Acid rain
Acid rain (or more accurately acid precipitation) occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.

Additives are chemicals added to fuels in very small quantities to improve and maintain fuel quality. Detergents and corrosion inhibitors are examples of gasoline additives.

Air toxics
Air toxics are toxic air pollutants including benzene, formaldehyde, acetaldehyde, 1-3 butadiene, and polycyclic organic matter (POM). Benzene is a constituent of motor vehicle exhaust, evaporative, and fueling emissions. The other compounds are exhaust pollutants.

Aromatics are hydrocarbons based on the ringed six-carbon benzene series or related organic groups. Benzene, toluene and xylene are the principal aromatics represent one of the heaviest fractions in gasoline.

A barrel is a unit of volume measurement used for petroleum and its products. 1 barrel (bbl) = 42 U.S. gallons or 35 British gallons.

Benzene is an aromatic hydrocarbon which is a colorless, volatile, flammable liquid. This common gasoline component is identified as being toxic. Benzene is a known carcinogen.

Bi-fuel vehicle
A Bi-fuel vehicle is a vehicle with two separate fuel systems designed to run on either an alternative fuel, or gasoline or diesel, using only one fuel at a time.

Biofuels are liquid fuels made of biomass for transportation purpose. Biofuels include biogas, pure plant oil, biodiesel, bioethanol and their derivates.

Calorific value
The calorific value defines the amount of heat produced by the complete combustion of a unit weight of fuel. Usually expressed in calories per gram or BTU's per pound, the latter being numerically 1.8 times the former.

Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO2. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle. Atmospheric carbon dioxide derives from multiple natural sources including volcanic outgassing, the combustion of organic matter and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide come mainly from the burning of various fossil fuels for heating, power generation and transport use. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to oxygen during a process called photosynthesis, using both the carbon and part of the oxygen to construct carbohydrates. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms, forming a cycle.

Carbon monoxide, with the chemical formula CO, is a colourless, odourless, and tasteless gas. It is the product of the incomplete combustion of carbon-containing compounds, notably in internal-combustion engines. It has significant fuel value, burning in air with a characteristic blue flame, producing carbon dioxide. Despite its serious toxicity, CO is extremely useful and underpins much modern technology, being a precursor to a myriad of useful — even life-saving — products. It has one carbon atom and one oxygen atom.

Cetane number
The cetane number (Cetane Rating, Cetane Index) is a measure of ignition quality of diesel fuel. The higher the cetane number, the easier the fuel ignites when it is injected into the engine. Biodiesel has a higher cetane number than petrol diesel because of its higher oxygen content. This means that engines run smoother and create less noise when running on biodiesel. It is the equivalent to the octane number of gasoline.

Climate change
Climate change refers to the variation in the Earth's global climate or in regional climates over time. It describes changes in the variability or average state of the atmosphere over time scales ranging from decades to millions of years. These changes can be caused by processes internal to the Earth, external forces (e.g. variations in sunlight intensity) or, more recently, human activities. In recent usage, especially in the context of environmental policy, the term "climate change" often refers only to changes in modern climate, including the rise in average surface temperature known as global warming.

 Converted vehicle
A converted vehicle is a vehicle originally designed to operate on gasoline or diesel that has been modified or altered to run on an alternative fuel.

Crude oil
Petroleum or crude oil is a naturally occurring liquid found in formations in the Earth consisting of a complex mixture of hydrocarbons (mostly alkanes) of various lengths. In its naturally occurring form, it may contain other nonmetallic elements such as sulfur, oxygen, and nitrogen. It is usually black or dark brown (although it may be yellowish or even greenish) but varies greatly in appearance, depending on its composition.

Density is the term meaning the mass of a unit of volume. Its numerical expression varies with the units selected.

The diesel engine is a type of internal combustion engine. It is a compression ignition engine, in which the fuel ignites as it is injected into the engine. By contrast, in the gasoline engine the fuel is mixed first and then ignited by a spark plug. Also, diesels generally have high compression ratios, to enable compression ignition, whereas in gasoline-burning engines, compression ignition is undesirable.The engine operates using the diesel cycle.

Emission factor
An emission factor can be defined as the average emission rate of a given pollutant for a given source, relative to units of activity. Emission factors can be used to derive estimates of gas emissions (for instance, greenhouse gas emissions) based on the amount of fuel combusted or on industrial production levels. The level of precision of the resulting estimates depends significantly on the activity in question. Carbon dioxide (CO2) emissions from the combustion of fuel can be estimated with a high degree of certainty regardless of how the fuel is used as these emissions depend almost exclusively on the carbon content of the fuel, which is generally known with a high degree of certainty. In contrast, the levels of non-CO2 emissions from combustion depend on the precise nature of the activity in which the fuel is being combusted. For instance, methane (CH4) emissions from transport depend on a vehicle's type, whether the vehicle has been fitted with emissions controls and so on. Therefore, because of the uncertainties involved, the estimates of non-CO2 emissions from transport using emission factors are much less precise than estimates of CO2 emissions.

Emission standards
Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment.

Emission trading
Emissions trading (or cap and trade) is an administrative approach used to control pollution by providing economic incentives for achieving reductions in the emissions of pollutants. In such a plan, a central authority (usually a government agency) sets a limit or cap on the amount of a pollutant that can be emitted. Companies or other groups that emit the pollutant are given credits or allowances which represent the right to emit a specific amount. The total amount of credits cannot exceed the cap, limiting total emissions to that level. Companies that pollute beyond their allowances must buy credits from those who pollute less than their allowances. This transfer is referred to as a trade. In effect, the buyer is being fined for polluting, while the seller is being rewarded for having reduced emissions. The more firms that need to buy credits, the higher the price of credits becomes, which makes reducing emissions cost-effective in comparison.The overall goal of an emissions trading plan is to reduce pollution.

Flexible-fuel vehicle (FFV)
A Flexible-Fuel Vehicle (FFV) is a vehicle with a common fuel tank designed to run on varying blends of unleaded gasoline with either ethanol or methanol.

Fossil fuel
Fossil fuels are hydrocarbon deposits, such as petroleum, coal, or natural gas, derived from living matter of a previous geologic time and used for fuel. Combustion of fossil fuels emits large amounts of CO2 into the atmosphere.

Fuel cell
A fuel cell is an electrochemical energy conversion device. It produces electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continously as long as the necessary flows are maintained. Fuel cells differ from batteries in that they consume reactants, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable. Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.

Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines.

Gasoline gallon equivalent (GGE)
The gasoline gallon equivalent is a unit for measuring alternative fuels so that they can be compared with gasoline on an energy equivalent basis. This is required because the different fuels have different energy densities.

Global warming
Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades and its projected continuation. Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations, which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Greenhouse gases are released by activities such as the burning of fossil fuels, land clearing and agriculture. Other phenomena such as solar variation and volcanoes have had smaller but non-negligible effects on global mean temperature. An increase in global temperatures can in turn cause other changes, including a rising sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.

Greenhouse effect
In common parlance, the term "greenhouse effect" may be used to refer either to the natural greenhouse effect, due to naturally occurring greenhouse gases, or to the enhanced (anthropogenic) greenhouse effect, which results from gases emitted as a result of human activities.

Greenhouse gases
Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases.

Hybrid electric vehicle (HEV)
A hybrid electric vehicle is a vehicle powered by two or more energy sources, one of which is electricity. HEVs may combine the engine and fuel of a conventional vehicle with the batteries and electric motor of an electric vehicle in a single drivetrain.

Kyoto protocol
The Kyoto Protocol to the United Nations Framework Convention on Climate Change is an amendment to the international treaty on climate change, assigning mandatory targets for the reduction of greenhouse gas emissions to signatory nations. The Kyoto Protocol is an agreement made under the United Nations Framework Convention on Climate Change (UNFCCC). Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other greenhouse gases or engage in emissions trading if they maintain or increase emissions of these gases. The Kyoto Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.

Liquid fuels
Liquid fuels are those combustible or energy-generating molecules that can be harnessed to create mechanical energy, usually producing kinetic energy; they also must take the shape of their container. Most liquid fuels, in widespread use, are or derived from fossil fuels; however, there are several types, such as hydrogen fuel (for automotive uses), which are also categorized as a liquid fuel.

Motor octane
Motor octane is the octane as tested in a single-cylinder octane test engine at more severe operating conditions. Motor octane number (MON) affects high-speed and part-throttle knock and performance under load, passing, climbing, and other operating conditions.

Natural gas
Natural gas is a mixture of fossil gaseous hydrocarbons, primarily methane, occurring naturally in the Earth and used principally as a fuel.

Nitrogen-oxides (NOx)
NOx is a generic term for the various nitrogen oxides produced during combustion. They are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems. In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule of reactant (in this case fuel and oxygen) it is said to be "running at stoichiometry". When this burns, combustion temperatures reach a high enough level to drive endothermic reactions between nitrogen and oxygen in the flame, yielding various oxides of nitrogen, the results of which can be seen over major cities such as Los Angeles, California in the summer in the form of brown clouds of smog.

Octane enhancer
An octane enhancer is any substance such as MTBE, ETBE, toluene, or xylene that is added to gasoline to increase octane and reduce engine knock.

Octane rating
Octane rating (Octane number) is a measure of a fuel's resistance to selfignition, hence a measure as well of the antiknock properties of the fuel.

Ozone (O3)
Tropospheric ozone (smog) is formed when volatile organic compounds (VOCs), oxygen, and NOx react in the presence of sunlight (not to be confused with stratospheric ozone, which is found in the upper atmosphere and protects the earth from the sun's ultraviolet rays). Though beneficial in the upper atmosphere, ground-level ozone is a respiratory irritant and considered a pollutant.

Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines.

The term spark-ignition is normally used to refer to internal combustion engines where the fuel-air mixture is ignited with a spark. The term contrasts with compression-ignition engines, where the heat from compression alone ignites the mixture. Spark-ignition engines can be either two-stroke or four-stroke, and are commonly referred to as "gasoline engines" in US English and "petrol engines" in British English. However, these terms are not preferred, since spark-ignition engines can (and increasingly are) run on fuels other than gasoline, such as autogas (LPG), methanol, ethanol, compressed natural gas (CNG), hydrogen, and (in drag racing) nitromethane. A four-stroke spark-ignition engine is an Otto cycle engine. Until recently, a major distinction between spark-ignition and compression-ignition engines has been where the fuel is mixed - spark-ignition engines mix fuel outside the cylinders and compression-ignition engines mix fuel inside the cylinders. However, both two-stroke and four-stroke spark-ignition engines are increasingly being designed with direct injection, eliminating this distinction.

Volatile organic compounds (VOC)
Volatile organic compounds are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere. (The term VOC is also occasionally used as an abbreviation, especially in biological contexts, for "volatile organic carbon".) A wide range of carbon-based molecules, such as aldehydes, ketones, and hydrocarbons are VOC's. The term often is used in a legal or regulatory context and in such cases the precise definition is a matter of law. These definitions can be contradictory and may contain "loopholes"; e.g. exceptions, exemptions, and exclusions. Others believe the concept that a volatile organic compound is any organic that participates in a photoreaction, as found in the EPA's definition, is very broad and vague. Organics that are not volatile, as described above, can fall into that definition. The term may refer both to well characterized organic compounds and to mixtures of variable composition.

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