08/14/2003

Buchi, Birmann, and back pressure

There is a peak pressure relating to combustion.  This is caused by the
maximum temperature.  If all combustion was to take place at constant volume,
then the peak temperature would cause peak pressure at TDC when all the charge
burned.

But, that's not how it works.  It takes time for the charge to burn to
completion.  The unburned charge acts as a heat sink for the burned charge,
and keeps it from reaching maximum temperature until the charge is nearly
fully consumed.  More charge - the longer the time for the charge to reach
peak temperature ( which is approximately the same regardless the amount of
fuel/air as long as the mixture is the same - pressure matters little on peak
temperature. ).

Working against rising pressure with charge, is the motion of the piston.  All
combustion is not constant volume, but is working against expanding volume as
the piston moves away from TDC.

The "slow burn" of the charge combined with cylinder expansion give the effect
described - that the Peak Pressure is not materially affected by
Supercharging, but the duration of the burn and pressure pulse is lengthened.
This is not a bad thing, as although the pressure either stays close to
constant or begins to fall, the longer the charge burns, the larger the lever
arm and the more power is transmitted to the crankshaft.

On two of the three types of turbocharged engine exhaust's, fresh charge can
and will be blown out the exhaust under normal conditions and a scavenging
cam.

Buchi exhausts ALWAYS create a depression on the exhaust such that the exhaust
pressure is several pounds below the intake through out the power range.

A proper Birrman style will create the same conditions in the power band as a
Buchi does throughout the normal operation.

Both of these exhausts require thought and an understanding of acoustics and
how they work in conjection with the turbo.  Neither acoustics have any
relationship to a NA engine.

The turbo "myth" motor with excessive back pressure is a lazy design requiring
the least thinking and just sticking a single turbo up the tail pipe like a
potatoes.  Unfortunately, much of the world tries to tie constipated turbo
results as godlike axioms for any turbo engine.  

Note that unless you specifically design for a buchi or birmann, the exhaust
back pressure with a turbo will always be higher than the intake.