Editing Stirling engine (section)

{{Short description|Closed-cycle regenerative heat engine}}
{{Use dmy dates|date=June 2020}}
[[File:HPS_Eigenbau2.jpg|thumb|250x250px|A model of a Stirling engine showing its simplicity.  Unlike the steam engine or internal combustion engine, it has no valves or timing train.  The heat source (not shown)  would be placed under the brass cylinder.]]

A '''Stirling engine''' is a [[heat engine]] that is operated by the cyclic expansion and contraction of air or other gas (the ''[[working fluid]]'') by exposing it to different temperatures, resulting in a net conversion of [[heat]] energy to mechanical [[Work (physics)|work]].<ref name="G. Walker 1980 page 1" /><ref name="W.R. Martini 1983, p.6" /> 

More specifically, the Stirling engine is a closed-cycle regenerative heat engine, with a permanent [[gas]]eous working fluid. ''Closed-cycle'', in this context, means a [[thermodynamic system]] in which the working fluid is permanently contained within the system. ''Regenerative'' describes the use of a specific type of internal [[heat exchanger]] and thermal store, known as the [[Regenerative heat exchanger|''regenerator'']]. Strictly speaking, the inclusion of the regenerator is what differentiates a Stirling engine from other closed-cycle [[hot air engine]]s.<ref name="haeinventors-s01" />

In the Stirling engine, a working fluid (e.g. air) is heated by energy supplied from outside the engine's interior space (cylinder).  As the fluid expands,  mechanical work is extracted by a piston, which is coupled to a displacer.  The displacer moves the working fluid to a different location within the engine, where it is cooled, which creates a partial vacuum at the working cylinder, and more mechanical work is extracted. The displacer moves the cooled fluid back to the hot part of the engine, and the cycle continues.

A unique feature is the regenerator, which acts as a temporary heat store by retaining heat within the machine rather than dumping it into the heat sink, thereby increasing its efficiency.   

The heat is supplied from the outside, so the hot area of the engine can be warmed with any external heat source. Similarly, the cooler part of the engine can be maintained by an external heat sink, such as running water or air flow. The gas is permanently retained in the engine, allowing a gas with the most-suitable properties to be used, such as helium or hydrogen. There are no intake and no exhaust gas flows so the machine is practically silent.

The machine is reversible so that if the shaft is turned by an external power source a temperature difference will develop across the machine; in this way it acts as a [[heat pump]].

The Stirling engine was invented by Scotsman  [[Robert Stirling]]<ref name="haestirling1816engine" /> in 1816 as an industrial [[Prime mover (engine)|prime mover]] to rival the [[steam engine]], and its practical use was largely confined to low-power domestic applications for over a century.<ref name="Finkelstein-2001-2-3" /> 

Contemporary [[Renewable energy commercialization|investment in renewable energy]], especially [[solar energy]], has given rise to its application within concentrated solar power and as a heat pump.