Gas expands when heated, and contracts when cooled. Stirling engines move the gas from the hot side of the engine, where it expands, to the cold side, where it contracts.
When there is a temperature difference between upper space of the displacer and lower space of it, the engine pressure is changed by the movement of the displacer. The pressure incluse when the displacer is located upper part of the cylinder. The pressure decrease when the displacer is located lower part of the cylinder. The displacer has a duty of displacing the air and changing the engine pressure. It does not have a duty of operating the crank shaft and the engine.
When the engine pressure is become the max pressure by the duty of the displacer, a power piston is located the direction which add the operating force to the crank shaft. The direction is 90 degree. The displacer type Stirling engine is operated by the power of the power piston.
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