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Internal combustion engine calculations.

  1. Dec 31, 2013 #1
    Hello everybody,

    I am working on the whole vehicle calculations, engine, brakes, suspension and engine simulation, using that software, take a look:


    That software, you input the values at the top, and it calculates and outputs the results shown in the bottom of the screen

    my work is to get the formulas which are used to get those ouputs, and i am just stuck with the intake air density, of those inputs in the screen i sent.

    well, here are the formulas i used for "outside air density" calculations, and it gave me 0.0771 Ibs/cubic.ft, see how i got it, to help you understanding what i am requiring, for the "intake air density".

    3- outside air density:
    By substituting in the formula: Density = P / (R * T)
    Where p: is the outside air pressure (Ibs/inch^2),

    R: is the Gas constant = 53.35 (ft-lb)/ (lbm•R), and T: is the outside air temperature in °R,

    And to convert P from (Ibs/inch^2) to (Ibs/ ft^2), we divide by (0.083) ^2, so we get:

    Air pressure P = 14.68 /.083^2 = 2130.933 (Ibs/ft^2), so we get:

    Density = 2130.933/ (53.35*518.67) = 0.077I Ibs/cf.

    i am sure we will use same formula, but how can i calculate or get the intake air pressure to substitute in that formula, as the R is known, and T: is the intake air temperature is given as 80 F in the inputs.

    hope anyone familiar with internal combustion engines help me with that,
    will appreciate your help a lot,
  2. jcsd
  3. Dec 31, 2013 #2


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    Let me save you some time on some of the conversions:

    If you want to convert pressure in psi to psf, multiply by 144 ( = 12^2) rather than divide by 0.083^2 (= 1/12^2)
    There are 144 in^2 in 1 ft^2.

    I'm not sure I understand your question. Air density varies with temperature and altitude. I notice you have specified an altitude of 1000 feet. Are you trying to calculate the air pressure at that altitude?
  4. Jan 1, 2014 #3
    Thanks steamKing for the reply, and no worries about the conversions, as long it gives same result, or very close to the software results

    simply, what i am trying to calculate is the intake air density using that screen inputs which i sent its link, so how would calculate it, and getting same results of that screen, which is 0.002472 slugs/cu.ft, or 0.079452 Ibs/cu.ft, or 0.000040 grams/cc, so any help?

    i hope you understand now, and that i could let you understand what i need,
    Thanks again
  5. Jan 1, 2014 #4


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    Homework Helper

    Are you trying to calculate the ambient pressure or the intake manifold pressure which will be significantly less, depending on the intake setup and throttle position?
  6. Jan 1, 2014 #5
    the intake manifold pressure , what is i am trying to calculate rcgldr, how to calculate using the software inputs?
  7. Jan 2, 2014 #6
    Do you develop those applications, or are you simply trying to work out what the underlying equations are?
  8. Jan 3, 2014 #7
    hello kozy, thanks for the reply,

    well, simply i am trying to work out the equations used for calculations, to substitute in and get same results as the software outputs,

    i gave an example, for the outside air density calculations, i got the formula, substituted in it and got the results of 0.0771 Ibs/cubic feet

    what i amstuck with is the inside air density, which formula used, and how to get the output of 0.079452 Ibs/cubic feet, look again at the link i sent, to see that result there, in same line of the "intake air density"

    so can you help with how to get that value?, or anyone else here in the forum can help with that?

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