Re: Theory of superchargers / turbos / energy conversion devices

[Otter <kdsebe0@xxxxxxxxxxx>]

>'Cause its not efficient.  

Nothing is.  Second law of thermodynamics, etropy, and all that.

>a turbocharger isn't a purely mechanical device.  

Well, I'm not exactly missing it, just wanting to avoid its traps and
pitfalls.

>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, this is not a description of a turbocharger, but any turbine in
the entire world.  

>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.

Of course.  The more potential energy you have, the more you can extract as
work.

>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.

But car turbos _aren't_ efficient.  Not after you consider the flow losses
and various rates these things must spin at.  Turbines and compressors
really like to be driven in an appropriate range.  There are tricks to
force the turbo to run within a certain speed, like wastegates, but these
expel rather than use the potential enthalpy of the gas.

>Introducing more inefficiencies into the system.

Into the inital system, but removing many if you run the piping straighter.
 Also, you're gaining the advantage of not having turbo lag, so you might
be more efficient than a supercharger with none of the drawbacks of a turbo.

>  You gotta convert
>the exhaust gas KE and pressure into electric energy with a 
>turbine (lots of energy wasted) 

Mmmmm...  I don't think this is quite right.  A generator that is kept cool
can be quite efficient; same with motors.  This is why people want to use
electricity in ultra efficient cars.  And exhaust gas kinetic energy is
generally ignored when you are calculating turbine output, as kinetic
energy related work is about 1000 times less than enthalpy(combination of
pressure and temperature) derived work.

>You gotta go exhaust press & velocity -> mechanical -> electrical ->
>mechanical -> intake charge press.
>With a turbo, you go exhuast press & vel -> mechanical -> intake 
>charge press.  

True, a much more simple system, but I'd like to see a mathematical
discussion of the flow losses related to a turbine.  Unfortunately, I know
next to nothing about fluid dynamics, whereas I know a good deal about
simple thermodynamics and turbines.

>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?

No idea on these numbers, but I know that turbines can be _much_ more
efficient that that today.  I believe the best of them are in the high
90's(>95%).  If 72% were all we were getting, then someone could come out
with a much more efficient turbine and rule the market.  Maybe I'm
misunderstanding you: 72% efficiency for the entire system?  Turbine and
compressor?  That would make much more sense.

>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!?!?

But if you suck some energy out of the exaust and use it to generate
elecrticity, you increase the engine's efficiency becuase the alternator no
longer requires as much torque to spin.  And then you can use it in a low
psi boost to help increase the efficiency even more by negating some flow
losses related to a normally aspirated engine.

You're probably right about everything, but I want to *understand* the
numbers behind why this won't work.  I just get this nagging feeling that
you would have such an incredible boost factor if you could just get rid of
all those danged bends in the pipes!  Also, if you could replace the
alternator, you would save a lot of power (an alternator takes more torque
the more current it must deliver).

Thanks, Jason.

Kenn
http://www.euromajic.com

------------------------------