How Horsepower Works

The word “horsepower” is constantly mentioned regarding car and industrial engines. This edition of How Stuff Works defines the term, and explains how to measure it.

Definition

The term horsepower was coined by engineer James Watt, who is most famous for his work on improving the performance of steam engines. We are also reminded of him every day when talking about 60-W light bulbs and kilowatts.

The story goes that Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. He ultimately pegged the measurement of one horsepower at 33,000 ft-lb of work/min. Any combination of weight lifted and distance moved in one minute totaling 33,000 is one horsepower. This strange, arbitrary unit of measure is still used today.

Horsepower can be converted into several other units. For example, 1 hp is equivalent to 746 W or 2545 Btus/hr. In other words, if you took a 1-hp horse and put it on a treadmill, it could operate a generator producing a continuous 746 W. If you took that 746 W and ran it through an electric heater, it would produce 2545 Btus/hr (Btu is the amount of energy needed to raise the temperature of one pound of water one degree F). One Btu is equal to 1055 joules, 252 gram-calories, or 0.252 food calories. Presumably, the horse would burn 641 calories in 1 hr doing the work if it were 100% efficient.

Measurement

A dynamometer is used to determine the horsepower of an engine. The dynamometer places a load on the engine and measures the amount of power it can produce against that load at different rpms.

For example, you might hook an engine to a dynamometer, floor it, apply enough load to keep the engine at 7000 rpm, and record the load. Additional load is applied to knock the rpm down to 6500, 6000, 5500, and so on. You can also start at a low rpm and work upward.

The dynamometer is actually measuring engine torque (in ft-lb). To understand the concept of torque, imagine that you are removing a bolt using a socket wrench with a 2-ft-long handle. You apply 50 lb of force to that handle. In other words, you are applying a torque, or turning force, of 100 ft-lb to the bolt. You could get the same 100 ft-lb of torque by applying 1 lb of force to the end of a 100-ft handle or 100 lb of force to a 1-ft handle.

Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft. A dynamometer measures this torque. Multiply the torque by rpms/5252 to determine horsepower.

Plotting horsepower versus the rpm values produces a horsepower curve for the engine. The graph below reveals than all engines have a peak horsepower, which is the rpm value at which the power available from the engine is at its maximum.

An engine also has a peak torque at a specific rpm. You will often see this expressed in a brochure or a review in a magazine as “320 hp @ 6500 rpm, 290 ft-lb torque @ 5000 rpm.” When someone says an engine has “lots of low-end torque,” they mean the peak torque occurs at a fairly low rpm value, like 2000 or 3000.

The curve also reveals where the engine has maximum power.

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