Use this comprehensive glossary to define terms commonly used in the alternative fuels and advanced vehicles industry. If you have questions about specific technologies or fuels, contact the Technical Response Service at 1-800-254-6735.

Click on the appropriate letter.

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ASTM International

A nonprofit organization that develops and delivers international standards. ASTM standards, test methods, specifications, and procedures are recognized as definitive guidelines for fuel quality.

Advanced Technology Vehicles

A vehicle that combines new engine, power, or drivetrain systems to significantly improve fuel economy. This includes hybrid power systems and fuel cells, as well as some specialized electric vehicles.

Alternative Fuel Vehicles

A dedicated, flexible fuel, or dual-fuel vehicle designed to operate on at least one alternative fuel.

Alternative Fuels

The Energy Policy Act of 1992 defines an alternative fuel as:

  • Biodiesel (B100)
  • Natural gas and liquid fuels domestically produced from natural gas
  • Propane (liquefied petroleum gas)
  • Electricity
  • Hydrogen
  • Blends of 85% or more of methanol, denatured ethanol, and other alcohols with gasoline or other fuels
  • Methanol, denatured ethanol, and other alcohols
  • Coal-derived, domestically produced liquid fuels
  • Fuels (other than alcohol) derived from biological materials
  • P-Series fuels

Anaerobic Digestion

Anaerobic digestion is series of processes in which microorganisms break down biodegradable material in the absence of oxygen.

Autoignition Temperature

The lowest temperature at which a flammable gas vapor will ignite spontaneously, without a source of ignition, after several minutes of exposure to sources of heat.

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Battery pack

Stores energy generated from regenerative braking and provides supplemental power to the electric traction motor.


Chemical reactions in living organisms.

Biochemical Conversion

Using enzymes and catalysts to change biological substances chemically to produce energy products. An example is digestion of organic wastes or sewage by microorganisms to produce methane.


Plant matter such as trees, grasses, agricultural residue, algae, and other biological material.

Blender Pump

A fuel dispenser that draws fuel from two separate storage tanks and can dispense preprogrammed blends of those two fuels.

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Cetane Number

Cetane number relates to the fuels susceptibility to self-ignite. The higher the cetane number, the greater the fuel's tendency to self-ignite.

Clean Air Act Amendments of 1990

Amendments to the Clean Air Act of 1970 creating two gasoline standards to reduce vehicle emissions in highly polluted cities by requiring gasoline to contain cleaner-burning additives, such as ethanol.

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A high-level gasoline-ethanol blend containing 51% to 83% ethanol, depending on geography and season.

Electric Grid

An interconnected system that maintains an instantaneous balance between supply and demand (generation and load) while moving electricity from generation source to customer.


Electric current used as a power source. Electricity can be produced from a variety of feedstocks, including oil, coal, nuclear, hydro, natural gas, wind, and solar.


Electrolysis is a method by which an electric current splits water into hydrogen and oxygen. If the electricity used is from renewable sources, such as solar or wind, the resulting hydrogen will be considered renewable as well.

Emission Control Technologies

Equipment used in diesel-powered vehicles to reduce exhaust emissions, such as particulate matter and nitrogen oxides. New engines and vehicles can be designed with these technologies, and used engines can be retrofitted to use this equipment.

Energy Policy Act of 1992

Passed by Congress to enhance U.S. energy security by requiring federal, state, and alternative fuel provider fleets to implement petroleum-reduction measures. Learn more about EPAct Transportation Regulatory Activities.

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Any material converted to another form of fuel or energy product. An example is using cornstarch to produce ethanol.

Flash Point

The minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid.

Flow Improvers

A chemical used in fuel to reduce friction and increase performance.

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Gasoline Gallon Equivalent

The amount of fuel it takes to equal the energy content of one liquid gallon of gasoline where one gasoline gallon equivalent (GGE) equals 120,167 British thermal units (BTUs).

Gross Vehicle Weight Rating

The maximum weight of a vehicle, including curb weight (the weight of the vehicle on its own) and payload (the weight of cargo).

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Higher Heating Value

The heating value is the amount of heat released during the combustion of a specific substance, usually a fuel or food. The higher heating value is determined by bringing all the products of combustion back to the original pre-combustion temperature, condensing any water vapor generated. This value assumes the entire water component is liquid in the products of combustion and that heat can be used.

Higher Lubricity

The ability to reduce friction—usually in fuel pumps and fuel injectors.

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Incremental Price

The additional price of an alternative fuel vehicle over a similar conventional vehicle.

Internal Combustion Engine

A conventional vehicle motor that burns fossil fuel in a chamber in the presence of air.


Two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties.

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Kilowatt Hours

A measurement for electricity use.

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Legacy Chargers

Legacy electric charging systems such as inductive paddles. Learn more about types of chargers.

Load Aggregators

A large group or block (aggregate) of consumers joined together to leverage their combined purchasing power when negotiating rates for energy services.

Lower Heating Value

The heating value is the amount of heat released during the combustion of a specific substance, usually a fuel or food. The lower heating value is determined by subtracting the heat of vaporization of water from the higher heating value for a particular substance, treating any water as a vapor.

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Natural gas sensors

These monitor the pressure of the fuel supply and relay that information to the Electronic Control Module.

Negative Energy Balance

When producing a fuel takes more energy than the amount of energy the fuel provides.

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Onboard charger

Takes the incoming AC electricity supplied via the Charge Port and converts it to DC power for charging the Traction Battery. It regulates battery characteristics such as voltage, current, temperature, and state of charge while charging the pack.

Overfill protection device

This device shuts off the flow of fuel into a fuel tank after 80% capacity has been reached.


A cleaner-burning additive in a fuel—usually containing hydrogen, carbon, and oxygen. Examples are ethers and alchohols, such as ethanol and methanol.

Oxygenated Fuels

Fuels blended with an additive—usually ether or ethanol—to increase oxygen content, allowing more-thorough combustion for reduced carbon monoxide emissions.

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Passenger-Miles per Gallon

Pmpg is the vehicle miles per gallon multiplied by the number of passengers traveling in the vehicle.

Petroleum Administration for Defense Districts

The Petroleum Administration for Defense Districts (PADD) are groupings of U.S. states and the District of Columbia. These districts are defined by the U.S. Energy Information Administration and used to report fuel prices.

Map of Petroleum Administration for Defense Districts. PADD 1 (East Coast) includes three parts. PADD 1A (New England) includes Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. PADD 1B (Central Atlantic) includes Delaware, District of Columbia, Maryland, New Jersey, New York, and Pennsylvania. PADD 1C (Lower Atlantic) includes Florida, Georgia, North Carolina, South Carolina, Virginia, and West Virginia. PADD 2 (Midwest) includes Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, Tennessee, and Wisconsin. PADD 3 (Gulf Coast) includes Alabama, Arkansas, Louisiana, Mississippi, New Mexico, and Texas. PADD 4 (Rocky Mountain) includes Colorado, Idaho, Montana, Utah, and Wyoming. PADD 5 (West Coast) includes Alaska, Arizona, California, Hawaii, Nevada, Oregon, and Washington.

Positive Energy Balance

When producing a fuel takes less energy than the amount of energy the fuel provides.

Power electronics controller

This unit manages the flow of electrical energy delivered by the Traction Battery, controlling the speed of the Electric Traction Motor and the torque it produces.

Pressure gauge

Measures and displays the fuel pressure within the tank.

Pressure relief valve

This device incorporates a valve to limit the pressure within the fuel tank. At a preset pressure level, the valve opens and fuel is vented from the tank.

Pump Octane Number

This number represents the ability of a fuel to resist knocking when ignited in the cylinder of an internal-combustion engine. The number here is the average of the research octane number and motor octane number.

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Regenerative Braking

A feature of hybrid and plug-in electric vehicles that captures energy normally lost during braking by using the electric motor as a generator and storing the captured energy in the battery.

Renewable Fuels Standard

A regulation created under the Energy Policy Act of 2005 and implemented by the U.S. Environmental Protection Agency to ensure transportation fuel sold in the United States contains a minimum volume of renewable fuel.

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Selective Catalytic Reduction

Selective catalytic reduction (SCR) is a means of converting nitrogen oxides (NOx) into nitrogen (N2) and water (H20). This is done using a reductant, such as urea or ammonia, and a catalyst.

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Tailpipe Emissions

Emissions produced through fuel combustion during a vehicle's operation.


Animal fat that can be used to produce biodiesel.

Tank valve

This primary, manual valve stops fuel from entering or leaving the tank.

Technical Response Service

For assistance with technical questions about alternative fuels and advanced vehicles, email the Technical Response Service at or call 1-800-254-6735.

Thermal system (cooling)

This system maintains a proper operating temperature range of the engine, electric motor, power electronics, or other components.


Heat and pressure-based chemical reactions that produce energy. Through gasification (heating biomass by partial oxidation to produce synthesis gas) and pyrolysis (heating biomass in the absence of oxygen to produce liquid oil), biomass feedstocks can be converted to alcohol and hydrocarbon fuels, chemicals, and power.

Traction battery pack

Stores electricity for use by the electric traction motor.


In this process, the feedstock chemically reacts with an alcohol (usually methanol) in the presence of a catalyst, like lye. The products are glycerin and the biodiesel fuel or FAME (fatty acid methyl esters).


Transfers mechanical power from the engine and/or Electric Traction Motor to drive the wheels.

Transportation Sector

An energy-consuming sector that consists of all vehicles whose primary purpose is transporting people and/or goods from one physical location to another. Included are automobiles; trucks; buses; recreational vehicles; motorcycles; trains, subways, and other rail vehicles; aircraft; ships, barges, and other waterborne vehicles; and pipelines. Vehicles whose primary purpose is not transportation (e.g., construction cranes and bulldozers, farming vehicles, and warehouse tractors and forklifts) are not included.

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Vapor Pressure

Vapor pressure is the pressure of the vapor resulting from evaporation of a fuel above a sample of the liquid in a closed container. It is used to measure volatility, an important property of transportation fuels.

Vehicle Categories

A system of classifying vehicles (e.g., passenger cars, commercial vehicles, trailers, off-road vehicles, and special-purpose vehicles).

Vehicle Weight Class

The size of vehicles. Includes light-duty, medium-duty, and heavy-duty vehicles based on their gross vehicle weight rating (the weight of a vehicle on its own plus the weight of cargo).

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Yellow Grease

Second-hand cooking oil that can be used to produce biodiesel.

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Analysis of energy use and emissions from the primary energy source through vehicle operation.