From: John De Armond
X-Source: The Hotrod Mailing list
Date: Jul 1992
Subject: Turbos and heat soak

>In the chapter on lubrication, however, he makes mention of a problem
>that I've seen mentioned in many other places, but never quite accepted.
>He describes what happens after a turbo vehicle has had a hard run, and
>then the driver suddenly reduces speed, stops, and shuts the engine off
>right away. According to MacInnes (and others?) the sudden loss of oil
>pressure leaves the bearings in a condition where they will burn up as
>the turbo shaft continues to merrily spin on at 100,000 rpm. I don't
>understand how this is possible.

It's not.  I've run a turbo engine on a dyno with no tail stack on
the turbine exit (for no other reason than I'd not fabricated one yet)
and I could watch the turbine wheel.  The turbine stops almost immediately
when the engine does.  Coastdown is a bit slower when the engine is
running for what might be a not so obvious reason.  If you've ever
capped the intake of a squirrel cage fan and noted how the fan speeds
up, this is the condition that exists during coastdown assuming an
upstream throttle.  The blower has a high vacuum on it and thus little
drag.  The turbine partially evacuates the exhaust manifold so it is
also running in a partial vacuum.  When the engine is stopped, a different
situation exists.  The turbine is still in a partial vacuum but the
blower is now at atmospheric.  It stops very fast.

As you surmised, the real problem is cookoff.  I'm not really sure
a short idle interval really helps.  The temperature decreases asymtotically
with time so once the first flash of visible heat is gone, much more
time is needed to cool to a safe zone.  Water and/or oil cooling are
the real answer.  My solution is simply not to push the car hard for
several minutes before shutdown.  The flow of relatively cool exhaust
flow helps speed the cooling process.

>Anyway, maybe I'm wrong on all this, but I noticed something else that
>bothered me even more. Nowhere in the chapter on lubrication does
>MacInnes mention the problem of heat-soak from the exhaust manifold,
>and the resultant problems of oil coking and varnishing on shafts,
>bearings, and oil feed lines. This, to me, seems like a much more
>important consideration than the scant few seconds that a turbo
>*might* have to endure moderate RPM's with no oil pressure.

Heat soak from the manifold is not a particular problem.  The gasket is
a relatively good insulator so not much heat gets transfered.  The
problem is the turbine housing and the wheel itself which is at least as
hot as the manifold.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Cooling off diesel engine
Date: Fri, 06 Apr 2007 23:34:11 -0400
Message-ID: <th3e131thqtfpsfe8vnejo27079r9tgk48@4ax.com>

On 6 Apr 2007 18:21:17 -0700, "Yukon" <Loyza3@gmail.com> wrote:

>I drive diesel F350.  My manual suggests, after a long drive,  to let
>the engine idle for a few minutes to allow it to cool off.  I don't
>always do that.  Is it really necessary?   If yes, then how long?   I
>have a hunch that most drivers don't do that.  y

Not a long drive but after a long pull.  The reason is to cool the
turbo exhaust side.  If the engine is shut off immediately after a
hard pull, the turbine side can be near red hot.  That heat soaks
across the bearing capsule and decomposes what oil there is in the
capsule - called "coking" because the residue resembles coke.  Over
time the coke builds up until oil flow is restricted and bearing
failure follows.  In unusual cases where the turbo is very hot and
shutdown is immediate after a hard pull, enough heat can be conducted
along the turbine shaft to damage the bearing immediately.

Because a diesel's air flow is not throttled, cooling starts the
instant the throttle is released after a hard pull.  Because the
volume of air flowing through the engine is high, cooling is fairly
rapid.  If you pull a long hill, maybe sit at a traffic light for a
bit and make a turn into a parking lot at the top, the cooling is
happening as you pull around to find a parking place.  If you spend a
couple minutes finding the place to park, parking and getting your
stuff together, that's probably long enough.

The best way to know when and how long to idle is to have an exhaust
temperature gauge.  That's pretty much standard equipment on big
trucks and IMO, should be on all diesels.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Cooling off diesel engine
Date: Sat, 07 Apr 2007 12:27:39 -0400
Message-ID: <a1hf13dgpqicetua9rsh93vm050qjqsn2n@4ax.com>

Thank you, Matt.  I really appreciate that.

I started wrecking, er, using turbos on gas hotrods back in the early
70s.  I learned the hard way :-)  Then I found Innis's book and
started doing it right.  Only a little diesel experience but my recent
truck driving gig taught me a lot.  Even though I knew it would, it
was fascinating to watch the EGT drop like panties after the prom the
instant I lifted the throttle.

Now I have a question for you.  Which side of the turbo should the EGT
be on with a diesel?  On gas engines I've always installed them on the
inlet side, the reasoning being that I wanted to limit the turbine
wheel temperature, since there's quite a bit of combustion still going
on in the manifold.  I've seen the manifold actually get hotter/redder
closer to the turbo.

My big rig had the EGT installed between the compound turbos - outlet
of the first and inlet of the second.  That doesn't seem like the
placement for the best picture of what's going on.

John

On Sat, 07 Apr 2007 09:02:59 -0400, Matt Colie <mattcolie@nospam.net>
wrote:

>John,
>
>This is one of the best descriptions of the entire situation that I have
>seen on the web ever.  I was reading the thread to see if someone put
>down the complete and correct answer so I didn't have to.  Well, I
>don't.  As a diesel engineer for about four decades, I can assure you
>that John knows the story.
>
>Thanks
>
>Matt Colie

From: Matt Colie <mattcolie@nospam.net>
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Cooling off diesel engine
Message-ID: <R5YRh.1556$ZU4.1171@newsfe04.lga>
Date: Sat, 07 Apr 2007 22:05:29 -0400

Neon John wrote:
> Thank you, Matt.  I really appreciate that.
>
> I started wrecking, er, using turbos on gas hotrods back in the early
> 70s.  I learned the hard way :-)  Then I found Innis's book and
> started doing it right.  Only a little diesel experience but my recent
> truck driving gig taught me a lot.  Even though I knew it would, it
> was fascinating to watch the EGT drop like panties after the prom the
> instant I lifted the throttle.
>
> Now I have a question for you.  Which side of the turbo should the EGT
> be on with a diesel?  On gas engines I've always installed them on the
> inlet side, the reasoning being that I wanted to limit the turbine
> wheel temperature, since there's quite a bit of combustion still going
> on in the manifold.  I've seen the manifold actually get hotter/redder
> closer to the turbo.

John,

The reasoning you used is completely sound and quite correct for a SI
engine.  Diesels have so much lower A/F ratio due to the smoke limit, so
that same condition does not exist.  That is why they have had to screw
around with diesels so much to make them light any kind of a catalyst.

This is also why the turbo with a jacket water cooled bearing are a real
good idea for passcar motors.

Except for aircraft, where it is used to set mixture, the EGT is largely
an underway diagnostic.  So, you are more interested in the change than
the actual value.  As far as using it to confirm the inlet to the
turbine wheel, it is your only hope.  A highway engine should be
calibrated so it can't get in trouble that way.  If someone messes with
the calibration to get more power, he is on his own and he can't do it
without leaving tracks that the manufacture's rep (BTDT) will look for
as soon as there is a warranty claim.

Most of the ship engines have had a "lota position" switch (cylinders +
2) so I could pick each individual cylinder and the collected at the
turbine outlet.  That is real nice information, but of very little value
in a practical sense.
>
> My big rig had the EGT installed between the compound turbos - outlet
> of the first and inlet of the second.  That doesn't seem like the
> placement for the best picture of what's going on.
When I asked my Cummins guy why they did that with the NT compound.  He
said that their thinking was that a difficulty with either turbo would
show up there first.  There is some expansion through the turbine that
should reduce the measured temperature, but it should still run right
with the inlet at almost any load.
>
> John
>

I think (hope) I answered all your questions.  If I didn't please try
again.  I'll be back tomorrow.

Matt Colie