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Thermodynamic equation for combustion in a spark ignition engine

  1. Jan 20, 2010 #1
    if i have calorific value of mixture, volume at time of compression, bore diameter, can i know the pressure being exerted on the pistons due to the combustion?
  2. jcsd
  3. Jan 21, 2010 #2


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    No, there are lots more variables here, sorry. Think about compression ratio, stroke, spark timing, burn delay, end gas temperature, fuel composition, fuel temperature, fuel mixing ratio, engine speed, torque, piston bowl design, cylinder head design, thermal conductivity of the block, piston and head, air fuel ratio, expansion ratio......
  4. Jan 21, 2010 #3
    Plug and chug baby. This is precisely the reason for practical tests.
    Short of making lots of assumptions, there is no way to have an equation to show the pressure.

    A VERY rough and ready method is to constuct an air standard cycle, and simply assume that the peak combustion pressure is 75% of maximum, that occurs approx 13 deg aTDC.

    Disclaimer: I cannot stress just how 'pulled out of the backside' this method is. But it's good if you really are stuck for a better option. This is similar to a method they used in the 1930s to design piston engines aircraft (I can't remember the name of the book that the method was in)
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