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Posted for "George Braly" <gwbraly@gami.com>:
I have compressor maps for lots of aircraft centrifugal compressors - - and
only one of them gets to 80% at any point on the map.
It is unlikely that the fixed rpm ratio version of that unit would operate at
that optimum efficiency for more than a small portion of its operating life.
Typical turbine/compressor combinations I have seen operate at around 70% combined efficiency. Of course a fraction of that efficiency is reclaimed from the proverbial "waste heat" of the exhaust. Centrifical compressors are commonly 80% or better. <<
I have compressor maps for lots of aircraft centrifugal compressors - - and
only one of them gets to 80% at any point on the map.
It is unlikely that the fixed rpm ratio version of that unit would operate at
that optimum efficiency for more than a small portion of its operating life.
>> 4) To get the air temperature back down to tolerable (detonation
margins) ranges requires an intercooler All sorts of engines run without
intercoolers, without damage. Detonation is a function of temperature AND
pressure in the cylinder at, or near, full compression, not the temperature in
the intake. Besides, if you are flying up high, where an add on system makes
sence, the air starts out colder so it isn't all that much warmer than it
would have been at ground level.
The principle reason for adding an intercooler is the same as for adding a
supercharger, to make the intake air denser and increase the mass flow through
the engine. <<
Well, after reading that, I am sort of beginning to wonder that NACA never
knew what it was doing.
The primary reason in an aircraft to use an intercooler is not to gain more
charge density. It is a LOT easier to do that with the compressor.
The primary reason is to provide adequate detonation margins at higher power
settings.
If you think that one of the primary component causes of detonation is NOT
the temperature of the induction air - - - then I would suggest that you
take another look at the hard data.
I have been teaching this precise subject matter on Saturday mornings every
other month for the last 34 months. This includes about 600 mechanics and
pilots over the last couple of years, including as recently as last Saturday
morning when I had the definitive NACA data on induction air temperature vs
detonation free BMEP plotted and up on the over head screen in the class
room.
In addition to teaching the subject, I have measured the results, directly,
on the test stand on a TIO-540J2BD engine - - with and without an
intercooler.
Regards, George
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