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Hydrocarbons are organic chemical compounds containing only hydrogen and carbon atoms. Automotive fuels such as gasoline are made mostly of two elements, hydrogen and carbon. They have the chemical symbols H and C. This type of fuel is hydrocarbon (HC) fuel. During complete combustion in the engine, these two elements unite with a third element, the gas oxygen. Oxygen usually in the form of free oxygen makes up about 20 percent of the earth's atmosphere,.