Turbo cooling 101 - the long and nerdy
QSHIPQ at aol.com
QSHIPQ at aol.com
Tue Dec 30 08:49:38 EST 2003
Not good information Phil. Comments inserted...
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
>This airflow is IMPORTANT. Most of the turbo overheat problems I see have
>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
>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
>help - the only underbonnet airflow in such cases is provided by the
alternator fan, and it's
> 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
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