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Torque of a small gasoline engine

  1. Oct 10, 2014 #1
    Does someone know how much torque a 20hp gasoline engine produce at 3500rpm? An equation that I can use to figure this out would be helpful also.
  2. jcsd
  3. Oct 10, 2014 #2
    Found the formula! 5,252×20÷3,800=27.6421052632 I used 3800rpm instead of 3500rpm - (note so not to confuse anyone)
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    hp rpm torque

    Attached Files:

  4. Oct 10, 2014 #3


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    HP = T * RPM / 5252, where T is torque in lb-ft

    For your motor, HP = 20, RPM = 3500, therefore doing a little algebra, T = 20 * 5252 / 3500 = 30 lb-ft of torque produced by the motor at this output condition.

    Since 1 HP = 33,000 lb-ft/min, and 1 revolution of the output shaft = 2 pi radians, then the factor 5252 = 33,000 / (2 pi).

    The formula works for any motor where the output is in HP and the speed is in RPM.
  5. Oct 14, 2014 #4


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    Keep in mind though that this only works if the motor is producing 20hp at 3500rpm. What is colloquially called a "20hp" motor is really a motor which, when operated at its peak performance point, outputs 20hp. If peak performance occurs at some other point than 3500rpm, the torque at 3500rpm will be lower than your calculated number.
  6. Nov 22, 2014 #5
    Bear also in mind the fact that the torque produced by any IC engine fluctuates quite substantially. On a single cylinder engine, the instantaneous torque may actually reverse. So, while the power calculation (based on average torque and average speed) can give you an average torque value, instantaneous values will be all over the place.
  7. Nov 23, 2014 #6
    Indeed it does. Here's a calculated torque output from a single cylinder engine showing negative torque on compression and exhaust and exhaust strokes.

    The peak torque is showing around 430lbft, however this is actually representative of a single cylinder in a four cylinder engine producing an average torque value of a mere 138lbft.

    Attached Files:

  8. Nov 23, 2014 #7
    Kozy, thanks for posting the pressure - crank angle curve. That looks just about right.

    It also show how much higher the instantaneous maximum torque is compared to the average torque, as well as showing the torque reversal.
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