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Homework Help: Basic thermo question: heat engine efficiency

  1. Sep 6, 2012 #1
    Is it strictly true that an engine will be more efficient if operating in a hotter environment... e.g. your car is more efficient in the summer than the winter? Just going through thermo 1, not a HW question, but it just seems to be true to me because the simple newton's law of cooling says there will be less heat transfer and thermo says therefore there is more work output / less loss associated with the cycle.

    Of course this doesn't take into account efficiency of fuel (for your car). I have no idea how it would affect combustion. Maybe next semester in chem I'll get a better idea ;)

    Probably a simple question, but I'm curious :) Thanks.
  2. jcsd
  3. Sep 6, 2012 #2
    Interesting, so the TH or hot reservoir would be the heat we add to the engine though combustion. That means the TL or cold reservoir is the surrounding air. And we see by the equation that as TL -> TH the efficiency drops to zero.

    This makes sense because if the combustion cannot add heat, then there is no work done.

    Which means engines are more efficient the colder it is, due to a higher rate of heat transfer in (more heat to work with, more work produced so to speak).

    So this means the Qout is unrelated to the reservoirs, it's simply what's left over that's not converted to work. While it's Qin that is directly related to the reservoirs. Seems I had it totally backwards :)

    Last edited: Sep 7, 2012
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