[[Image:Four stroke engine diagram.jpg|thumbnail|right|Internal combustion piston engine

Components of a typical, four-stroke cycle, internal combustion, gasoline piston engine.

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A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the Stirling engine for niche applications. Internal combustion engines are further classified in two ways: either a spark-ignition (SI) engine, where the spark plug initiates the combustion; or a compression-ignition (CI) engine, where the air within the cylinder is compressed, thus heating it, so that the heated air ignites fuel that is injected then, in a diesel engine, or earlier, in a hot bulb engine.

Common features in all types

thumb|3D render of a piston engine

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 (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (Stirling engine). The hot gases expand, pushing the piston to the bottom of the cylinder. This position is also known as the bottom dead center (BDC), or where the piston forms the largest volume in the cylinder. The piston is returned to the cylinder top (top dead center) (TDC) by a flywheel, the power from other pistons connected to the same shaft or (in a double acting cylinder) by the same process acting on the other side of the piston. This is where the piston forms the smallest volume in the cylinder. In most types the expanded or "exhausted" gases are removed from the cylinder by this stroke. The exception is the Stirling engine, which repeatedly heats and cools the same sealed quantity of gas. The stroke is simply the distance between the TDC and the BDC, or the greatest distance that the piston can travel in one direction.

In some designs the piston may be powered in both directions in the cylinder, in which case it is said to be double-acting.

[[Image:Steam engine nomenclature.png|thumb|left|300px|Steam piston engine<br />A labeled schematic diagram of a typical single-cylinder, simple expansion, double-acting high pressure steam engine. Power takeoff from the engine is by way of a belt.

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In most types, the linear movement of the piston is converted to a rotating movement via a connecting rod and a crankshaft or by a swashplate or other suitable mechanism. A flywheel is often used to ensure smooth rotation or to store energy to carry the engine through an un-powered part of the cycle. The more cylinders a reciprocating engine has, generally, the more vibration-free (smoothly) it can operate. The power of a reciprocating engine is proportional to the volume of the combined pistons' displacement.

A seal must be made between the sliding piston and the walls of the cylinder so that the high pressure gas above the piston does not leak past it and reduce the efficiency of the engine. This seal is usually provided by one or more piston rings. These are rings made of a hard metal, and are sprung into a circular groove in the piston head. The rings fit closely in the groove and press lightly against the cylinder wall to form a seal, and more heavily when higher combustion pressure moves around to their inner surfaces.

It is common to classify such engines by the number and alignment of cylinders and total volume of displacement of gas by the pistons moving in the cylinders usually measured in cubic centimetres (cm<sup>3</sup> or cc) or litres (l) or (L) (US: liter). For example, for internal combustion engines, single and two-cylinder designs are common in smaller vehicles such as motorcycles, while automobiles typically have between four and eight, and locomotives and ships may have a dozen cylinders or more. Cylinder capacities may range from 10&nbsp;cm<sup>3</sup> or less in model engines up to thousands of liters in ships' engines.

The compression ratio affects the performance in most types of reciprocating engine. It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

The bore/stroke ratio is the ratio of the diameter of the piston, or "bore", to the length of travel within the cylinder, or "stroke". If this is around 1 the engine is said to be "square". If it is greater than 1, i.e. the bore is larger than the stroke, it is "oversquare". If it is less than 1, i.e. the stroke is larger than the bore, it is "undersquare".

Cylinders may be aligned in line, in a V configuration, horizontally opposite each other, or radially around the crankshaft. Opposed-piston engines put two pistons working at opposite ends of the same cylinder and this has been extended into triangular arrangements such as the Napier Deltic. Some designs have set the cylinders in motion around the shaft, such as the rotary engine.

thumb|250px|Stirling piston engine [[Rhombic drive|Rhombic Drive – Beta Stirling Engine Design, showing the second displacer piston (green) within the cylinder, which shunts the working gas between the hot and cold ends, but produces no power itself.

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In some steam engines, the cylinders may be of varying size with the smallest bore cylinder working the highest pressure steam. This is then fed through one or more, increasingly larger bore cylinders successively, to extract power from the steam at increasingly lower pressures. These engines are called compound engines.

Aside from looking at the power that the engine can produce, the mean effective pressure (MEP), can also be used in comparing the power output and performance of reciprocating engines of the same size. The mean effective pressure is the fictitious pressure which would produce the same amount of net work that was produced during the power stroke cycle. This is shown by:

: <math>W_{net}= \text{MEP}\cdot A_pS = \text{MEP} \cdot V_d</math>

where <math>A_p</math> is the total piston area of the engine, <math>S</math> is the stroke length of the pistons, and <math>V_d</math> is the total displacement volume of the engine. Therefore:

: <math>\text{MEP} = \frac{W_{net{V_d}</math>

Whichever engine with the larger value of MEP produces more net work per cycle and performs more efficiently.

Torpedoes may use a working gas produced by high test peroxide or Otto fuel II, which pressurize without combustion. The Mark 46 torpedo, for example, can travel underwater at fuelled by Otto fuel without oxidant.

Reciprocating quantum heat engine

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 quantum mechanics. Quantum refrigerators are devices that consume power with the purpose to pump heat from a cold to a hot reservoir.

In a reciprocating quantum heat engine, the working medium is a quantum system such as spin systems or a harmonic oscillator. The Carnot cycle and Otto cycle are the ones most studied. The quantum versions obey the laws of thermodynamics. In addition, these models can justify the assumptions of endoreversible thermodynamics. A theoretical study has shown that it is possible and practical to build a reciprocating engine that is composed of a single oscillating atom. This is an area for future research and could have applications in nanotechnology.

Miscellaneous engines

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:

  • Bourke engine
  • Free-piston engine
  • IRIS engine
  • Opposed-piston engine
  • Axial engine
  • Cam engine
  • Revolving cylinder engine
  • Swing-piston engine
  • Thermo-magnetic motor

See also

  • Heat engine for a view of the thermodynamics involved in these engines.
  • For a contrasting approach using no pistons, see the pistonless rotary engine.
  • For an historical perspective see Timeline of heat engine technology.
  • Steam engine
  • Steam locomotive
  • Stirling engine
  • Internal combustion engine
  • Otto cycle
  • Diesel cycle
  • Engine configuration for a discussion of the layout of the major components of a reciprocating piston internal combustion engine.
  • Diesel engine
  • Gasoline engine
  • Oil field engine

Notes

  • Combustion video – in-cylinder combustion in an optically accessible, two-stroke engine
  • HowStuffWorks: How Car Engines Work
  • Reciprocating Engines at Infoplease.
  • Piston Engines at the US Centennial of Flight Commission.