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Please note: This summary is provided to help you understand the regulations. Consult the references provided for links to the full text of the regulations.

Engine Emissions (Nonroad and Auxiliary Equipment)

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ForkliftThis page covers emissions regulations that apply to engines used in nonroad equipment including auxiliary equipment on vehicles. Examples include:

  • Forklift trucks
  • Portable generators
  • Construction equipment
  • Grounds maintenance equipment
  • Transport refrigeration units

For information on other types of engines, see references in the More Resources section below.

Who is covered by the regulations?

Engine manufacturers bear the primary responsibility for meeting the standards, but in many cases owners and operators are required to maintain the equipment so that it continues to meet the standards. In some states, including California, some types of engines must be registered with the state.

What is the purpose of the regulations?

Internal combustion engines generate:

  • Toxic emissions when fuel is not burned completely
  • Excess greenhouse gas emissions when engines are not operated efficiently

In a 1991 study, various small nonroad engines were found to generate toxic emissions at about one to four times the rate per hour of operation as compared to the engine powering a standard passenger vehicle, and emission rates from larger equipment such as tractors and outboard motors could exceed ten times the rate from a vehicle. In densely populated regions, emissions from nonroad sources can add up to a significant burden. In several large metropolitan areas, the total contribution to the atmospheric concentration of some key pollutants from small and nonroad engines was found to be greater than the contribution of those pollutants coming from all of the heavy-duty highway traffic operating in the area.


Nonroad engines are in widespread use in the transportation sector, as in many other sectors. This section provides an overview of regulations that apply generally, and includes some extra detail on forklift trucks, RICE engines (e.g., generators) and transport refrigeration units, engines that are commonly used in specifically transportation-related applications.

General Overview: Engines can be classified in many different ways: spark-ignition vs. diesel, mobile vs. stationary, large vs. small, etc. Different types of engines can present different emission profiles and different control problems. Accordingly, several different sets of standards have been developed for various engine types and applications. Although the burden of compliance falls most directly on manufacturers, owners and operators are required to take steps to assure that the engines remain in compliance with the standards during their service life. The operator's obligations are somewhat different depending on the type of engine, the applications involved, the emissions status of the facility (stationary engines installed at facilities which are large emitters, termed "major sources", operate under more stringent rules), the model year of the engine, and other factors.

In general, owners and operators of spark-ignition engines must keep maintenance records, and owners and operators of stationary diesel engines must use fuel compliant with fuel standards, and must install back-pressure monitoring devices on particulate filters. Some large engines require filing a notification with EPA. A comprehensive list of compliance requirements for stationary engines can be found at the Combustion Portal website.

Here is additional information on three applications of special interest for transportation facilities:

Forklift Trucks powered by large spark-ignition engines (over 25 horsepower) fall under a general federal regulation that also applies to such items as generators and air compressors. The rules apply most directly to manufacturers and importers, who are required to certify that engines sold in the United States comply with the relevant emissions standards. In California, anyone who operates four or more forklifts is considered a "fleet operator", and is required to maintain records documenting both that the engines comply with the rules, and that the fuel purchased for the engines is compatible with compliant operation. The records do not have to be reported, but can be requested at any time by the state for inspection. Operators of "medium fleets" (4 to 25 vehicles) and of "large fleets" (26 or more vehicles) are required to meet specific emissions targets as listed in the final regulation order (2011).

Stationary Reciprocating Internal Combustion Engines (RICE) are used by the transportation industry to drive process equipment such as compressors, pumps, and for standby generators.

There are two basic types of stationary reciprocating engines - spark ignition and compression ignition and these are separately regulated. Spark ignition engines use fuels such as gasoline and natural gas. Compression ignition engines mainly use diesel fuel. "Spark ignition" engines are further subdivided by power cycle, i.e., two vs. four stroke, and whether the engine is "rich burn" (burning with a higher amount of fuel as compared to air) or "lean burn" (less fuel compared to air) engine.

Manufacturers and owners of stationary reciprocating internal combustion engines are affected by federal air pollution regulations promulgated in stages starting in 2004. RICE rules were most recently amended in 2013. To find out more about how EPA regulates stationary engines and the applicability of the RICE Rule, see the EPA Region 1 RICE page.

Transport Refrigeration Units (TRUs) are defined by the California Air Resources Board (CARB) as: "refrigeration systems powered by diesel internal combustion engines designed to refrigerate or heat perishable products that are transported in various containers, including semi-trailers, truck vans, shipping containers, and rail cars." Starting in 2009, all in-use TRUs based in California must be registered with the state. Over the next ten years, a series of increasingly stringent emission standards are being phased in. In some cases, operators will need emission controls to remain in compliance with the standards. CARB provides a compliance guide with additional details.

Auxiliary Power Units (APUs), small engines used to provide power for in-cab temperature control and other amenities while the truck is stopped, operate with lower emissions than an idling truck engine.

More Resources

Detailed information on California regulations covering offroad diesel engines is provided by California Air Resources Board, as well as pages on forklift trucks and transport refrigeration units.

The DieselNet website provides detailed summaries of federal and California emissions standards (as well as Canadian, Mexican, and global standards) applying to diesel engines.

Emissions standards for stationary engines (both diesel and spark-ignition) are summarized on the Reciprocating Internal Combustion Engines (RICE) page provided by the Combustion Portal website. The page lists the compliance obligations of owners and operators separately from the obligations of manufacturers.

For regulations that apply to engines used to power road vehicles, see the TERC pages on:

   •  Engine Emissions (Truck, Idling) Summary Full Text
   •  Engine Emissions (On-board Diagnostics) Summary Full Text
   •  Engine Emissions (Truck, Tailpipe) Summary Full Text
   •  Engine Tampering Summary Full Text
   •  Greenhouse Gas Emissions Summary Full Text
   •  SmartWay Summary Exit TERC

More information on regulations applying to locomotives and marine engines can be found on the DieselNet website.

For regulations that apply to engine fuels, see the TERC pages on:

   •  Diesel Exhaust Fluid (DEF) Summary Full Text
   •  Diesel Fuel Requirements (Truck) Summary Full Text
   •  National Clean Diesel Program Summary Exit TERC