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Homework Help: I need help calculating the heat exhausted from an engine.

  1. Apr 20, 2016 #1
    1. The problem statement, all variables and given/known data
    A 112 hp car engine operates at 11% efficiency. Assume the engine's water temperature of 85°C is its cold-temperature (exhaust) reservoir and 445°C is its thermal "intake" temperature (the temperature of the exploding gas-air mixture).
    (a) What is the ratio of its efficiency relative to its maximum possible (Carnot) efficiency?
    eactual / eCarnot =
    (b) Calculate how much power (in watts) goes into moving the car.
    (c) Calculate how much heat, in joules and in kcal, is exhausted to the air in 1.0 h

    2. Relevant equations
    For a) eideal=1-(TL/TH)
    For b) (hp)(W)
    For c) I need help

    3. The attempt at a solution
    I got the answers for a and b.

    I have no clue how to do c. I need some guidance.

  2. jcsd
  3. Apr 20, 2016 #2


    User Avatar
    Science Advisor

    Show your working and units for a) & b).

    As for c), do you understand how a car radiator works? The hot engine water is cooled by atmospheric air, thus, waste heat ultimately ends up in the atmosphere.

    you are given "W" and the efficiency and that's all you need to find the power going to the cold sink "Qc". If you don't know where to start, draw a diagram and apply conservation of energy.
    Then multiply by the appropriate time to get the required energy.
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