There may be one or more pistons. Each piston is inside a cylinder, into which a gas is introduced, either already under pressure (e.g. steam engine), or heated inside the cylinder either by ignition of a fuel air mixture (

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An early known example of rotary to reciprocating motion is the crank mechanism. The earliest hand-operated cranks appeared in China during the Han Dynasty(202 BC–220 AD). The Chinese used the crank-and-connecting rod for operating querns as far

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For piston engines, an engine's capacity is the engine displacement, in other words the volume swept by all the pistons of an engine in a single movement. It is generally measured in litres(l) or cubic inches (c.i.d., cu in, or in³) for larger engines, and cubic

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Reciprocating engines can be characterized by their specific power, which is typically given in kilowatts per litre of engine displacement (in the U.S. also horsepowerper cubic inch). The result offers an approximation of the peak power output of an engine. This

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Quantum heat engines are devices that generate power from heat that flows from a hot to a cold reservoir. The mechanism of operation of the engine can be described by the laws of

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There are a large number of unusual varieties of piston engines that have various claimed advantages, many of which see little if any current use:
• Free-piston engine

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Reciprocating engines that are powered by compressed air, steam or other hot gases are still used in some applications such as to drive many modern torpedoes or as pollution-free motive

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