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

3
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 iso-
choric. The working piston is in its lower rest position
(bottom dead centre) at this point (see Fig. 1). The dis-
placement 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
P
V
P2
Fig. 6 Compression