The Stirling Principle

Operating principle

The working principle of all types of piston engines is to compress a cold gas in a cylinder, apply heat to increase the gas temperature and thereby the pressure, and finally expand the hot gas in the cylinder. During the expansion force is transmitted to a crankshaft.

Closed circle process

The Stirling engine is based on a closed cycle, where the working gas (normally pressurised He, H2, N2 or air) alternately is compressed in a cold cylinder volume and expanded in a hot cylinder volume.

The heat input from the combustion of fuel is transferred from the outside to the working gas at a high temperature (typically 700°C - 750°C). The heat, which is not converted into work on the shaft, is rejected into the cooling water at 40°C - 85°C.

The single steps

The basic principle of the Stirling cycle can be illustrated in a pV-diagram (pressure / volume). The steps in the diagram are explained below. The shown engine has two pistons in one cylinder, which is the simplest layout of the Stirling engine.




Aktion 1-2

Power piston moves up
Result:Compression of working gas at low temperature and pressure = Power input


Aktion 2-3

Displacer piston moves down moving the working gas from cold volume to hot volume.
Result:Increase of temperature and pressure.



Aktion 3-1

Power- and displacer piston moves down.
Result: Expansion of hot working gas at high temperature and pressure = Power output



Aktion 4-1

Displacer piston moves up moving the working gas from hot volume to cold volume
Result: Decrease of temperature and pressure.