Page 8 The Fundamentals

Previous page______ Index______ Next page

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.