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The idealised Stirling cycle is a thermodynamic cycle with two isochores and two isotherms. Theoretically it is the most efficient thermodynamic cycle practically possible, however technical issues limit its efficiency when applied - a simpler mechanism is favored over attaining a close fit to the theoretical cycle.
The regenerator is a reverse flow heat exchanger, which tends to improve thermal efficiency wherever it is found in technology and nature. The effect of thermal regeneration contributes greatly to the overall efficiency and power produced by a Stirling engine. The regenerator was the key feature invented by Robert Stirling in 1816 which greatly improved his machine and distinguished it from other “hot air engines”, although several of Stirling’s later machines did not use regenerators. In modern designs, the regenerator is often absent from low temperature difference (LTD) designs, but is almost always used in high temperature difference (HTD) Stirling engines.
The regenerator is typically a mass of fine metal wire, preferably with low porosity to reduce unswept volume, and with the wire axes perpendicular to the gas flow, as in a stack of wire meshes. The regenerator is located in the path of the gas between the hot and cold heat exchangers. As the gas cycles between the hot and cold spaces, over 90% of its heat is temporarily transferred to and from the regenerator. The regenerator essentially recycles unused heat, and thus reduces the heat flow requirements of the hot and cold heat exchangers.
There is a performance trade off, particularly for high power density, HTD engines, where the regenerator must be carefully designed to obtain high heat transfer with low viscous pumping losses.
In LTD engines, the potential for heat regeneration is minimal, and also flow losses must be minimized. In LTD beta and gamma designs, the displacer piston acts as a simple regenerator. The displacer piston does not have a seal, and instead has a loose fit within the cylinder. The resulting air gap between the piston and the cylinder allows the gas to flow around the displacer as it moves through the cylinder. The surfaces of the displacer and cylinder provide some regeneration with minimal flow losses, thus achieving near optimal performance for an LTD engine.
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