Long disc of turbo stuff again... :-)

["Graydon D. Stuckey" <graydon@apollo.kettering.edu>]

On Sun, 14 Jun 1998 QSHIPQ@aol.com wrote:

> Be careful here Graydon.  Pulse theory of exhaust has little to do with
> sequence or grouping (that's a runner length calculation, already done for you

I'll agree with this.

> by audi).  Pulse theory and collectors have to do with the relationship of the
> AREA of the effective valve area, the runners, the nozzle and the collector/s.
> If, after the calculation of these relationships, you determine that
> increasing collector size is necessary, not so sure that the triple fins are
> where I'd be looking.

Each of those sections in the triple fin are much smaller than the 
ports feeding the EM.  I believe that if Audi did their homework on this 
EM then only one pulse at a time should travel through the collector.  
However, even if they did this correctly (and I doubt it - I suspect that 
other compromises were made for packaging here) the 3 little pie shaped 
outlets are too small.  They are little more than half the size of the EM 
inlet ports an dthere is only threeof them where we have 5 inlet ports.  
That's too small.   That would require that the gas 
increase its velocity almost twice in order to get through without heat
losses.   Since you never get anything for nothing, then we have a throttle 
here.  That causes an energy loss before the hot gas gets to the turbo.

> it sits.  Remember, you want exhaust VELOCITY, higher velocity means more
> energy to spin the turbine.  Velocity is more a function of HEAT than of

Just to get some definitions straight here, velocity is not equal to
energy. Heat = energy.  Velocity is a property of the gas as it passes 
through the exhaust manifold.  Just to recap our thermo here...  For a 
subsonic flow (I haven't calculated the mach number, but I'm pretty sure 
that we are dealing with subsonic flow here, at least in the EM)...

in a converging duct...

Pressure decreases kPa
Velocity increases m/s
Molar mass increases kg/kmol
Temperature decreases deg C or K
density decreases kg/m^3

At first glance this appears counterintuitive, but remember a converging 
duct is basically a venturi and we all remember from our carb class that 
the venturi generates low pressure which draws fuel into the throat of 
the carburettor.

It also occurrs to me that the above describes why the scroll gets 
smaller as it travels around the perimeter of the turbo impeller.  It 
accelerates the gas to a velocity that can generate very high velocities 
in the turbo impeller.  Its like spraying an air gun at a ball bearing in 
your hand (not recommended BTW)  The high velocity flow out of the air 
gun can accelerate the bearing to extremely high rpms hich can actually 
cause the bearing to explode.

> volume.  You increase volume, you decrease velocity.  

Correct.  In a diverging duct (again with subsonic flow)

Pressure increases kPa
Velocity decreases m/s
Molar mass decreases kg/kmol
Temperature increases deg C or K
density increases kg/m^3

> You increase heat,
> given volume, you increase velocity.  

Correct, I suppose, but we can't increase the amount of heat available.  
That's a relative constant.

> Can you improve what is there?  I
> suppose you could, buying the 2 piece exhaust manifold gets you the most.  But
> 'restriction' isn't necessarily a bad thing.  Betcha higher gains could be

If the "restriction" were a perfect converging/diverging nozzle, we would 
probably have a nozzle efficiency in the region of 90% to 99%, but even 
that means a loss, so why leave it there?   If the nozzle were close to 
99%, then you might be right, look elsewhere for bigger bang for the buck.

> you.  But then, you have increased runner length which means heat becomes a

I don't think that a runner length in the region of one or two feet will 
be an issue when you see 4 - 6 foot runners on all out racing engines all 
the time.  Check out Bell's book for a few pix.

> proprietary, a lot of time could be spent making a lot of wrongs, before
> getting close to right, and maybe never 'better'.  Without a dyno to test and
> measure each changed variable, a lot of time could easily equal a lot of

Maybe you're right.  I don't have the testing facility to check it out.

> nothing.   Here, I'd give audi the BOD on the EM design and go to other places
> for real and measureable gains.  

I don't have much faith in the Audi engineers any more than the Cadillac 
engineers that I work with.  Most modern autos are designed with far too 
many compromises.  There's rocket science in our labs, but it just 
doesn't make it to the street.  :-(

Later,
Graydon D. Stuckey

"There's alot more to Jazz than just wrong notes"