How Stirling Engines work

What is a Stirling engine?

A Stirling engine is a special type of 'heat engine' that converts small temperature differences into motion, and they are sometimes called 'hot-air' or 'caloric' engines.

How does a Stirling engine work?

All Stirling engines are powered by a temperature difference. In our low temperature engines this difference is normally achieved by warming the bottom plate to above room temperature and allowing the top plate to stay at room temperature.

In operation the engine cyclically heats and cools the air inside. This process is shown in the simplified cut-away diagrams below.

It helps to remember that the large displacer disc just moves (displaces) the air from top to bottom and back again, and it is the small black piston that actually drives the flywheel.

Figure 1: With the large displacer disc at the top, most of the air inside the main chamber is at the bottom, where it is heated by the warm bottom plate. As it warms, it expands, and as it expands it pushes the small black piston upwards, which drives the flywheel round.

Figure 2: As the flywheel turns, the large displacer disc is moved (by means of a crank and connecting rod) to the bottom of the chamber. As the displacer moves down, the air that was at the bottom of the main chamber is displaced around the outside of the displacer to the top of the chamber. So with the displacer at the bottom, most of the air is at the top, where it is cooled by the cool top plate. As it cools, it contracts, which pulls the small black piston downwards, and drives the flywheel around some more, and so the cycle continues.

Figure 1	Figure 2