[s-cars] Fwd: Turbo cooling 101

[Kirby Smith]

I agree with Scott.  I cooked my Corvair turbo (TRW) bearings when
racing in the 1960s due to shutting down the engine (and thus the oil
cooling) immediately after laps in a misguided attempt to keep my cold
plugs clean.

Even if one were to generate such a massive airflow over the turbine
housing that it was significantly cooled (and thereby probably not as
thermodynamically efficient), the turbine itself would remain hot, and
conduct heat into the center of the bearing.

kirby


QSHIPQ at aol.com wrote:
> 
> 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
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