Page 19 – How NOT to Measure Engine Power

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Earlier, I pleaded that newcomers to Stirling engines do not innovate with their first engine.

Now, I plead that when it comes to measuring the power of an engine,
that you do not fall prey to this tempting idea.

Do not hook up a generator to your engine, connect some 20 watt car light bulbs
and claim that because three of them are alight that the engine makes 60 watts.

The are many ways in which this not producing correct results

How is the engine shaft connected to the generator?
A drive belt is the worst. Drive belts slip. During a revolution of the engine, the force
from the engine changes quite a lot. This change in tension causes slippage.
This is why toothed drive belts are used widely.
But engine power is used in flexing belts.
Some of the engine power is lost - before it even gets to the generator.

The DC Generator. How efficient is it? Most of man’s engines are less than 50% efficient.

Worse, my understanding of the DC generator is that its efficiency varies with its speed.
Low speeds – low efficiency. 2000 rpm – maybe 40% efficiency.

The 12 volt, 20 watt car sidelight bulbs. Yes, if you put 12v through them, they can take
20 watts without burning out the filament.
The 20w rating is the maximum load they are designed for.

A bulb may be glowing quite brightly but it is certainly not using 20 watts.
It might have 8 volts across it and say 1.2 amps going through it. That’s 9.6 watts.

Do not claim that with three bulbs alight, the engine is making 60 watts.

It is not.

Gaspard de Prony sorted this all out back in 1821.

The “Prony brake” uses action and reaction to produce force that is easily measured.

This where we get the term brake horsepower – BHP.

Our engines are more into brake wattage rather than horse power and, unfortunately,
for these low power values, we have to make our own equipment.

The principle is simple – a clamp is fixed around the rotating shaft.
It grips the shaft
and acts as a brake.

But the brake shoes want to spin with the shaft.

A lever is attached to the brake and if the shaft is tightly clamped,
then the lever
needs quite a lot of force to stop it spinning.

If the end of that lever pushes on some scales,
then we can measure the force that the brake is placing on the shaft.

The power is calculated from measuring the rpm of the engine,
the weight exerted at the end of the lever,
and the length of the lever.