Page 23 – Self Adjustment

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We are used to the idea that we plug something in, turn it on and there it is – done deal!

A multimeter. Select the item to be read – say volts. Set the range, apply the probes.

Not so when measuring a running Stirling engine.

Having waited to get it to thermal equilibrium, the engine is running steadily under fairly
heavy load. Say at 370 revs. The red point, bottom left, on the busy chart.

You release the load, and a minute later, you take another reading.
The revs leap up to almost 400 rpm.
Another minute just over 400 rpm
Then they fall back to just under 400rpm

The load is then released further. The revs jump to 425
And a minute later, the watts and revs fall slightly.

What is this zig-zag pattern about?
With each release of load, the revs soar, the watts rise – then they both fall back.

When the engine is under heavy load, the revs are low. Heat is being poured into the hot cap
at a steady rate by the burner, but at low revs, that heat is being taken out at a slow rate.
This lets the hot cap rise to a higher temperature.

As soon as the load is reduced, that enhanced hot cap temperature make the revs soar.
But those higher revs take heat out of the hot cap more rapidly.
The hot cap temperature drops – and the revs. Until steady state is reached.

We see this repeatedly and reliably when in the boat in a lock.
The engine is sat there – no revs, but with the burner turned down low.
The hot cap temperature rises gently.
When it is time to leave the lock, the hot cap has a higher temperature and there is a surge
of power as the engine crops the extra stored heat in the hot cap.

So we have, left to right, in red on the chart, successive small power surges.
Each time the load is changed, the engine self adjusts.

On the far right, around 525 rpm, the engine is now running fast with low load.
The reverse pass now starts, in green, with successive increases in load.


Again, a zig-zag chart as, with every increase in load, the revs fall.
Each time the hot cap temperature rises with the lower speed removal of the heat.
And the revs (and power) rise slowly to regain equilibrium with each new setting.
A clear example is around 470 rpm. It took 5 minutes of readings to reach 74 watts.
The blue line is a computer generated curve to fit between all of the points.
Reality lies between the two passes.

Nota Bene: Take multiple readings at each load setting. Self adjustment takes time