Re: 91 200'isms

[Jeremy R King <kingjer@eng.auburn.edu>]

Y'all are forgetting/neglecting a pretty basic principle.  A turbocharger 
is DRIVEN by exhaust gasses.  The compressor's job is to get more MASS 
into the combustion chamber, which is compressed, combusted, and then 
dumped into the exhaust stream.  If you start with a lower ambient 
pressure (lower atmospheric pressure), the inlet air is necessarily LESS 
DENSE than at higher atmospheric pressures.  Less dense air means less 
mass of air (specifically for a given velocity).  Less mass coming out of 
the cylinder will result in less mass to push the turbine side (exhaust 
side) of the turbo.  Result - the turbo will spin more slowly and 
compress less than at higher atmospheric pressures.  Find a textbook and 
look at a P-v diagram for an internal combustion engine.  

Imagine having a propeller of some sort (maybe one of those colorful foil 
things on a stick like you'd get at the fair or something).  Blowing on it 
will not spin it as fast as spraying it with a water hose.  The water is 
more dense and has more kinetic energy (1/2 MV^2) than the air.  Well 
substitute less mass for M in that equation and you'll see the effect of 
high altitude on the system.

Jeremy
'86 VW Quantum GL5
Auburn University, Alabama, USA
Hometown - Reidville, South Carolina, USA

On Sat, 16 Mar 1996, Albert Ng wrote:

> 
> Yeah, you are right. In low ambient pressure the wastegate should keep 
> close longer to maintain the manifold pressure. But for what I mean is,  
> at a certain altitude(maybe more higher) the situation won't happen the 
> same as you menthioned. The wastegate will fully open (by altitude sensor) 
> to perevent the turbo overrevving since the turbo restriction is reduce 
> by the thin air density. 
> 
> I really can't recall where I read this piece of info, but I think 
> some member on the list can probably tell us more about this. :-)
> 
> 
> Albert  Ng
> '87 5kcst
> 
> On Sat, 16 Mar 1996, Robert Myers wrote:
> 
> > Albert,
> > 
> > It would make sense for there to be some sort of ambient air pressure
> > measurement to allow better boost control.  But at lower ambient pressures
> > you'd want the wastegate to stay closed longer, not open sooner.  Part of
> > the "spring" holding the wastegate closed is atmospheric back pressure.  At
> > lower ambient pressures (higher altitude) you need a stiffer spring to
> > maintain the same absolute manifold pressure.
> > 
> > Is this 18 cents short of a paradigm?  :-)
> > 
> > At 12:41 PM 3/16/96 -0500, you wrote:
> > >
> > >Just my little something about this topic. I heard from somewhere a while 
> > >ago that all the Audi 5kt car has a altitude sensor. If the car is operating 
> > >at a certain altitude, the sensor will kick on and send signal to somewhere 
> > >to fully open the wastegate. I don't know why they doing this, but, by my
> >           ^^^^^^^^^^^^^^^^^^
> > 
> >                  ???
> > 
> > >guess, is that they want to keep the turbo spin slower since on a higher 
> > >altitude the air density is lessen and the turbo will spin fast.  
> > >
> > >
> > >Albert Ng
> > >'87 5kcst
> > 
> > ---snip---
> > 
> > ___
> > 	Bob 
> > ********************************************************************
> > *  Robert L. Myers <rmyers@wvit.wvnet.edu> 304-442-1046 (FAX)      *
> > *  Chair, Department of Chemistry          304-442-3358 (office)   *
> > *  WVU Institute of Technology             304-574-2372 (home)     *
> > *  Montgomery, WV  25136                   304-442-3109 (secretary)*
> > *            Obligatory Quattro and Sleddog Content:               *
> > *        My Siberian Huskies like to ride in my '89 200TQ          *
> > ********************************************************************
> > 
> > 
>