Now for the raw basics as to
what makes a Stirling engine work.
The diagram opposite
is the same type of Stirling engine (there are three basic
type originally patented by the Reverend Robert Stirling in
It’s know as a beta engine – where
the piston is underneath a plunger
(known as the displacer)
inside the same
cylinder as the piston.
is a very compact arrangement and ensures that the entire quantity
of the trapped air
is either fully heated or fully cooled.
One of the “snags” about Stirling engines is
called “dead space”. This is space,
within the engine, which contains air that is neither fully
nor fully cooled. This air acts as a sort of sponge, reducing the
power of the engine.
The linkage at
the bottom has two con rods. When the piston moves and turns the
the linkage makes the plunger, the displacer, move
ahead of the piston.
So, when the piston is at the top
of its, travel – top dead centre – TDC – the
moving rapidly downwards. It is mid stroke.
displacer is a plunger. When it moves, the air has to go round it.
When the displacer is
mid stroke and travelling downwards,
the air is being moved to the top of the cylinder.
of the cylinder is being heated – from the outside –
and this heats the air inside.
The molecules in the air,
getting heated, are bouncing around with far more vigour
energy. They bump into each other and make pressure which
is transmitted, extremely
quickly, to the piston –
which just happens to be ready to move downwards under the force
of the hot pressurised air. The shaft turns.
the reverse happens, the displacer moves upwards. The air gets out
of the way and
moves downwards to the end that is not heated.
The cold end. The molecules “calm down” and the
pressure reduces. The piston can return to start the cycle all
In summary, repeated heating and cooling of
the air creates a pressure oscillation which drives the piston and