How Stirling Engines Work

Displacer-type Stirling Engine

Instead of having two pistons, a displacer-type engine has one piston and a displacer. The displacer serves to control when the gas chamber is heated and when it is cooled. This type of Stirling engine is sometimes used in classroom demonstrations. You can even buy a kit to build one yourself!

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In order to run, the engine above requires a temperature difference between the top and the bottom of the large cylinder. In this case, the difference between the temperature of your hand and the air around it is enough to run the engine.

In the figure on this page, you can see two pistons:

  1. The power piston - This is the smaller piston at the top of the engine. It is a tightly-sealed piston that moves up as the gas inside the engine expands.
  2. 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. There are two positions:

  • When the displacer is near the top of the large cylinder, most of the gas inside the engine is heated by the heat source and it expands. 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, extracting energy from the gas's expansion and contraction.

Next, we'll take a look at a two-piston Stirling engine.