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Carnot efficiency

  1. Jul 23, 2009 #1
    the carnot engine does not obey any law of thermodynamics,because it has
    100% efficiency,now do we have any engine that has come close to this hypothetical
    engine?(in terms of efficiency ) can this process of carnot be used for a 6 stroke engine?(hypothetically)
     
  2. jcsd
  3. Jul 23, 2009 #2
    Which thermodynamic law did Carnot engine break?? And where did you read that its efficiency is 100%?
     
  4. Jul 23, 2009 #3
    Here's a http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html" [Broken] where you practice some calculations.
     
    Last edited by a moderator: May 4, 2017
  5. Jul 24, 2009 #4
    well my engineering book of thermodynamics says that carnot engine(imaginary)
    is supposed to have 100% efficiency,the perfect engine.
    thus it disobeys the 2nd law,which states that heat cannot be totally converted
    to work?
     
  6. Jul 24, 2009 #5
    It's also a fact that it's impossible to construct a Carnot cycle engine in real life for that very reason. Carnot only really demonstrates the 1st law, its a purely paper concept and so doesnt have to obey the 2nd law as its idealised. It's also why other cycles are compared to Carnot efficieny.

    Also a Carnot engine isnt actually 100% efficient, its just the best possible efficiency (which is why it says 100%) between two heat reservoirs. The Carnot efficieny is 1- Tc/Th.
     
    Last edited: Jul 24, 2009
  7. Jul 24, 2009 #6

    russ_watters

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    Staff: Mentor

    Could you quote the passage for us? You clearly misread it.
     
  8. Jul 24, 2009 #7

    russ_watters

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    Staff: Mentor

    That doesn't make sense - why would the book say that an engine with 65% efficiency (for example) is 100% efficient? I think what we have here is a complete misread.
     
  9. Jul 24, 2009 #8
    As between the two temperatues, a Carnot Cycle will extract the maximum possible efficiency compared to a Rankine cycle for example. Thats the only reason why I can think that a book would say its 100% efficient. The book chose a poor (and misleading) choice of words if it does indeed say that.

    Like you said though, its probably a misread.
     
    Last edited: Jul 24, 2009
  10. Jul 24, 2009 #9

    stewartcs

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    The thermal efficiency is always less than unity. Carnot is just the best theoretical efficiency (not practical though). It's typically used in thermodynamics to help understand the general concept of the second law.

    It doesn't break any laws of thermodynamics, but it can not exist practically since there is no such thing as a truly reversible process.

    CS
     
  11. Jul 25, 2009 #10
    the lines go like this
    " is it that we cannot achieve 100%efficiency due to practical difficulties or is there
    any theoritical problem about it? to decide about this sadi carnot imagined the
    concept of an ideal engine in which no energy loss takes place"

    so do you mean to say even theoritically ,100%efficiency is not possible,but they
    have not listed any loss in case of carnot engine.
     
  12. Jul 25, 2009 #11

    russ_watters

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    Staff: Mentor

    Yeah, you misread that, though it may be a little poorly worded. It is saying that Carnot envisioned how the perfect thermodynamic enegine could work. "No losses" doesn't mean no heat of rejection (just no friction, heat transfer, etc.) and isn't meant to imply 100% efficiency.

    Anyway, yes, it means 100% efficiency is not possible: Carnot efficiency is the maximum that is theoretically possible.
     
  13. Jul 27, 2009 #12
    Maybe think of it this way: If you could reject heat to a sink at absolute zero, then your Carnot engine would be 100% efficient.

    With normal everyday heat sinks at something like normal environmental temperatures (500 to 550 R), the efficiency is going to be alot less.
     
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