[Diy_efi] RE: Throttling intake air -- references

[Brian Michalk]

I said I would get a reference to back up my claim that a throttle in fron
of the compressor inlet is a bad thing, and WOT throttle operation is more
efficient.  I must have buried my Corky Bell, and Hugh Macinnes books, but I
do have:
"Scientific Design of Exhaust & Intake Systems" by Philip H. Smith, FIMechE
MSAE and John C. Morrison, BSc PhD MIMechE.

On page 20, they show a graph of engine thermal efficiency vs intake
pressure and exhaust back pressure.  The EBP ranges from 6psi absolute
(about 8 psi vacuum) to 20 psi absolute.  At WOT, or intake pressure equal
to ambient, or 14PSI absolute, with EBP=6psi, efficiency is a little better
than 34%.  With EBP of 20PSI, efficiency is just under 32%.

At part throttle, or 6PSI absolute intake pressure, with 6psi EBP,
efficiency is 33%.  At 20PSI EBP, efficiency is a miserable 24%.

The book goes on to say, and typos are mine:
"Thus it is evident that as the weight of residual gases increases in the
head due to back-pressure, this not only reduces the effective volume of
fresh charge but also increases its temperature at the beginning of
compression, by transferring heat thereto.  When in particular the induction
manifold pressure is low and the exhaust back-pressure is high, the pumping
work constitutes a disproportionate amount of the power available, reducing
the net output.

Therefore my interpretation of the graphs and the text means this:
If you raise the EBP without a corresponding increase in MAP, you lose
power.  Therefore it does not make any sense (efficiency-wise) to have a
turbo working against a partially closed intake throttle body.

 Brian Michalk  <http://www.michalk.com>
Life is what you make of it ... never wish you had done something.
Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
musician, home-brewer, entrepreneur and mostly single


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