Imagine a single cylinder, 4
stroke, motorcycle engine. Block off the inlet and outlet
Remove the spark plug and block off its hole.
is trapped inside the cylinder.
Imagine that we could
heat the gas inside the cylinder – above the piston.
the temperature of the gas rises, pressure is created. In just the
same way as a petrol explosion – but far, far weaker and
slower. That pressure pushes the piston - the shaft turns.
imagine that we can cool the gas above the piston.
shrinks. The pressure reduces and the piston now returns.
the process of heating and cooling. The piston pushes and then
This is the Stirling engine.
how could we heat the gas, and then cool it – more
Let’s make the cylinder much longer.
Heat the top end, from the outside, and cool the other end
– say with a water jacket.
So, the top end of the
cylinder is a source of heating and the other end, a source of
If a large plunger is now placed inside the
cylinder, then it can be used to move the air from one end to the
When the plunger is at the bottom, there is no room
for the air at that end. The air has to be at the top –
where it gets heated.
When the plunger is moved to the
hot end, then the air has to move out of the way and the only
place to go is the other, cold, end. Where it is cooled and
We now have a mechanism to cause the air to be
heated and cooled and “work” the piston and
crankshaft. It was initially called a plunger, but now it’s
called “the displacer”.
The air is displaced by
There is one last thing that
needs to be added.
And this is the “clever”
If a bundle of thin wire is put half way between the hot
end and the cold end, then, as the air is displaced, or moved,
then it has to pass through the wire mesh.
When the hot
air is moving through the wires, it looses heat to the wire. It is
cooled – before it even gets to the water cooling
Then, when the air is moved back from the cold
end, it now has to pass through the wire bundle again. The wires
are warm. They heat the air on its way to the hot end.
this “to and fro” movement of the gas through the
heat is transferred between the gas and the
The heat in the gas is repeatedly
The bundle of wires is known as the
This idea of the regenerator
was the brain child of the Reverend Robert Stirling.
He is rightly famous.
He and his
younger brother, James went on to build various engines.
“Dundee” Stirling engine produced 40 hp (Horsepower).
In the late 1800's,
John Ericsson and Alexander Rider produced many thousands of Stirling
engine water pumps in the USA.
Two of these engines can
be seen at the Hereford Water Works museum in the UK.