Page 31 – Pressure and Helium

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When the internal pressure in the engine is raised, there are more gas molecules to provide
more force to push the piston. Some engines won’t run until they are pressurised.
Pressurisation is needed to overcome the power loss caused by dead space.

”I’ll make a pump to self pressurise the engine” says the innovative Stirling engine maker.

This is an issue where engineering enthusiasm and user requirement clash.

The Stirling boat user wants to use his boat now. Not wait while the engine self pressurises.

And it will be a long wait.

At the very time when the engine is at its weakest – it has yet to warm up – that is the time
when fast pressurisation is needed. Because the engine is still weak, the pump has to be
rather small. A small pump will take an even longer time to raise the pressure.

Philips had this problem with their Stirling generator set. What they did was to turn the
carrying frame into a pressure reservoir. To start the engine they released the pressure into
the gas circuit then started the engine. When it was warmed up, the first task was to engage
the pump and re-pressurise the reservoir. Then the engine was ready to generate electricity.

”Sir, the answer to your engineering problem is “more” engineering!”
Have a self pressurising pump and a more complicated (and longer) startup procedure
Not a good idea.

We need pressurisation. For £30 (in the UK) you can buy 400 bar litres of pressurised gas.
Let’s say your engine has 1 litre of volume inside. That’s 100 charges at 4 bar.
And it takes 4 or 5 seconds to charge the engine.

If you want engineering challenge – make an engine that doesn’t leak.

The idea is that once pressurised, the gas should stay in there.

The whole idea of the Stirling engine is to heat the gas quickly and cool it quickly.

But air (80% nitrogen) is an insulator.
It takes time for it to accept heat – or loose it.
We wear woollen jumpers in winter to trap air, insulate ourselves and keep warm.

Helium conducts heat more rapidly than nitrogen.
The gas inside the engine heats, and cools, more quickly and the engine runs faster.
The chart shows the extra power generated, and the higher speed for peak power.

In the UK, 2020, a 2 litre cylinder of helium at 200 bar costs £30

Don’t pressurise with air or nitrogen.