for adverse competitive disadvantages or environmental risks from either unequal investments to generate credits or producing large volumes of credits from engines that could easily meet the requirements of the 2026 Service Class Pull Ahead Credits pathway. Finally, engines certified using 2026 Service Class Pull Ahead Credits in 2027 through 2034 will need to meet a FEL of 50 mg/hp-hr or less; this requirement helps to ensure that these credits are used only to certify engines that are at least as low emitting as the engines that generated the credits.

The second pathway (Full Credits) acknowledges the emissions reductions that could be achieved prior to the start of new standards if manufacturers certify to a FEL lower than today's standard and meet all other MY 2027 and later requirements, although without doing so for an entire engine service class. This pathway is similar to our proposed transitional credit program and is consistent with input from commenters who highlighted the importance of meeting MY 2027 and later requirements such as the low-load cycle and off-cycle standards to ensure real-world reductions on the road. As proposed, all heavy-duty engine service classes, including heavy-duty natural gas engines in the respective service classes, can participate in this pathway.

The third pathway (Partial Credits) incentivizes manufacturers to produce engines that meet several of the key final requirements for MY 2027 and later, including the LLC and off-cycle standards for NO_X , while meeting the existing useful life and warranty periods. This pathway allows manufacturers to adopt new emissions control technologies without demonstrating durability over the longer useful life periods required in MY 2027 and later, or certifying to the longer warranty periods in the final rule. We expect that some manufacturers may already be planning to produce such engines in order to comply with 2024 California Omnibus program; however, this transitional pathway would incentivize manufacturers to produce greater volumes of these engines than they would otherwise do to comply in states adopting the Omnibus standards. Some commenters were concerned that the proposed transitional credit program would result in “windfall credits” due to manufacturers generating credits from engines produced to comply with more stringent state standards. As discussed in IV.G, the final program will not allow manufacturers to generate credits from engines certified to meet state standards that are different from the Federal standards. The Partial Credits pathway thus avoids “windfall credits” because manufacturers are not allowed to generate credits from engines produced to meet the more stringent 2024 Omnibus requirements, but rather are incentivized to produce cleaner engines that would benefit areas of the country where such engines may not otherwise be made available ( i.e., outside of states adopting the Omnibus program). Further, because engines participating in this pathway will be certified to shorter useful life periods, they will generate fewer credits than engines participating in the third and fourth pathways (Full Credits and 2026 Service Class Pull Ahead Credits).

The first, second, and third pathways all include meeting the LLC requirements for MY 2027 and later. One commenter suggested meeting the LLC would require manufacturers to simply meet a lower numeric standard than the current standard; however, EPA disagrees. Certifying to the LLC will require more than simply meeting a lower numeric standard since the LLC is a new test cycle that requires demonstration of emissions control in additional engine operations ( i.e., low load) compared to today's test cycles (see preamble Section III and section 3 of the Response to Comments document and for more discussion on the LLC).

Finally, the fourth pathway (Discounted Credits) allows manufacturers to generate credits for use in MY 2027 and later with engines that are not designed to meet the LLC and off-cycle standards and so could provide additional compliance flexibility for meeting the final standards; however, since the engines are not meeting the full requirements of the MY 2027 and later program the credits are discounted and will expire before credits generated in the other transitional credit pathways. This Discounted Credits pathway includes consideration of input from one commenter who stated that it would be infeasible for manufacturers to comply with the new LLC and off-cycle test procedures in MY 2024 in order to generate credits under the proposed credit program; they further argued that for manufacturers relying on credits to comply with the final standards, the proposed transitional credit program would not provide the lead time required by the CAA. As described in Section III of this preamble, the new standards in the final rule are feasible without the ABT program and without the use of transitional credits; participation in ABT is voluntary and is intended to provide additional flexibility to manufacturers through an optional compliance pathway. While manufacturers have the option of generating NO_X emissions credits under the transitional credit program in the final rule, they are not required to do so. The four-year lead time requirement under CAA 202(a)(3) does not apply to these ABT provisions.

Nevertheless, the final rule allows credits generated under this Discounted Credits pathway to incentivize improvements in emissions controls, even if the engines are not certified to the full MY2027 and later requirements. Credits will be discounted by 40 percent to account for differences in NO_X emissions during low-load and off-cycle operations between current engines and engines certifying to the model year 2027 and later requirements. While we expect that manufacturers certifying to a FEL below the current 200 mg/hp-hr standard will reflect improvement in emissions control over the FTP and SET duty-cycles, the discount applied to the credits accounts for the fact that these engines are not required to maintain the same level of emissions control over all operations of the off-cycle standards, or during the low-load operations of the LLC. For example, a manufacturer certifying a HHDE engine family to a FEL of 150 mg/hp-hr and all other MY 2010 requirements with a U.S.-directed production volume of 50,000 engines in 2024 would generate approximately 5,000 credits (see Equation IV-1), which they would then multiply by 0.6 to result in a final credit value of 3,000 credits. See the final, revised from proposal, interim provision in 40 CFR 1036.150(a)(1) for additional details on the calculation of discounted credits.

Credits generated under this Discounted Credits pathway could be used in MY 2027 through MY 2029. The combination of the discount and limited number of model years in which manufacturers are allowed to use these credits is consistent with our past practice and helps to addresses some commenters' concerns about allowing legacy engines to generate credits, or credits generated under the transitional credit program eroding emissions reductions expected from the rule (55 FR 30584-30585, July 26,1990). There are two primary ways that the Discounted Credits pathway results in positive public health impacts. First, an immediate added benefit to the environment is the discounting of credits, which ensures that there will be a reduction of the overall emission level. The 40 percent discount provides a significant public health benefit, while not being so substantial that it would discourage the voluntary initiatives and innovation the transitional ABT program is designed to elicit. Second, consistent with the benefits of the overall transitional credit program, when the “time value” of benefits ( i.e., their present value) is taken into account, benefits realized in the near term are worth more to society than those deferred to a later time. The earlier expiration date of credits in the Discounted Pathway reflects that these credits are intended to help manufacturers transition in the early years of the program, but we don't think they are appropriate for use in later years of the program. The earlier expiration of credits is also consistent with comments that we should finalize a 3-year credit life for transitional credits ( i.e., credits can be used for 3-years once the new standards begin).

As discussed earlier in this Section IV.G.7, credits generated under the first pathway (2026 Service Class Pull Ahead Credits) can be used for eight years, through MY 2034; we selected this expiration date to balance incentivizing manufacturers to participate in the 2026 Credits pathway and thereby realize the potential for additional, early emissions reductions, with continuing to encourage the introduction of improved emissions controls, particularly as the heavy-duty fleet continues to transition into zero emissions technologies. As stated in the preceding paragraphs, all credits generated in the second and third pathways can be used through MY 2032. Our rationale for this expiration date is two-fold. First, providing a six-year credit life from when the new standards begin provides a longer credit life than provided in the final ABT program for credits generated in MY 2027 and later; similar to the first pathway, this longer credit life incentivizes manufacturers to produce engines that emit lower levels of NO_X earlier than required. Second, the six-year credit life balances additional flexibility for manufacturers to transition over all of their product lines with the environmental and human health benefits of early emissions reductions. This transitional period acknowledges that resource constraints may make it challenging to convert over all product lines immediately when new standards begin, but maintains emission reductions projected from program by requiring the use of credits to certify engines that emit above the level of the new standard. While some commenters stated that manufacturers will have been complying with the CARB Omnibus program starting in 2024, we acknowledge that complying with the 2027 and later Federal standards will require another step in technology and thus think it is appropriate to provide additional flexibility for manufacturers to transition to the new standards through the use of emissions credits in the ABT program.

This section describes how to generate credits for MY 2026 and earlier engines that are certified to standards under 40 CFR part 86, subpart A. As noted in Section III.A.3, we are allowing manufacturers to continue to certify engines to the existing standards for the first part of model year 2027. While those engines continue to be subject to standards under 40 CFR part 86, subpart A, we are not allowing those engines to generate credits that carry forward for certifying engines under 40 CFR part 1036. Manufacturers may only generate NO_X emissions credits under transitional credit pathways for MY 2024-2026 engines since one purpose of transitional credits is to incentivize emission reductions in the model years leading up to MY 2027. To the extent manufacturers choose to split MY 2027, the engines produced in the first part of the split MY are produced very close in time to when the new standards will apply, and thus we expect that rather than incentivizing earlier emission reductions, providing an allowance to generate NO_X emission credits would incentivize production at higher volumes during the first part of the split MY than would otherwise occur ( i.e., incentivizing more of the MY 2027 production before the final standards apply). The higher production volume of engines in the first part of the split MY could thereby result in additional NO_X emission credits without additional emission reductions that would otherwise occur. See preamble Section III.A.3 for details on the split model year provision in this final rule.

8. Early Adoption Incentives

EPA is choosing not to finalize the Early Adoption Incentives program as proposed. This includes a decision not to include emissions credit multipliers in the final ABT program. Rather, we are finalizing a revised version of the transitional credit program under the ABT program as described above in Section IV.G.7. In this Section IV.G.8 we briefly describe the proposed Early Adoption Incentives program, stakeholder comments on the proposed Early Adoption Incentives program, and then discuss EPA's responses to comments along with our rationale for choosing not to finalize the Early Adoption Incentives program.

We proposed an early adoption incentive program that would allow manufacturers who demonstrated early compliance with all of the final MY 2027 standards (or MY 2031 standards under proposed Option 1) to include Early Adoption Multiplier values of 1.5 or 2.0 when calculating NO_X emissions credits. In the proposed Early Adoption Incentives program, manufacturers could generate credits in MYs 2024 through 2026 and use those credits in MYs 2027 and later.

We requested comment on all aspects of our proposed early adoption incentive program. We were aware that some aspects of the proposed requirements could be challenging to meet ahead of the required compliance dates, and thus requested comment on any needed flexibilities that we should include in the early adoption incentive program in the final rule. See 87 FR 17555, March 28, 2022, for additional discussion on the proposed Early Adoption Incentives program, including specifics of our requests for comment.

Several commenters provided general comments on the proposed Early Adoption Incentive program. Although many of the commenters generally supported incentives such as emissions credit multipliers to encourage early investments in emissions reductions technology, several were concerned that the emissions credit multipliers would result in an excess of credits that would undermine some of the benefits of the rule; other commenters were concerned that the multipliers would incentivize some technologies ( e.g., hybrid powertrains, natural gas engines) over others ( e.g., battery-electric vehicles).

As described in preamble Section IV.G.7, the revised transitional credit program that we are finalizing provides discounted credits for engines that do not comply with all of the MY 2027 and later requirements. In addition, after consideration of comments responding to our request for comment about incentivizing early reductions through our proposed transitional and Early Adoption Incentive program, the final transitional credit program includes an additional pathway that incentivizes manufacturers to produce engines that meet several of the key final requirements for MY 2027 and later, including the LLC and off-cycle standards for NO_X, while meeting the current useful life and warranty periods. We expect that this transitional credit pathway will incentivize manufacturers to produce greater volumes of the same or similar engines that they plan to produce to comply with the MY 2024 Omnibus requirements. By choosing not to finalize the Early Adoption Incentives program and instead finalizing a modified version of the Transitional Credit program, we are avoiding the potential concern some commenters raised that the credit multipliers would result in a higher volume or magnitude of higher-emitting MY 2027 and later engines compared to a program without emission credit multipliers. We believe the Transitional Credit program we are finalizing will better balance incentivizing emissions reduction technologies prior to MY 2027 against avoiding an excess of emissions credits that leads to much greater volumes or magnitudes of higher-emitting engines in MYs 2027 and later. Moreover, by not finalizing the Early Adoption Incentive program we are avoiding any concerns that the emissions credit multipliers would incentivize some technologies over others (see section 12.5 of the Response to Comments and preamble Section IV.G.10 for additional discussion on battery-electric and fuel cell electric vehicles in the final rule; see section 3 of the Response to Comments for discussion on additional technology pathways).

9. Production Volume Allowance

After further consideration, including consideration of public comments, EPA is finalizing an interim production volume allowance for MYs 2027 through 2029 in 40 CFR 1036.150(k) that is consistent with our request for comment in the proposal, but different in several key aspects. In particular, the production volume allowance we are finalizing allows manufacturers to use NO_X emissions credits to certify a limited volume of heavy heavy-duty engines compliant with pre-MY 2027 requirements in MYs 2027 through 2029. In addition, since we are requiring the use of credits to certify MY 2010 compliant heavy heavy-duty engines in the early years of the final program, and to aid in implementation, we are choosing to not limit the applications that are eligible for this production volume allowance. Finally, the production volume allowance in the final rule will be five percent of the average U.S.-directed production volumes of Heavy HDE over three model years, see 40 CFR 1036.801, and thus excludes engines certified to different emission standards in CA or other states adopting the Omnibus program. In this section, IV.G.9, we summarize our request for comment on a production volume allowance, related stakeholder comments, and EPA's responses to comments along with our rationale for the production volume allowance in the final rule.

In the proposal we stated that we were considering a flexibility to allow engine manufacturers, for model years 2027 through 2029 only, to certify up to five percent of their total production volume of heavy-duty highway CI engines in a given model year to the current, pre-MY 2027 engine provisions of 40 CFR part 86, subpart A. We stated the allowance would be limited to Medium HDE or Heavy HDE engine families that manufacturers show would be used in low volume, specialty vocational vehicles. We noted that such an allowance from the MY 2027 criteria pollutant standards may be necessary to provide engine and vehicle manufacturers additional lead time and flexibility to redesign some low sales volume products to accommodate the technologies needed to meet the proposed more stringent engine emission standards.

We requested comment on the potential option of a three-year allowance from the proposed MY 2027 criteria pollutant standards for engines installed in specialty vocational vehicles, including whether and why the flexibility would be warranted and whether 5 percent of a manufacturers engine production volume is an appropriate value for such an interim provision. In addition, we requested comment on whether the flexibility should be limited to specific vocational vehicle regulatory subcategories and the engines used in them.

Several commenters provided perspectives on our request for comment on providing an additional flexibility that would allow manufacturers to certify up to five percent of their total production volume of 2027 through 2029 MY medium and heavy HDEs to the current Federal engine provisions. Many environmental and state organizations opposed the potential production volume allowance, while most manufacturers and one supplier generally supported the potential allowance although they suggested changes to the parameters included in the proposal.

Commenters opposing the production volume allowance had two primary concerns. First, they stated that the production volume flexibility is not needed because there is enough lead time between now and MY 2027 to develop the technologies and overcome any packaging challenges. One commenter further noted that the CARB Omnibus standards would already be in effect in 15 percent of the market. Second, commenters argued that the production volume allowance would result in high NO_X emissions and adverse health effects, particularly in high-risk areas, which would undermine the effectiveness of the rule to reduce emissions and protect public health. One commenter noted that HHDEs last for many years before being scrapped and that the production volume allowance, combined with other flexibilities in the proposal, could result in significant emissions impacts for many years to follow, which would create extreme difficulty for California and other impacted states to achieve air quality goals. Another commenter estimated that in MY 2027 through 2029, the production volume allowance would result in 20,000 vehicles emitting nearly 6 times more NO_X on the FTP cycle than proposed Option 1, and that these vehicles could represent 20-25 percent of the total NO_X emissions from MY 2027 through 2029 vehicles. Still another commenter stated that the production volume allowance would result in up to a 45 percent increase in NO_X emissions inventory for each applicable model year's production from a manufacturer with products in a single useful life and power rating category; the commenter noted that the emissions inventory impact could be even greater if a manufacturer used the five percent allowance for engines with longer useful life periods and higher power ratings. One commenter opposing the production volume allowance stated that EPA should not exempt any engines from complying with the adopted new emission standards for any amount of time. Other commenters opposing the production volume allowance stated that if EPA chose to finalize a production allowance then emissions from those engines should be offset with ABT emission credits to protect vulnerable impacted communities. Finally, one commenter opposing the production volume allowance state that if EPA chose to finalize the production allowance then the Agency should provide strong technical justification for each engine category subject to the provision.

Commenters generally supporting the production volume allowance suggested several ways to further limit the flexibility, or suggested additional flexibilities based on the CARB Omnibus program. For instance, several engine manufacturers and their trade association suggested limiting the provision to include only engines with low annual miles traveled to minimize the emissions inventory impacts. These commenters suggested limiting the allowance to engines with greater than or equal to 525 hp or 510 hp in specific vehicle applications, namely: Heavy-haul tractors and custom chassis motor homes, concrete mixers, and emergency vehicles. Two engine manufacturers further suggested the production volume allowance include vehicles where aftertreatment is mounted off the frame rails, or that EPA review and approve applications demonstrating severe packaging constraints for low volume, highly specialized vocational applications. Another engine manufacturer argued that manufacturers need to be able to carry over some existing engines into MY 2027 and later for a few years in order to adequately manage investments and prioritize ultra-low NO_X and ZEV technology adoption in the applications that make the most sense. They further stated that EPA should consider alternate credit program options that can be used to truly manage investment and to prioritize appropriate applications by allowing manufacturers to leverage credits to stage development programs. One engine manufacturer and one supplier suggested EPA consider programs similar to the CARB Omnibus' separate certification paths for `legacy engines,' emergency vehicles, and low-volume high horsepower engines. Additional details on comments received on the request for comment on a potential production volume allowance are available in section 12.7 of the Response to Comments.

After considering comments on the proposed production volume allowance, we are finalizing an allowance in MY 2027 through 2029 for manufacturers to certify up to five percent of their Heavy HDE U.S.-directed production volume averaged over three model years (MY 2023 through 2025) as compliant with the standards and other requirements of MY 2026 ( i.e., the current, pre-MY 2027 engine provisions of 40 CFR part 86, subpart A). As explained earlier in this Section IV.G, U.S.-directed production volume excludes engines certified to different state emission standards ( e.g., would exclude engines certified to CARB Omnibus standards if EPA grants the pending waiver request), and thus would be a smaller total volume than all Heavy HDE engine production in a given model year. By finalizing a production volume allowance based on the average U.S.-directed production volume over three model years (MY 2023 through 2025), rather than an allowance that varies by production volume in each of the model years included in the allowance period (MY 2027 through 2029), we are providing greater certainty to manufacturers and other stakeholders regarding the number of engines that could be produced under this allowance. Further, we avoid the potential for economic conditions in any one year to unduly influence the volume of engines that could be certified under this allowance. Based on EPA certification data, we estimate that five percent MY 2021 Heavy HDE would result in approximately 12,000 engines per year permitted under this allowance.

We are limiting the final production volume allowance to Heavy HDE, rather than Heavy HDE and Medium HDE as proposed, because comments from manufacturers generally pointed to Heavy HDE applications or otherwise suggested limiting the allowance to larger engines ( e.g., greater than 510 hp). After considering comments on the vehicle categories to include in the production volume allowance, we are choosing not to specify the vehicle categories for engines certified under this production volume. Our rationale includes three main factors. First, we are requiring manufacturers to use credits to certify engines under the production volume allowance, which will inherently result in the production of lower-emitting engines to generate the necessary credits. We believe requiring emission credits to certify engines under the production volume allowance better protects the expected emission reductions from the final rule than limiting the production allowance to specific vehicle categories. Our approach is consistent with some commenters' recommendation to finalize a program that required the use of emission credits to protect vulnerable impacted communities by ensuring that lower-emitting engines are produced earlier to generate the credits necessary to produce engines certified under this allowance. Second, a variety of vehicle categories were identified in comments as vehicle categories for which manufacturers may need additional lead time and flexibility to redesign to accommodate the technologies needed to meet the final emission standards. We expect that the specific vehicle category(ies) for which additional lead time and flexibility is of interest will vary by manufacturer, and thus are choosing not to specify vehicle categories to avoid competitive disruptions. Finally, we are choosing not to limit the production volume allowance to specific vehicle categories to simplify and streamline implementation; the specific vehicle in which an engine will be installed is not always known when an engine is produced, which would make implementing restrictions on engines installed in specific vehicle categories challenging for both EPA and manufacturers.

We continue to believe it is important to ensure that technology turns over in a timely manner and that manufacturers do not continue producing large numbers of higher-emitting pre-MY 2027 compliant engines once the MY 2027 standards are in place. The combination of a limited production volume ( i.e., five percent of the average U.S.-directed production volume over three model years, (MY 2023 through 2025, in MYs 2027 through 2029) and a requirement to use credits will prevent the production of large numbers of these higher emitting engines, while providing additional flexibility for manufacturers to redesign engine product lines to accommodate the technologies needed to meet the final emission standards.

For engines certified under the production volume allowance, manufacturers would need to meet the standards and related requirements that apply for model year 2026 engines under 40 CFR part 86, subpart A. Engine families must be certified as separate engine families that qualify for carryover certification, which means that the engine family would still be properly represented by test data submitted in an earlier model year.

Manufacturers would need to declare a NO_X family emission limit (FEL) that is at or below the standard specified in 40 CFR 86.007-11 and calculate negative credits by comparing the declared NO_X FEL to the FTP emission standard for model year 2027 engines. In addition, manufacturers would calculate negative credits using a value for useful life of 650,000 miles to align with the credit calculation for engines that will be generating credits under 40 CFR part 1036 starting in model year 2027 (see Equation IV-I for credit calculation). The inclusion of useful life and work produced over the FTP in the calculation of credits addresses some commenters' concern regarding the production of engines with higher power ratings and longer useful life periods under the production volume allowance. Manufacturers would need to demonstrate compliance with credit accounting based on the same ABT reporting requirements that apply for certified engines under 40 CFR part 1036.

See 40 CFR 1036.150(k) for additional details on the limited production volume allowance in the final rule.

10. Zero Emission Vehicle NO_X Emission Credits

After further consideration, including consideration of public comments, EPA is not finalizing the proposed allowance for manufacturers to generate NO_X emissions credits from heavy-duty zero emissions vehicles (ZEVs). Rather, the current 40 CFR 86.016-1(d)(4), which specifies that heavy-duty ZEVs may not generate NO_X or PM emission credits, will continue to apply through MY 2026, after which 40 CFR 1037.1 will apply. The final 40 CFR 1037.1 migrates without revisions the text of 40 CFR 86.016-1(d)(4), rather than the revisions we proposed to allow manufacturers to generate NO_X emissions credits from ZEVs. In this Section IV.G.10, we briefly describe the proposal to allow manufacturers to generate NO_X emissions credits from ZEVs; the comments received on the proposal to allow ZEV NO_X credits; and EPA's response to those comments, which includes our rationale for the approach to ZEV NO_X credits in the final rule.

We proposed that if manufacturers met certain requirements, then they could generate NO_X emissions credits from battery-electric vehicles, BEVs, and fuel cell electric vehicles, FCEVs; we refer to BEVs and FCEVs collectively as zero emissions vehicles, ZEVs. Under the proposal, manufacturers would calculate the value of NO_X emission credits generated from ZEVs using the same equation provided for engine emission credits (see Equation IV-1 in final preamble Section IV.G.2). To generate the inputs to the equation, we proposed that manufacturers would submit test data at the time of certification, which is consistent with requirements for CI and SI engine manufacturers to generate NO_X emissions credits. We proposed that vehicle manufacturers, rather than powertrain manufacturers, would generate vehicle credits for ZEVs since vehicle manufacturers already certify ZEVs to GHG standards under 40 CFR part 1037. To ensure that ZEV NO_X credits were calculated accurately, and reflected the environmental and public health benefits of vehicles with zero tailpipe emissions over their full useful life, we proposed that in MY 2024 and beyond, ZEVs used to generate NO_X emission credits would need to meet certain battery and fuel cell performance requirements over the useful life period ( i.e., durability requirements).

We requested comment on the general proposed approach of allowing ZEVs to generate NO_X credits, which could then be used in the heavy-duty engine ABT program. We also requested comment on several specific aspects of our proposal. See 87 FR 17558, March 28, 2022, for additional discussion on the proposal to allow manufacturers to generate NO_X emissions credits from ZEVs if those vehicles met the specified requirements.

Numerous commenters provided feedback on EPA's proposal to allow manufacturers to generate NO_X emissions credits from ZEVs. The majority of commenters oppose allowing manufacturers to generate NO_X emissions credits from ZEVs. Several additional commenters oppose ZEV NO_X emissions credits unless there were restrictions on the credits ( e.g., shorter credit life, sunsetting credit generation in 2026). Other commenters support allowing manufacturers to generate NO_X emissions credits from electric vehicles. Arguments from each of these commenter groups are summarized immediately below.

Commenters opposing NO_X emissions credits for ZEVs present several lines of argument, including the potential for: (1) Substantial impacts on the emissions reductions expected from the proposed rule, which could also result in disproportionate impacts in disadvantaged communities already overburdened with pollution; (2) a lack of improvements in conventional engine technologies; and (3) ZEV NO_X credits to result higher emissions from internal combustion engines, rather than further incentivizing additional ZEVs (further noting that other State and Federal actions are providing more meaningful and less environmentally costly HD ZEV incentives). Stakeholders opposing NO_X emissions credits from ZEVs were generally environmental or state organizations, or suppliers of heavy-duty engine and vehicle components.

In contrast, several commenters support allowing manufacturers to generate these credits. Many of these commenters are heavy-duty engine and vehicle manufacturers. Commenters supporting an allowance to generate NO_X emissions credits from ZEVs also provided several lines of argument, including the potential for: (1) ZEVs to help meet emissions reductions and air quality goals; (2) ZEV NO_X credits to be essential to the achievability of the standards for some manufacturers; and (3) ZEV NO_X credits to allow manufacturers to manage investments across different products and ultimately result in increased ZEV deployment. Each of these topic areas is discussed further in section 12.5 of the Response to Comments document.

Three considerations resulted in our decision not to finalize at this time the allowance for manufacturers to generate NO_X emissions credits from heavy-duty ZEVs. First, the standards in the final rule are technology-forcing, yet achievable for MY2027 and later internal combustion engines without this flexibility. Second, since the final standards are not based on projected utilization of ZEV technology, and given that we believe there will be increased penetration of ZEVs in the HD fleet by MY2027 and later, we are concerned that allowing NO_X emissions credits would result in fewer emissions reductions than intended from this rule. For example, by allowing manufacturers to generate ZEV NO_X credits, EPA would be allowing higher emissions (through engines using credits to emit up to the FEL cap) in MY 2027 and later, without requiring commensurate emissions reductions (through additional ZEVs beyond those already entering the market without this rule), which could be particularly impactful in communities already overburdened by pollution. Third, we continue to believe that testing requirements to ensure continued battery and fuel cell performance over the useful life of a ZEV may be important to ensure the zero-emissions tailpipe performance for which they are generating NO_X credits; however, after further consideration, including consideration of public comments, we believe it is appropriate to take additional time to work with industry and other stakeholders on any test procedures and other specifications for ZEV battery and fuel cell performance over the useful life period of the ZEV (see section 12.6 of the Response to Comments document for additional detail on comments and EPA responses to comments on the proposed ZEV testing and useful life and warranty requirements).

In section 12.6 of the Response to Comments document, we further discuss each of these considerations in our decision not to finalize the allowance for manufacturers to generate NO_X emissions credits from ZEVs. Additional detail on comments received and EPA responses to comments, including comments on more specific aspects of comments on the proposed allowance for ZEV NO_X emissions credits, such as testing, useful life, and warranty requirements for ZEVs, are also available in section 12.6 of the Response to Comments document. Our responses to comments on the proposed vehicle certification for ZEVs are summarized in preamble Section III, with additional detail in section 12.6.3 of the Response to Comments document.

V. Program Costs

In Chapter 3 of the RIA, we differentiate between direct, indirect, and operating costs when estimating the costs of the rule. “Direct” costs represent the direct manufacturing costs of the technologies we expect to be used to comply with the final standards over the final useful lives; these costs accrue to the manufacturer. In this section we use those costs to estimate the year-over-year manufacturing costs going forward from the first year of implementation. “Indirect” costs, i.e., research and development (R&D), administrative costs, marketing, and other costs of running a company, are associated with the application of the expected technologies and also accrue to the manufacturer. Like direct costs, indirect costs are expected to increase under the final standards, in part due to the useful life provisions. Indirect costs are also expected to increase under the final program due to the warranty provisions. We term the sum of these direct and indirect costs “technology costs” or “technology package costs.” They represent the costs incurred by manufacturers— i.e., regulated entities—to comply with the final program. “Operating” costs represent the costs of using the technology in the field. Operating costs include, for example, changes in diesel exhaust fluid (DEF) consumption or fuel consumption. These costs accrue to the owner/operator of MY 2027 and later heavy-duty vehicles. We present total costs associated with the final program in Section V.C. All costs are presented in 2017 dollars consistent with the proposed cost analysis, unless noted otherwise.

We requested comment on all aspects of the cost analysis. In particular, we requested comment on our estimation of warranty and research and development costs via use of scalars applied to indirect cost contributors (see Section V.A.2) and our estimates of emission repair cost impacts (see Section V.B.3). We also requested that comments include supporting data and/or alternative approaches that we could have considered when developing estimates for the final rulemaking.

In response to our requests, we received many helpful comments, although lack of data in conjunction with some comments made it challenging to evaluate the changes suggested by the commenter. After careful consideration of the comments we received, we have made several changes to the final cost analysis relative to the proposal. Those changes are summarized in Table V-1. Note that, throughout this discussion of costs, we use the term regulatory class which defines vehicles with similar emission standards (see Chapter 5.2.2 of the RIA); we use the term regulatory class for consistency with our MOVES model and its classification system so that our costs align with our inventory estimates and the associated benefits discussed in Sections VI, VII and VIII.

A. Technology Package Costs

Commenters' primary comment with respect to our proposed technology package costs dealt with the need to replace the emission control system due to the combination of the low NO_X standards with the long warranty and useful life provisions under proposed Option 1. Another comment with respect to our proposed technology package costs dealt with the estimated warranty costs, including both the methodology used and the magnitude of the cost estimated by EPA. As explained in Sections III and IV, the final program neither imposes numeric NO_X standards as stringent as, nor does the final rule for heavy HDE contain warranty and useful life provisions as long as, proposed Option 1. We address these comments in more detail in section 18 of the RTC. EPA considers the concerns raised in first of these comments to be obviated by the final emission standards and regulatory useful life values, in light of which we foresee no need for a routine replacement of the entire emission control system to maintain in-use compliance as suggested by some commenters. Regarding the second, as discussed in more detail in Section V.A.2 and section 18 of the RTC, EPA has updated the warranty cost methodology, including based on information submitted by commenters, and this has resulted in different costs associated with warranty.

Individual technology piece costs are presented in Chapter 3 of the RIA. The direct manufacturing costs (DMC) presented in RIA Chapter 3 use a different dollar basis than the cost analysis, and as such, the DMC values presented here have been adjusted to 2017 dollars. Following the first year of implementation, the costs also account for a learning effect to represent the cost reductions expected to occur via the “learning by doing” phenomenon. This provides a year-over-year cost for each technology package—where a technology package consists of the entire emission-control system—as it is applied to new engine sales. We then apply industry standard “retail price equivalent” (RPE) markup factors, with adjustments discussed in the rest of this section, to estimate indirect costs associated with each technology package. Both the learning effects applied to direct costs and the application of markup factors to estimate indirect costs are consistent with the cost estimation approaches used in EPA's past transportation-related regulatory programs. The sum of the direct and indirect costs represents our estimate of technology costs per vehicle on a year-over-year basis. These technology costs multiplied by estimated sales then represent the total technology costs associated with the final program.

This cost calculation approach presumes that the expected technologies will be purchased by original equipment manufacturers (OEMs) from their suppliers. So, while the DMC estimates include the indirect costs and profits incurred by the supplier, the indirect cost markups we apply cover the indirect costs incurred by OEMs to incorporate the new technologies into their vehicles and to cover profit margins typical of the heavy-duty truck industry. We discuss the indirect costs in more detail in Section V.A.2.

1. Direct Manufacturing Costs

To produce a unit of output, manufacturers incur direct and indirect costs. Direct costs include cost of materials and labor costs to manufacture that unit. Indirect costs are discussed in the following section. The direct manufacturing costs presented here include individual technology costs for emission-related engine components and exhaust aftertreatment systems (EAS).

Notably, for this analysis we include not only the marginal increased costs associated with the standards, but also the emission control system costs for the baseline, or no action, case. Throughout this discussion, we refer to baseline case costs, or baseline costs, which reflect our cost estimate of emission-related engine systems and the exhaust aftertreatment system absent impacts of this final rule. This inclusion of baseline system costs contrasts with EPA's approach in recent greenhouse gas rules or the light-duty Tier 3 criteria pollutant rule where we estimated costs relative to a baseline case, which obviated the need to estimate baseline costs. We have included baseline costs in this analysis because the new emissions warranty and regulatory useful life provisions are expected to have some impact on not only the new technology added to comply with the final standards, but also on emission control technologies already developed and in use. The new warranty and useful life provisions will increase costs not only for the new technology added in response to the new standards, but also for the technology already in place (to which the new technology is added) because the new warranty and useful life provisions will apply to the entire emission-control system, not just the new technology added in response to the new standards. The baseline direct manufacturing costs detailed in this section are intended to reflect that portion of baseline case engine hardware and aftertreatment systems for which new indirect costs will be incurred due to the new warranty and useful life provisions, even apart from changes in the level of emission standards.

As done in the NPRM, we have estimated the baseline engine costs based on studies done by the International Council on Clean Transportation (ICCT), as discussed in more detail in Chapter 7 of the RIA. As discussed there, the baseline engine costs consist of turbocharging, fuel system, exhaust gas recirculation, etc. These costs represent those for technologies that will be subject to new, longer warranty and useful life provisions under this final rule. For costs associated with the action case, we have used FEV-conducted teardown-based costs as presented in Chapter 3 of the RIA for newly added cylinder deactivation systems, and for the exhaust aftertreatment system (EAS) costs. The direct manufacturing costs for the baseline engine+aftertreatment and for the final program are shown for diesel engines in Table V-2, gasoline engines in Table V-3 and CNG engines in Table V-4. Costs are shown for regulatory classes included in the cost analysis and follow the categorization approach used in our MOVES model. Please refer to Chapter 6 of the RIA for a description of the regulatory classes and why the tables that follow include or do not include each regulatory class. In short, where MOVES has regulatory class populations and associated emission inventories, our cost analysis estimates costs. Note also that, throughout this section, we use several acronyms, including heavy-duty engine (HDE), exhaust gas recirculation (EGR), exhaust aftertreatment system (EAS), and compressed natural gas (CNG).

The direct costs are then adjusted to account for learning effects going forward from the first year of implementation. We describe in detail in Chapter 7 of the RIA the approach used to apply learning effects in this analysis. Learning effects were applied on a technology package cost basis, and MOVES-projected sales volumes were used to determine first-year sales and cumulative sales. The resultant direct manufacturing costs and how those costs decrease over time are presented in Section V.A.3.

2. Indirect Costs

The indirect costs presented here are all the costs estimated to be incurred by manufacturers of new heavy-duty engines and vehicles associated with producing the unit of output that are not direct costs. For example, they may be related to production (such as research and development (R&D)), corporate operations (such as salaries, pensions, and health care costs for corporate staff), or selling (such as transportation, dealer support, and marketing). Indirect costs are generally recovered by allocating a share of the indirect costs to each unit of good sold. Although direct costs can be allocated to each unit of good sold, it is more challenging to account for indirect costs allocated to a unit of goods sold. To ensure that regulatory analyses capture the changes in indirect costs, markup factors (which relate total indirect costs to total direct costs) have been developed and used by EPA and other stakeholders. These factors are often referred to as retail price equivalent (RPE) multipliers. RPE multipliers provide, at an aggregate level, the relative shares of revenues, where:

Revenue = Direct Costs + Indirect Costs

Revenue/Direct Costs = 1 + Indirect Costs/Direct Costs = Retail Price Equivalent (RPE)

Resulting in:

Indirect Costs = Direct Costs × (RPE−1)

If the relationship between revenues and direct costs ( i.e., RPE) can be shown to equal an average value over time, then an estimate of direct costs can be multiplied by that average value to estimate revenues, or total costs. Further, that difference between estimated revenues, or total costs, and estimated direct costs can be taken as the indirect costs. EPA has frequently used these multipliers to predict the resultant impact on costs associated with manufacturers' responses to regulatory requirements and we are using that approach in this analysis to account for most of the indirect cost contributions. The exception is the warranty cost as described in this section.

The cost analysis estimates indirect costs by applying the RPE markup factor used in past rulemakings (such as those setting greenhouse gas standards for heavy-duty trucks). The markup factors are based on financial filings with the Securities and Exchange Commission for several engine and engine/truck manufacturers in the heavy-duty industry. The RPE factors for the HD truck industry are shown in Table V-5. Also shown in Table V-5 are the RPE factors for light-duty vehicle manufacturers.

For this analysis, EPA based indirect cost estimates for diesel and CNG regulatory classes on the HD Truck Industry RPE values shown in Table V-5. For gasoline regulatory classes, we used the LD Vehicle Industry values shown in Table V-5 since they more closely represent the cost structure of manufacturers in that industry—Ford, General Motors, and Stellantis.

Of the cost contributors listed in Table V-5, Warranty and R&D are the elements of indirect costs that the final rule requirements are expected to impact. As discussed in Section IV of this preamble, EPA is lengthening the required warranty period, which we expect to increase the contribution of warranty costs to indirect costs. EPA is also tightening the numeric standards and extending the regulatory useful life, which we expect to result in increased R&D expenses as compliant systems are developed. All other indirect cost elements—those encapsulated by the “Other” category, including General and Administrative Costs, Retirement Costs, Healthcare Costs, and other overhead costs—as well as Profits, are expected to scale according to their historical levels of contribution.

As mentioned, Warranty and R&D are the elements of indirect costs that are expected to be impacted. Warranty expenses are the costs that a business expects to or has already incurred for the repair or replacement of goods that it has sold. The total amount of warranty expense is limited by the warranty period that a business typically allows. After the warranty period for a product has expired, a business no longer incurs a warranty liability; thus, a longer warranty period results in a longer period of liability for a product. At the time of sale, a warranty liability account is adjusted to reflect the expected costs of any potential future warranty claims. If and when warranty claims are made by customers, the warranty liability account is debited and a warranty claims account is credited to cover warranty claim expenses.

In the proposed analysis, to address the expected increased indirect cost contributions associated with warranty (increased funding of the warranty liability account) due to the proposed longer warranty requirements, we applied scaling factors commensurate with the changes in proposed Option 1 or Option 2 to the number of miles included in the warranty period ( i.e., VMT-based scaling factors). Industry commenters took exception to this approach, arguing that it resulted in underestimated costs associated with warranty. To support their comments, one commenter submitted data that showed costs associated with actual warranty claims for roughly 250,000 heavy heavy-duty vehicles. The following figure includes the chart from their comments, which are also in the public docket for this rule.

Figure V-1 Warranty Costs Submitted as Part of the Comments From An Industry Association; See EPA-HQ-OAR-2019-0055-1203-A1, Page 151

EPA considers this comment and supporting information to be persuasive, not only because it represents data, but also because it represents data from three manufacturers and over 250,000 vehicles; thus, we switched from a VMT-based scaling approach to a years-based approach to better take into account this information. However, the data are for heavy HDE, so it is not possible to determine an appropriate cost per year for light or medium HDE from the data directly. Also, the data represent actual warranty claims without any mention of the warranty claims rate ( i.e., the share of engines sold that are making the warranty claims represented in the data). This latter issue makes it difficult to determine the costs that might be imposed on all new engines sold to cover the future warranty claims for the relatively smaller fraction of engines that incur warranty repair. In other words, if all heavy HDE purchases are helping to fund a warranty liability account, it is unclear if the $1,000 per year per engine is the right amount or if $1,000 per year is needed on only that percent of engines that will incur warranty repair. In the end, warranty costs imposed on new engine sales should be largely recouped by purchasers of those engines in the form of reduced emission repair expenses. EPA believes it is unlikely that a manufacturer would use their warranty program as a profit generator under the $1,000 per engine approach, especially in a market as competitive as the HD engine and vehicle industry. The possibility exists that the costs associated with the longer warranty coverage required by this rule will (1) converge towards those of the better performing OEMs; and (2) drop over time via something analogous to the learning by doing phenomenon described earlier. If true, we have probably overestimated the costs estimated here as attributable to this rule.

Thus, after careful consideration of these comments regarding warranty, and the engineering judgement of EPA subject matter experts, we revised our approach to estimating warranty costs, and for the final rule we have estimated warranty costs assuming a cost of $1,000 (2018 dollars or $977 in 2017 dollars) per estimated number of years of warranty coverage for a heavy heavy-duty diesel engine or heavy-duty vehicle equipped with such an engine. For other regulatory (engine) classes, we have scaled that value by the ratio of their estimated baseline emission-control system direct cost to the estimated emission-control system direct cost of the baseline heavy heavy-duty diesel engine. We use the baseline heavy heavy-duty diesel engine direct cost here because it should be consistent with the data behind the $1,000 per year value. The resulting emission-related warranty costs per year for a MY 2027 HD engine are as shown in Table V-6.

As noted, we have used the estimated number of years of warranty coverage, not the regulated number of years. In other words, a long-haul tractor accumulating over 100,000 miles per year will reach any regulated warranty mileage prior to a refuse truck accumulating under 40,000 miles per year, assuming both are in the same regulatory class and, therefore, have the same warranty provisions. In all cases, we estimate the number of years of warranty coverage by determining the minimum number of years to reach either the number of years, the number of miles, or the number of hours of operation covered by the EPA emissions-related warranty. We provide more detail on this in Chapter 7 of the final RIA.

Lastly, with respect to warranty, we have estimated that many of the regulated products are sold today with a warranty period longer than the EPA required emissions-related warranty period. In the proposal, we calculated baseline warranty costs only for the required warranty periods. In the final analysis, we calculate baseline warranty costs based on the warranty periods for which engines are actually sold. For diesel and CNG heavy HDE, we assume all are sold with warranties covering 250,000 miles, and for diesel and CNG medium HDE, we assume half are sold with warranties covering 150,000 miles. For all other engines and associated fuel types, we have not estimated any use of extended warranties in the baseline.

We use these annual warranty costs for both the baseline and the final standards despite the addition of new technology associated with this final rule. We believe this is reasonable for two reasons: (1) The source data included several years of data during which there must have been new technology introductions, yet annual costs appear to have remained generally steady; and, (2) the R&D we expect to be done, discussed next, is expected to improve overall durability, which should serve to help maintain historical annual costs.

For R&D, we have maintained the approach used in the proposal, although it is applied using the final useful life provisions. For example, for R&D on a Class 8 truck, the final standards would extend regulatory useful life from 10 years, 22,000 hours, or 435,000 miles, to 11 years, 32,000 hours, or 650,000 miles. We have applied a scaling factor of 1.49 (650/435) to the 0.05 R&D contribution factor for MYs 2027 and later. We apply this same methodology to estimating R&D for other vehicle categories. We estimate that once the development efforts into longer useful life are complete, increased expenditures will return to their normal levels of contribution. Therefore, we have implemented R&D scalars for three years (2027 through 2029). In MY 2030 and later, the R&D scaling factors are no longer applied.

The VMT-based scaling factors applied to R&D cost contributors used in our cost analysis of final standards are shown in Table V-7 for diesel and CNG regulatory classes and in Table V-8 for gasoline regulatory classes.

Lastly, as mentioned in Section V.A.1, the markups for estimating indirect costs are applied to our estimates of the absolute direct manufacturing costs for emission-control technology shown in Table V-2, Table V-3 and Table V-4, not just the incremental costs associated with the final program ( i.e., the Baseline + Final costs). Table V-9 provides an illustrative example using a baseline technology cost of $5000, a final incremental cost of $1000, and an indirect cost R&D contribution of 0.05 with a simple scalar of 1.5 associated with a longer useful life period. In this case, the costs could be calculated according to two approaches, as shown in Table V-9. By including the baseline costs, we are estimating new R&D costs in the final standards, as illustrated by the example where including baseline costs results in R&D costs of $450 while excluding baseline costs results in R&D costs of $75.

3. Technology Costs per Vehicle

The following tables present the technology costs estimated for the final program on a per-vehicle basis for MY 2027. Reflected in these tables are learning effects on direct manufacturing costs and scaling effects associated with final program requirements. The sum is also shown and reflects the direct plus indirect cost per vehicle in the specific model year. Note that the indirect costs shown include warranty, R&D, “other,” and profit, the latter two which scale with direct costs via the indirect cost contribution factor. While direct costs do not change across the different vehicle types ( i.e., long-haul versus short-haul combination), the indirect costs do vary because differing miles driven and operating hours between types of vehicles result in different warranty and useful life estimates in actual use. These differences impact the estimated warranty and R&D costs.

We show costs per vehicle here, but it is important to note that these are costs and not prices. We are not estimating how manufacturers might price their products. Manufacturers may pass costs along to purchasers via price increases in a manner consistent with what we show here. However, manufacturers may also price certain products higher than what we show while pricing others lower—the higher-priced products thereby subsidizing the lower-priced products. This is true in any market, not just the heavy-duty highway industry. This may be especially true with respect to the indirect costs we have estimated because, for example, R&D done to improve emission durability can readily transfer across different engines whereas hardware added to an engine is uniquely tied to that engine.

Importantly, we present costs here for MY2027 vehicles, but these costs continue for every model year going forward from there. Consistent with the learning impacts described in section V.A.2, the costs per vehicle decrease slightly over time, but only the increased R&D costs are expected to decrease significantly. Increased R&D is estimated to occur for three years following and including MY2027 ( i.e., MY2027-29), after which time its contribution to indirect costs returns to lower values as shown in Table V.4.

B. Operating Costs

We have estimated three impacts on operating costs expected to be incurred by users of new MY 2027 and later heavy-duty vehicles: Increased diesel exhaust fluid (DEF) consumption by diesel vehicles due to increased DEF dose rates to enable compliance with more stringent NO_X standards; decreased fuel costs for gasoline vehicles due to new onboard refueling vapor recovery systems that allow burning (in engine) of otherwise evaporated hydrocarbon emissions; emission repair impacts brought about by longer warranty and useful life provisions; and the potential higher emission-related repair costs for vehicles equipped with the new technology. For the repair impacts, we expect that the longer duration warranty period will result in lower owner/operator-incurred repair costs due to fewer repairs being paid for by owners/operators since more costs will be borne by the manufacturer, and that the longer duration useful life periods will result in increased emission control system durability. We have estimated the net effect on repair costs and describe our approach, along with increased DEF consumption and reduced gasoline consumption, in this section. Additional details on our methodology and estimates of operating costs are included in RIA Chapter 7.2.

1. Costs Associated With Increased Diesel Exhaust Fluid (DEF) Consumption in Diesel Engines

Consistent with the approach used to estimate technology costs, we have estimated both baseline case DEF consumption and DEF consumption under the final program. For the baseline case, we estimated DEF consumption using the relationship between DEF dose rate and the reduction in NO_X over the SCR catalyst. The relationship between DEF dose rate and NO_X reduction across the SCR catalyst is based on methodology presented in the Technical Support Document to the 2012 Nonconformance Penalty rule (the NCP Technical Support Document, or NCP TSD). The relationship of DEF dose rate to NO_X reduction used in that methodology considered FTP emissions and, as such, the DEF dose rate increased as FTP tailpipe emissions decreased. The DEF dose rate used in this analysis is 5.18 percent of fuel consumed.

To estimate DEF consumption impacts under the final program, which involves not only the new FTP emission standards but also the new SET and LLC standards along with new off-cycle standards, we developed a new approach to estimate DEF consumption for the proposal, which we also applied in this final rule. For this analysis, we scaled DEF consumption with the NO_X reductions achieved under the final emission standards. This was done by considering the molar mass of NO_X, the molar mass of urea, the mass concentration of urea in DEF, along with the density of DEF, to estimate the theoretical gallons of DEF consumed per ton of NO_X reduced. We estimated theoretical DEF consumption per ton of NO_X reduced at 442 gallons/ton, which we then adjusted based on testing to 527 gallons/ton, the value used in this analysis. We describe this in more detail in Section 7.2.1 of the RIA.

These two DEF consumption metrics—dose rate per gallon for an engine meeting the baseline emission standards and any additional DEF consumption per ton of NO_X reduced to achieve the final emission standards over the final useful lives—were used to estimate total DEF consumption. These DEF consumption rates were then multiplied by DEF price per gallon, adjusted to 2017 dollars from the DEF prices presented in the NCP TSD, to arrive at the impacts on DEF costs for diesel engines. These are shown for MY2027 diesel vehicles in Table V-17. Because these are operating costs which occur over time, we present them at both 3 and 7 percent discount rates.

2. Costs Associated With Changes in Fuel Consumption on Gasoline Engines

We have estimated a decrease in fuel costs, i.e., fuel savings, associated with the final ORVR requirements on gasoline engines. Due to the ORVR systems, evaporative emissions that would otherwise be emitted into the atmosphere will be trapped and subsequently burned in the engine. We describe the approach taken to estimate these impacts in Chapter 7.2.2 of the RIA. These newly captured evaporative emissions are converted to gallons and then multiplied by AEO 2019 reference case gasoline prices (converted to 2017 dollars) to arrive at the monetized impacts. These impacts are shown in Table V-18. In the aggregate, we estimate that the ORVR requirements in the final program will result in an annual reduction of approximately 0.3 million (calendar year 2027) to 4.9 million (calendar year 2045) gallons of gasoline, representing roughly 0.1 percent of gasoline consumption from impacted vehicles.

3. Emission-Related Repair Cost Impacts Associated With the Final Program

The final extended warranty and useful life requirements will have an impact on emission-related repair costs incurred by truck owners. Researchers have noted the relationships among quality, reliability, and warranty for a variety of goods. Wu, for instance, examines how analyzing warranty data can provide “early warnings” on product problems that can then be used for design modifications. Guajardo et al. describe one of the motives for warranties to be “incentives for the seller to improve product quality”; specifically for light-duty vehicles, they find that buyers consider warranties to substitute for product quality, and to complement service quality. Murthy and Jack, for new products, and Saidi-Mehrabad et al. for second-hand products, consider the role of warranties in improving a buyer's confidence in quality of the good.

On the one hand, we expect owner-incurred emission repair costs to decrease due to the final program because the longer emission warranty requirements will result in more repair costs covered by the OEMs. Further, we expect improved serviceability in an effort by OEMs to decrease the repair costs that they will incur. We also expect that the longer useful life periods in the final standards will result in more durable parts to ensure regulatory compliance over the longer timeframe. On the other hand, we also expect that the more costly emission control systems required by the final program may result in higher repair costs which might increase owner-incurred costs outside the warranty and/or useful life periods.

As discussed in Section V.A.2, we have estimated increased OEM costs associated with increased warranty liability ( i.e., longer warranty periods), and for more durable parts resulting from the longer useful life periods. These costs are accounted for via increased warranty costs and increased research and development (R&D) costs. We also included additional aftertreatment costs in the direct manufacturing costs to address the increased useful life requirements ( e.g., larger catalyst volume; see Chapters 2 and 3 of the RIA for detailed discussions). We estimate that the new useful life and warranty provisions will help to reduce emission repair costs during the emission warranty and regulatory useful life periods, and possibly beyond.

In the proposal, to estimate impacts on emission repair costs, we began with an emission repair cost curve derived from an industry white paper. Some commenters took exception to the approach we took, preferring instead that we use what they consider to be a more established repair and maintenance cost estimate from the American Transportation Research Institute. After careful consideration of the ATRI data, we derived a cost per mile value for repair and maintenance based on the 10 years of data gathered and presented. We chose to use the ATRI data in place of the data used in the proposal because it constituted 10 years of data from an annually prepared study compared to the one year of data behind the study used in the proposal.

Because the ATRI data represent heavy HD combination vehicles it was necessary for us to scale the ATRI values for applicability to HD vehicles with different sized engines having different emission-control system costs. We have done this in the same way as was discussed earlier for scaling of warranty cost (see Table V-6). Given that future engines and vehicles will be equipped with new, more costly technology, it is possible that the repair costs for vehicles under the final program will be higher than the repair costs in the baseline. We have included such an increase for the period beyond useful life. This is perhaps conservative because it seems reasonable to assume that the R&D used to improve durability during the useful life period would also improve durability beyond it. Nonetheless, we also think it is reasonable to include an increase in repair costs, relative to the baseline case, because the period beyond useful life is of marginally less concern to manufacturers. Lastly, since our warranty and useful life provisions pertain to emissions-related systems and their repair, we adjusted the ATRI values by 10.8 percent to arrive at an emission-related repair cost. The 10.8 percent value was similarly used in the proposal and was derived by EPA using data in the Fleet Advantage Whitepaper. Table V-19 shows how we have scaled the repair and maintenance costs derived from the ATRI study.

Importantly, during the warranty period, there are no emission-related repair costs incurred by owner/operators since those will be covered under warranty.

We present more details in Chapter 7 of the RIA behind the emission-repair cost values we are using, the scaling used and the 10.8 percent emission-related repair adjustment factor and how it was derived. As done for warranty costs, we have used estimated ages for when warranty and useful life are reached, using the required miles, ages and hours along with the estimated miles driven and hours of operation for each specific type of vehicle. This means that warranty and useful life ages are reached in different years for different vehicles, even if they belong to the same service class and have the same regulatory warranty and useful life periods. For example, we expect warranty and useful life ages to be attained at different points in time by a long-haul combination truck driving over 100,000 miles per year or over 2,000 hours per year and a refuse truck driven around 40,000 miles per year or operating less than 1,000 hours per year. The resultant MY2027 lifetime emission-related repair costs are shown in Table V-20 for diesel HD vehicles, in Table V-21 for gasoline HD vehicles, and in Table V-22 for CNG HD vehicles. Since these costs occur over time, we present them using both a 3 percent and a 7 percent discount rate. Note that these costs assume that all emission-related repair costs are paid by manufacturers during the warranty period, and beyond the warranty period the emission-related repair costs are incurred by owners/operators.

C. Program Costs

Using the cost elements outlined in Sections V.A and V.B, we have estimated the costs associated with the final program. Costs are presented in more detail in Chapter 7 of the RIA. As noted earlier, costs are presented in 2017 dollars in undiscounted annual values along with present values (PV) and equivalent annualized values (EAV) at both 3 and 7 percent discount rates with values discounted to the 2027 calendar year.

VI. Estimated Emissions Reductions From the Final Program

The final program, which is described in detail in Sections III and IV, is expected to reduce emissions from highway heavy-duty engines in several ways. We project the final emission standards for heavy-duty CI engines will reduce tailpipe emissions of NO_X; the combination of the final low-load test cycle and off-cycle test procedure for CI engines will help to ensure that the reductions in tailpipe emissions are achieved in-use, not only under high-speed, on-highway conditions, but also under low-load and idle conditions. We also project reduced tailpipe emissions of NO_X, CO, PM, VOCs, and associated air toxics from the final emission standards for heavy-duty SI engines, particularly under cold-start and high-load operating conditions. The longer emission warranty and regulatory useful life requirements for heavy-duty CI and SI engines in the final rule will help maintain the expected emission reductions for all pollutants, including primary exhaust PM_2.5, throughout the useful life of the engine. The onboard refueling vapor recovery requirements for heavy-duty SI engines in the final rule will reduce VOCs and associated air toxics. See RIA Chapter 5.3 for details on projected emission reductions of each pollutant.

Section VI.A provides an overview of the methods used to estimate emission reductions from our final program. All the projected emission reductions from the final program are outlined in Section VI.B, with more details provided in the RIA Chapter 5. Section VI.C presents projected emission reductions from the final program by engine operations and processes ( e.g., medium-to-high load or low-load engine operations).

A. Emission Inventory Methodology

To estimate the emission reductions from the final program, we used the current public version of EPA's Motor Vehicle Emission Simulator (MOVES) model, MOVES3. MOVES3 includes all the model updates previously made for the version of the MOVES model used for the NPRM analysis (“MOVES CTI NPRM”), as well as other more recent updates. Detailed descriptions of the underlying data and analyses that informed the model updates are discussed in Chapter 5.2 of the RIA and documented in peer-reviewed technical reports referenced in the RIA. Inputs developed to model the national emission inventories for the final program are also discussed in Chapter 5.2.2 of the RIA.

B. Estimated Emission Reductions From the Final Program

As discussed in Sections III and IV, the final program includes new, more stringent numeric emission standards, as well as longer regulatory useful life and emissions warranty periods compared to today's standards. Our estimates of the emission impacts of the final program in calendar years 2030, 2040, and 2045 are presented in Table VI-1. As shown in Table VI-1, we estimate that the final program will reduce NO_X emissions from highway heavy-duty vehicles by 48 percent nationwide in 2045. We also estimate an eight percent reduction in primary exhaust PM_2.5 from highway heavy-duty vehicles. VOC emissions from heavy-duty vehicles will be 23 percent lower. Emissions of CO from heavy-duty vehicles are estimated to decrease by 18 percent. Reductions in heavy-duty vehicle emissions of other pollutants, including air toxics, range from an estimated reduction of about 28 percent for benzene to about seven percent change in acetaldehyde. RIA Chapter 5.5.2 includes additional details on the emission reductions by vehicle fuel type; Chapter 5.5.4 provides our estimates of criteria pollutant emissions reductions for calendar years 2027 through 2045.

As the final program is implemented, emission reductions are expected to increase over time as the fleet turns over to new, compliant engines. We estimate no change in CO₂ emissions from the final program, based on data in our feasibility and cost analyses of the final program (see Section III for more discussion).