Page 8 – The Fundamentals
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
types)
as the
type originally patented by the Reverend Robert Stirling in
1816.
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.
This
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
something
called “dead space”. This is space,
within the engine, which contains air that is neither fully
heated
nor fully cooled. This air acts as a sort of sponge, reducing the
power of the engine.
What happens?
The linkage at
the bottom has two con rods. When the piston moves and turns the
shaft,
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
displacer is
moving rapidly downwards. It is mid stroke.
The
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.
The top
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
and
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.
Then
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
over again.
In summary, repeated heating and cooling of
the air creates a pressure oscillation which drives the piston and
the shaft.
