Displacer

The displacer is a special-purpose piston, used in Beta and Gamma type Stirling engines, to move the working gas back and forth between the hot and cold heat exchangers. Depending on the type of engine design, the displacer may or may not be sealed to the cylinder, i.e. it is a loose fit within the cylinder and allows the working gas to pass around it as it moves to occupy the part of the cylinder beyond.

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A low-temperature difference (LTD) Stirling engine, typically does not have a distinct regenerator component, so in that case, the engine just runs on the small amount of regeneration provided by the displacer itself and the nearby cylinder wall.