electric superchargers

[Ed Mellinger]

A little back-of-the-envelope arithmetic:

Your 3 liter 4 stroke eats 1.5 liters of air per revolution, 3000 liters
per minute at 6000 rpm, or 150 liters/sec.  That's .15 cubic meter/sec.

5 psi boost is about 35 kilopascals which is also 35 kilonewtons per
square meter.

Multiply .15 m**3/sec times 35,000 N/m**2 and you've got 5,200 N-m/sec
after you cancel the units.  Often called "PV Work".

A newton-meter is a Joule and a Joule per second is a Watt... so the raw
power required to compress your input charge is 5.2 kilowatts.  That's
about 7 horsepower.  Now figure 60% blower efficiency (a number I made
up, but you get the idea) and you need almost 12 shaft hp going into
your blower.  For comparison, your starter motor (not the best
comparison, but a cheap motor) is typically rated 1-2 hp.  An
inverter-driven induction or PM motor would be nicer... figure 5-10 lb
per HP for ones you can buy off the shelf.  Gives one some respect for a
little turbo you can hold in the palm of your hand, no?

Figure an 80% efficient motor (a number I didn't make up) and we're up
to about 11 kW going from your alternator into your motor terminals...
that would be close to 800 amps at 14 volts; 120V or the new aircraft
standard 270V (F-22) would be better.  Figure an 85% efficient
alternator and you're going to take about 17 shaft hp from your engine.

I don't know what size alternator you've got, mine's 90 amps which at
14V is about 1.3 kW.  So to run this blower steady state, you need an
alternator about 9 times the size/weight of what you've got now.

But of course you only need the boost for acceleration and hill
climbing, at cruise you are running manifold *vacuum*.  If you have 
a battery that will put out 500 cranking amps at 10V, you've got 5 kW
available for several seconds.  Two of 'em would get you through a drag
race.  The steep part of the Grapevine takes a couple of *minutes* to
climb... want to option your sports car with several deep-cycle forklift
batteries?

Nice idea, though.  There are certainly high performance technologies on
the horizon (or already here) that could cut component weights by a
factor of 3 or 4... but if you have to ask you can't afford 'em (the
F-22 could!).  It just takes a lot of power to force-feed an internal
combustion engine.

Ed Mellinger