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Emission Standards

United States

Regulatory Authorities

Federal Standards. US federal emission standards for engines and vehicles, including emission standards for greenhouse gas (GHG) emissions, are established by the US Environmental Protection Agency (EPA). The EPA authority to regulate engine emissions—and the air quality in general—is based on the Clean Air Act (CAA), most recently amended in 1990.

Fuel economy standards are developed by the National Highway Traffic Safety Administration (NHTSA), an agency within the US Department of Transportation (DOT).

The development of engine emission standards occurs according to the procedures of the US rulemaking process. New regulations are first published as proposed rules. Following a period of public discussion, the new rule is finalized and signed into law. New regulatory proposals and regulations are published in the Federal Register. Consolidated regulations become a part of the Code of Federal Regulations (CFR).

California Standards. The State of California has the right to adopt its own emission regulations, which are often more stringent than the federal rules. Engine and vehicle emission regulations are adopted by the California Air Resources Board (ARB), a regulatory body within the California EPA.

California is the only state vested with the authority to develop its own emission regulations. Other states have a choice to either implement the federal emission standards, or else to adopt California requirements (CAA section 177).

Regulated Engines and Vehicles

Emission Standards for New Engines and Vehicles

The following categories of new engines and/or vehicles are subject to emission standards in the USA:

GHG & Fuel Economy

Fuel economy in new light-duty vehicles has been regulated since the 1970’s by CAFE standards administered by the National Highway Traffic Safety Administration (NHTSA), an agency within the Department of Transportation (DOT).

The first greenhouse gas regulations for motor vehicles were adopted in 2002 in California. At the federal level, GHG emission standards and harmonized CAFE legislation for light-duty vehicles were adopted in joint regulatory actions by the EPA and the NHSTA in 2010 and 2012. GHG/fuel economy regulation for heavy-duty trucks was adopted in 2011.

On-Board Diagnostics (OBD)

On-Board Diagnostic requirements—California and federal—apply to light-duty vehicles, as well as to increasing number of categories of heavy-duty engines. OBD regulations ensure compliance with emission standards by setting requirements to monitor selected emission system components (e.g., catalytic converters) or in-use emission levels, and to alert the driver/operator—such as by a dashboard-mounted malfunction indicator light—when a problem is detected.

In-Use Engine Regulations

In addition to new engine emission regulations, there is a growing number of programs—mandatory or incentive-based—to reduce emissions from in-use diesel engines. These initiatives are being implemented by all levels of government: federal, state, and local. We provide an overview of the following diesel programs:

Vehicle Weight Classes

Some of the commonly used US vehicle weight classifications are summarized in the following tables.

Table 1
Vehicle weight classifications by the US FHA and US Census Bureau
Gross vehicle weight rating (lbs)Federal Highway AdministrationUS Census Bureau
Vehicle ClassGVWR CategoryVIUS Classes
≤ 6,000Class 1: ≤ 6,000 lbsLight Duty ≤ 10,000 lbsLight Duty ≤ 10,000 lbs
10,000Class 2: 6,001-10,000 lbs
14,000Class 3: 10,001-14,000 lbsMedium Duty 10,001-26,000 lbsMedium Duty 10,001-19,500 lbs
16,000Class 4: 14,001-16,000 lbs
19,500Class 5: 16,001-19,500 lbs
26,000Class 6: 19,501-26,000 lbsLight Heavy Duty 19,501-26,000 lbs
33,000Class 7: 26,001-33,000 lbsHeavy Duty ≥ 26,001 lbsHeavy Duty ≥ 26,001 lbs
> 33,000Class 8: > 33,000 lbs
Table 2
Vehicle weight classifications by the US EPA
Gross vehicle weight rating (lbs)EPA Emissions Classifications
Heavy Duty Vehicles and EnginesLight Duty Vehicles
HD TrucksHD EnginesGeneral trucksPassenger Vehicles
≤ 6,000Light Duty Trucks 1 & 2: ≤ 6,000 lbsLight Light Duty Trucks: ≤ 6,000 lbsLight Duty Trucks ≤ 8,500 lbsLight Duty Vehicles ≤ 8,500 lbs
8,500Light Duty Trucks 3 & 4: 6,001-8,500 lbsHeavy Light Duty Trucks: 6,001-8,500 lbs
10,000Heavy Duty Vehicle 2b: 8,501-10,000 lbsLight Heavy Duty Engines: 8,501-19,500 lbsHeavy Duty Vehicle
Heavy Duty Engine
≥ 8,500 lbs
Medium Duty Passenger Vehicles 8,501-10,000 lbs
14,000Heavy Duty Vehicle 3: 10,001-14,000 lbs
16,000Heavy Duty Vehicle 4: 14,001-16,000 lbs
19,500Heavy Duty Vehicle 5: 16,001-19,500 lbs
26,000Heavy Duty Vehicle 6: 19,501-26,000 lbsMedium Heavy Duty Engines: 19,501-33,000 lbs
33,000Heavy Duty Vehicle 7: 26,001-33,000 lbs
60,000Heavy Duty Vehicle 8a: 33,001-60,000 lbsHeavy Heavy Duty Engines
Urban Bus:
≥ 33,001 lbs
> 60,000Heavy Duty Vehicle 8b: ≥ 60,001 lbs

Auxiliary Emission Control Devices and Defeat Devices

Under some operating conditions, components of the emission control system can be shut-off or deactivated. This is usually done for reasons including: ensuring engine start-up, protection of the vehicle against damage or accident and preventing the unwanted shut-down of emergency vehicles or equipment. Deactivating components of the emission control system is carried out using what is called an Auxiliary Emission Control Device (AECD). EPA regulations define an AECD as:

any element of design which senses temperature, vehicle speed, engine RPM, transmission gear, manifold vacuum, or any other parameter for the purpose of activating, modulating, delaying, or deactivating the operation of any part of the emission control system.

The EPA definition for emission control system covers all components that are used to control emissions including: aftertreatment devices, engine modifications, sensors, actuators, EGR system and so on.

A defeat device is an AECD that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use. Defeat devices are prohibited. In order for manufacturers to certify their vehicles and engines, during the application for certification, they must submit a list of AECDs, justify their use, explain how they work and demonstrate that the AECDs are not defeat devices.

While there are some differences, the definitions of AECD, emission control system and defeat device as well their approval is relatively consistent for light-, medium- and heavy-duty vehicles and engines as well as nonroad engines.