Page 28 – The Strain Gauge

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Gaspard de Prony’s brake concept is still true but instead of brake shoes to load the engine,
we can now use electrical resistance load.

Looking at the photo, a DC generator is mounted on gimbals. The generator shaft
must protrude from both ends of the generator to enable ball bearing mountings.

In this way, the generator can spin with the reaction to the force made by generating current.

To restrain this spinning movement, a strain gauge is mounted between a post
and a clamp gripping the body of the generator.

The Stirling engine is then coupled mechanically to the generator shaft.

The generator output is then taken either to a bank of resistors or to a set of filament lamps.

Earlier, I was critical of the use of generators because their efficiency varies with rpm.
With this method the generator efficiency does not matter.
The strain gauge reading is thing that matters.

If the generator is very inefficient, the generator will get hot and only a little electricity will be produced. If it is less inefficient, there will be somewhat less heat and somewhat more electricity. Either way the load on the strain gauge will be the same – and true.

Should you have a bank of resistors, or a set of filament bulbs? I favour the use of car
sidelight bulbs. There is heat generated by both, but the light bulbs can handle it.
With higher wattage engines I have had to water cool the resistor bank.

The distance of the strain gauge form the generator shaft is the prony brake radius.
Readings are smoother and are more accurate than with scales and a friction brake.

And there is no smell of burning (or smoke) from a wooden prony clamp!