Building a Large Prony Brake

How Much Power Does That Engine Really Produce? A Working Prony Brake Provides the Answer

| January/February 2003

Working Out Design Considerations and Initial Details: Part One of a Two Part Series

Dennis Jacowski gives Mike Johnson's 50 HP Case a work out on Bruce Babcock's Prony brake at the 2002 Mad River Steam & Gas Show in Urbana, Ohio.


About three years ago, I built a small Prony brake to take to engine shows so people could see how small hit-and-miss gas engines perform working under a load. A Prony brake belted to an engine not only supplies a uniform load on the engine, it also measures the engine's horsepower output, and it demonstrates how horsepower was measured 100 years ago. Running my small Prony brake, I noticed it wasn't just engine owners who were interested in the brake many spectators stopped to examine the device, ask questions and watch it in operation. At the first show where I exhibited the brake, I recall an elderly gentleman proclaiming, 'Now I know what they mean when they say 'brake' horsepower!' My small Prony brake is described in detail in 'The Design, Construction and Use of a Small Prony Brake' in the July 2000 issue of Gas Engine Magazine.

A Short History of the Prony Brake

The concept of rating engines, water wheels and windmills in terms of horsepower dates back to the early 1700s, but it wasn't until the late 1700s that anyone made a real effort to determine just how many foot-pounds per minute a horse was capable of producing. Fittingly, it was James Watt, the father of the modern steam engine, who was the first person to make this determination. Watt was selling steam pumping engines, and he needed a reliable way to calculate the output of his engines so potential customers would have an idea of their capacity. Watt studied mine ponies lifting coal at a coal mine, and he found that, on average, a mine pony could perform 22,000 foot-pounds of work every minute. Put another way, he found that a horse exerting one 'horsepower' could lift 220 pounds of coal 100 feet in one minute.

Watt, however, wanted to make sure customers would have no reason to complain of the power output of his engines, so he arbitrarily increased this figure by 50 percent. His final formula determined that one horsepower was equal to 33,000 foot-pounds of work a minute. This meant a five horsepower Watt engine would do significantly more work than five horses. It's interesting to note that Watt's number inflation created at least a certain level of trouble for users of gas and steam engines for over 125 years, as builders of engines continued the practice of under-rating their engines.

Watt applied his horsepower formula to his pumping engines, and by using the number of gallons pumped, the height the water was raised and the time required for a given volume he was able to calculate equivalent horsepower output. However, when Watt began building 'rotative' engines (engines with a crank and flywheel) he did not have a reliable method of calculating power output. Some time around 1800 his assistant, John Southern, invented the steam engine indicator, and Watt may have used this to calculate the input horsepower of his engines. But because of the low efficiency of his engines and the primitive construction of the indicator, he could probably only get a rough estimate of the power delivered to a rotating load. It is, however, from this exercise that we get the term 'indicated horsepower.'


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