Page 69 - Piston Engine-Based Power Plants

Stirling Engines and Free Piston Engines 61

and can be compared to the intake, compression, power and exhaust
phases of an internal combustion engine. During these stages the gas-
eous working fluid behaves according to the gas laws that relate
volume, pressure and temperature.

Fig. 6.1 shows a simplified Alpha Stirling engine at different stages
of its cycle. As the figures indicate, the engine has two linked pistons
which drive the working fluid, the gas, between a hot source and a
cold sink. The working fluid in most modern Stirling engines is either
hydrogen or helium which are good absorbers of heat. However both
gases diffuse easily through seals and can be difficult to contain. Some
recent machines have used nitrogen instead although its thermal prop-
erties are not as good as those of the other two gases. The gas inside
the engine is highly compressed, often to 20 MPa, or 200 times atmo-
spheric pressure.

Crankshaft
and linked Hot source
pistons

Cold sink

(A) (B)

(C)
(D)
Figure 6.1 The Alpha Stirling engine cycle. Source: Wikimedia.