Re: Beating Carnot Engines

From: Anders Sandberg (asa@nada.kth.se)
Date: Fri Feb 28 2003 - 02:56:37 MST

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    OK, here is a version of the message with some useful physical information
    rather than the hype of the home university:

    (if he really goes beyond the Carnot cycle, then there is real news - but
    breaking thermodynamics is hard, since you have to break probability
    theory)

    The quantum afterburner
                       4 February 2002
    http://physicsweb.org/article/news/6/2/2

    Quantum mechanics could be used to improve the efficiency of heat engines,
    according to Marlan Scully of Texas A&M University in the US. Scully has
    calculated that exhaust energy from an engine could be used to power a
    laser. This would improve engine performance beyond that of the ideal Otto
    cycle without violating the laws of thermodynamics (M Scully 2002 Phys.
    Rev. Lett. 88 050602).

    Scully considered a simplified version of the Otto cycle, a sequence of
    operations similar to that carried out by a standard car engine. In this
    four-step cycle, a hot gas expands in a cylinder doing useful work before
    it loses heat to its surroundings; then it is compressed and heated again.

    But Scully added two extra steps. After the heat has dissipated the gas is
    passed through a laser-maser cavity at constant temperature, where it
    deposits useful energy. This cavity is then reheated once the system as a
    whole has been reheated, again at a constant temperature.

    The crux of Scully's system is that the heating of the cavity does not
    raise the kinetic energy of the atoms in the gas, as the normal heating
    stage does. Instead it increases the internal energy levels of the atoms,
    making them emit photons. Scully calculated that this liberated quantum
    energy was greater than the extra work that would have been produced if
    the additional heat had instead been ploughed into a normal four-stage
    cycle.

    Scully points out that this result does not violate any laws of
    thermodynamics. He also analysed an equivalent system in a Carnot cycle -
    which, unlike the Otto cycle, runs at the highest efficiency permitted by
    thermodynamics - and found that quantum mechanics could not be used to
    improve the engine's performance.

    He hints that he has already worked out how to test his idea in the lab
    and that he has conceived several novel laser systems. His idea has been
    dubbed a 'quantum afterburner' because of its parallel with the devices
    that extract useful energy from the exhaust of a jet engine.

    -- 
    -----------------------------------------------------------------------
    Anders Sandberg                                      Towards Ascension!
    asa@nada.kth.se                            http://www.nada.kth.se/~asa/
    GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y
    


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