09/17/2003

Low Pressure Supercharging

The early turbo blowers were used on large diesels.  A low loss compared to
centrifugal roots that positively pumped air.

Conjugate upon your navel por favour.  Consider that a large diesel turbo has
a large slow moving efficient turbine blade.  Failing that, apparently,
getting a replacement blade in ridiculous a/r's for a LARGE turbo is cheap and
easy according to Dave who checked into it.

Take a GMC-6-71 as an example.  Figure it for 429 cubic inches of air per
revolution.  Thus driving it at crankshaft speed on a four cycle engine
delivers enough air at normal pressure for 854 ci.  

Say that's on an engine that crank 6000 rpm and 400 cubic engines - this will
give us approaching 15 psi.  That will do the dirty deed on many an engine and
have an enormous grin factor.

If we put a twenty to one gear train ( paper and drawings in daves to scan
pile ) on the front, that's 120,000 rpm.  With the diesel turbo large, I doubt
that's going to push more than 60000 rpm, so 10 to 1 would work.  With a Eaton
off a t-bird ( which I keep either trying to steal or find in a junk yard ),
which turns 12000 rpm, on a smaller engine we would be down to a 5 to 1
geartrain.

The advantage of a roots is that is starts moving air much quicker and at a
lower rpm than a centrifugal supercharger.  And it takes less power to turn
than a same capacity centrifugal, heats the air less, and has higher
mechanical efficiency.  Per Pye - Air Ministry and Naca Testing of Roots vs
Centrifugal superchargers.  Another trick of roots is that air moving thru the
rotors pulled by the vacuum will rotate them at a significant rate of speed.
So the turbine is adding to an already moving load.  

If however, we were to invest in ONE over-running clutch, we could drive both
ends.  One, with the turbine, the other with a belt and clutch.  We pick some
gear ratio and when the turbine kicks in, it accelerates the roots past that
level up to maximum boost.  It could also be combined with a standard clutch
to kick it out.  

Sorting this out would give you the effect of positive displacement at all
rpms and the advantages of turbocharging. "exhaust turbine driven
supercharger" is the definition of turbochargers.  You did know the compressor
side on a turbo is a centrifugal didn't you?

With a over-riding clutch gear driven method, it would be very wise and "free"
to drive the water pump off one of the shafts on the roots side.  Power / heat
production is basically linear with air flow and air flow is basically linear
with regard to the engine as the speed of the roots.  Thus cooling would
closely approximate air flow needs and roots speed.