A Beta type Stirling engine has a single power piston and a displacer within the same cylinder and on the same shaft. The displacer piston is a loose fit and and is used only to move the working gas from the hot heat exchanger to the cold heat exchanger.

When the working gas is pushed to the hot end of the cylinder it expands and pushes the power piston. When it is pushed to the cold end of the cylinder it contracts and the momentum of the machine, pushes the power piston the other way to compress the gas. The beta type avoids the technical problems of hot moving seals (as in the alpha type).
1. Power piston (dark grey) has compressed the gas, the displacer piston (light grey) has moved so that most of the gas is adjacent to the hot heat exchanger.

2. The heated gas increases in pressure and pushes the power piston to the farthest limit of the power stroke.

3. The displacer piston now moves, shunting the gas to the cold end of the cylinder.

4. The cooled gas is now compressed by the flywheel momentum. This takes less energy, since its pressure drops when it is cooled.