Page 18 – The Regenerator

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The hot cap feeds heat to the gas. The expansion stroke is enabled.

When that gas is passed through the regenerator, that heat is deposited in the regenerator.
The gas continues on its way but is now cool and the pressure is reduced.

In an atmospheric engine, the pressure drops below atmospheric pressure and a partial
vacuum is created. The return “vacuum” stroke is enabled.

Then the gas passes back through the regenerator and picks up heat.
The heat in the gas is regenerated from the heat in the regenerator wires.

The gas then passes through slots in the hot cap, pressure is rising and the expansion
stroke will be enabled again.

The amount of heat needed from the hot cap is just that needed to deliver work for the shaft
and replace the heat losses. The regenerator maintains a reservoir of heat inside the engine.

The metal in the regenerator should be as thin as possible.

The heat transferred to it is at the surface of the metal. We don’t want that heat being transferred into the body of the metal. We want it on the surface, unable to go anywhere,
so that it is ready to heat the gas on its return.

Without regenerator wires, there would be a natural temperature gradient from the hot
end of the engine down to the cold end. That gradient is most likely a big enough
temperature difference to be able to run the engine.

But wouldn’t it be good if we could make that temperature difference bigger?

That’s exactly what the regenerator wires do.

As John Ericsson said of his Caloric ship:-
“Bundles of wire are worth shiploads of coal”.

How much regenerator wire should be used?

A single sheet of wire gauze for the gas to pass through – would not be enough.

That sheet would be heated to almost the full temperature of the gas,
and the gas having passed through – it would be very little cooled.

Two sheets of gauze wire.
On arrival at the second sheet, the gas has been cooled a very little bit.
The gas will transmit its heat to the wire mesh, but because it is not quite so hot,
not so much heat will be transferred into the wire.

The same for the third sheet of gauze – and thereafter, ever diminishing returns.

There is a temperature gradient across the regenerator. This photo shows the regenerator bundle from a Whispergen Stirling engine. You can see the heat burn at the top end – but not through to the bottom. There is another factor. As the gas path through the gauze gets longer, it takes more effort to push the gas through the mesh. That extra pumping effort takes more power from the engine. Eventually the extra pumping effort will outweigh the power gained from a bigger temperature difference.

Get the engine on the bench. Measure its power.
Add/remove regenerator. Measure again. Find the balance point.