The calculations for supercharging

[Jon Pike <jhpike@domain.elided>]

> 1 litre is 61.02 cu. inches.  So our motor turning at 3000 rpm will ingest 3051 cu. inches
> per second, or approx. 183060 cu. inches per minute.  There are 1728 cu. inches in a cu.
> ft, therefore the motor ingests 105.93 cfm.  At 6000 rpm  - assuming the same pumping
> efficiency - the motor ingests twice this amount of air = 211.8 cfm.
>
> > From tables for typical air compressors using vane blades:  An electrical  blower with a
> 100 cfm throughput at 0.7 inches of water  (0.025 psi) requires approx. 75 watts.
>
> There are 12 inches of water per foot. 1 psi = 2.307 ft water, 1 BAR of boost is equal to
> 14.5 psi or 33.45 ft water gauge.
>
> Therefore to develop 1 BAR of boost with a 100 cfm throughput requires: 43.5kW = 58.3 HP
>
> Someone please check my calculations!
Hmmm... they sound good John,  till you get to the fan calculations.  I 
think you can't scale it in a linear fashion..
I'd guess it gets worse as you try to make more pressure.  Either way,  
you sure are illustrating how inefficent a
basic fan is for compressing air.  
A turbo has a very different,  more centrifugal kind of air mover,  that 
operates at extreme speeds..  a blower is
more of a volume pump than a fan..   you get the picture.   Hmmm..  how 
about a 500cc motorcycle engine driven
off the crankshaft as a poor man's blower??  (yikes!! "Alfa Romeo, 
blown by Kawasaki")  Hmmm.. probably need
to run a four times the engine's speed..  24,000 rpm anybody?  Put a 
shrapenal shield around that "blower"!!

Jet engines,  or other turbine powerplants,  have banks and banks of 
fans to make sufficent pressure.

Anyway,  point I was getting at,  you need MUCH less HP,  if you have a 
more efficient way to move air than that
basic fan..  
But with 423 CFM being needed for our theoretical 2L motor at 6000 
rpm... and you wanted pressure
with that...  don't forget that if you want 1 bar (14.7 psi) you have to 
cram 846 CFM (twice the volume) of atmospheric
air in there for that 423 @ 1bar..  it gets to be a serious challange, 
no matter how efficent your airmover is!  
Aha...  just remembered I had a reference book,  it had a formula for 
theoretical HP to compress air..  after plugging in
the various amounts, 14.7 psi @ 423 CFM,  it indicated a little under 21 
HP.   That's with a "perfect" compressor.

Yep.. a lot of work for that little electric fan..

Jon
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