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Homework Help: Heat Engines and exhaust heat

  1. Jun 14, 2010 #1
    1. The problem statement, all variables and given/known data

    A heat engine operates between 40°C and 380°C. Being a real engine, its
    efficiency is only 60% of that theoretically possible for a Carnot engine at these
    temperatures. If it absorbs heat at a rate of 60 kW, at what rate does it exhaust
    heat?



    2. Relevant equations

    e = 1 - TL/TH --> Carnot Efficiency
    e = 1 - QL/QH --> Efficiency


    3. The attempt at a solution

    1) First I changed the temperatures to Kelvins, then I calculated the Carnot efficiency:

    e = 1 - (313/657)
    = 1 - 0.48
    = 0.52

    2) Then I determined the actual efficiency to be:

    0.52 x 0.60 = 0.31 or 31%

    3) Finally I plugged in the efficiency in 2 to the 2nd equation above:

    0.31 = 1 - (QL/60 kW)
    60(0.31) = 60 - QL
    18.72 = 60 - QL
    QL = 41.3 kW

    So therefore the engine exhausts heat at 41.3 kW or 41.3 kJ/s. I think I did this right, but I'm the type of guy who always manages to forget something or other.

    Does this look right?
     
  2. jcsd
  3. Jun 14, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    Yes that looks right to me.
     
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