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Additives
Chemicals added to fuel in very small quantities to improve and maintain fuel quality and/or to lower emissions.
Aftercooling / Intercooling
Cooling the engine intake air after the turbocharger and prior to introduction into the cylinder. Aftercooling increases engine power and lowers NOx emissions.
Aftertreatment Devices
Devices which remove pollutants from exhaust gases after the gas leaves combustion chamber (e.g., catalytic converters or diesel particulate filters). The term “exhaust gas aftertreatment” is considered derogatory by some in the emission control industry, but there is no consensus on the use of such alternatives as “post-combustion treatment” or “exhaust emission control”.
Air Quality Management District (AQMD)
Administrative districts organized in California to control air pollution. Nationwide in the U.S., AQMDs are parallel to the areas designated for classification against the National Ambient Air Quality Standards (NAAQS). Generally, AQMDs and their national parallel encompass multiple jurisdictions and closely follow the definition of Consolidated Metropolitan Statistical Areas and Metropolitan Statistical Areas.
Air Toxics
Toxic air pollutants, as classified by pertinent regulations. Examples of substances classified as air toxics by the US Clean Air Act include acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and polycyclic organic matter (POM). California air toxics regulations also classify diesel exhaust particulates as a toxic air contaminant.
Alternative Fuel
Fuel other than petroleum diesel or gasoline.
American Society for Testing and Materials (ASTM)
A non-profit organization that establishes specifications and standard test methods for a broad range of materials and products. ASTM standards are recognized as definitive guidelines for quality of motor fuels.
Articulated Pistons
Two-piece pistons incorporating an entirely separate piston crown or dome with a separate skirt, and linking the two together with the piston pin. Many 1994 and later engines incorporate steel crown/aluminum skirt articulated pistons.
Bi-Fueled Vehicle
A vehicle with two separated fuel systems designed to run on either conventional fuel or an alternative fuel using only one fuel at a time.
Biodiesel
The mono alkyl esters of long chain fatty acids derived from renewable lipid feedstocks, such as vegetable oils and animal fats, for use in compression ignition (diesel) engines. Manufactured by transestrification of the organic feedstock by methanol.
Brake Mean Effective Pressure (BMEP)
The work accomplished during one engine cycle divided by the engine swept volume. It is essentially the engine torque normalized by the engine displacement. The word “brake” denotes the actual torque/power available at the engine flywheel as measured on a dynamometer. Thus, BMEP is a measure of the useful power output of the engine.
Brake Specific Fuel Consumption (BSFC)
BSFC is the ratio of the engine fuel consumption to the engine power output (as measured at the flywheel). BSFC has units of grams of fuel per kilowatt-hour (g/kWh) or pounds mass of fuel per brake horsepower-hour (lb/bhp-hr). BSFC is a measure of engine efficiency.
California Air Resources Board (CARB)
A state regulatory agency charged with regulating the air quality in California.
Carbon Dioxide (CO2)
A colorless, odorless, non-toxic gas. It is one of main products of fossil-fuel combustion. Carbon dioxide is a greenhouse gas that contributes to the potential for global warming.
Carbon Monoxide (CO)
A colorless, odorless and toxic gas. It blocks the lungs’ ability to obtain oxygen. CO is produced by incomplete combustion of fossil fuels and is a major part of air pollution. Compression ignition (diesel) engines generate significantly lower CO emissions than spark ignited engines.
Carcinogens
Substances known to cause cancer.
Catalyst
A substance which influences the rate of a chemical reaction but is not one of the original reactants or final products, i.e. it is not consumed or altered in the reaction. Catalysts are used in many processes in the chemical and petroleum industries. Emission control catalysts are used to promote reactions that change exhaust pollutants from internal combustion engines into harmless substances.
Cetane Index
A calculated value, derived from fuel density and volatility, giving a reasonably close approximation to cetane number.
Cetane Number
A measure of ignition quality of diesel fuel. The higher the cetane number the easier the fuel ignites when injected into an engine. Cetane number is determined by an engine test using two reference fuel blends of known cetane numbers. The reference fuels are prepared by blending normal cetane (n-hexadecane), having a value of 100, with heptamethyl nonane, having a value of 15.
CFR (Cooperative Fuel Research) Engine
A single cylinder, overhead valve, variable compression ratio engine used for measuring octane or cetane quality.
Clean Air Act (CAA)
In the U.S., the fundamental legislation to control air pollution. The original Clean Air Act was signed in 1963. The law set emissions standards for stationary sources, such as factories and power plants. Criteria pollutants included lead, ozone, CO, SO2, NOx and PM, as well as air toxics. The CAA was amended several times, most recently in 1990. The Amendments of 1970 introduced motor vehicle emission standards for automobiles and trucks.
Clean-Fuel Vehicle (CFV)
A vehicle that has been certified to meet clean-fuel standards of the Clean Air Act Amendments of 1990.
Cloud Point (CP)
A measure of the ability of a diesel fuel to operate under cold weather conditions. Defined as the temperature at which wax first becomes visible when diesel fuel is cooled under standardized test conditions (ASTM D2500).
Cold Filter Plugging Point (CFPP)
A measure of the ability of a diesel fuel to operate under cold weather conditions. Defined as the lowest temperature at which diesel fuel will pass through a fine wire mesh screen of the test apparatus.
Common Rail Injection
A diesel fuel injection system employing a common pressure accumulator, called the rail, which is mounted along the engine block. The rail is fed by a high pressure fuel pump. The injectors, which are fed from the common rail, are activated by solenoid valves. The solenoid valves and the fuel pump are electronically controlled. In the common rail injection system the injection pressure is independent from engine speed and load. Therefore, the injection parameters can be freely controlled. Usually a pilot injection is introduced, which allows for reductions in engine noise and NOx emissions.
Compressed Natural Gas (CNG)
Natural gas compressed to a volume and density that is practical as a portable fuel supply.
Compression Ignition (CI)
The form of ignition that initiates combustion in a diesel engine. The rapid compression of air within the cylinders generates the heat required to ignite the fuel as it is injected.
Converted Vehicle
A vehicle, originally designed to operated on gasoline or diesel, that has been modified or altered to operate on an alternative fuel.
Cordierite
A ceramic material of the formula 2MgO-2Al2O3-5SiO2 which is used for automotive flow-through catalyst substrates and ceramic wall-flow diesel filters.
Dedicated Vehicle
A vehicle designed to operate solely on one alternative fuel, as opposed to a converted vehicle which was later altered to use an alternative fuel.
Diesel Oxidation Catalyst (DOC)
Catalyst promoting oxidation processes in diesel exhaust. Usually designed to reduce emissions of the organic fraction of diesel particulates, gas-phase hydrocarbons, and carbon monoxide.
Diesel Particulate Filter (DPF)
A device which physically captures diesel particulates preventing their discharge from the tailpipe. Collected particulates need to be removed from the filter, usually by continuous or periodic oxidation in a process called “regeneration”.
Diesel Particulate Matter (DPM)
Sub-micron size particles found in diesel exhaust. Most emission regulations specify DPM measurement methods in which particulates are sampled on filters from cooled exhaust gas. The cooling causes condensation of vapors in the gas sampling train. Thus, the DPM is composed of both solid and liquid particles and is generally classified into three fractions: (1) inorganic carbon (soot), (2) organic fraction (often referred to as SOF or VOF), and (3) sulfate fraction (hydrated sulfuric acid).
Dimethyl Ether (DME)
The simplest ether CH3-O-CH3. Can be manufactured from natural gas or from a renewable organic feedstock. DME is a prospective alternative diesel fuel.
Direct Injection (DI)
In diesel engines with direct injection the combustion chamber is not divided and fuel is injected directly to the cylinder.
DOD
The U.S. Department of Defense.
DOE
The U.S. Department of Energy.
DOT
The U.S. Department of Transportation.
Dual-Fuel Vehicle
A vehicle designed to operate on a combination of alternative fuel, such as compressed natural gas (CNG) or liquefied petroleum gas (LPG), and conventional fuel, such as diesel or gasoline. These vehicles have two separate fuel systems, which inject both fuels simultaneously into the engine combustion chamber.

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