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Calculation of torque of engine (mathematically)

  1. Mar 17, 2016 #1
    We have tried to convert a petrol engine into a compressed air engine back in our graduation project. At that time we only measured the RPM of engine using tachometer and pressure of compressed air using pressure gauge. Now, we need to know the power of wngine at those readings. As the setup is long gone and we cannot duplicate the trials again, can you please suggest me how to determine the torque by mathematical calculations? I am thinking of using following series of formula:

    Torque (of engine) = Force (on piston) x Radius (of crank travel)
    Force (on piston) = Pressure (of air) / Area (of piston head)

    Please tell me that can this calculation be justified?
  2. jcsd
  3. Mar 17, 2016 #2
    There are a lot of unknowns and variables that will affect the engines torque, but unless the engine was tested on a dyno or used to propel a vehicle fitted with the appropriate sensors, then it will be practically impossible to accurately determine its torque using only RPM and cylinder pressure.

    Number of pistons, rotational mass, frictional losses, volumetric efficiency, crank angle, parasitic losses(If 4 stroke multiple cylinder engine, then some cylinders will use some power during compression stroke), cylinder leakage... These are just a few things that need consideration when trying to determine just how much torque is generated.

    Best of luck with your project
    J Mc
  4. Mar 17, 2016 #3
    He says its a compressed air engine with gauge pressure measued... so he can get bloody close.

    OP the general approach is right depending on how you applied the compressed air. A pneumatic shot, a constant feed.

    Draw the problem out (ie a piston and conrod). Why is directly applying Fxd not quite right in this case?
  5. Mar 17, 2016 #4


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    From an earlier thread :

    Output power = Area of piston * Length of stroke * Mean effective pressure * number of power strokes/rev * Number of revs/second .

    Output Torque = Output Power / ( Number of revs/second * 2 * Pi )

    Mean effective pressure can be taken as about 0.85 * supply pressure .

    Number of power strokes/rev is 1 for a single cylinder single acting engine .

    The above assumes that the practical arrangement of engine and flywheel allow it to work smoothly and continuously .
    Last edited: Mar 17, 2016
  6. Mar 17, 2016 #5


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    The 0.85 factor assumes admission of air for most of power stroke . When there is early cut off this factor has to be reduced . If you have no certain information about cut off then use 0.4 .

    Probably not nescessary for present purposes but given detail drawings of the engine a good estimate of the shape of the actual P-V diagram for the power stroke could be drawn .
    Last edited: Mar 17, 2016
  7. Mar 18, 2016 #6
    It sounds like from your original post that you now need to present some data for an experiment that has been disposed of. If that is true the formulas that Nidum presented should work very well for you although as an interpretation not measured data. If you are attempting to move forward with some design process based on the experiment I believe that you will encounter some variations. More than likely this comes into play from the conversion used. Peak torque is concurrent with the point where a combustion engine is operating at peak volumetric efficiency. Four stroke engines typically operate at around 0.8 for a 2 valve, 0.85 for a 4 valve, and 0.9 for a 5 valve. These values being with a naturally aspirated engine. When you introduce forced induction the values are consistently above 1. I am assuming that your compressed air engine had no actual combustion but only ran on the force created by induction air acting on the piston. This change makes the actual values challenging to accurately predict. However if you have cam timing and overlap data one could possibly interpolate where peak cylinder pressures would occur.
  8. May 13, 2016 #7
    What is "number revs/second"
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