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How to calculate the power of an engine

Updated February 21, 2017

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.

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  1. 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 480hp at 2,000 RPM. You can calculate torque and horsepower expressions at both speeds.

  2. 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 = 480hp x 5,252/2,000 = 1,260 foot-pounds of torque.

  3. 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 = 411hp.

  4. 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 411hp, while at 2,000 RPM, the engine only produces 1,260 foot-pounds of torque while delivering 480hp. 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.

  5. Define the high-speed engine application. A Formula One race car may produce 1,200hp 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.

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

  7. 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 466.48hp, not even close to half of the horsepower rating of this racing engine, where speed is everything.

  8. Tip

    Always insist on high-torque diesel engines for lugging heavy loads, and high-output, high-speed engines for winning the weekend races.

    Warning

    Always obey speed laws with powerful engines on public roads.

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Things You'll Need

  • Calculator or spreadsheet

About the Author

Pauline Gill is a retired teacher with more than 25 years of experience teaching English to high school students. She holds a bachelor's degree in language arts and a Master of Education degree. Gill is also an award-winning fiction author.

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