Technical standards

Model years 1987–2003

MY 1987–2003 California emission standards for heavy-duty diesel truck and bus engines are summarized in the following table. Applicable from 1994­ to 2006, sulfur content in the certification fuel was reduced to 500 ppm weight percent (wt).

Useful life and warranty periods: Compliance with emission standards must be demonstrated over the useful life of the engine. The regulation was adopted as follows, for both U.S. and California:

• LHDDE: 8 years or 110,000 miles (whichever occurs first)
• MHDDE: 8 years or 185,000 miles
• HHDDE: 8 years or 290,000 miles

Applicable federal standards: 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 NO_x standard (1998–present). The emission warranty period is 5 years or 100,000 miles (5 years or 100,000 miles and/or 3,000 hours in California), but no less than the basic mechanical warranty for the engine family.

Consent Decrees: In 1998, a court settlement was reached between the Environmental Protection Agency (EPA), Department of Justice, CARB, and engine manufacturers (Caterpillar, Cummins, Detroit Diesel, Volvo, Mack Trucks/Renault and Navistar) over the issue of high NO_x emissions from heavy-duty diesel engines during certain driving modes. Since the early 1990s, 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. 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 NO_x emissions
• Supplemental Emission Test (steady state) with a limit equal to the federal test procedure (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

Clean Fuel Fleet Program: The Voluntary Clean Fuel Fleet (CFF) program was a federal standard that applied to 1998–2003 MY engines, both CI and SI, > 8,500 lbs. GVWR. In addition to the CFF standard, vehicles must have met applicable conventional standards for other pollutants.

Model years 2004 and later

In 2004, EPA signed emission standards for MY 2007 and later heavy-duty highway engines (CARB adopted virtually identical 2007 heavy-duty engine standards in 2001). The rule included emission standards and diesel fuel regulations. For additional information on heavy-duty emissions, see the US Heavy-duty Emissions page.

Optional NO_x standards (THROUGH MY 2023)

In 2013, CARB adopted an optional low-NO_x standard for on-road heavy-duty engines. In this program, manufacturers can voluntarily opt to certify their engines at levels below the 0.2 grams per horsepower-hour federal standard, which has been in place since 2010. There are three levels of certification: 0.1 g/hp-hr, 0.05 g/hp-hr, and 0.02 g/hp-hr, which correspond to reductions below the current federal standard of 50%, 75%, and 90%, respectively. A significant number of engines are certified well below the 0.2 g/hp-hr limit, which gives the ARB confidence that the optional NO_x levels are achievable targets in the near-term.

With this optional NO_x standard, engines that are certified below the standard could utilize incentive efforts such as the Carl Moyer Program , which provides funding for vehicles and equipment that provide emission benefits beyond what is required by regulation.

MY 2024 and beyond

In December of 2021, CARB adopted a new Heavy-Duty Engine and Vehicle Omnibus Regulation to mandate NO_x standards. This regulation phases in more stringent emissions standards starting in MY 2024 and updates them again in MY 2027 to achieve a 90 percent reduction in NO_x emissions by 2031. These changes will lead to an estimated 30% overall fleet-wide reduction in heavy-duty NO_x. Emissions by 2050. Although CARB and the EPA have historically harmonized standards and new regulations concerning heavy-duty vehicles, CARB’s proposal was released before the EPA announced updates of the continuation of the Advanced Noticed of Proposed Rulemaking for the Cleaner Trucks Initiative. . The standards are more stringent than previous national-level regulations but are weaker than those imposed by the CARB rule.

This adopted regulation also enumerates new testing procedures starting with MY 2024, replacing the NTE (Not-to-Exceed) method with one of two options (manufacturer’s choice): a 3-bin moving average window (3B-MAW) method (described in section 86.1370.B.6), or a ramped modal cycle (RMC) which combines multiple test modes into a single test (described in section 86.1362 of U.S. EPA’s Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and Other Technical Amendments). The 3-bin moving average window test sorts windows into three bins based on the CO₂ output during the testing window normalized to the Family Certification Level (FCL) of the engine family, which is the CO₂ emission rate at the maximum power output. The normalized rate is sorted into idle, low-load, and medium to high load windows which are compared to the FTP limit. Every window will need to fall below 2 times the limit for the corresponding cycle from 2024 to 2029 and 1.5 times the limit starting in 2030.

For Otto engines, windows will not be binned by CO2 emissions, and all will be compared to the FTP emissions limit.

Emissions standards

Exhaust emission standards (g/bhp-hr) for light, medium, and heavy heavy-duty engines are the same from MY 2024 to MY 2026:

Light and medium heavy-duty vehicles have the same exhaust emissions standards (g/bhp-hr) for MY 2027 and subsequent models:

Heavy heavy-duty vehicles have an additional phase-in period from MY 2027 to 2030 before final standards are set starting in MY 2031. The MY 2031 NOx standards are slightly less stringent to account for the longer useful life that comes into effect for MY 2031:

Under the updated regulation, heavy-duty engines must comply with emission standards for the duration of their useful life. The useful life requirements will increase as model years advance, and as technological advancements make longer useful lifetimes feasible. The next phases are shown below for light, medium, and heavy heavy-duty diesel vehicles and heavy-duty Otto vehicles. The useful life periods are tracked in terms of miles, years, or hours—whichever metric comes first.

AVERAGING, BANKING, AND TRADING

CARB’s Low NO_x Omnibus regulation proposal has also specified California provisions for averaging, banking, and trading credits to increase flexibility of meeting the new standards. Manufacturers may participate in California’s ABT program starting with MY 2022 vehicles, but all manufacturers are required to enroll in the program beginning with MY 2024 vehicles.

A fraction of federal credits generated through the federal ABT program from MY 2010 through MY 2021 may be transferred to the CA program only for MY 2022.

CA ABT credits may only be used up to five model years after the year the credits were generated, except for credits generated by zero-emission vehicles. For zero-emission vehicles, credits are available from MY 2022 through MY 2026. Starting in MY 2027, banked zero-emission credits will no longer be available.