Re: Beating Carnot Engines

From: Mike Lorrey (mlorrey@yahoo.com)
Date: Mon Mar 03 2003 - 20:26:24 MST

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    --- spike66 <spike66@attbi.com> wrote:
    > Chuck Kuecker wrote:
    > >
    > > An afterburner, on a jet engine, does not "extract" energy from the
    >
    > > exhaust - it dumps a large amount of fuel into the exhaust,
    > providing a
    > > rocket-like thrust augmentation. Afterburners suck huge amounts of
    > fuel,
    > > rather inefficently, which is why they are used only when
    > absolutely
    > > necessary...
    >
    > The term "rather inefficiently" is an understatement.
    > If one looks at the curve for additional thrust vs
    > additional fuel use, it is enormously inefficient. It
    > amazes me that it would work at all: the afterburner
    > tosses unburned fuel into the exhaust stream.

    Not entirely exactly. There is a flame holder inside the AB chamber
    which provides eddy space.

    >
    > Perhaps what they really had in mind is if a plane is going
    > into combat it needs to lighten its fuel load somehow.
    > Burning it inefficiently is no worse than just throwing
    > it overboard unburned I suppose. An F-4 jockey told
    > me the afterburners could run for a little less than
    > 5 minutes full tanks to empty.

    Note that that is an F-4, which uses turbojet engines, which to the
    uninitiated, has no bypass air that bypasses the combustion chambers.
    Turbofan engines, as are used in all modern fighter aircraft (F-15,
    F-16, F-22, etc), have signficant ratios of bypass air, and thus have
    more oxygen available to burn the fuel dumped into the AB chamber. A
    turbofan thus has far more efficient burning than a turbojet in the AB
    stage.

    The newest supercruise plane ( the F-22 ) has a rather modest AB stages
    since they are capable of flying supersonically without engaging the
    AB, they don't need to boost their thrust all that much.

    =====
    Mike Lorrey
    "Live Free or Die, Death is not the Worst of Evils."
                                                         - Gen. John Stark
    "Pacifists are Objectively Pro-Fascist." - George Orwell
    "Treason doth never Prosper. What is the Reason?
    For if it Prosper, none Dare call it Treason..." - Ovid

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