The Stirling engine is different from the internal combustion engine since it operates as a closed-cycle heat engine. The gasous working fluid undergoes cyclic compression and expansion at different temperatures such that there is a conversion of heat energy into mechanical work. The working fluid is contained within the system and usually is air, hydrogen or helium.

The working fluid (gas) is compressed in the colder section of the engine and expanded in the hotter section. If an regenerative heat exchanger is included or attached this will increase the thermal efficiency of the engine compared to simpler engines.

The Stirling engine`s four ideal thermodynamic processes:

Isothermal Expansion – The working fluid (gas) undergoes near-isothermal expansion absorbing heat from the heat source.

Isovolumetric (isochoric or constant-Volume) heat-removal – The working fluid (gas) passes through the regenerator, where it cools, transferring heat to the regenerator for use in the cycle to follow.

Isothermal Compression – The working fluid (gas) undergoes near-isothermal compression ransferring heat to the cold sink

Isovolumetric (isochoric constant-volume) heat-addition – The gas passes in opposite direction through the regenerator where it recovers the majority of the heat transferred during the Isovolumetric heat-removal , heating up on its way to the expansion space.

Configurations

• The alpha configuration. – This configuration consist of two power pistons, one in a hot cylinder and one in a cold cylinder, the working fluid (gas) is driven between the two cyslinders by the pistons.

• The beta configuration. – This configuration has a one cylinder with a hot end and a cold end, containing a power piston and a ‘displacer’ that drives the gas between the hot and cold ends.

• The gamma configuration. – This configuration has two cylinders: one containing a displacer, with a hot and a cold end, and one for the power piston; they are joined to form a single space with the same pressure in both cylinders.

The actual efficiency of Stirling machines is linked to the environmental temperature and internal losses. Higher efficiency is obtained when the ambient temperature is lower. As with other external combustion engines, Stirling engines can use more or less any heat sources.