California Air Resources Board

CARB staff is starting work on potential amendments to the off-road diesel engine standards, in what we are calling the Tier 5 rulemaking. The Tier 5 rulemaking aims to reduce oxides of nitrogen (NO_x) and particulate matter (PM) emissions from new, off‑road compression-ignition (CI) engines compared to what is allowed by today’s Tier 4 final emission standards. It will likely include more stringent exhaust standards for all power categories, including those that do not currently utilize exhaust aftertreatment such as diesel particulate filters (DPFs) and selective catalytic reduction. As of model year 2020, more than half of all new off-road CI engine families continue to be certified in California to the Tier 4 final emission standards without DPFs.

Staff is considering possible elements to achieve NO_x standards up to 90 percent more stringent, and PM standards up to 75 percent more stringent than today’s Tier 4 standards. First-time carbon dioxide (CO₂) emission standards for off-road engines may also be proposed. Other possible elements include enhancing in-use compliance, proposing more representative useful life periods, and developing a low load test cycle. Staff is also investigating first-time, off-road, on-board diagnostics requirements and encouraging the development of zero-emission off-road equipment. Staff plans to bring a proposal to the Board in 2025, with implementation of the Tier 5 standards expected to begin in 2028.

Need for Federal Action

Under the Clean Air Act, the United States Environmental Protection Agency (U.S. EPA) preempts California from regulating agricultural and construction equipment less than 175 horsepower. Therefore, California must depend on U.S. EPA to strengthen emission standards for such preempted off-road equipment, which is responsible for nearly 30 percent of NO_x emissions from off-road CI engines (excluding locomotive, ocean-going vessels, and commercial harbor craft).

Research Activities

CARB is currently funding several research projects to assess the feasibility of lower NO_x , PM, and CO₂ emission standards, and developing more representative useful-life periods. Southwest Research Institute (SwRI) is a major contractor assisting CARB in their research efforts.