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Fuel Regulations

USA: Fuels


Combustion engine fuel properties in the United States are affected by numerous factors including legal and commercial considerations. Legal requirements can be imposed at the federal and state levels while commercial considerations include requirements by vehicle manufacturers, fuel supply and distribution entities as well as fuel producers. Fuels must minimize harm to people, the environment, equipment, and the fuel supply chain. Vehicles and other equipment that burn fuel must operate reliably over a wide range of environmental conditions, fuels must be fungible and disruptions in their supply and distribution must be minimized.

To ensure these requirements are met, acceptable properties of many common fuels in the US are collected into fuel standards that are maintained by organizations such as ASTM. While ASTM standards are not mandatory at a federal level, many states require that fuels meet these standards.

Diesel Fuels

Historically, the quality of diesel fuels in the United States was specified by the ASTM D975 standard.

With the growing importance of alternative diesel fuels, standards have also been developed for biodiesel fuels and their blends. ASTM standards related to biodiesel include:

  • D6751, which covers six grades of biodiesel (B100) for use as a blend component with middle distillate fuels.
  • D7467, which covers three fuel blend grades of 6 volume percent to 20 volume percent (%) biodiesel.
  • D975, which allows up to 5% biodiesel.

Regulatory requirements for diesel fuels exist at the federal and state levels. The federal Clean Air Act (CAA) allows EPA to specify fuel properties that affect emissions. Diesel fuel properties that fall under this authority include the concentrations of sulfur and aromatics [6143]. Federal requirements also exist for blending renewables into diesel fuel through the Renewable Fuel Standard (RFS).

Additional requirements can be imposed at the state level. For example, California regulates additional fuel properties such as API gravity, lubricity, polycyclic aromatics, nitrogen and cetane number [6144]. California’s Low Carbon Fuel Standard (LCFS) also places requirements on the renewable content of diesel fuel that differ from those imposed at the federal level.

Not all regulatory requirements are related to emissions. For example, fire codes and insurance requirements set minimum requirements for flash point temperature.

Gasoline Fuels

Gasoline standards in the US are detailed in ASTM D4814. This standard applies to low level blends of gasoline and oxygenates such as alcohols and ethers. Requirements for denatured ethanol blended into gasoline are contained in ASTM D4806. For fuels in which ethanol is the primary component, from 51% to 83% ethanol, ASTM D5798 applies.

Regulatory requirements for gasoline are considerably more complex than for diesel fuel. At the federal level, these include limits on sulfur, Reid vapor pressure (RVP), lead, phosphorous and benzene. Additionally, some areas of the US require the use of reformulated gasoline (RFG) that has additional requirements to limit volatile organic compounds (VOCs) during the high ozone season and toxic air pollutants all year round [6143]. State level regulatory requirements also exist such as those in California that can have stricter requirements than those at the federal level. Regulatory requirements for renewable content also apply to gasoline such as the federal RFS and California’s LCFS.

For many years, most US gasoline contained up to 10% ethanol. This "limit” came about via the RVP requirements for which the Clean Air Act allowed a 1.0 psi waiver for federal gasoline containing 10% ethanol during the high ozone season, i.e., for E10 gasoline, the RVP could be 1.0 psi higher in the summer than for gasoline containing less than 10% ethanol. For numerous reasons including the increased renewable volume requirements from the RFS, there have been efforts to increase the blend level of ethanol in gasoline to E15 year-round. While E15 can be used in the low ozone season without concerns over its impact on RVP, the Clean Air Act wording makes its use during the high ozone season challenging. In 2019 the EPA finalized rules that allowed the 1.0 psi waiver to apply to federal gasoline blends from E10 to E15. However, this was overruled in 2021 and the waiver applies again only to E10. Measures to circumvent this limit include “ethanol-friendly” states asking the EPA to remove the 1.0 psi waiver for E10 in their states to create parity for E10 and E15 [6139]. The effect on vapor pressure of ethanol addition to gasoline is non-linear and both 10% and 15% ethanol raise the vapor pressure by a similar amount.

Less commonly known as an on-road fuel, E98 is commonly used as a racing fuel. There have been efforts to promote its use in heavy-duty trucks. Typically, E98 is denatured ethanol conforming to ASTM D4806 containing from 95% to 98% commercial ethanol and 2% to 5% denaturant. Denaturants are typically hydrocarbons such as gasoline or gasoline blending components.

The Alcohol and Tobacco Tax and Trade Bureau (TTB) of the US Treasury Department requires that fuel ethanol be made unfit for human consumption by adding a denaturant. They specify numerous recipes for different denaturants, only a few of which are compatible with using ethanol as an engine fuel [6140].

California also has specifications for several alcohol blends including E85, E100, M85 and M100 [6141]. The latter three have been recommended for withdrawal [6142]. One difference between CARB’s E85 specification and ASTM D5798 is the minimum ethanol content. CARB sets the minimum at 79% while in ASTM D5798, it is 51%.

Natural Gas

Natural gas quality as a vehicle fuel in the US is defined in several standards, recommended practices and regulations. These include ASTM D8080, SAE J1616, and a CARB specification [4105].

Regarding the CARB specification, in 2019 it was reported that the California Department of Food and Agriculture (CDFA) is expected to adopt ASTM D8080 to promote the use of renewable natural gas, However, ASTM D8080 is less stringent than the CARB specification and CDFA cannot adopt a specification that is less stringent than the CARB specification. To address his issue, CARB may have to first rescind its CNG specification [6142].


Liquid petroleum gas (LPG) for engine applications is specified in ASTM D1835. CARB also has a specification for which changes were proposed in 2019 to reflect the composition of renewable propane [6141][6142].