Stirling Engines

Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency. Stirling engines are powered by the expansion of a gas when heated, followed by the compression of the gas when cooled. The Stirling engine contains a fixed amount of gas that is transferred back and forth between a “cold” end (often room temperature) and a “hot” end (often heated by a kerosene or alcohol burner). The “displacer piston” moves the gas between the two ends and the “power piston” changes the internal volume as the gas expands and contracts.

Air in the engine is cyclically heated (by an alcohol burner) and expands to push the power piston (shown in blue) to the right. As the power piston moves to the right, the yellow linkage forces the loose-fitting, red “piston” (on the left half of the machine) to displace air to the cooler side of the engine. The air on the cool side loses heat to the outside world and contracts, pulling the blue piston to the left. The air is again displaced, sending it back to the hotter region of the engine, and the cycle repeats.

The Stirling engine cycle can also be used “in reverse”, to convert rotating motion into a temperature differential (and thus provide refrigeration).