petrol: see gasoline.

Most widely used form of internal-combustion engine, found in most automobiles and many other vehicles. Gasoline engines vary significantly in size, weight per unit of power generated, and arrangement of components. The principal type is the reciprocating-piston engine. In four-stroke engines, each cycle requires four strokes of the piston—intake, compression, power (expansion), and exhaust—and two revolutions of the crankshaft. In a two-stroke cycle, the compression and power strokes of the four-stroke cycle are carried out without the inlet and exhaust strokes, in one upstroke and one downstroke of the piston and one revolution of the crankshaft. The size, weight, and cost of the engine per horsepower are therefore less, and two-stroke-cycle engines are used in smaller motorcycles, most marine motors, and many handheld landscaping tools (e.g., hedge trimmers and chain saws). Seealso compression ratio; piston and cylinder; rotary engine.

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British petrol

Mixture of volatile, flammable hydrocarbons derived from petroleum, used as fuel for internal-combustion engines and as a solvent for oils and fats. Gasoline became the preferred automobile fuel because it releases a great deal of energy when burned, it mixed readily with air in a carburetor, and it initially was cheap due to a large supply. Costs have now increased greatly except where subsidized. Gasoline was first produced by distillation. Later processes increased the yield from crude oil by splitting large molecules into smaller ones. Still other methods, such as conversion of straight-chain hydrocarbons into their branched-chain isomers, followed. The resulting gasoline is a complex mixture of hundreds of hydrocarbons. A gasoline's octane number indicates its ability to resist knocking (premature combustion) and can be altered by changing the proportions of certain components. The compound tetraethyl lead, once used to reduce knocking, has been banned as toxic. Other additives include detergents, antifreezes, and antioxidants. Since the mid-20th century gasoline fumes have been recognized as a major component of urban air pollution. Efforts to reduce dependence on gasoline, which is a nonrenewable resource, include use of gasohol, a 9:1 mix of gasoline and ethanol, and the development of electric automobiles.

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See Gasoline
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