> Back pressure is EXTREMELY dependent on exhaust manifold and turbine
    > configuration. There is NO typical value! Mistral has measured
    > full-power
    > backpressures up to 1.8 bar (23-24 psi) which is unacceptable,
   resulting
    > in
    > a significant loss of engine power, and as low as 1.2 bar (17
   psi) which
    > does not optimize turbine efficiency. By changing manifold
   configurations
    > (pulse or constant-pressure system), turbine scroll (single or
    > twin-scroll)
    > and turbine inlet configuration (A/R ratio), back pressure and
   turbine
    > efficiency can be adjusted to the desired compromise. Believe me, it
    > ain't
    > trivial.
    > I recommend highly the following reading: Diesel Engine Reference
   Book,
    > Chapter 2, The Theory of Turbo charging, p. 28-71); it doesn't
   have to do
    > with diesel engines only and it is quite comprehensive.
    >
    > Francois Badoux

   Hi Francois!
   Can Francois or anyone tell me what the A and the R in A/R represents?
   Also with the low bar/psi which doesn't give optimum turbo
   efficiency - does
   the turbo need optimum efficiency for turbo normalizing at altitude. I
   thought we would be looking at running the turbo less efficiently (with
   reduced back pressure) to save the turbo from overspinning and
   overheating?
   George ( down under)


   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.

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