Measure the Power of your Engine
Back in 1772, James Watt wanted to compare the power of horse driven machinery with steam machinery.
Over a four hour period, he observed that a “brewery horse” could lift 180 lbs
by turning and coiling rope around a 12 ft radius mill wheel, 144 times per hour.
Multiplying up, that is 32,572 ft lbs per minute.
Presumably coal, or water, was being raised out of a mine shaft.
A year later, 1773, Messrs Watt and Boulton agreed that the horsepower shoud be 33,000 ft lbs per minute.
(Easier to calculate?)
So, the weight was lifted a vertical distance of 2 x pi x Radius x No of revolutions
and the rate at which Ft lbs of work was done is:
Weight in lbs x 2 x pi x 12 ft x no of revolutions / minutes taken
But to convert to HP - divide by 33,000
HP = Lbs weight x 2 pi Foot Radius x RPM / 33,000
It would be nice if our Stirling engines could make horsepower, but we are not there yet.
There are 746 watts to a horsepower – so it is more convenient to rate our engines in watts.
Converting to kilograms and centimetres, the formula becomes:-
Watts = WEIGHT Kg
x DIAMETER cms
Rather than raising weights in a mineshaft, Gaspard de Prony devised his Prony Brake.
Courtesy of action, and its equal and opposite reaction,
a brake clamped to an engine shaft with a lever, will exert force at the end of that lever.
And the force, or weight, can be measured with a scale.
But, if you have yet to catch up with “metric matters”, the Imperial formula is
Watts = WEIGHT ounces
x RADIUS inches
/ 1 million
(It should be divide by 1,008,405.
But I'll divide by 1 million)