3B Scientific Stirling Engine D - Manual (Page 9)

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6. Operating principle
The principle of a how a Stirling engine works can be
divided, in simplified form, into the following four
processes:
Heating:
During the heating phase, the displacement piston
(P1) moves upwards so that air is displaced down
into the heated part of the displacement cylinder.
Temperature and pressure both rise in a fashion
that is almost isochoric. The working piston is in its
lower rest position (bottom dead centre) at this
point (see Fig. 1). The displacement piston moves in
advance of the working piston till it reaches its top
dead centre position. This is the point where the
air is at its lowest volume, but highest temperature
and pressure (see Fig. 2).
Expansion:
The heated air expands almost isothermally, thus
forcing the working piston (P2) upwards. In the
process, mechanical work is transferred via the shaft
to the rotor. The volume of air increases as the air
absorbs heat and the pressure reduces (see Fig. 3).
Cooling:
Cooling occurs while the working piston is at top
dead centre and the displacement piston (P1) is on
its downstroke, forcing air to move into the upper
part of the displacement cylinder. The air then
cools and the upper plate absorbs heat. The dis-
placement piston finally reaches bottom dead
centre (see Figs. 4 and 5).
Compression:
The cooler air is compressed isothermally by the
working piston moving downwards. The mechani-
cal work needed for this is supplied by the rotor
acting as a flywheel (see Fig. 6).
P
V
P1
Fig. 1 Heating
P
V
Fig. 2 Heating
P
V
P2
Fig. 3 Expansion
P
V
P1
Fig. 4 Cooling
P
V
Fig. 5 Cooling