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Measuring engine torque

  1. Feb 24, 2009 #1


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    i've got an engine to work with, and i have the basic specs of it like peak hp @ an rpm. but, i don't have figures for torque. I need to know how i can measure the torque of the engine with different fuels. I calculated the torque of the engine from their hp figure, but i'm having trouble figuring out a way i can measure the torque once i put in various types of fuels (biodiesels).

    the engine won't be in a vehicle, and i don't have access to a dyno. i know its possible to calculate torque from acceleration, speed, and time, but i can't seem to find it/figure it out.

    thanks in advance
  2. jcsd
  3. Feb 24, 2009 #2
    Your going to need a dyno if you want to do any meaningful measurements. Without putting a load on the engine there is just no way that you can accurately compare performance of different fuels or even take any actual torque measurements.

    The only way I can think of doing is to connect the output shaft to an oversize flywheel in a torque transducer in between. Throttle up the engine and record the angular acceleration.
  4. Feb 24, 2009 #3


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    I don't even know how you have calculated peak torque from the peak horsepower figure. You definitely need some driven equipment (preferably a dyno) to measure torque. A heavy flywheel might give you a pretty good estimate unless the engine is turbocharged (which, given that you're talking about biodiesels, is a definite possibility).
  5. Feb 24, 2009 #4
    You can't do it eoe, not without practical data.

    Fuel is the worst possible variable as an accurate combustion model is incredibly difficult to find for simulations and requires vigorous validation.
  6. Feb 24, 2009 #5


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    i calculated the torque from looking at the formula for power. and i used the engine's measured stroke as the distance, but now i'm thinking that doesn't really work? all i'll have is a hand held tachometer (with a laser) that i will measure off the flywheel. and the engine isn't turbo'd, its a small engine rated at 6.5hp. If i can't measure torque, i can just think of another way to determine which fuel will let the engine be more powerful.

    would it work if i were to time how fast it takes the engine to go from idle rpm to peak rpm (3600) with each fuel? I think i'd have to bolt something on the flywheel to put the engine under load in order to time it better.

    thanks for the replies
  7. Feb 24, 2009 #6
    Time to get building your own dyno then. A cheap and cheerful option would be to use a heavy flywheel and brake it somehow with a known amount of force. This will give the engine a load and you can measure the rpm drop, an engine with higher torque output will keep the flywheel spinning at a constant rate under a higher load.

    You will nead to repeat this at several different rpm to get an indiation of the variation of torque. Also be sure to note the rpm fluctuations as you will also have some indication of the cyclic variation.

    Edit found this site, not read it all but it looks useful: http://www.dyno-dynamometer.com/how_dyno_works.htm [Broken]
    Last edited by a moderator: May 4, 2017
  8. Feb 24, 2009 #7


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    thanks chris, that site is extremely useful. I think i've got it figured out now. it has a part about an "intertia dyno" which is just like what i was describing, so i won't need to provide a load for on the flywheel besides its own weight. i really appreciate all the help
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