Note: In December 2021, the California Air Resources Board (CARB) finalized their heavy-duty (HD) Low NOx Omnibus Regulation. The regulation, which was presented for consideration in 2020, achieves a 90% reduction from current NOx emissions limits by 2027 and establishes a more robust certification method that includes better in-use testing protocols. The standard also effectively doubles current equipment useful life and warranty requirements for HDVs.
The current U.S. Environmental Protection Agency (EPA) emissions standards for heavy-duty highway engines, model year (MY) 2007 and later, were phased in over a period of several years. Diesel particulate filter-forcing particulate matter (PM) emission limits took full effect in 2007, whereas limits for nitrogen oxides (NOx) and non-methane hydrocarbons (NMHC) were phased in between 2007 and 2010. Steady real-world NOx reductions have been observed for diesel engines over MYs 2010 to 2014. Fully phased-in, the U.S. 2010 standards are roughly equivalent in terms of stringency and required technology to Euro VI standards.
Conventional pollutant emission limits
U.S. Environmental Protection Agency (EPA)
2007 Heavy-Duty Highway Rule, fully phased-in as of 2010
Engines that are used in vehicles with a gross vehicle weight rating (GVWR) > 8,500 lbs.
Federal regulation of heavy-duty engine emissions in the United States began in 1974. More stringent regulations were phased-in beginning in 1988, with the most recent set of standards phased-in between 2007 and 2010. The current PM limits took effect in 2007, while NOx limits were phased-in between 2007 and 2010. The fully phased-in 2010 standards are also referred to as the U.S. 2010 standards.
Heavy-duty engine emissions are regulated by the EPA, though California maintains its own regulation per its authority codified in the U.S. Clean Air Act. Historically, U.S. EPA and CARB have harmonized standards, but it is unclear if U.S. EPA’s Cleaner Trucks Initiative will follow CARB’s forthcoming Low NOx Omnibus Regulation.
Ultralow-sulfur diesel fuel (ULSD) with 15 ppm sulfur content was introduced as a technology enabler for the US 2007 standards. ULSD paved the way for advanced, sulfur-intolerant exhaust emission control technologies, such as catalytic diesel particulate filters and NOx catalysts, which are necessary to meet the current, stringent emission standards in the United States.
In January 2020, U.S. EPA released an Advance Notice of Proposed Rulemaking (ANPRM) for the Cleaner Trucks Initiative to update HDV NOx standards. On March 28, 2022, EPA published the proposal to which the ANPRM refers. The proposed rule would significantly reduce emissions of smog- and soot-forming nitrogen oxides (NOx) from heavy-duty gasoline and diesel engines and set more stringent greenhouse gas (GHG) standards for certain commercial vehicle categories.
Model year (MY) 1988–2003 U.S. EPA emission standards for heavy-duty diesel truck and bus engines are summarized in the following tables. Applicable to the 1994 standards and beyond, sulfur content in the certification fuel was reduced to 500 ppm wt.
|Heavy-duty diesel truck engines|
|Urban bus engines|
|a The in-use PM standard is 0.07|
Useful life and warranty periods: Compliance with emission standards had to be demonstrated over the useful life of the engine, which was adopted as follows (federal & California):
- Light heavy-duty diesel engines (LHDDE): 8 years or 110,000 miles (whichever occurred first)
- Medium heavy-duty diesel engines (MHDDE): 8 years or 185,000 miles
- Heavy heavy-duty diesel engines (HHDDE): 8 years or 290,000 miles
Federal useful life requirements were later increased to 10 years, with no change to the above mileage numbers, for the urban bus PM standard (1994–present) and for the NOx standard (1998–present). The emission warranty period was 5 years/100,000 miles (5 years or 100,000 miles or 3,000 hours in California), but no less than the basic mechanical warranty for the engine family.
Clean Fuel Fleet Program, 1998–2003: Voluntary Clean Fuel Fleet (CFF) program was a federal standard that applied to 1998–2003 model year engines, both Compression Ignition (CI) and Spark Ignition (SI), > 8,500 lbs. GVWR. In addition to the CFF standard, vehicles had to meet applicable conventional standards for other pollutants.
|LEV (Federal fuel)||–||3.8||–||–|
|LEV (California fuel)||–||3.5||–||–|
|a LEV — low-emission vehicle; ILEV — inherently low-emission vehicle; ULEV – ultralow-emission vehicle; ZEV — zero-emission vehicle|
In October 1997, EPA adopted new emission standards for MY 2004 and later heavy-duty diesel truck and bus engines. These standards reflect the provisions of a 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. Manufacturers had the flexibility to certify their engines to one of the two options shown below.
|Option||NMHC + NOx||NMHC|
All emission standards other than NMHC and NOx applying to 1998 and later MY heavy-duty engines would continue at their 1998 levels.
Useful life and warranty periods: EPA established revised useful engine lives, with significantly extended requirements for the heavy heavy-duty diesel engine service class, as follows:
- LHDDE: 110,000 miles or 10 years
- MHDDE: 185,000 miles or 10 years
- HHDDE: 435,000 miles or 10 years or 22,000 hours
The emission warranty remained at 5 years or 100,000 miles. With the exception of turbocharged and supercharged diesel fueled engines, discharge of crankcase emissions was not allowed for any new 2004 or later model year engines.
The federal 2004 standards for highway trucks were harmonized with California standards, with the intent that manufacturers could use a single engine or machine design for both markets. However, California certifications for MYs 2005–2007 additionally required the Supplemental Emission Test (SET), and Not-to-Exceed (NTE) limits of 1.25 × Federal Test Procedure (FTP) standards (see “Testing” for more information). California also adopted more stringent standards for MY 2004–2006 engines for public urban bus fleets.
Consent Decrees: In 1998, a court settlement was 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.”
Provisions of the Consent Decree included the following:
- Civil penalties for engine manufacturers and requirements to allocate funds for pollution research
- Upgrading existing engines to lower NOx emissions
- SET (steady-state) 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 by October 2002, 15 months ahead of time
2007 and later
On December 21, 2000, EPA signed emission standards for MY 2007 and later heavy-duty highway engines (CARB adopted virtually identical 2007 heavy-duty engine standards in October 2001). The rule includes two components: (1) emission standards, and (2) diesel fuel regulations. Legislative text can be found here. Additional information can be found on the EPA website for the heavy trucks, buses, and engines.
Emission standards are as follows:
The PM emission standard took full effect in the 2007 heavy-duty engine model year. The NOx and NMHC standards were phased in for diesel engines between 2007 and 2010. The phase-in was 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). Very few engines meeting the 0.20 g/bhp-hr NOx requirement appeared before 2010. In 2007, most manufacturers opted instead to meet a Family Emission Limit (FEL) around 1.2-1.5 g/bhp-hr NOx for most of their engines with a few manufacturers still certifying some of their engines as high as 2.5 g/bhp-hr NOx + NMHC.
In addition to transient FTP testing, emission certification requirements also include:
- SET test, with limits equal to the FTP standards, and
- NTE testing with limits of 1.5 × FTP standards for engines meeting a NOx FEL of 1.5 g/bhp-hr or less and 1.25 × FTP standards. for engines with a NOx FEL higher than 1.5 g/bhp-hr.
Effective for MY 2007, the regulation maintained the earlier crankcase emission control exception for turbocharged heavy-duty diesel fueled engines but required that if they are emitted to 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.
Emission standards for Otto-cycle heavy-duty engines and vehicles came into effect for MYs 2008 and later. The standards are as follows:
Fuel: The diesel fuel regulation limits the sulfur content in on-highway diesel fuel to 15 ppm (wt), down from the previous 500 ppm. Phased in over the year, all providers were required to start producing the 15 ppm sulfur fuel beginning in 2006. Refiners had temporary compliance option that allowed them to continue producing 500 ppm fuel in 20% of the volume of diesel fuel they produce through 2009. In addition, refiners could participate in an averaging, banking and trading program with other refiners in their geographic area. For more information on fuel requirements, see the U.S. fuels page.
Technology choice: Engine manufacturers chose to utilize catalyzed particle filters in order to comply with the PM standard. To meet the interim NOx standard, manufacturers employed cooled exhaust gas recirculation to achieve compliance instead of using NOx absorbers as expected by EPA. Most manufacturers use selective catalytic reduction (SCR) technology to meet NOx emission limits, which were fully phased in 2010.
Chassis certification option: Medium-duty vehicles can also certify as complete vehicles, with testing completed on a chassis dynamometer rather testing the engine separately. The standards are intended to be roughly comparable to the engine-based standards in the same size range. The following standards apply for vehicles in the 8,500–10,000 lb. gross vehicle weight range that are not classified as medium-duty passenger vehicles:
For standards in the 10,000–14,000 lb. gross vehicle weight range, the standards are as follows:
PROPOSED CONTROL OF AIR POLLUTION FROM NEW MOTOR VEHICLES: HEAVY-DUTY ENGINE AND VEHICLE STANDARDS (MY 2027 AND LATER)
On March 28th 2022, the EPA published a proposed regulation that would significantly reduce the emission of greenhouse gases and other criteria air pollutants from heavy-duty vehicles and engines starting in model year (MY) 2027. The proposal introduces two regulatory options and an alternative policy that incorporate changes to four key aspects of heavy-duty vehicle regulation.
Updating test procedures: The proposed regulation would update testing procedures to accommodate a low-load cycle and implement standards that cover stop-and-go and idle operation. The policy will also update procedures and standards for vehicles on the road and lower standards for existing lab duty cycles that cover FTP and SET operation.
Extending regulatory useful life: The EPA acknowledges that most engines remain in the field (operational life) for twice their current useful life across all heavy-duty classes. A longer regulatory useful life ensures that engines are built to comply with standards for a greater duration of their operational life. The two proposed options would increase regulatory useful life to 66-88% of operational life, up from 48% under current policy.
Updating numeric emissions: The proposed standards reflect developments in selective catalytic reduction SCR technology and the use of existing cylinder deactivation hardware. These technologies are well-understood, already in production, and have the potential to reduce NOx emissions by up to 90% or more for diesel engines over a wide range of engine operations. Reductions from the two options are shown in the table below:
|*Useful life and warranty period vary between options.|
|Reduction from current FTP and SET driving standard (%)||Key differences between Options*|
|Proposed Option 1 (MY 2031)||90% for Light, Medium and Spark Ignition HDE90% for Heavy HDE at intermediate useful life||
Provides phase-in to lower levels of standards, whereas proposed Option 2 begins fully in 2027.
Includes more stringent numeric standards for all duty cycles than proposed Option 2
|Proposed Option 2||75% for all HDE||Standards could be met with less improvement to the durability of the technology than required for the proposed Option 1|
Extending emissions warranty: The EPA proposed lengthening HDV emissions warranty to 49% of operational life in MY 2027 and 66% in MY 2031 under Option 1 and a flat 38% under Option 2. Both scenarios are an increase from 11% of operational life under current regulation.
Emission standards apply to new diesel engines used in heavy-duty highway vehicles. The current federal definition of a compression-ignition (diesel) engine 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 the diesel-cycle operation (this allows 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 > 8,500 lbs. in the federal jurisdiction and above 14,000 lbs. in California (MY 1995 and later). Diesel engines used in heavy-duty vehicles are further divided into service classes by GVWR:
- 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 (phased-in beginning 2004) 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 legislation. 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.
Current federal 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 < 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, first introduced in 1998, include the following:
- 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 typically higher than the FTP limits
These tests were introduced for most signatories of the 1998 Consent Decrees between the EPA and engine manufacturers for the period 1998–2004. Federal regulations require supplemental testing from all engine manufacturers effective 2007. In California, tests are required for all engines effective MY 2005.