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Discrepancy in my calculations!

  1. Mar 30, 2008 #1
    I am having a problem with some thermodynamics calculations. I am not very experienced with them and it certainly shows in my frustration with them! Anyway the check that I am using is incrementally simulated work compared to theoretical energy in.

    Example 31KJ in 133KJ mechanical work out. I am obviously way off considering other calculations predicted 42% thermal eff. if you ignore over unity. (diesel cycle)

    I am having trouble formulating intelligent questions that might help me understand what I am doing wrong but here it goes:


    p = pressure, y = 1.4,Cr = compressed volume ratio, Ps = initial pressure, Ta = temp added = 1000,
    Ti = initial temperature = 300K
    Vr = volume ratio

    Vr = Cr
    if Cr = 1, then T = Ti * Cr^(y-1) = 300K, T = T + Ta, Cr = (T/Ti)^(1/(y-1)) = 39.089

    if the volume did not increase, p = 14.7 * cr^(1.4) = 2490 psi

    is this not correct? or am I missing something?

    now if Vr = .5 then Cr = Cr/Vr = 78

    1714K = Ti * Cr^(y-1)
    6550 PSI = 14.7 * Cr^(1.4)
    How does this second part look?

    Thank you for your help, I appreciate it.
    Last edited: Mar 30, 2008
  2. jcsd
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