Page 3 – Comparison: The Four Stroke and the Stirling


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Imagine a single cylinder, 4 stroke, motorcycle engine. Block off the inlet and outlet valves.
Remove the spark plug and block off its hole.

Air is trapped inside the cylinder.

Imagine that we could heat the gas inside the cylinder – above the piston.
As 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.

Now imagine that we can cool the gas above the piston.
It shrinks. The pressure reduces and the piston now returns.

Repeat the process of heating and cooling. The piston pushes and then returns. Repeatedly.

This is the Stirling engine.

But how could we heat the gas, and then cool it – more quickly?

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 cooling.

If a large plunger is now placed inside the cylinder, then it can be used to move the air from one end to the other.
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 shrinks.

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 its presence.



There is one last thing that needs to be added.

And this is the “clever” bit.
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 jacket.

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.

With this “to and fro” movement of the gas through the wires,
heat is transferred between the gas and the wires.

The heat in the gas is repeatedly regenerated.

The bundle of wires is known as the “regenerator”.

This idea of the regenerator was the brain child of the Reverend Robert Stirling.
Back in1816.

He is rightly famous.

He and his younger brother, James went on to build various engines.

The “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.