George,

    The "A" in A/R is the cross-sectional area of the scroll "tube,"
    while the "R" is the radius, measured from the center of the scroll
    to the turbine shaft. The A/R ratio basically tells you the size and
    shape of the scroll that is cast into the housing.

    A lower A/R ratio is a more restrictive housing so the exhaust gases
    will speed up and increase the amount the gases will expand, which
    drives the turbine. This restriction also means more back pressure.

    I think what Reeves and Francois were getting at is that you have to
    have a positive pressure gradient from the engine's intake tract to
    the turbine inlet. This will help the volumetric efficiency of the
    engine by using the lower pressure in the exhaust tract to draw in
    more intake charge. Even if your engine is boosted, if your exhaust
    tract pressure is too high, your supercharger or turbocharger will
    be working against this pressure and wasting power.

    But you also want to keep in mind the pressure gradient across the
    turbine. Obviously the higher the pressure gradient across the
    turbine, the more quickly the gas will accelerate across the
    turbine, resulting in more kinetic energy and more work at the
    turbine shaft. In aircraft operations this is significant because
    the atmoshperic pressure goes down as we climb.

    So I think the goal is to have a boosted intake tract that keeps
    pressure a little above the turbine inlet in order to maximize
    engine power. But you also want to maximize the turbine efficiency,
    which means you also want to keep this pressure enough above ambient
    that your turbine will do some work. I think that's what Francois
    was getting at when he said too little back pressure is also not good.

    I think it boils down to two spearate pressure gradients -- one
    across the engine and another across the turbine. You want to keep
    both positive for best results. Of course as you climb higher the
    pressure gradient across the turbine will keep increasing --
    provided your intake boost keeps up -- so you could have a system
    with very little back pressure at sea level that will still work
    well at altitude.

    I think the principles are the same whether turbocharging or
    turbocompounding.

    Regards,

    Gordon.

Thanks Gordon!
George ( down under)




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