Engine

Engine, machine that transforms energy into useful mechanical work. The most familiar engines are heat engines, which transform heat energy, obtained by burning fuel, into a force that turns wheels, propellers, turbines, and so on. Other types of engines include hydroelectric plants, which use the energy of falling water to spin rotors that generate electricity, and windmills, which harness the energy of the wind. Engines may be classified by the fuel they use (gasoline engine), by the way they burn their fuel (internal or external combustion), and by the way they produce motion (reciprocating, rotary, or reaction). In internal combustion engines combustion takes place inside the engine cylinders. The gaseous products of combustion press against pistons in the cylinders to produce reciprocating, or back-and-forth, motion. Jet and rocket engines work on the principle of reaction. They burn fuel in a combustion chamber to produce hot gases that leave the engine at high velocity through a nozzle. Reaction to the backward stream of gases thrusts the engine forward. Jet engines take in oxygen from the atmosphere to burn their fuel; rockets carry their own oxygen supply as well as their fuel and are thus independent of the atmosphere. The industrial equivalent of the jet engine is the gas turbine in which a turbine drives a shaft. Gas turbines are used mainly to drive generators, but they also power locomotives, ships, and even some experimental automobiles. Steam engines and steam turbines are external combustion engines, since they burn their fuel in a furnace outside the engine itself. The heat is used to produce steam in a boiler, and the steam drives a piston back and forth or spins the blades of a turbine. Steam turbines, which are highly efficient, are the major propulsion units in ships and the most widely used engines in the world's electricity-generating stations.

See also: Combustion; Steam engine; Turbine.