[Diy_efi] RE: Banjo Dis. Pic

Brian Michalk michalk at awpi.com
Wed Jan 15 21:18:37 GMT 2003


> > Regulating either a draw through OR a blow through throttle in
> this manner
> > would, however, create a situation where the compressor was
> being run at a
> > higher pressure ratio than otherwise necessary at times, thus
> raising the
> > IAT, and obviously , therefore, lowering efficiency. And, in
> the case of a
> > blow through, pushing the compressor closer to, if not into, surge.
>
> Wouldn't the IAC lower as the pressure dropped across the throttle plate?
>
> Grant Beaty
>

Yes, the ideal gas law applies to the pressure drop across the throttle, but
you have to take the throttle/compressor as a complete unit.  Making the
compressor run at a higher ratio heats the air to a higher temperature.  A
compressor may be 85% efficient at it's sweet spot.  Running at a higher
pressure ratio implies more power into the compressor.  Let's assume 14psi
goes into the throttle, and 7psi comes out of the throttle.  Let's assume
the turbo compresses this air to 21PSI.  Say we put 10 horsepower(it's a lot
of air) into the compressor at this higher pressure ratio, and it's 85%
efficient.  That means that 1.5 horsepower went into heat whipping up the
air.  Take compressed, hot air and expand it through an expansion valve
(thus allowing adiabatic cooling)to the ambient room pressure.  If you put a
temperature probe in the stream, it will read hotter than the surrounding
air temperature because of that 1.5hp in heat, and the other unavoidable
heat penalties.

Now, assume a compressor with a wide open throttle.  Assume the room is at
14PSI, and after the throttle the pressure still near 14PSI.  We want a deck
pressure identical to above at 21 PSI, so the pressure ratio is half of the
above example.  Assume that the compressor is still 85% efficient and we
only deliver five horsepower to the compressor.  The pressure ratio across
the compressor is lower, but the MAP is the same as the first case.  .85 * 5
horsepower now comes out to .75 horsepower that went into heat.

Notice how I never mentioned wastegate.  This could be a squirrel cage fan,
or any system that compresses air.  PV=nRT applies universally.  If you want
your intake manifold air temperature to be as low as it can be in order to
achieve maximum air density, then you treat your intake air with kid gloves
so as not to heat it up.


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