[urq] Turbo cooling 101 - the long and nerdy

[QSHIPQ at aol.com]

Not good information Phil.  Comments inserted...

SJ

In a message dated 12/30/2003 5:10:36 AM Central Standard Time, 
quattro at isham-research.com writes:
>All turbos are air-cooled.  The heat removed by liquids is insignificant.

No.  Get SAE 880258- The Development of a Severe Turbocharger Bench Engine 
Test, 970922 - Development of Modern Engine Lubrication Systems,   860103 - The 
Third Generation Turbocharged Engine for the Audi 5000 CS and the 5000CS 
Quattro.  All these SAE articles are co/authored by Audi AG, btw. 

There are basically 3 generations of the "K26 turbo" as we know it.  The 
earliest generation (which excluded audis k26) had no oil spray in the center 
housing.  Gen II (audi "oil coolers") had oil spray onto the center shaft and 
shaft walls to cool the center bearing assembly and help prevent heat soak.  The 
Gen III (current audi flock and latest k24/26 designs) has Gen II plus water 
cooled center bearing jacket assembly.  The design of Gen III was to help 
prevent post down heat soak problems in the turbo, reduce temp spikes associated 
with high turbo loads, and to reduce the temperature of the piston rings (running 
AND shutdown function).  Even SAE 860103 (fig 9) shows significant reduction 
of Piston Ring and Center housing temps with the addition of Water cooling.  

Trivia:  In the I5 turbo water feed runs at around 6-10L per minute thru the 
turbo and the oil flow is 10L per min at 1000rpm and 25L per min at 4500rpm.  
That seems a bit more than "insignificant" to me.

>On the 10V I5s, there are two carefully designed air passages that take 
forced air from the
>front grill and direct it down the side of the engine.  Some plays over the 
turbo, some over
>the exhaust manifold.

Insignificant IMO/E.  The best efficiency of a turbo runs the hottest hot 
side and manifold (increase temp = increase velocity) and the coldest cold side 
(decrease temp = increase charge air density).  That means airflow is a mixed 
bag.  I removed the euros and installed the quad 4 - 2 on ly 83 and the temps 
of the turbo weren't significant.  The temps to the intake snout at the airbox 
was.

>This airflow is IMPORTANT.  Most of the turbo overheat problems I see have 
occured >because
>this airflow has been impeded - the most common culprit is an after-market 
dump valve, >though
>sometimes I find misguided people have put shielding between the manifolds.

Call me shirley... misguided.  FYI, the shielding is a great performance 
upgrade if done properly on the 10vt non crossflow head designs.  It's there to 
reflect exhaust manifold heat off the intake manifold.  Significant temperature 
effects right where you want them, at the tip of the fuel injector.

>The later engines have a water jacket around the centre bearing.  This is 
properly fed only >at
>rest, after the engine has been switched off for around a minute and subject 
to the water
>temperature at the head union (where the water in the bearing jacket 
convects to) >exceeding a
>certain temperature.  The electric pump actually pumps water BACKWARDS 
around the >circuit.
>There is no meaningful flow in this circuit when the engine is running.

Not with you.  See above.

>The cool-down advice applies equally to the original and to the so-called 
"water-cooled"
>turbos.  There's absolutely NO WAY a pipe less than 1/4" in diameter can 
carry enough >water to
>deal with the kind of heat generated at the hot end of a turbo.  At least 
two orders of
>magnitude, possibly three.

Get the articles above Phil.  There have been several revisions to both oil 
and water passages and placement to optimize their effectiveness in cooling.  
The reason the pipes are small is because bigger isn't better, you want as much 
heat exchange within a small housing as possible.

>Equally, cool-down has to be done with the vehicle in motion.  Idling while 
parked doesn't
>help - the only underbonnet airflow in such cases is provided by the 
alternator fan, and it's
>pathetic.
>--
>  Phil Payne


I disagree completely.  Cool down refers to taking a hot turbo and reducing 
the temperatures of it as fast and as efficiently as you can.  Water cooling 
after shutdown is only using half the available cooling, even with the vehicle 
not in motion.  Idle the car after a hot run is the best thing you can do, 
regardless of underhood temps.  The underhood temps will never reach 1400degrees, 
so "aiflow" is insignificant to the objective of turbo cooling.  The airflow 
is *less* with no motion, but so is engine load.  An easy trade, well 
documented.

HTH

Scott Justusson
QSHIPQ Performance Tuning
'91 v8
'84 RS2URQ Project
'83 Urq