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How to calculate BMEP from P-V Graphs?

  1. Apr 24, 2009 #1
    I have a P-V trace for a wide open throttle and a part load. Is it possible to calculate the BMEP from these graphs?
     
  2. jcsd
  3. Apr 24, 2009 #2

    Ranger Mike

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    see Horsepower - Please help - Confused! posted on Feb 14 2009

    BMEP = ( Hp x 13000) / ( liter displacement x RPM)
     
  4. Apr 26, 2009 #3
    that doesnt really help considering all i am given are values of different pressures and volumes
     
  5. Apr 26, 2009 #4

    Q_Goest

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    If you have pressure and volume you may integrate PdV to determine work, then divide by time to determine power.

    For example, if you have a table of values that give pressure and volume, you should be able to take average pressure (gage pressure, not absolute) and change in volume, multiply to obtain work, then sum all work done over a single cycle (360 degrees of crankshaft rotation for a 2 stroke, 720 degrees for a 4 stroke) and divide by time it takes for that cycle to get power.
     
  6. Apr 28, 2009 #5

    brewnog

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    Sure, integrate to find the area enclosed by the PV curve; this gives you the IMEP.
     
  7. Apr 28, 2009 #6
    Q_Goest is bang on the money with this one, I used a similar type of thing in my masters disseration. Designing a vintage V8 racing engine using simulation techniques. huzzah.

    Integrating real curves is a *****, either find a program that can make approximate curves from a set of data points or break out the graph paper.
     
  8. Apr 29, 2009 #7
    If you have no better software than Excel, you can use a trapezoidal numerical integration approach reasonably well so long as the x-interval is pretty small compared to the sample length.

    But when it comes to a manual method, use a planimeter! I've mentioned before with PV diagrams, they're absolutely fantastic and much better than counting squares :)
     
  9. Apr 29, 2009 #8
    Those are two marvellous suggestions. I feel especially silly about the numerical integration in excel as that is probably the easiest way to do it, I should have known that but never even thought to use the trapezoid rule. :(
     
  10. Apr 29, 2009 #9

    Q_Goest

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    Actually, if you used the average pressure between two datapoints then you used the trapezoid rule! :)
     
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