We are used to the idea that
we plug something in, turn it on and there it is – done
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
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
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
With each release of load, the revs soar, the
watts rise – then they both fall back.
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.
the hot cap rise to a higher temperature.
As soon as
the load is reduced, that enhanced hot cap temperature make the
But those higher revs take heat out of the hot cap
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.
engine is sat there – no revs, but with the burner turned
The hot cap temperature rises gently.
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.
we have, left to right, in red on the chart, successive small
Each time the load is changed, the engine self
the far right, around 525 rpm, the engine is now running fast with
reverse pass now starts, in green, with successive increases in
a zig-zag chart as, with every increase in load, the revs
time the hot cap temperature rises with the lower speed removal of
the revs (and power) rise slowly to regain equilibrium with each
clear example is around 470 rpm. It took 5 minutes of readings to
reach 74 watts.
blue line is a computer generated curve to fit between all of the
Reality lies between the two passes.
Bene: Take multiple
readings at each load setting. Self adjustment takes time