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Emission Standards

United States: 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 significantly more stringent than the current EPA emission requirements.

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The major regulatory steps can be summarized as follows:

  • The first set of emission standards for heavy-duty engines were adopted beginning 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 NOx 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 lagged behind schedule and 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 tightens FTP NOx limits to 0.050 g/bhp·hr from 2024 and to 0.02 g/bhp·hr from 2027, introduces a new LLC certification cycle and a corresponding set of NOx limits, and significantly extends emission durability and warranty requirements.
  • In 2022, the EPA re-opened its low NOx regulatory process and proposed new emission standards for heavy-duty engines that would be to some degree harmonized with the CARB low NOx rule.

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 (highway) vehicles, such as trucks and buses. These standards apply to diesel fueled engines, as well as to compression-ignition (CI) engines fueled by natural gas and other alternative fuels. A number of definitions of the compression-ignition or diesel-cycle engine are used in various US and California engine and vehicle regulations. One definition used by the US EPA in heavy-duty engine regulations is 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 corresponds with lean combustion and diesel-cycle operation. This allows for the possibility that a natural gas-fueled engine equipped with a spark plug is considered a compression-ignition engine.

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 transient 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 added in several steps, beginning in 1998. These include:

  • Supplemental Emission Test (SET): A steady-state test to ensure that heavy-duty engine emissions are controlled during steady-state type driving, such as a line-haul truck operating on a freeway. More recent emission standrds require the ramped mode version (RMC-SET) of the test. SET emission limits are numerically equal to the FTP limits.
  • Not-to-Exceed (NTE) 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.
  • Low Load Cycle (LLC): A transient dynamometer test cycle representing low speed and load driving conditions, such as in slow urban traffic. LTC emission limits are higher than FTP limits.

The SET and NTE tests were introduced for most signees of the 1998 Consent Decrees between the EPA and engine manufacturers for the period of 1998-2004. Federal regulations require the supplemental testing for all engine manufacturers from 2007. In California, the tests are required for all engines effective model year 2005. NTE limits are applicable to engine certifications only; they are not applicable to chassis-certified vehicles.

The LLC test was introduced by the California low NOx regulation and is applicable from 2024.

Emission Standards

Summary of Emission Limits

The emission limits for heavy-duty engines were tightened in a number of steps, as shown in Table 1. The current US federal emission standards for heavy-duty engines were phased-in over the period of 2007-2010. The table also covers the California Low NOx Standards—optional from 2015 and mandatory from 2024/2027.

Table 1
US EPA & California emission standards for heavy-duty CI engines, FTP, g/bhp·hr
GeneralUrban Bus
  1. NMHC for 2004 and later standards
  2. For methanol-fueled engines, the standard is for total hydrocarbon equivalent (THCE).
  3. California: NMHC = 1.2 g/bhp·hr, in addition to the THC limit.
  4. California: NOx = 6.0 g/bhp·hr
  5. California: Urban bus NOx = 4.0 g/bhp·hr
  6. California only, no federal PM limit.
  7. California standard 0.10 g/bhp·hr
  8. In-use PM standard 0.07 g/bhp·hr
  9. Alternative standard: NMHC+NOx = 2.5 g/bhp·hr and NMHC = 0.5 g/bhp·hr
  10. Under the 1998 Consent Decrees, several manufacturers supplied 2004 compliant engines from October 2002.
  11. NOx and NMHC standards were phased-in on a percent-of-sales basis: 50% in 2007-2009 and 100% in 2010. Most manufacturers certified their 2007-2009 engines to a NOx limit of about 1.2 g/bhp·hr, based on a fleet average calculation.
  12. Optional. Manufacturers may choose to certify engines to the California Optional Low NOx Standards of 0.10, 0.05 or 0.02 g/bhp·hr
  13. California only, not applicable at the federal level.

In addition to the limits shown above, the following emission standards 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.

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

Historically, the first sets of emission standards were adopted at the federal level beginning from 1974. Since 1987, California standards required on several occasions that PM and NOx emission limits be introduced in California a few years ahead of the federal schedule (Table 1).

Some of the regulatory emission challenges of that period that required the development of new emission technologies were:

  • 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.

Table 2
Clean Fuel Fleet Program for heavy-duty SI and CI engines (1998-2003), g/bhp·hr
LEV (Federal Fuel) 3.8  
LEV (California Fuel) 3.5  
ILEV14.42.5 0.050
* LEV - low emission vehicle; ILEV - inherently low emission vehicle; ULEV - ultra low emission vehicle; ZEV - zero emission vehicle

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 NOx—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 NOx 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 and Later

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 NOx 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 are 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 NOx catalysts that have been widely introduced to meet the 2007/2010 emission requirements.

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 additionally 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.

California Low NOx Regulation

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 supercedes 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).

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.

Table 3
Emission standards for 2024-2026 heavy-duty diesel engines, g/bhp·hr
Model YearTestNOx*NMHCCOPM
2024-2026FTP & RMC0.0500.1415.50.005
* Manufacturers may certify engines to either of two alternative, optional NOx standards:
  1. More stringent optional NOx standards: 0.020 g/bhp·hr (FTP/RMC) and 0.080 g/bhp·hr (LLC).
  2. Less stringent NOx standards for 50-state-directed engines: 0.100 g/bhp·hr (FTP/RMC) and 0.300 g/bhp·hr (LLC). Manufacturers must meet these standards on a nationwide basis.

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.

Table 4
Emission standards for 2027 and later light heavy-duty and medium heavy-duty diesel engines (LHDDE/MHDDE), g/bhp·hr
Model YearTestNOx*NMHCCOPM
2027+FTP & RMC0.0200.1415.50.005
* Optional NOx standards: 0.010 g/bhp·hr (FTP/RMC) and 0.025 g/bhp·hr (LLC).

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.

Table 5
Emission standards for 2027 and later heavy heavy-duty diesel engines (HHDDE), g/bhp·hr
Model YearTestNOx*NMHCCOPM
2027-2030FTP & RMC0.0200.0350.1415.50.005
2031+FTP & RMC0.0200.0400.1415.50.005
* Optional FUL NOx standards: 0.010 g/bhp·hr (FTP/RMC) and 0.025 g/bhp·hr (LLC).

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

Table 6
Emission standards for heavy-duty Otto cycle engines, FTP, g/bhp·hr
* Optional NOx standards: 0.020 g/bhp·hr (2024) and 0.010 g/bhp·hr (2027).
a 0.10 g/bhp·hr for optional 50-state-directed engines

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.