Page 10Practical Heat Matters

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The whole idea is to get the hot cap as hot as possible.
Then the heat of the metal can heat the gas inside the engine cylinder.

Heat travels by three methods.
Radiation. We feel the heat of the sunshine. That is the heat of radiation.
Conduction. Saucepans have long handles. Hold the far end – or get burned!
Convection. Hot air molecules, blown by electric fan heaters, touch our skin and warm us.

It is this convection that heats the air inside the Stirling. The displacer has
only a narrow
gap between itself and the cylinder wall. This maximises the chance that molecules touch
the metal of the hot cap wall and pick up heat.

Unfortunately, a red hot hot cap is also radiating heat. When it is red hot we can actually
see the red heat. It radiates
in all directions. Even outward and away from the engine.
Heat is being lost!

But that heat can be reflected back towards the engine and its hot cap.
You may have noticed that, at the end of a marathon race, runners are often clad in a sort
of cloak. They wrap a sheet of thin reflective aluminium around themselves.
This reflects their body heat back at themselves and keeps them warm.
Cooking foil uses the same idea.

If we shroud the hot cap of our engines with aluminium, the heat is not lost.
It is reflected back to where we want it.
On the measurement bench, we have seen 10% extra engine power from this.

And when out there on the water, in a breeze, the hot cap looses less heat to convection
from the breeze.

A double win!

The burner is very important for the engine.

Out in a boat on the water, the space in the boat is limited. This affects what fuel
we use. We could use wood – but the size of the wood basket needed is far bigger
than the equivalent coal bucket. A propane cylinder is just a lot more convenient.
(Propane delivers more heat flow than butane)

It’s not just economy of space – there’s also cost.
We have long used small propane cylinders.
Sadly, they are expensive. But there has been a recent development!

Refillable LPG cylinders are now available. In the UK they are sold with the brand name of “Safefill”. The cylinder is bought, not rented and it can be refilled just like the tank in an LPG car.
At a fraction of the cost. A day’s boating is costing us £2.50 (in 2019).

Another heat issue.
When starting an engine, it takes time for heat to build up and enable engine starting. So, just follow one of the golden rules of boating.
Never cast off before starting the engine. Always start the engine – then cast off.

The sequence for a Stirling boat is
1. Light the burner.
2. While the heat builds, load the picnic – and the booze? Then passengers.
3. Time has now passed and heat has built in the engine. It’s time to start it.
4. Cast off

On return to the mooring, it’s time to stop the engine.
But this is a hot Stirling – it wants to run. You can turn the burner off,
but the hot cap is still hot and the engine continues run.
Our practice is to leave it running. Tied up – even in gear with the propeller pushing.
It’s a quick way to get the heat out of the hot end and stop the engine.