California Air Resources Board

CARB staff has proposed the heavy-duty omnibus regulation, which would establish oxides of nitrogen (NO_x) engine emission standards 90 percent lower than today's. This regulation would implement a key measure in the 2016 Mobile Source Strategy, which is 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. 

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. CARB staff and U.S. EPA are currently collaborating on their rulemaking sharing data and ideas and funding research efforts together.

Research Activities

CARB is currently funding several research projects, hereinafter referred to as the Stages 1, 2, and 3 Heavy-Duty Low NO_x programs, to assess the feasibility of lower NO_x emissions. The Stage 1 project was a $1.6 million research contract with Southwest Research Institute (SwRI) to assess the feasibility of lower NO_x emissions on a diesel and a natural gas heavy-duty engine through a combination of engine calibration, exhaust thermal management strategies, and advanced aftertreatment systems. Additional support was provided by the Manufacturers of Emissions Control Association (MECA) on advanced aftertreatment components, Volvo on the contribution and support of the diesel engine, and SwRI on the contribution and support of the Cummins 12 L stoichiometric natural gas engine. View Report

The Stage 2 program was a $1.05 million follow-on project to the Stage 1 program with SwRI as the research contractor and the National Renewable Research Laboratory, or NREL, as a subcontractor. Additional support for this program was provided by MECA, SwRI, E-Controls, and Volvo. Stage 2's objectives were to develop a new low load certification cycle to better represent diesel engine operation in urban environments. The diesel engine and advanced aftertreatment system developed in SwRI’s Stage 1 Low NO_x Demonstration program was optimized for improved emission control during low load duty cycles. Stage 2 also examine a different "load" metrics for estimating work during low load operations. Current metrics used to evaluate in-use emissions are inaccurate at low engine loads. Additional supplemental work, called the Stage 1b program, involved aging of fresh Stage 1 aftertreatment parts and was conducted concurrently with the Stage 2 program. Stage 1b was funded by MECA and the South Coast Air Quality Management District (SCAQMD). Stage 2 and Stage 1b were finalized in May 2020.

The Stage 3 program is a follow-on to the Stages 1 and 2 programs. Stage 3’s objective is to demonstrate low NO_x emissions on an alternative engine platform, a 2017 model year Cummins X15, on certification cycles and the low load cycles developed in the Stage 2 program. Stage 3b is a supplement to Stage 3. It involves adding engine hardware technologies designed to reduce GHG emissions and improve the performance capabilities of advanced aftertreatment systems when engines operate under sustained low loads. Engine hardware investigated include cylinder deactivation, SuperTurbo, charge air cooler bypass, exhaust gas recirculation (EGR) cooler bypass, turbocharger bypass, and exhaust manifold insulation. The total research contract expenditure for Stage 3 and 3b was $2.7 million and was supported by CARB, U.S. EPA, MECA, SCAQMD, Port of Los Angeles, the Clean High Efficiency Diesel Engine VII Consortium (CHEDE VII), Cummins, and Eaton. Stage 3 and 3b are expected to be finalized by August 2020.

In addition to the above low NO_x research programs, CARB had also contracted with NREL to conduct a cost analysis for compliance with CARB’s plan to propose lower NO_x exhaust emission standards of 0.02 g/bhp-hr NO_x on current certification test cycles and a proposed low load certification test cycle, as well as plans to increase the useful life and warranty period requirements. The study considered diesel- and Otto-cycle on-road heavy-duty engines used in vehicles greater than 14,000 pounds gross vehicle weight rating. The compliance cost analysis has now been completed and the final report can be accessed now.