California Air Resources Board

The Heavy-Duty Low NOx Omnibus (Omnibus) regulation implements two key measures in the 2016  State SIP Strategy. The two SIP measures which include the “Low-NOx Engine Standard” and the “Lower In-Use Emission Performance Level” are critical for attaining federal health-based air quality standards for ozone in 2031 in the South Coast and San Joaquin Valley air basins, and fine particulate matter (PM2.5) standards in the next decade.

In 2013, California established optional low-NO_x standards with the most aggressive being 0.02 g/bhp-hr, which is 90 percent below the current standard. The optional NO_x standards were developed to pave the way for mandatory standards by encouraging manufacturers to develop and certify low NO_x engines and incentivizing the purchase of certified low NO_x engines. As a result, since 2016, a number of natural gas- and propane-fueled Otto-cycle engine families have been certified to the most stringent optional low NO_x standards of 0.02 g/bhp-hr.

In its public hearing of August 27, 2020, CARB staff proposed, and the Board approved for adoption the Omnibus regulation, which established stringent NOx engine emission standards that are 90 percent below current levels on existing certification cycles and lower NOx standards on new certification cycles to control emissions over a broader range of vehicle operation, including idling, low load, and highway operation. In addition, the regulation revised the heavy-duty in-use testing program to make it more effective in ensuring compliance with the in-use emission standards over a broader range of vehicle operation and lengthened the useful life and emissions warranty period requirements to reflect the longevity of heavy-duty vehicles. The regulation was approved by the Office of Administrative Law (OAL) on December 22, 2021, with an effective date of December 22, 2021.

In August 2023, CARB staff proposed to amend the Omnibus regulation to provide additional compliance flexibility to engine manufacturers while ensuring the proposed amendments will not reduce the emissions benefits of the program. On December 28, 2023, CARB approved Executive Order R-23-006 adopting the proposed amendments. OAL approved the rulemaking and filed it with the Secretary of State on May 31, 2024 with an effective date of May 31, 2024.

Need for Federal Action

As vehicles purchased outside the state account for a significant portion of the heavy-duty vehicle miles travelled in the South Coast on any given day, federal NO_x engine emission standards are necessary in order to achieve emission reductions from vehicles operating in California that were purchased outside the state. Recognizing the contribution of heavy-duty trucks to the NO_x inventory nationwide, more than 20 organizations, including state and local air agencies from across the country, petitioned U.S. EPA in the summer of 2016 to develop more stringent NO_x emission standards for on-road heavy-duty engines. In response, on November 13, 2018, U.S. EPA announced the “Cleaner Trucks Initiative” to develop regulations to reduce NO_x emissions from on-road heavy-duty vehicles and engines. On January 6, 2020, U.S. EPA released an Advance Notice of Proposed Rule soliciting pre-proposal comments on the Cleaner Trucks Initiative. On August 5, 2021, U.S. EPA announced the “Clean Trucks Plan”, which consists of two major heavy-duty vehicle and engine regulations to reduce NOx emissions starting with the model year (MY) 2027 and greenhouse gas emissions starting with MY 2030. Accordingly, On March 28, 2022, U.S. EPA published a proposed rule that would set new, more stringent NOx standards to reduce pollution from heavy-duty vehicles and engines starting in MY 2027. The final rule was published in the Federal Register on January 24, 2023.

Research Activities

To support the rulemaking, CARB in partnership with federal and local air agencies, and the heavy-duty engine industry have funded over $ 5 million worth of research contracts with South Research Institute (SwRI) to evaluate various engine and emission control strategies to reduce NOx emissions from heavy-duty engines by 90 percent without or with minimal GHG impacts. The results from these contracts referred to as the Stage 1, Stage 2, and Stage 3 Heavy-Duty Low NOx Programs formed the bases for supporting the Omnibus regulation. In addition, CARB had also contracted with the National Renewable Energy Laboratory to conduct a cost analysis for compliance with CARB’s proposed lower NOx exhaust emission standards on current certification test cycles and a new low-load certification test cycle, as well as cost associated with increasing the useful life and emission warranty period requirements.