How to Calculate Motor Efficiency

••• luxury car - model toy car image by alma_sacra from

When designing engines and motors, engineers aim for high efficiencies. The transformation of energy from potential to mechanical within engines causes a large percentage of it to be lost as heat due to friction and physical deformation. The easiest way to calculate the efficiency of an engine is to divide its work output by the energy input. This is easy to do once some basic measurements of the motor's performance are taken.

    Write this equation for the work done by a motor:

    Work = mass x acceleration x distance (Keep in mind that Force = mass x acceleration.)

    Work is measured in joules (J), mass in kilograms (kg), acceleration in meters per square second and distance in meters (m).

    Calculate the amount of work that is done by your motor.

    For example, consider a 600 kilogram (kg) car that takes 10 seconds (s) to accelerate to 45 m/s over a distance of 225 m

    Work = 600 x (45/10) x 225 = 607,500 J

    The car's motor has performed 607,500 J of work.

    Measure the amount of energy that was supplied to the motor to accomplish the work.

    Imagine that the 600 kg car burned 69 milliliters (mL) of gasoline to accelerate as it did. Gasoline in the United States has an energy content of approximately 35,000,000 joules per liter (J/L), so the energy in 69 mL can be calculated thusly:

    Energy in 69 mL of gasoline = 35,000,000 J/L x 0.069 L = 2,400,000 J

    Calculate the energy efficiency of the motor by dividing the work produced by the energy supplied and turning the answer into a percentage.

    The car's motor produced 607,000 J of work and used 2,400,000 J of energy to do it, so:

    Energy Efficiency = 607,000 / 2,400,000 x 100 percent = 25.3 %

    The car's motor is 25.3% efficient.


About the Author

Timothy Banas has a master's degree in biophysics and was a high school science teacher in Chicago for seven years. He has since been working as a trading systems analyst, standardized test item developer, and freelance writer. As a freelancer, he has written articles on everything from personal finances to computer technology.

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