Glossary
A | B | C | D | E | F | G | H | I | J | K | L | M
N | O | P | Q | R | S | T | U | V | W | X | Y | Z
B
Biodiesel: A renewable diesel fuel substitute that can be made by chemically combining a natural oil or fat with an alcohol. Many vegetable oils, animal fats, and recycled cooking greases can be transformed into biodiesel fuel. Biodiesel is a nontoxic, biodegradable fuel that has been proven to provide lower exhaust emissions even when used in a blend called B20, which consists of 20% biodiesel and 80% petroleum-based diesel fuel.
C
Corporate Average Fuel Economy (CAFE): A standard, enacted by Congress in 1975, mandates that all manufacturers marketing vehicles in the United States meet average fuel economy requirements for each class of vehicles they sell.
California Air Resources Board (CARB): A state agency charged with regulating air quality in California. Air quality regulations established by CARB are often stricter than those set by the federal government.
Compression ignition (CI): The type of ignition typically used in a diesel engine, which rapidly compresses the air within the cylinders to generate enough heat to ignite the fuel as it is injected. This is why diesel engines do not need spark plugs for ignition.
Catalyst: A substance that accelerates a chemical change without actually entering into the chemical reaction.
Compressed natural gas (CNG): Natural gas that has been pressurized. It is held in a container (tank) and expands when released for use as a fuel.
D
Diesel engine: Any internal combustion engine in which air is compressed to a sufficiently high temperature to ignite fuel injected into a cylinder. The high in-cylinder temperature is a product of a higher compression ratio than typical spark-ignited engines. A diesel engine differs from other internal combustion engines, including gasoline engines, in that it does not employ an ignition device, such as an electric spark. For this reason, it is often called a compression-ignition engine.
Diesel fuel: A petroleum product containing components of crude oil that have heavy hydrocarbons containing at least 12-16 carbon atoms per molecule. These heavier fractions are taken from crude oil after the more volatile portions used in gasoline are removed. Diesel fuel has a higher heating value than gasoline because more energy is released when its longer-chain molecules are broken.
Direct injection (DI): Also called "direct fuel injection"; a process in which a fuel is injected directly into an engine cylinder, as is typical in a diesel engine . Most other internal combustion engines use either throttle body injection, in which the air and fuel are mixed before the air enters the cylinder, or port fuel injection, in which the fuel is injected just in front of the cylinder intake valve.
E
E85: An alternative engine fuel mixture of 85% denatured ethanol and 15% gasoline, by volume.
Electricity: An electrical charge or current generated by friction, induction, or chemical change. Electricity produced by onboard batteries or a fuel cell can be used to power vehicles.
Emission standards: All new vehicles for sale in the United States must be certified to meet either federal emissions standards, set by the U.S. Environmental Protection Agency (EPA), or California standards, set by the California Air Resources Board (CARB). These exhaust emissions standards limit the amounts of key pollutants coming from a vehicle's tailpipe and any leaks in its fuel system. California standards are progressively more stringent by pollutant. Specific standards are listed below:
- Tier 1 (T1): In place since 1994 for cars and trucks, these standards are the least stringent and are being replaced by Tier 2 standards.
- Tier 2 (T2): New emission standards that will result in cars, minivans, sport utility vehicles (SUVs), and light-duty trucks that are 77-95% cleaner than those on the road today. Optional in model years 2001-2003, they are required to be phased in beginning in 2004. By 2009, all new cars and trucks must meet Tier 2 standards, which for the first time ever require minivans, SUVs, and light-duty trucks to meet the same emission standards as cars.
- TLEV: Transitional Low Emission Vehicle standards, which are more stringent than Tier 1 standards for hydrocarbons.
- LEV: Low Emission Vehicle standards, which are more stringent than TLEV standards for both hydrocarbons and oxides of nitrogen.
- ULEV: Ultra Low Emission Vehicle standards, which are more stringent than LEV standards for hydrocarbons.
- SULEV: Super Ultra Low Emission Vehicle standards, which are even more stringent than ULEV, for both hydrocarbons and oxides of nitrogen.
- ZEV: Zero Emission Vehicle standard, which permits no vehicle level emissions.
Ethanol: An alcohol made by fermenting sugars derived from starches in plants, such as corn or sugar cane (CH3CH2OH, ethyl or grain alcohol). When modified, it can be used as a fuel. When used as a vehicle fuel, ethanol is commonly used in a blend of 85% ethanol and 15% gasoline, called "E85." It is also sold at conventional gasoline stations as "gasohol" - a mixture of 10% ethanol and 90% gasoline.
Evaporative emissions: Hydrocarbon vapors that escape from a gasoline storage tank, a vehicle's fuel tank during refueling, or a vehicle's fuel system.
Exhaust aftertreatment: A method of controlling emissions, such as oxides of nitrogen, from internal combustion engines by applying air pollution control technologies to the engine exhaust (as opposed to treatments applied to the engine's intake air or during in-cylinder combustion stages).
Exhaust gas recirculation (EGR): An emissions control technique that reuses engine exhaust gases as part of the intake air supply to help reduce harmful emissions (especially oxides of nitrogen).
Exhaust emissions: Also called "tailpipe emissions"; any substance, gas, or particle that results from fuel combustion.
F
Fuel cycle: The activities required to produce, distribute, and consume petroleum- or biomass-based transportation fuels, including overall energy use, the emissions associated with petroleum refining or biomass farming, production, and combustion.
Fuel economy: Typically the ratio of vehicle miles traveled divided by the gallons of fuel consumed.
Fuel efficiency: The measure of how much stored energy in the fuel is actually converted into propelling a vehicle (Fuel energy • Fuel mass flow rate ÷ power at tire interface).
Fuel cell: An electrochemical "engine" without moving parts that combines hydrogen (supplied) and oxygen (from the air) and converts the resulting chemical energy into electricity without employing combustion. The only by-products are water and useful heat. Principal components of a fuel cell include catalytically activated electrodes for the fuel (anode) and the oxidant (cathode) and an electrolyte to conduct ions between the two electrodes.
Fuel flexible: The ability of a vehicle or an engine to operate on a wide range of fuels, blends (e.g., gasoline mixed with alcohol), or multiple fuels carried onboard the vehicle (e.g., gasoline and compressed natural gas).
Fuel processor: See "fuel reformer."
Fuel reformer: An onboard device containing catalysts that can convert or "reform" hydrocarbon fuels, such as gasoline or diesel fuel, into a hydrogen-rich stream to power a fuel cell. Fuel reformers are an attractive alternative to storing hydrogen onboard fuel-cell-powered vehicles because no infrastructure currently exists to deliver hydrogen fuel to consumers and bulky onboard hydrogen storage tanks would have a negative effect on the driving range of the vehicle.
G
Glow plug: An electrically heated element that helps raise in-cylinder temperatures when a compression ignition engine is cold and the compression process may not raise the air to a high enough temperature for auto-ignition of fuel.
Greenhouse gases: Gases, such as carbon dioxide, nitrous oxide, methane, and water vapor, that capture heat from the sun in the lower atmosphere of the Earth. As the amount of greenhouse gases in the atmosphere increases, more heat is captured, which can lead to global warming (the "greenhouse effect").
GREET Model: Greenhouse gases, Regulated Emissions and Energy use in Transportation model. A computer program intended as an analytical tool for researchers and industry practitioners to help estimate the fuel-cycle energy use and emissions associated with alternative transportation fuels and advanced transportation technologies. (More information...)
H
HEV: See "hybrid electric vehicle."
Homogeneous charge compression ignition (HCCI): An engine in which fuel is homogeneously mixed with air and exhaust before it enters the cylinder and is then auto-ignited simultaneously as the piston nears top dead center. The start of ignition relies on the heat generated as the piston compresses the air/fuel/exhaust mixture. Complete control of both intake and exhaust valve timing and intake air temperature is necessary to achieve the appropriate mixture and temperature in the cylinder. This engine promises to increase part-load efficiency dramatically by approaching diesel efficiency because it has no throttling device.
Hybrid electric vehicle (HEV): A vehicle that combines two or more sources of power, such as an internal combustion engine and a battery with an electric motor/generator, or a fuel cell and a battery with an electric motor/generator. In most cases, the engine or fuel cell is the primary power source for the vehicle. During high load conditions, such as for passing and steep inclines, the electric motor provides additional power. Some HEVs also recapture energy produced during braking (a process called "regenerative braking") to help recharge the battery.
An HEV generally does not need to be plugged in for charging — it recharges itself during operation. One exception to the "plug-less" HEV concept is the FutureTruck vehicle designed by the team from the University of California, Davis, which developed a plug-in HEV to reduce vehicle tailpipe emissions and maximize vehicle energy efficiency. This design philosophy permits a more powerful battery and electric-drive system, which increases the "electric only" driving distance of the vehicle and means some of the energy the vehicle uses is "off board" energy (electricity) that can come from renewable sources. The fact that the vehicle can be driven a longer distance in electric only (50 miles or so) means that it is more attractive in high-pollution areas.
There are many ways to combine the engine or fuel cell, battery, and motor/generator. The basic hybrid configurations are as follows:
- Series: The engine never directly powers the car. Instead, the engine drives the generator, and the generator can either charge the batteries or power an electric motor that drives the wheels.
- Parallel: The engine (powered by fuel from the fuel tank) connects to the transmission, as does the electric motor (powered by electricity from the batteries). Thus, either or both the engine and the electric motor can supply power to the wheels, switching back and forth as driving conditions vary.
- Power split: The power from the engine is simultaneously directed to the wheels by two paths — an electrical path that uses a generator and motor(s) and a mechanical path. This configuration offers benefits from both series and parallel hybrid configurations.
- Combination: The engine and electric motor(s) can be connected or disconnected to mimic either a series or parallel hybrid configuration.
Hydrocarbons: Chemical compounds, such as those found in vehicle emissions, that contain only the elements carbon and hydrogen.
Hydrogen: The simplest and lightest of the elements exists naturally as a colorless, odorless gas or in combination with other elements. In transportation applications, it is primarily used to power fuel cells.
I
Internal combustion engine (ICE): A heat engine in which the combustion that generates the heat takes place inside the engine. The crank-slider reciprocating piston engine typically found in automobiles is an internal combustion engine.
L
Lean combustion limit: The point at which combustion cannot occur because the fuel-air mixture contains too little fuel, resulting in unstable combustion and increased hydrocarbon emissions from misfires. Engine operation with a "lean" fuel-air mixture (using as little fuel as possible) reduces oxides of nitrogen and hydrocarbon emissions and improves fuel efficiency.
Liquefied natural gas (LNG): Natural gas that has been condensed to a liquid; often used as an alternative transportation fuel.
Liquefied petroleum gas (LPG): Also called "propane"; Any of several gases extracted from natural gas or petroleum and then liquefied under pressure; commonly used as an alternative transportation fuel.
M
M85: A mixture of 85% methanol and 15% unleaded gasoline, by volume.
Methanol: An odorless, clear liquid alcohol made from natural gas, coal, or biomass such as wood fiber (CH3OH, methyl or wood alcohol), which can be used as a fuel for engines.
Miles per gallon equivalent (mpge): The average number of miles a vehicle can travel on a gallon equivalent of an alternative fuel, or an amount of fuel equal in energy content to one gallon of gasoline; often used to compare the energy content per "gallon" of various fuels or to compare vehicle miles per "gallon" using the British Thermal Units (BTUs) in a gallon of gasoline as the base gallon. A gallon of gasoline has about 114,000 BTUs and a gallon of propane has about 84,000 BTUs, so the gasoline gallon equivalent (GGE) of propane is 1.35 gallons.
N
Natural gas: A mixture of gaseous hydrocarbons, primarily methane, occurring naturally in the earth, that is used principally as a fuel to produce heat or electricity, or to power a vehicle. For vehicle use, natural gas is compressed in special tanks. It can also be liquefied (see "liquefied natural gas") by cooling.
NOx: See "oxides of nitrogen."
O
Original Equipment Manufacturer (OEM): A manufacturer that provides the original design and materials to make product. Vehicle OEMs include the major U.S. auto manufacturers.
Oxygenates: Additives used to increase the oxygen content of gasoline so that the fuel will burn more completely in the engine, reducing tailpipe emissions of carbon monoxide. Ethanol fuel, a blend of gasoline with up to 10% by volume of anhydrous ethanol, is one type of oxygenated gasoline.
Oxygenated gasoline: Gasoline, other than reformulated gasoline, having an oxygen content of 2.7% or higher by weight. "Gasohol" is one example.
Oxides of nitrogen: A by-product of the combustion process, which has been shown to contribute to acid rain and, perhaps, global warming.
P
Payload: The weight of cargo a vehicle is rated to carry.
Particulate matter (PM): Solid or liquid particles found in the air that originate from a variety of mobile and stationary sources (diesel trucks, wood-burning stoves, power plants, etc.). Some particles are large or dark enough to be seen as soot or smoke, while others are so small they can be detected only with an electron microscope. Chemical and physical compositions vary widely. Particulate matter can be directly emitted, or it can be formed in the atmosphere when gaseous pollutants, such as sulfur dioxide and oxides of nitrogen, react to form fine particles.
R
Reformulated gasoline (RFG): A gasoline that has been modified to include oxygenates. In cities that do not meet the air-quality requirements for ozone set forth in the Federal Clean Air Act, only reformulated gasoline can be sold during months when ozone pollution levels are highest.
Regenerative braking: The process of recovering some of an electric or hybrid electric vehicle's kinetic energy during braking by using the wheels to drive a traction motor that functions as a generator to produce electric power that is stored for later use.
S
Spark-ignition engine: An internal combustion engine in which the fuel mixture is ignited electrically (e.g., by a spark plug).
T
Turbocharger: A turbine driven by engine exhaust gases that, in turn, powers a compressor in the intake tract of an engine that forces greater amounts of air into the cylinders of an engine to create more power. The effect is similar to increasing engine displacement.
