There are three major types of Stirling engines, that are distinguished by the way they move the air between the hot and cold areas:
configuration has two power pistons, one in a hot cylinder, one in a cold cylinder, and the gas is driven between the two by the pistons; it is typically in a V-formation with the pistons joined at the same point on a crankshaft.
configuration has a single 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. It is typically used with a rhombic drive to achieve the phase difference between the displacer and power pistons, but they can be joined 90 degrees out of phase on a crankshaft.
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 pistons are typically in parallel and joined 90 degrees out of phase on a crankshaft.
The ideal Stirling cycle consists of four thermodynamic processes acting on the working fluid:
Isothermal 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 isovolumetric or isochoric) heat-removal. The gas is passed through the regenerator, where it cools, transferring heat to the regenerator for use in the next cycle.
Isothermal 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 (isochoric) heat-addition. The gas passes back through the regenerator where it recovers much of the heat transferred in 2, heating up on its way to the expansion space.
Theoretical thermal efficiency equals that of the hypothetical Carnot cycle – i.e. the highest efficiency attainable by any heat engine.
(Source: https://en.wikipedia.org/wiki/Stirling_engine#/media/File:Stirling_Cycle_color.png )
The thermodynamic cycle of a Stirling engine can be driven in reverse with the aid of an outside power source. This will cause one side to be heated and the other side of the engine to be cooled. Simply put, a Stirling engine can be used as a heat pump. By spinning the engine through its mechanical cycles, the gas inside is compressed and expanded, heated and cooled, respectively. Cooling with the Stirling cycle is currently used commercially for cryogenics and refrigeration.