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Computing Cylinder Pressure from Temperature Trace

  1. Jun 28, 2015 #1
    I'm just trying to put together a very basic engine model with extremely limited combustion. Basically, I am modeling isentropic compression, combustion (using LHV to calculate the delta T), and isentropic expansion.

    At the moment, I am calculating a peak cylinder temperature under motoring of ~400 C and a peak cylinder temperature under firing (5 deg BTDC) of ~2800 C. This puts my exhaust gas temperature around ~1200 C, which all seems to make sense. I am using a compression ratio of 8:1 and air standard conditions.

    However, when I try to convert these numbers to a cylinder pressure, my ~2800 C point is exploding to ~3700 bar! Obviously, this is not correct. I am using an isentropic expansion equation (P2/P1 = (T2/T1)^(g/g-1)).

    Any ideas? Can anyone double check my reasoning and let me know what you get?
  2. jcsd
  3. Jul 4, 2015 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Jul 4, 2015 #3

    Randy Beikmann

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    Gold Member

    Part of the discrepancy might be that in a real engine you don't have isentropic compression or expansion, where PV^k=constant, k being 1.40. Considering heat losses, it's polytropic expansion and compression, so PV^n=constant, where n is about 1.30.
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