Page 9Essential Knowledge

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If you want to drive a car, there’s quite a lot that has to be taught and learned.

You take lessons from a driving instructor, have to take a written examination and a practical
driving test on the road. That’s the law in the UK.

An important part of this “learning curve” is about the engine and the car – and how to use it.
Pulling away from a standing start has to be learned. Changing gear with a manual gearbox.
Knowledge about stopping distances being the square of your speed.

Similarly, there are “things you need to know” about the use of a Stirling engine.
Because Stirlings have little in common with our everyday experience of petrol engines.

Let’s start with the lawnmower and its pull cord. (A pretty abysmal situation
but a worldwide “that’s how it is” pretty poor setup).

The bigger the “pull” that you can achieve – the better. Sudden, fast and strong.
The reason is this four stroke business. The spark plug will only spark every fourth stroke.
Every other
revolution of the engine. If you can spin the engine twice – there is only one
chance that the plug will spark and start the engine.
If you can get it to spin for four revolutions, then there are two chances that it might start.
you have to provide the effort to compress the gas to get the engine through that fourth stroke – the compression stroke.

Stirlings are very different. There is no spark plug. Or four strokes to the cycle.
Once the hot end of the engine is hot enough – it
wants to run!
As soon as it is running, there is no compression stroke. But for the first turn – there is
compression. It is necessary to lever, or pull the engine past the first contraction stroke.
The cold air inside the engine has not been cooled/contracted below atmospheric pressure.
Once that cold gas has been shifted to the hot end and heated - then the expansion stroke
will push – and then the rest follows. Cooling causes a partial vacuum and the contraction
stroke keeps the piston moving.
So, absolutely no need for a huge spin to get revs – and energy – into the flywheel.

To start a Stirling you must WAIT for it to get hot enough.

It’s like making a piece of toast.
It takes time for the heat to get to the piece of bread and do the job.

When it is hot enough, the engine
will run. Just wait.

Cranking away at a Stirling when it is not hot enough is counter productive.

When a Stirling is running, heat is taken from the hot end and turned into rotational
energy. The temperature of the hot end is
So, when the hot cap is not hot enough, and you crank the engine, you are actually making the hot end even less hot – and less able
to run.


What NOT to do

When a Stirling is running, it is essential to keep an eye on the state of the cooling water system.
It’s obvious that the hot end of the engine must be kept hot.
The burner must be alight – to keep the engine running.
Likewise, excess heat must be removed from the cold end by the cooling system.
The cold end must be kept cold.

If the cooling system fails, then the engine power will fade. The cold end of the engine
just gets hotter and hotter. Too hot. The engine will slow and stop.

The problem is that once heat has got into the cold end. It takes quite a long time to
get the heat out. The cooling system will have to run for quite a while to restore
the coldness of the cold end.

It is essential to keep the water in the cooling system and flowing well.