Editing Stirling engine (section)

== Theory ==
{{Main|Stirling cycle}}
[[File:Stirling cycle pV.svg|thumb|A [[Pressure volume diagram|pressure/volume graph]] of the idealized Stirling cycle.]]

The idealised Stirling cycle consists of four [[thermodynamic processes]] acting on the working fluid:

# [[Isothermal]] [[Thermal expansion|expansion]]. The expansion-space and associated heat exchanger are maintained at a constant high temperature, and the gas undergoes near-isothermal expansion absorbing heat from the hot source.
# Constant-volume (known as [[isometric process|isovolumetric]] or [[isochoric process|isochoric]]) heat-removal. The gas is passed through the [[regenerative heat exchanger|regenerator]], where it cools, transferring heat to the regenerator for use in the next cycle.
# [[Isothermal]] [[Compression ratio|compression]]. The compression space and associated heat exchanger are maintained at a constant low temperature so the gas undergoes near-isothermal compression rejecting heat to the cold sink
# Constant-volume (known as [[isometric process|isovolumetric]] or [[isochoric process|isochoric]]) heat-addition. The gas passes back through the regenerator where it recovers much of the heat transferred in process 2, heating up on its way to the expansion space.

With the ideal, maximally efficient, Stirling engine, for the thermal reservoirs the ratio of the heat in to the heat out is the efficiency of the ideal Carnot cycle.  This is the Carnot efficiency, which is the ratio of the Kelvin temperatures of the cold to the hot reservoir.  With the ideal, maximally efficient, Carnot cycle, the isochores (constant volume) are replaced by adiabats (no net heat transfer because no heat transfer).  For the ideal Stirling cycle, whatever heat enters during the isochoric leg where the temperature increases is totally released during the isochoric leg where the temperature decreases (no net heat transfer). 

The engine is designed so the working gas is generally compressed in the colder portion of the engine and expanded in the hotter portion resulting in a net conversion of heat into [[work (thermodynamics)|work]].<ref name="W.R. Martini 1983, p.6" /> An internal [[regenerative heat exchanger]] increases the Stirling engine's thermal efficiency compared to simpler [[hot air engine]]s lacking this feature.

The Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed, thus converting thermal [[energy]] into mechanical energy. The greater the temperature difference between the hot and cold sources, the greater the thermal efficiency. The maximum theoretical efficiency is equivalent to that of the [[Carnot cycle]], but the efficiency of real engines is less than this value because of friction and other losses.{{citation needed|date=July 2020}}

Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "working fluid", most commonly [[air]], [[hydrogen]] or [[helium]]. In normal operation, the engine is sealed and no gas enters or leaves; no valves are required, unlike other types of piston engines. The Stirling engine, like most heat engines, cycles through four main processes: cooling, compression, heating, and expansion. This is accomplished by moving the gas back and forth between hot and cold [[heat exchangers]], often with a [[regenerative heat exchanger|regenerator]] between the heater and cooler. The hot heat exchanger is in thermal contact with an external heat source, such as a fuel burner, and the cold heat exchanger is in thermal contact with an external heat sink, such as air fins. A change in gas temperature causes a corresponding change in gas pressure, while the motion of the piston makes the gas alternately expand and compress.{{citation needed|date=July 2020}}

The gas follows the behaviour described by the [[gas laws]] that describe how a gas's [[pressure]], [[temperature]], and [[volume]] are related. When the gas is heated, the pressure rises (because it is in a sealed chamber) and this pressure then acts on the power [[piston]] to produce a power stroke. When the gas is cooled the pressure drops and this drop means that the piston needs to do less work to compress the gas on the return stroke. The difference in work between the strokes yields a net positive power output.{{citation needed|date=July 2020}}

When one side of the piston is open to the atmosphere, the operation is slightly different. As the sealed volume of working gas comes in contact with the hot side, it expands, doing work on both the piston and on the atmosphere. When the working gas contacts the cold side, its pressure drops below atmospheric pressure and the atmosphere pushes on the piston and does work on the gas.{{citation needed|date=July 2020}}