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Homework Help: Efficiency question

  1. Apr 1, 2014 #1
    A question is being asked about a combustion chamber that has a moving piston; "an engine."
    "Which expression expresses the efficiency of the engine?"
    The answer given is:
    (Pressure(from burning))(Volume)/(moles)(delta H)

    While this looks some thing like PV = nRT, I don't fully understand it. What's the "efficiency" of something?
    I think that the delta H is representing the "temp released per mole" which is then multiplied by number of moles. That leaves "efficiency" to be something like an "R" value.
    In the explained answer section it mentions something about Work = Fd = P*delta V. P*delta V is what's on top of the fraction. This is divided by the "heat released." Why?
    Any input will be helpful, thanks.
  2. jcsd
  3. Apr 2, 2014 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    For a heat engine, the efficiency is defined as the mechanical work output/(heat) energy input.

    The work output is the work done by the piston. This work is the pressure inside the cylinder x the change in volume (P = force/area and ΔV = piston displacement x area).

    The heat input is the heat flow from the reaction at constant pressure, which is the number of moles of reactant x ΔH of the reaction (per mole).

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