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Homework Help: Thermodynamics - Cycle efficiency

  1. Nov 23, 2012 #1
    1. The problem statement, all variables and given/known data

    Compute the efficiency of the following cycle:


    2. Relevant equations

    3. The attempt at a solution

    I know there's an isothermic process between a and b, where heat comes into the system, let's call it Qh.
    I also know that between b and c heat will leave the system, let's call it Qc.
    The process from c to a is adiabatic.

    The efficiency, n, is equal to:

    η = 1- Qc / Qh = 1 - Qc / T2(S2-S1)

    The problem is that I don't know how to compute Qc...
    Any help would be highly appreciated.
    Also, if anyone has a good book on the subject I'd appreciate.

  2. jcsd
  3. Nov 23, 2012 #2


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    The area enclosed by a cycle on a T vs S graph has a physical interpretation that will help to determine the efficiency.
  4. Nov 23, 2012 #3


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  5. Nov 23, 2012 #4
    Thank you very much! Sadly I had no idea it had a physical interpretation..
  6. Jan 1, 2013 #5
    Don't use Carnot efficiency equation for the cycle; but use a more fundamental and simpler equation: efficiency = (W/Qh) = (work output from the engine / heat input to the engine). The numerator is obtained from the area of the triangle. You already know how to calculate (the denominator) the heat input to the engine (T2*delta S). The ratio of the two numbers gives the efficiency.

    Caution: Cycles involving sloping straight line paths can sometimes be dangerous! For eg the efficiency calculation of similar cyclic triangular paths on PV plane (instead of TS plane) is a very complicated and controversial issue.
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