The Stirling engine is a heat engine that is vastly different from the internal-combustion enginediesel engine. But today, Stirling engines are used only in some very specialized applications, like in submarines or auxiliary power generators for yachts, where quiet operation is important. Although there hasn't been a successful mass-market application for the Stirling engine, some very high-power inventors are working on it. in your car. Invented by Robert Stirling in 1816, the Stirling engine has the potential to be much more efficient than a gasoline or

Displacer-type Stirling Engine

A Stirling engine uses the Stirling cycle,­ which is unlike the cycles used in internal-combustion engines.

  • The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.
  • The Stirling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine.

There are hundreds of ways to put together a Stirling engine. In this article, we'll learn about the Stirling cycle and see how two different configurations of this engine work.

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!

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.