Theory of superchargers / turbos / energy conversion devices

["J. Leung" <jleung20@xxxxxxxxx>]

Kenn Sebesta asks:

>
>Okie, I've got a serious theory question about turbines and >superchargers.:
>
>Why not use a turbine on the exhaust to generate electricity and then use
>the excess electricity to power a compressor motor?  This way you avoid the
>inefficiency of having the motor *directly* power the super, and you get
>rid of the annoying turbo lag!  Simply have a decent sized capacitor bank
>to handle low rpm power surges, and the turbine can recharge it during
>routine driving.  

I'm gonna guess:

'Cause its not efficient.  That's why.  The thing your missing is that
a turbocharger isn't a purely mechanical device.  Yes, a turbocharger is
spun by exhaust gases shooting out the manifold, but it is also takes the
energy in a high pressure super high heat gas (out the manifold) and
converts the energy in the gas to work (spinning the turbine) and ejects
the exhaust gas as a low pressure, relatively low heat gas.

Actually, that's why on a turbo car, you want as little back pressure
as possible, to create the biggest differential in pressure possible
for the exhaust gas before/after the turbo.

So really, the turbocharger is  a device that converts kinetic energy
and gas-pressure energy into mechanical energy (spinning the turbine.)
That's why the turbocharger is so efficient.

If you want to drive a supercharger with an electric motor fed by an alternator
driven by an exhaust gas driven turbine, you are:
 
Introducing more inefficiencies into the system.  You gotta convert
the exhaust gas KE and pressure into electric energy with a 
turbine (lots of energy wasted) and then you gotta convert that
electric energy back into mechanical work of a compressor wheel
(lots of energy wasted again on an electric motor as heat and noise.)

You gotta go exhaust press & velocity -> mechanical -> electrical ->
mechanical -> intake charge press.

With a turbo, you go exhuast press & vel -> mechanical -> intake 
charge press.  

You're adding in two more steps.


I'm gonna SWAG and say you'd be lucky to see even a 20% energy
efficiency.  What's a turbocharger, something like 72%, where
only 28% of the energy into the system is wasted as heat and noise and
overcoming frictional losses?

Remember that no energy conversion device is 100% efficient.  Energy
is always wasted as heat and noise.  Just think...a gas engine converts
potential energy in fuel to mechanical work, and its only, what, 15%
efficient!?!? 

Well, I don't have a Mechanical Engineering degree, so what do I know?

;-)

Jason Leung
92 332i

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