It is a tightly-sealed piston that moves up as the gas inside the engine expands. The displacer . This is the large piston in the drawing. This piston is very loose in its cylinder, so air can move easily between the heated and cooled sections of the engine as the piston moves up and down. The displacer moves up and down to control whether the gas in the engine is being heated or cooled.
Pressure builds inside the engine, forcing the power piston up.
When the displacer is near the bottom of the large cylinder, most of the gas inside the engine cools and contracts. This causes the pressure to drop, making it easier for the power piston to move down and compress the gas. The engine repeatedly heats and cools the gas inside in order to extract energy from the gas's expansion and contraction. Next, we'll take a look at a two-piston Stirling engine. Inc. All rights reserved.