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Thermodynamics exercise.

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
    An engine works with an ideal diatomic gas (γ=1.40) according to a thermodynamic cycle consisting in four
    processes that take the gas successively through the states 1, 2, 3 and 4. The processes 1 -> 2 and 3 -> 4 take
    place at constant pressure and the two other processes are adiabatic. The cycle’s lowest pressure is 1 atm and
    corresponds to the state 1.
    a. Find an expression for the cycle’s efficiency exclusively in terms of the temperatures of the states 1,
    2, 3 and 4.
    b. If the efficiency is φ=0.40, what is the gas pressure in state 3?


    2. The attempt at a solution

    Ok, first part is easy (I guess) here's what I did http://imgur.com/hpBxIex if someone could give it a look I'd be very thankful. I'm sorry I cannot give you the P-V diagram because I don't know how to draw in this PC.
    The second part is where it gets tougher because I only have 2 pieces of information and I really cannot relate them. If someone could sketch the solution I'd be very thankful.
     
  2. jcsd
  3. Jan 18, 2015 #2

    TSny

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    OK for part (a). For part (b), think of a way to relate T and P for the adiabatic processes.
     
    Last edited: Jan 18, 2015
  4. Jan 19, 2015 #3
    Hello TSny, thx for replying, thing is my professor didn't give us a way to relate P and T however I found this formula in wikipedia but I'm not sure if it's correct: (It's valid iff the process is reversible) P1-γ Tγ=const. What do you think?
    The statement doesn't specify if the process is reversible actually...
     
  5. Jan 19, 2015 #4

    TSny

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    Yes, that's the correct formula. For these types of questions you can generally assume that the entire cycle is reversible.
     
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