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.
by three methods.
Radiation. We feel the heat of the
sunshine. That is the heat of radiation.
Saucepans have long handles. Hold the far end – or get
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
between itself and the cylinder wall. This maximises the chance
that molecules touch
the metal of the hot cap wall and pick
Unfortunately, a red hot hot cap is also
radiating heat. When it is red hot we can actually
red heat. It radiates in all
directions. Even outward and away from the engine.
Heat is being lost!
that heat can be reflected back towards the engine and its hot
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.
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.
is reflected back to where we want it.
On the measurement
bench, we have seen 10% extra engine power from this.
out there on the water, in a breeze, the hot cap looses less heat
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
than the equivalent coal bucket. A propane
cylinder is just a lot more convenient.
more heat flow than butane)
It’s not just
economy of space – there’s also cost.
long used small propane cylinders.
Sadly, they are
expensive. But there has been a recent development!
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.
a fraction of the cost. A day’s boating is costing us £2.50
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
Never cast off before starting the engine. Always
start the engine – then cast off.
for a Stirling boat is
1. Light the burner.
the heat builds, load the picnic – and the booze? Then
3. Time has now passed and heat has built in the
engine. It’s time to start it.
4. Cast off
return to the mooring, it’s time to stop the engine.
this is a hot Stirling – it wants to run. You can turn the
but the hot cap is still hot and the engine
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.