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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 the ARB. However, 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.

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, California ARB, 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 2020, the EPA and California ARB both published first regulatory documents pertaining to the so-called lower NOx emission standards, focused on further reductions of NOx emissions from heavy-duty engines and on achieving low emissions under real driving conditions. In January 2020, the EPA issued an ANPR for the Cleaner Trucks Initiative. In June 2020, California ARB proposed lower NOx emission regulation for heavy-duty engines. The ARB proposal envisions NOx limits (FTP) of 0.050 g/bhp-hr from 2024 and of 0.02 g/bhp-hr from 2027. The proposal also introduces a new low load certification cycle (LLC) and NOx limits, as well as extended useful life periods.

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.

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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, as follows.

  • Light heavy-duty diesel engines: 8,500 < LHDDE < 19,500 (14,000 < LHDDE < 19,500 in California, 1995+)
  • Medium heavy-duty diesel engines: 19,500 ≤ MHDDE ≤ 33,000
  • Heavy heavy-duty diesel engines (including urban bus): HHDDE > 33,000

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, phased-in from 1998 to 2007, 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. 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.

These 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 effective 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.

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 mandatory emission standards for heavy-duty engines were phased-in over the period of 2007-2010. The table also covers the 2015 California Optional Low NOx Standards.

Table 1
US EPA & California Emission Standards for Heavy-Duty CI Engines, 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

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.

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, California ARB 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. The California ARB 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].

California Optional Low NOx Standards. On October 21, 2014, California ARB adopted Optional Low NOx Standards for heavy-duty engines [3130]. Under the program, manufacturers may choose to certify their engines to three optional NOx emission standards: 0.10, 0.05 or 0.02 g/bhp·hr. Other pollutants must meet the conventional emission standards (Table 1).

Engine families certified to the optional NOx standards cannot be included in the ABT program for NOx. Instead, credits may be generated by an alternative mechanism proposed by the engine manufacturer and approved by the ARB.