Page 15 A Gas Tight Piston

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The hot cap and its fins are the source of the energy for the hot gas inside the engine.
The piston is the device that captures that energy and transmits it to the shaft.

To do so, it must be gas tight and have low friction.

World wide there are billions of pistons in internal combustion engines.
Surely, the Stirling engineer can pick and choose from all this proven technology?

Sadly, the internal combustion (IC) engine piston has far too much friction.
The pressure generated by the diesel or petrol engine is colossal. Multiple piston rings can
be needed to stop the gas from seeping past the piston. And they must grip the cylinder walls
strongly – with friction – to stay gas tight.

The pressure differential between top and bottom of the Stirling piston is hundreds of times smaller than in the IC engine. In the UK Stirling boats, various solutions have been found.

They all work, but some call on specialist skills or materials.

What I suggest is that, out of simplicity, we should use proven technology from yesteryear.

Something like 20,000 Ericsson and Rider Stirling engines were made in the late 1800s.

And a few are still working 150 years later.

They use a leather seal.

Take a trip to the UK Hereford Water Works museum. They have two 6ft high Rider engines.
They are called Hayward Tyler engines because they were made under license in the UK.

These are workhorses. They were used all day long, every day. The leather seal works.

We use them successfully in the boat engines.

The beauty of a leather seal is that it accommodates imperfect pistons and cylinders.

High precision is needed to make perfectly round, matching pistons and cylinders.

In private workshops, this can be achieved – but leather makes up for any discrepancy.

How is a leather cup seal made?

Buy an offcut from a leather goods market stall trader. Medium thickness.

Cut a disc, say 20mm larger than the piston diameter.

Soak the leather in water. It becomes flexible.

Make a mould to the size of the piston and cut a groove around the top.
The groove needs to be say 5 mm from the top.

Stretch the wet leather over the mould and over the sides, across the groove.

Twist grip a wire around the groove to hold the leather to shape.
Pull the leather skirt to remove folds.

Leave the leather to dry out overnight.

Cut around the clamping wire to reveal a circular cup seal.

Oil the cup seal with a light oil and make the leather flexible.

Cut a centre hole to allow a bolt to fix the seal to the top of the piston.

Hold the seal down onto the piston with a washer just smaller than the piston.
This supports the upturn of the cup seal when in use in the engine.