An engine is a mechanical device that produces some form of output from a given input. An engine whose purpose is to produce kinetic energy output from a fuel source is called a prime mover; alternatively, a motor is a device which produces kinetic energy from other forms of energy (such as electricity, a flow of hydraulic fluid or compressed air). A motor car (automobile) has a starter motor and motors to drive pumps (fuel, power steering, etc) – but the power plant that propels the car is called an engine. The term 'motor' was originally used to distinguish the new internal combustion engine -powered vehicles from earlier vehicles powered by a steam engine (as in steam roller and motor roller).

Military engines included siege engines, large catapults, trebuchets and battering rams.

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