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How does an electric dynamometer measure friction power?

  1. Jan 5, 2015 #1
    So I read that in a motoring test (to test the engine power), the engine is run to the desired speed by its own power. The power is absorbed by an electric dynamometer. Then, the fuel supply is cut-off and the dynamometer is converted to work as a motor to drive the engine at the same speed. After that the power supply to the motor becomes a measure of the friction power.
    My question is, How is this a measure of friction power? Shouldn't the power supply to motor be the same as the output power of the engine to overcome friction and also make it work at the same speed? how is it only a measure of friction power?
  2. jcsd
  3. Jan 5, 2015 #2


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    You measure the power needed to operate the engine at the given speed. The full power gets lost to friction.
    I don't understand that question.
  4. Jan 5, 2015 #3

    jack action

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    When an engine is idling at a given rpm (meaning no load, transmission in neutral) the power output is zero. This means that all the fuel burned is used to fight the friction power (which includes also the power for the oil pump, water pump, etc.). So if you cut the fuel, the energy required by an outside motor to maintain the same rpm will have to be the same as the friction power.
  5. Jan 20, 2015 #4
    That helped me very much Jack. I understand now that there is no load (delivered torque) so there is no brake power. Thanks :)
  6. Apr 20, 2015 #5
    hey jack so does that mean the friction power varies with rpm?
  7. Apr 20, 2015 #6

    jack action

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    Yes, friction power varies with rpm. In fact, the friction mean effective pressure (FMEP) is proportional to the mean piston speed (which in turn is directly linked to rpm).

    To get the friction power, you need to multiply the FMEP with the volumetric flow rate, which is also directly linked to rpm. That means that friction power varies with the square of rpm.
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