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
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
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
slots in the hot cap, pressure is rising and the expansion
will be enabled again.
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.
metal in the regenerator should be as thin as possible.
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.
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
temperature difference to be able to run the
But wouldn’t it be good if we could make
that temperature difference bigger?
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.
sheet would be heated to almost the full temperature of the
and the gas having passed through – it would be
very little cooled.
Two sheets of gauze wire.
arrival at the second sheet, the gas has been cooled a very little
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
another factor. As the
gas path through the
gauze gets longer,
it takes more effort to push the gas through the mesh.
extra pumping effort takes more power from
Eventually the extra pumping
effort will outweigh the power gained from a bigger temperature
the engine on the bench. Measure its power.
regenerator. Measure again. Find the balance point.