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Homework Help: Mean effective pressure in a diesel engine

  1. Jan 14, 2016 #1
    1. The problem statement, all variables and given/known data
    Air enters a diesel engine with a density of 1.0 kg/m3. The compression ratio is 21 .At steady state ,the air intake is 30 x10-3 Kg/s and the net work output is 15 kW .The mean effective pressure (in kPa) is ........

    2. Relevant equations
    Pm= W.D/ Vs
    C = V1/V2
    V1 =Vs +Vc

    ##\eta## = 1- ##\frac{1}{r^(\gamma-1)} \frac{ \rho^\gamma-1}{(\rho-1)\gamma}##
    Pm -mean effective pressure
    W.D -work done
    Vs -stroke volume
    Vc-clearance volume
    ##\gamma## -ratio of specific heats.
    ##\rho## -density (rho)
    ##\eta## -efficiency (eta)
    Mass flow rate -m.
    C- compression ratio
    3. The attempt at a solution

    Since density is 1 kg/m3 ,the efficiency becomes 100% according to the formula.
    because there is a (rho-1) term in the denominator of the second term.

    If the above is correct then W.D =Heat supplied = 15kW

    So m. /density = Volume flow rate = Vs ?

    if r= 21 ,then Vs =20Vc

    So volume flow rate = 30x10-3 = Vs ?

    So mean effective pressure =15000W/(30x10-3)
    = 500 kPa
    Is the above calcualtion correct ?
    but the answer given to me is 525 kPa
    Last edited by a moderator: Jan 15, 2016
  2. jcsd
  3. Jan 19, 2016 #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. Jan 20, 2016 #3
    I just want to know whether my calculation is right because i am not getting the answer i was given.
  5. Jan 31, 2016 #4
    Oops!! rho is not density , it is cut-off ratio.
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