Engine

Engine, machine for converting energy into motion or mechanical work. The energy is usually supplied in the form of a chemical fuel, such as oil or petrol (gasoline), steam, or electricity, and the mechanical work is most commonly delivered in the form of rotary motion of a shaft. Engines are usually classified according to the form of energy they utilize, as steam, compressed air, and petrol; the type of motion of their principal parts, as reciprocating and rotary; the place where the exchange from chemical to heat energy takes place, as internal combustion and external combustion; the method by which the engine is cooled, as air-cooled or water-cooled; the position of the cylinders of the engine, as V, in-line, and radial; the number of strokes of the piston for a complete cycle, as two-stroke and four-stroke; the type of cycle, as Otto (in ordinary petrol engines) and diesel; and the use for which the engine is intended, as car and aircraft engines. Engines are often called motors, although the term motor is sometimes restricted to engines that transform electrical energy into mechanical energy (see Electric Motors and Generators). Other specialized engines are the windmill, gas turbine, steam turbine, and rocket and jet engines.