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Homework Help: Torque and power produced in an engine

  1. Jun 7, 2010 #1
    1. The problem statement, all variables and given/known data
    Why does an IC engine generate about the same maximum torque over a range of engine speeds but the maximum power at only one speed?

    2. Relevant equations

    3. The attempt at a solution
    For max power, the best I can come up with is that since torque does not drop as fast as rpm is increasing, max power occurs after peak torque.

    As for why max torque can occur over a range of engine rpm, I have no clue.
  2. jcsd
  3. Jun 13, 2010 #2

    Filip Larsen

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    Gold Member

    I'm no expert, but with a bit of hand waving I would say that the torque is more or less directly related to the "average" pressure after ignition in each cylinder and since this pressure is more or less the same at the same point in the stroke-cycle for a cylinder for a wide range of rpm's, the torque must also be more or less the same.

    Your explanation for maximum power sounds correct.

    I hope a late answer is better than no answer at all. If nothing else, its an interesting question.
  4. Jun 13, 2010 #3
    Thanks. The thing is, there's no mention of this in our coursebook, and because our lecturers are old men, they don't check their emails. So you'll forgive me if I don't take your answer unless you're absolutely certain


    The lecturer just released the answers. You were right after all. My apologies for ever doubting you.
    Last edited: Jun 14, 2010
  5. Jun 14, 2010 #4

    Filip Larsen

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    Gold Member

    Being skeptical about unsupported claims is a hallmark of good science, so no need for apologies, my friend.

    Even though your professor released their answers, to support my claim I may add that I did manage to find a diagram labeled "Effect of engine speed on cylinder pressure with variable optimum ignition timing" in [1] depicting cylinder pressure vs crank-angle position for various engine speeds that show the peak pressure only drops very slowly as speed increases. The "variable optimum ignition timing" here means that ignition is started earlier on the crank-angle position for higher speeds to compensate for flame propagation time, so for a very simple internal combustion engines without such compensations the torque curve may not be as flat as it is for a modern or more advanced engine.

    [1] Advance Engine Technology, Heinz Heisler, Butterworth-Heinemann, 1995.
  6. Jun 14, 2010 #5
    Ah thankyou.
  7. Jul 7, 2010 #6
    Friend what I think is torque developed depends on amount of air and fuel gets into the engine cylenders. This is almost equal for a range of rpm. So max. torque is defined for a range of rpm
  8. Jul 8, 2010 #7
    Horsepower is a function of torque over time. Put simply, HP = Torque x RPM / 5252. (Google will provide numerous descriptions of the derivation.)
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