USA: Heavy-Duty Onroad Engines

Regulatory Background

US federal emission standards for heavy-duty engines are set by the EPA and California standards by CARB. In the heavy-duty engine sector, the federal and California emission standards have historically been well aligned; the differences were relatively minor and in most cases temporary. However, the CARB low NOx emission standards, applicable in California from 2024, are more stringent than the EPA emission requirements.

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The major regulatory steps can be summarized as follows:

• The first of US emission standards for heavy-duty engines applied from 1974. The standards were tightened on several occasions through the 1980s and 1990s. Some of the important steps included tightening of PM emission limits in 1991 (0.25 g/bhp·hr) and 1994 (0.10 g/bhp·hr), and of NOx limits in 1998 (4 g/bhp·hr).
• In October 1997, the EPA adopted new emission standards for model year 2004 and later heavy-duty diesel engines [801]. These standards reflected the provisions of the Statement of Principles (SOP) signed in 1995 by the EPA, CARB, and the manufacturers of heavy-duty diesel engines. The goal was to reduce NO_x emissions from highway heavy-duty engines to levels approximately 2.0 g/bhp·hr beginning in 2004.
• In December 2000, the EPA signed emission standards for model year 2007 and later heavy-duty highway engines, with a phase-in schedule over 2007-2010 [1044]. In addition to emission standards, the rule included ultra-low sulfur (15 ppm) diesel fuel regulations.
• In January 2020, the EPA issued an ANPR for the Cleaner Trucks Initiative. The envisioned regulations, so-called low NOx emission standards, focused on further reductions of NOx emissions from heavy-duty engines and on achieving low emissions under real driving conditions. However, the EPA had not issued a proposed rule. In April 2021, the EPA reclassified the rulemaking as a ‘long-term action’ and removed target dates from its regulatory agenda [5165].
• In August 2020, CARB adopted a low NOx emission regulation for heavy-duty engines a.k.a. the Omnibus regulation [4871]. The rule tightened FTP NOx limits to 0.050 g/bhp·hr from 2024 and to 0.02 g/bhp·hr from 2027, introduced a new LLC certification cycle and a corresponding set of NOx limits, and extended emission durability requirements to 800,000 miles.
• In March 2022, the EPA re-opened its low NOx regulatory process and in December 2022 finalized new emission standards for heavy-duty engines that become effective from 2027 [5724]. The standards are to some degree harmonized with the CARB low NOx rule, but are less stringent in terms of both emission limits and emission durability requirements—the FTP NOx limit is 0.035 g/bhp·hr, while the useful life period for heavy heavy-duty engines is 650,000 miles.
• In July 2023, CARB reached an agreement with the Truck and Engine Manufacturers Association (EMA), its on-highway members and Ford—known as the Clean Truck Partnership (CTP)—that committed CARB to propose amendments to the Omnibus regulation that would generally align the emissions standards, associated test procedures, and accompanying enforcement provisions for 2027 and subsequent MY engines and vehicles with the corresponding provisions in the federal EPA-NOx rule. The amendments were proposed in March 2024 and were scheduled to be finalized in late 2025.

In addition to emission standards for criteria pollutants, discussed below, heavy-duty engine and vehicle manufacturers must meet GHG emissions and fuel economy standards that became applicable from model year 2017.

Applicability and Testing

The emission standards discussed below apply to new engines used in heavy-duty onroad vehicles, such as trucks and buses. Two different sets of standards are applicable to diesel and to spark-ignited engines. The nomenclature and definitions used in various US EPA and California engine and vehicle regulations have evolved over time:

• For many years, the EPA regulations contained in 40 CFR Part 86 (A) had referred to “diesel heavy-duty engines” and “Otto-cycle heavy-duty engines”. At one time, the EPA definition was based on the engine cycle rather than the ignition mechanism, with the presence of a throttle as an indicator to distinguish between Diesel-cycle and Otto-cycle operation. Regulating power by controlling the fuel supply in lieu of a throttle corresponded to lean combustion and diesel-cycle operation. This allowed for the possibility that a natural gas-fueled engine equipped with a spark plug could be considered a diesel engine.
• In the current regulations, migrated to 40 CFR Part 1036, the EPA refers to these engines as “compression-ignition” (CI) and “spark-ignition” (SI), respectively [5724]. Therefore, the same standards apply to diesel fueled engines and to compression-ignition engines fueled by natural gas and other fuels. Heavy-duty engines not meeting the definition of compression-ignition or spark-ignition (e.g., gas turbine heavy-duty engines) are deemed to be compression-ignition engines.

Heavy-duty vehicles are defined as vehicles of GVWR (gross vehicle weight rating) of above 8,500 lbs in the federal jurisdiction and above 14,000 lbs in California (model year 1995 and later). Diesel engines used in heavy-duty vehicles are further divided into service classes by GVWR, including light heavy-duty diesel engines (LHDDE), medium heavy-duty diesel engines (MHDDE), and heavy heavy-duty diesel engines (HHDDE).

Under the federal light-duty Tier 2 regulation, vehicles of GVWR up to 10,000 lbs used for personal transportation have been re-classified as “medium-duty passenger vehicles” (MDPV, primarily larger SUVs and passenger vans) and are subject to the light-duty vehicle emission standards. Therefore, the same diesel engine model used for the 8,500 - 10,000 lbs vehicle category may be classified as either light- or heavy-duty and certified to different standards, depending on the application.

US and California regulations do not require that complete heavy-duty diesel vehicles be chassis certified, instead requiring certification of their engines (as an option, complete heavy-duty diesel vehicles under 14,000 lbs can be chassis certified). Consequently, the basic standards are expressed in g/bhp·hr and require emission testing over the FTP engine dynamometer cycle (however, chassis certification may be required for complete heavy-duty gasoline vehicles with pertinent emission standards expressed in g/mile).

Additional emission testing requirements have been introduced in two major steps:

• SET and NTE testing was introduced for most signees of the 1998 Consent Decrees between the EPA and engine manufacturers for the period of 1998-2004. California regulations require the tests for all engines effective 2005. Federal regulations require SET and NTE testing for all engines from 2007. In addition, under the EPA in-use compliance testing program launched in 2005, the EPA, CARB, and engine manufacturers measure in-use emissions of heavy-duty vehicles using PEMS, with compliance determined against NTE standards [5725].
  • SET (Supplemental Emission Test): A steady-state test to ensure that heavy-duty engine emissions are controlled during steady-state type driving and high load operation. More recent emission standards require the ramped mode version (RMC-SET) of the test. SET emission limits are numerically equal to the FTP limits.
  • NTE (Not-to-Exceed) testing: Driving of any type that could occur within the bounds of a pre-defined NTE control area, including operation under steady-state or transient conditions and under varying ambient conditions. NTE emission limits are higher than the corresponding FTP limits. NTE limits are applicable to engine certifications only; they are not applicable to chassis-certified vehicles.
• LLC and off-cycle MAW testing and the corresponding emission limits were introduced with the California low NOx regulation effective from 2024. At the federal level, LLC and MAW testing is applicable from 2027. The off-cycle MAW testing methodology and emission limits replace the prior NTE testing requirements and emission standards.
  • LLC (Low Load Cycle): A transient dynamometer test cycle representing low speed and load driving conditions, such as in slow urban traffic. LLC emission limits are higher than FTP limits.
  • Off-Cycle MAW Testing: A binned moving average windows (MAW) methodology for in-use vehicle emission testing using PEMS instruments. In the EPA version of the procedure, emissions are separated into two bins: bin 1 representing idle and very low load operation, and bin 2 representing higher power operation (including much of the operation covered by the prior NTE test). Bin 1 and bin 2 have two sets of respective emission standards. The CARB version of the test uses a 3-bin MAW methodology.

Emission Standards

Summary of Emission Limits

The first US emission standards for heavy-duty engines became effective in 1973 in California and in 1974 at the federal level. Since then, the standards were tightened on numerous occasions. The evolution of emission standards over the FTP test cycle is summarized in Table 1.

Further emission limits and testing requirements may be applicable, including not-to-exceed (NTE) limits and limits over the SET, LLC, and other test cycles, as discussed in the following sections. In addition, the following emission standards may apply:

• Smoke Opacity—Smoke opacity limits of 20% / 15% / 50% at acceleration/lug/peak modes, respectively, have been applicable since 1974.
• Idle CO Standard—An idle CO emission standard of 0.5% applies to compression-ignition engines fueled by diesel fuel since 1988, by methanol since 1990, and by natural gas and LPG since 1994.
• Idle NOx Standard—In California, idling NOx emission standards are applicable since 2007. At the federal level, an optional Clean Idle NOx certification is applicable from 2024 [5724].

ABT Program. Since 1991, the emission regulations include an emission averaging, banking, and trading (ABT) program for NOx and PM emissions, similar to those that have been a part of most US EPA emission control programs.

Model Year 1974-2003

The first set of emission standards for heavy-duty engines that became effective at the federal level in 1974 included only two standards, one for CO (40 g/bhp·hr) and one for HC+NOx (16 g/bhp·hr). These early limits required little, if any, emission reduction from the diesel engines manufactured at the time. More serious regulatory challenges that required the development of new emission technologies were brought by later updates to the emission regulations, including:

• 1991 PM emission standard of 0.25 g/bhp·hr
• 1994 PM emission standard of 0.10 g/bhp·hr
• Gradual tightening of the NOx limit to 4 g/bhp·hr (1998)

These challenges were generally met through in-cylinder emission control. However, the 1994 PM limit did trigger some usage of diesel oxidation catalysts, mostly on mechanically controlled heavy-duty engines.

A sulfur limit of 500 ppm in diesel fuel became effective in October 1993. This fuel, referred to as low sulfur diesel fuel, was introduced to enable the 1994 PM emission standard of 0.10 g/bhp·hr.

Optional Standards. Manufacturers could voluntarily certify engines to the Clean Fuel Fleet (CFF) emission standards shown in Table 2. It was a federal program that applied to 1998-2003 model year engines, both CI and SI, over 8,500 lbs GVWR. In addition to the CFF standards, vehicles had to meet the applicable conventional standards for other pollutants.

Model Year 2004-2006

The 2004 standards for heavy-duty engines—as adopted by the EPA in 1997 [801]—were harmonized with California standards, with the intent that manufacturers could use a single engine or machine design for both markets.

On-board diagnostic (OBD) requirements applicable to heavy-duty diesel engines and vehicles ≤ 14,000 lbs GVWR were phased-in from the 2005 through 2007 model years.

Discharge of crankcase emissions was not allowed for any new 2004 or later model year engines, with the exception of turbocharged or supercharged diesel fueled engines.

To achieve the 2004 emissions, most manufacturers introduced exhaust gas recirculation (EGR)—in many cases in conjunction with diesel oxidation catalysts—on heavy-duty diesel engines.

Consent Decrees. Most engine manufacturers were required to comply with the 2004 emission standards as early as from October 2002, 15 months ahead of the regulatory schedule. In October 1998, a series of court settlements were reached between the EPA, Department of Justice, CARB and engine manufacturers—Caterpillar, Cummins, Detroit Diesel, Volvo, Mack Trucks/Renault and Navistar—over the issue of high NOx emissions from heavy-duty diesel engines during certain driving modes. Since the early 1990’s, the manufacturers used engine control software that caused engines to switch to a more fuel efficient—but higher NO_x—driving mode during steady highway cruising. The EPA considered this engine control strategy an illegal “emission defeat device”. The provisions of the consent decrees included:

• Civil penalties for engine manufacturers and requirements to allocate funds for pollution research
• Upgrading existing engines to lower NO_x emissions
• Steady-state SET test with a limit equal to the FTP standard and NTE limits of 1.25 × FTP (with the exception of Navistar)
• Meeting the 2004 emission standards from October 2002

In the aftermath of the Consent Decrees, California certifications for all model year 2005-2007 engines required SET testing and NTE limits of 1.25 × FTP standards. California also adopted more stringent standards for MY 2004-2006 engines for public urban bus fleets.

Model Year 2007-2023/26

The EPA rule of December 21, 2000 [1044] included two components: (1) 2007 and later heavy-duty engine emission standards, and (2) diesel fuel regulations. CARB adopted virtually identical 2007 heavy-duty engine standards in October 2001.

The emission standards included new, very stringent limits for PM (0.01 g/bhp·hr) and NOx (0.20 g/bhp·hr). The PM emission standard took full effect in 2007. The NO_x standard was phased-in for diesel engines between 2007 and 2010. The phase-in was defined on a percent-of-sales basis: 50% from 2007 to 2009 and 100% in 2010 (gasoline engines were subject to these standards based on a phase-in requiring 50% compliance in 2008 and 100% compliance in 2009). In practice, very few engines meeting the 0.20 g/bhp·hr NOx limit actually appeared before 2010. In the 2007-2009 period, most manufacturers opted to meet a NOx family emission limit (FEL) of around 1.2 g/bhp·hr for most of their engines. Because of this compliance path during the NOx limit phase-in period, engines produced during 2007-2009 were technologically very different from those required to comply in 2010 and later when all engines needed to comply with the 0.2 g/bhp·hr NOx limit. While it is common to refer to “2010 standards” in a way that implies they are different from “2007 standards”, legally, there was not a standard for 2010 that differed from 2007.

Starting in 2007, manufacturers could choose to chassis certify complete heavy-duty diesel vehicles (HDV) with GVWR of 14,000 lb or less as an option to engine certification. The emission limits applicable to Otto cycle heavy-duty vehicles with the same GVWR applied. Diesel engines optionally certified as complete vehicles were not allowed to be included in any averaging, banking, or trading program for criteria emissions. However, they were included in the phase-in calculations that required 50% of engines to comply with the final 2010 NOx limit. Diesel engines thus certified were considered to be legally equivalent to a 0.20 g/bhp·hr NOx engine provided they met the 2008 Otto-cycle HDV limits (0.2 g/mile NOx and 0.02 g/mile PM for 8500 lb < GVWR ≤ 10000 lb and 0.4 g/mile NOx and 0.02 g/mile PM for 10000 lb < GVWR ≤ 14000 lb). After 2011, all manufacturers of complete HDVs with GVWR ≤ 14000 lb (primarily heavy pick-ups and utility vans) adopted this optional chassis certification approach because of the heavy-duty vehicle GHG regulations coming into effect for MY 2014.

In addition to the FTP testing, emission certification requirements include:

• SET test, with limits equal to the FTP standards, and
• NTE limits of 1.5 × FTP standards (or 1.25 × FTP for engines with NOx FEL > 1.5 g/bhp·hr).

The diesel fuel regulation limited the sulfur content in on-highway diesel fuel to 15 ppm (wt.), down from the previous 500 ppm. Refiners were required to start producing the 15 ppm S ultra low sulfur diesel (ULSD) fuel beginning from June 2006. The ULSD fuel has been introduced as a “technology enabler” to pave the way for sulfur-intolerant exhaust emission control technologies, such as catalytic diesel particulate filters and NO_x catalysts that have been widely introduced to meet the 2007/2010 emission requirements.

Spark-Ignition Engines. Emissions from spark-ignition heavy-duty engines are measured over the FTP test cycle. The following emission limits apply: NOx = 0.2 g/bhp·hr; HC = 0.14 g/bhp·hr; PM = 0.01 g/bhp·hr; CO = 14.4 g/bhp·hr.

Other Provisions. The 2007 emission standards and later amendments introduced a number of additional provisions:

• Crankcase Ventilation—Effective from 2007, the regulation maintains the earlier crankcase emission control exception for turbocharged heavy-duty diesel fueled engines but requires that if the emissions are discharged into the atmosphere, they be added to the exhaust emissions during all testing. In this case, the deterioration of crankcase emissions must also be accounted for in exhaust deterioration factors.
• DEF Refill Interval—For SCR-equipped heavy-duty diesel engines, a minimum DEF (urea solution) refill interval is defined as at least as far (in miles or hours) as the vehicle’s fuel capacity [3408].
• Ammonia Emissions—While ammonia emissions are unregulated, the EPA recommends that ammonia slip should be below 10 ppm average over the applicable test cycles [3693].
• Emergency Vehicles—Heavy-duty engines in fire trucks, ambulances and other types of emergency vehicles can be equipped with an AECD to override performance inducements related to the emission control system—for example, to allow engine operation without urea in the SCR system [3408].
• Idling Emissions California requires engines to certify to a NOx idling emission standard of 30 g/hr (in lieu of compliance with the standard, manufacturers may use a non-programmable 5-minute engine shutdown system). At the federal level, engines may certify to an optional Clean Idle NOx standard of 30 g/h, applicable from 2024 [5724].

California Low NOx Regulation (2024+)

On August 27, 2020, CARB approved the Heavy-Duty Low NOx Omnibus Regulation that requires a further 90% reduction of NOx emissions from heavy-duty onroad engines, to be phased-in over 2024-2031, and introduces a number of other requirements such as a new low load test cycle and extended emission durability periods [4871][5374].

The low NOx emission regulation supersedes the Optional Low NOx Standards for heavy-duty engines that were adopted in October 2014 [3130] and applicable from 2015. Under the program, manufacturers could certify their engines to three optional NOx emission standards: 0.10, 0.05 or 0.02 g/bhp·hr.

The mandatory low NOx standards apply to Diesel- and Otto-cycle engines in the heavy-duty category (GVWR > 14,000 lbs), as well as engines used in medium-duty vehicles of GVWR from 10,001 to 14,000 lbs. The rule requires manufacturers to comply with tougher NOx and PM emission standards, overhaul engine testing procedures, and extend engine useful life and warranty periods. The focus of the rule is to ensure durable reductions of NOx emissions from in-use vehicles, including vehicles operated at low engine load, in such real-world operations as slow urban traffic. The main provisions are:

• Lower NOx emission standards—including 0.050 g/bhp·hr from 2024 and 0.02 g/bhp·hr from 2027—over the FTP and the RMC-SET test cycles.
• A new low load certification cycle (LLC) and corresponding NOx emission limits that transition from 0.20 g/bhp·hr (2024) to 0.05 g/bhp·hr (2027, 435,000 mile useful life).
• New useful life periods of up to 800,000 miles, compared to the previous 435,000 mile requirements (HHDDE).
• New off-cycle emission testing procedure based on the binned MAW methodology.

As the new standards focus on reducing NOx emissions at low exhaust temperatures, they are expected to require improved NOx reduction technologies such as close-coupled SCR systems in addition to the main SCR catalyst, as well as advanced thermal management techniques.

The low NOx standards are implemented in two main stages: (1) MY 2024-2026 standards, and (2) MY 2027 and later standards. The 2024-2026 standards are listed in Table 3. The regulation also includes optional 50-state-directed engine standards. Manufacturers may meet these less stringent standards in California if they do so for all engine families they produce nationwide.

Emission standards for engines model year 2027 and later depend on the engine category. Standards for medium-duty, light heavy-duty (LHDDE), and medium heavy-duty (MHDDE) diesel engines are listed in Table 4.

Emission standards for heavy heavy-duty diesel engines, Table 5, include two sets of NOx emission limits: one to be met over the intermediate useful life (IUL) of 435,000 mi, and another for the full useful life (FUL) of 600,000/800,000 mi from 2027/2031, respectively.

For off-cycle emission testing using PEMS analyzers, a 3-bin moving average window (3B-MAW) methodology with time-based windows of a 300 second duration is used. The 300-second windows constructed from the second-by-second field data are grouped into one of three bins using the nominal “normalized average CO₂ rate” from the certification test cycles to identify the boundaries. Windows with a normalized average CO₂ rate of 6% or less are classified as idle and grouped into bin 1. Windows with a normalized average CO₂ rate greater than 6% but no more than 20% are classified as low-load operation and grouped int0 bin 2. Windows with a normalized average CO₂ rate greater 20% are classified as medium-/high-load operation and grouped into bin 3. Compliance within a bin is determined by comparing the measurement with the applicable emission standard for each bin—idle standard for bin 1, LLC standard for bin 2, and FTP standard for bin 3—multiplied by a conformity factor (CF). For 2024 through 2029 model year engines, CF = 2.0. For 2030 and subsequent model year engines, CF = 1.5.

Otto Cycle Engines. Emission standards for heavy-duty Otto cycle (HDO) engines are listed in Table 6.

Idling Emissions. Manufacturers must certify diesel engines to a NOx idling emission standard of 10 g/hr in 2024-2026, and 5 g/hr in 2027 and later model years. In lieu of meeting the standard, engines may be equipped with an engine shutdown system.

Model Year 2027 and Later

On December 20, 2022, the EPA adopted a final rule that set emission standards for heavy-duty engines and vehicles starting in model year 2027 [5724]. The regulation introduces new NOx, PM, HC, and CO emission standards for heavy-duty engines, including standards for the existing FTP and SET laboratory test cycles, as well as new standards based on the low load cycle (LLC) and the off-cycle MAW procedure, Table 7.

An ‘interim NOx compliance allowance’ of 15 mg/bhp·hr applies to any in-use testing of Medium HDE and Heavy HDE. This compliance allowance is added to the NOx standard that applies for each duty cycle and for off-cycle testing, with both field testing and laboratory testing (except for ‘bin 1’ off-cycle standard). With the allowance, the in-use NOx emission standards for Medium HDE and Heavy HDE effectively become 50/65/73 mg/bhp·hr over the FTP/LLC/MAW tests, respectively. Light HDE and Spark Ignition HDE must meet the above NOx standards in both certification and in-use testing.

For off-cycle emission testing using PEMS analyzers, the EPA finalized a two bin moving average window (MAW) methodology using time-based windows of a 300 second duration. The 300-second windows constructed from the second-by-second field data are grouped into one of two bins using the nominal “normalized average CO₂ rate” from the certification test cycles to identify the boundaries. Windows with a normalized average CO₂ rate of 6% or less (6% is equivalent to the average power of the LLC certification cycle) are classified as idle and binned together (bin 1). Windows with a normalized average CO₂ rate greater than 6% are classified as non-idle operation and binned together (bin 2).

The off-cycle MAW bin 1 (idle) NOx standard is 10 g/hr. There are no bin 1 standards for other pollutants. The off-cycle bin 1 NOx standard reflects a conformity factor of 1.0 times the Clean Idle NOx standard. The off-cycle bin 2 standards for all pollutants reflect a conformity factor of 1.5 times a weighted (25/75) average of the LLC and FTP/SET cycle standards.

The federal 2027 emission standards, while somewhat less stringent, are similar in their NOx reduction focus and in their structure to the California 2024/2027 low NOx regulation and will require similar emission control technologies—such as dual-dosing SCR systems and advanced exhaust gas thermal management. An alternative approach involves the use of conventional DPF+SCR aftertreatment in combination with novel combustion technologies, such as modern 2-stroke opposed piston engines.

Spark-Ignition Engines. Emissions from spark-ignition heavy-duty engines are measured over the SET and FTP test cycles. The emission limits are summarized in Table 8.

Heavy-duty vehicles with spark-ignition engines are also subject to a refueling (evaporative) standard of 0.20 grams hydrocarbon per gallon of dispensed fuel.

Other Provisions. The 2027 emission standards include several additional provisions:

• Crankcase Ventilation—Manufacturers have two options for controlling crankcase emissions, either (1) closing the crankcase, or (2) use an open crankcase and add crankcase criteria pollutant emissions to the tailpipe emissions.
• Idling Emissions—Manufacturers may certify engines to an optional Clean Idle NOx emission standard of 10 g/h.
• Maintenance—The regulation specifies minimum scheduled maintenance intervals (in miles or hours) for replacing critical emission-related components such as sensors, electronic control modules, EGR components, catalyst systems, particulate filtration systems, and more.
• Onboard Diagnostics (OBD)—The EPA regulation incorporates by reference the OBD requirements adopted by CARB on October 3, 2019 (California Code of Regulations, title 13, sections 1968.2, 1968.5, 1971.1, and 1971.5).