# How to Calculate the Power of an Engine

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Designers of different types of internal combustion engines focus on expressions of force and speed that best convey engine power for their own context of use. Race car designers concentrate on producing peak horsepower (hp) at extremely high RPM (revolutions per minute). Diesel truck engine builders tout load-lugging torque (T) produced at low RPM. What complicates the issue of internal combustion engine power is that peak torque and peak horsepower are usually not produced at the same RPM, as with electric motors. This can make a direct comparison of engines confusing.

## Calculating the Power of High-Torque Engines

• Define the high-torque engine application. For example, the diesel engine in a 40-ton cement truck may produce 1,800 foot-pounds of torque at 1,200 RPM, making it extremely useful for moving the truck up steep inclines. Yet it may produce its peak 480 hp at 2,000 RPM. You can calculate torque and horsepower expressions at both speeds.

• Calculate the output torque at 2,000 RPM where maximum horsepower is produced. According to the motor formulas, T = hp x 5,252/RPM. Plugging in the rated values, T = 480 hp x 5,252/2,000 = 1,260 foot-pounds of torque.

• Calculate the horsepower output at 1,200 RPM where maximum torque is produced. According to the motor formulas, hp = T x RPM/5,252. Substituting, hp = 1,800 foot-pounds x 1,200 RPM/5,252 = 411 hp.

• Compare the power output results. At 1,200 RPM, the heavy diesel engine produces 1,800 foot-pounds of torque to lug the heavy loads, and only 411 hp, while at 2,000 RPM, the engine only produces 1,260 foot-pounds of torque while delivering 480 hp. For climbing steep inclines, the driver would use lower gears to keep the engine at 1,200 RPM for the best pull, and on the highway would try to keep the engine closer to 2,000 RPM to build speed.

## Calculating the Power of High-Speed Engines

• Define the high-speed engine application. A Formula One race car may produce 1,200 hp at 16,000 RPM. Its maximum torque of 490 foot-pounds at 5,000 RPM is produced in an RPM area that is not even useful to the driver, who shifts gears to keep the engine in its maximum horsepower curve between 10,000 and 16,000 RPM.

• Calculate the output torque at 16,000 RPM where the 1,200 hp is produced. T = 1,200 x 5,252/16,000 = 393.9 foot-pounds.

• Calculate the horsepower produced at 5,000 RPM where the 490 foot-pounds of torque are produced. In this case, hp = 490 foot-pounds x 5,000 RPM/5,252, which is 466.48 hp, not even close to half of the horsepower rating of this racing engine, where speed is everything.

## Tips & Warnings

• Always insist on high-torque diesel engines for lugging heavy loads, and high-output, high-speed engines for winning the weekend races.
• Always obey speed laws with powerful engines on public roads.

## References

• Photo Credit Concrete truck in the process of pouring a job image by JKeen from Fotolia.com Engineering Trucks image by A74.FR Ben Fontaine from Fotolia.com line of trucks 1 image by Aaron Kohr from Fotolia.com racing car image by Goran Bogicevic from Fotolia.com hot rod engine image by itsallgood from Fotolia.com
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