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Reversibility of Otto Cycle

  1. Jun 24, 2009 #1
    Theoretically, we can have reversible isochoric expansion and isochoric compression. Is Otto Cycle reversible then?

    I read that the efficiency of all reversible engine between two heat reservoir would have the same efficiency. However, since the efficiency of an Otto Cycle is lower than Carnot Cycle, it cannot be reversible.

    How do I resolve this problem? Thanks in advanced.
  2. jcsd
  3. Jun 24, 2009 #2
    Are you referring to the process whereby the hot combustion products are allowed to expand, cool and perform work? Or to the preceding different process whereby chemical potential is let change directly into thermal energy, in a spontaneous (and somewhat uncontrolled in comparison to fuel cells) manner?
  4. Jun 24, 2009 #3
    I refer to the first process, but a hypothetical ideal one where it is reversible. After some thought, I think that in order for an ideal gas to expand under constant volume and yet be reversible, there will have to be many heat reservoir, with infinitesimal small difference in temperature. Hence the reversible Otto Cycle will not be operating between two heat reservoir.

    As for an Otto Engine operating between two heat reservoir, the process must be irreversible.

    Is my reasoning correct?
  5. Jun 24, 2009 #4
    Well a real Otto cycle is in no way reversible as you have the messy business of combustion and pumping losses.

    However the ideal air standard otto cycle can be considered reversible as each of the 4 processes are reversible. I dont get what you mean when you say that the gas expands under constant volume (it just seems a funny way of describing the process), are you referring only to the increase in pressure at heat addition/rejection? As contant volume heat addition is reversible between two heat reservoirs.

    I'm in the process of moving back home to all my books are packed so I can't read up to check.
    Last edited: Jun 24, 2009
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