Variable Compression, Variable Displacement you decide

[jb24 at chrysler.com]

Couple of more points about the operation of turbochargers.

1.  Turbos do work on the heat energy in exhaust to a degree.  It is
actually the pressure energy that high temp exhaust gases generate that
do the work.  The pressure ratio generated by the turbo compressor is
related to it's rpm, and turbo lag comes from the time needed to
accelerate the turbine to operating rpm.  At part throttle the exhaust
pressure is low, barely enough to overcome the drag of the turbo
bearings and keep the turbo spinning at a minimum rpm.  At idle, some
larger turbos don't spin at all.  When the throttle is whacked open,
the exhaust pressure builds and gets the turbo going from maybe 0 rpm
to operating rpm like 90,000-120,000 rpm in a second or two.  Therefore
if you are bogging the motor in top gear and whack the throttle, you
could get peak boost at very low rpm, assuming you stay at that rpm
long.

2.  Turbines are very efficient at energy conversion, somewhere between
70-80%.  Getting a clear picture of what is being converted is a little
harder.  That back pressure that spins up the turbo drops to only a few
psi across the turbo at full chat.  Also there is a substantial
temperature drop across the turbine.  A compressor will add temperature
above and beyond the heat of compression due to inefficiencies, and
take 20% more power than theoretically necessary to compress the
charge.  Now that is the difference between turbos and crank-driven
superchargers.  Turbos get that power by converting 80% of the exhaust
pressure into power to drive the compressor.  Crank-driven get all that
power from the crank, dumping all the energy of the exhaust down the pip
e.

John Bucknell is jb24 at chrysler.com
---------------------- Forwarded by John R Bucknell/JTE/Chrysler on
02/20/98 01:42 PM ---------------------------

        owner-diy_efi @ efi332.eng.ohio-state.edu
        02/20/98 10:52 AM
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Subject: Re: Variable Compression, Variable Displacement you decide

>
> > > Expert? Me?, No I am no expert - far from it. Its just my life's
work
> > > thats all. I do not believe in experts - (A drip under pressure
etc etc
> > > etc) - However I do seem to know more about the subject than you
do, so
> > > if you have any further questions, please put on your cone shaped
hat
> > > (Yunno - the one with the 'D' on it), sit in the corner, put your
hand
> > > in the air, and I will see if I can help you.
> > >
> > > Me - I just luv a bit of forced induction...
> >
> > I amr really not sure you do.  You seem to not realize where the
energy
> > from the turbo is coming from.  If the energy was being truely
wasted,
> > then turbo vehicles should get better gas mileage than any NA
vehicle
> > in existence, becasue more of the energy from the fuel is being
effectively
> > used, but turbo cars don't get any better than cars with na engines
> > of similar weight and hp.
> >
> >                                 Roger
>
> Roger - I give up with you. You really are a pratt. Nearly every
> statement you make is WRONG. Are you a professional wind-up artist?,
> Were you born that way, or did you have to train hard for it.
> Tony
> --
> Sent By Tony Cooper.
> email: tony.cooper at virgin.net
> Allow at least 10 working minutes for reply. ;)
>
>
>
Tony,

You where claiming every bit of the energy used to drive the turbo
was free.  That is not true. I will but the part about the heat
being used to drive the turbo - but you did not say that - someone
else did.  I will now buy that part of the energy (don't know
what part - guess around 1/2 is free) All you claimed was it was
all free all the hp used by a supercharger could be gotten completly
free with a turbo.  This statement is WRONG.   You could not explain
where the energy was coming from so it could be free.   That was
what I was trying to figure out.  And also,  if I only have the time
for "curt" appearing one word answers, I don't post, I wait until
I can post complete answers, and I check where the thread has gone.

   Roger