Off-Road Compression-Ignition (Diesel) Engines and Equipment

This page last reviewed October 20, 2008

This category consists of compression-ignition engines (a.k.a. diesel engines) that are found in a wide variety of off-road applications such as farming, construction, and industrial. Some familiar examples include tractors, excavators, dozers, scrapers, portable generators, transport refrigeration units (TRUs), irrigation pumps, welders, compressors, scrubbers, and sweepers. This category, however, does not include locomotives, commercial marine vessels, marine engines over 37 kilowatts (kW), or recreational vehicles.

The very first emission standards for new off-road diesel engines applied to engines under 19 kW and were adopted as part of the California requirements for 1995 and later small off-road engines. Subsequently, in 1992, the Board approved standards exclusively for
off-road diesel engines above 130 kW. Implementation of these standards, referred to as Tier 1 standards, began in 1996, and primarily targeted oxides of nitrogen (NOx) emission reductions. Since California does not have authority to set emission standards for new farm and construction engines under 130 kW according to the federal Clean Air Act, it must rely on the United States Environmental Protection Agency (U.S. EPA) to regulate those engines. In 2000, preempt engines and equipment represented about 71 percent of the total population of off-road diesel engines and equipment in California.

Second and third phases of more stringent emission standards were adopted in 2000, when ARB harmonized its off-road diesel program with U.S. EPA. These standards selectively apply to the full range of diesel off-road engine power categories. Tier 2 standards were originally intended to be equivalent in stringency to the 1991 on-road heavy-duty diesel engine standards, and are based on the emission control technologies used by those engines. They are scheduled to be completely phased-in by 2006. Tier 3 standards further reduce emissions of hydrocarbon (HC) and NOx and are scheduled to be completely phased-in by 2008. Both Tier 2 and Tier 3 standards include durability requirements to ensure compliance with the standards throughout the useful life of the engine.


On December 9, 2004, the Board adopted a fourth phase of emission standards (Tier 4) that are nearly identical to those finalized by the U.S. EPA on May 11, 2004, in its Clean Air Nonroad Diesel Rule. As such, engine manufacturers are now required to meet aftertreatment-based exhaust standards for particulate matter (PM) and NOx starting in 2011 that are over 90 percent lower than current levels, putting off-road engines on a virtual emissions par with on-road heavy-duty diesel engines.

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