Turbo cooling

[HP]

Hello! I did a search, but haven't found anything specifically about it, so:

We all know that we must wait some time before turning off the engine  in turbo'ed cars. Essencially right after a highway trip or a *sporting driving*. That, we all know.
What not all of us know (me) is why! ?
No one has ever explained it to me, but I guess it's mainly because of the high temperatures the oil probably reaches (under heavy load), so an amount of time is required for the oil that flows inside the turbo to cool down! :idea:

Am I right or wrong?

[zyewdall]

As I understand it, yes, it's because the oil in the bearings of the turbo will cook and turn to crud if it doesn't stay flowing for a few minutes till the turbo cools down and stops spinning at full speed.

There was in interesting discussion recently on another discussion board about "modern" subaru turbos, which apparently use water cooling, and can be shut down immediately without overheating.   :roll:

[RabbitJockey]

i think newer 1.8t turbos also use water cooling.  but yeah, it's so that your turb has time to spool down and cool as so that the oil doesn't harden and turn into a blockage the next time you drive.  which would take all cooling and lubricating from the turbo.  and obviously, nasty things occur after this

[HP]

That's what I thought. But to tell the truth, my car hasn't been carefully treated (relatively to this turbo cooling issue) for many years  :shock:  Only since it's under my care (late 2004)!... 'til then, my father (previous owner) didn't even knew about this!  :?  He did buy the car in 1996...
It has olny had turbo failure once, ever since! And obviously repaired at the time.
So, how dangerous is, actually, to procede with no care, over this turbo cooling question  :?:   :roll:




P.S.: I've read in this forum that the Audi's K24 were watercooled! How relevant is that, regarding this very issue (reading the posts above) :?:

[LeeG]

I think the danger depends on driving style.  If you drive up a mountain to work, you might want to cool it down before switching off.  If you exit the highway and putter through some side streets before you get to your destination, its probably cooled off when you get there.

[regcheeseman]

Isn't it more a case of letting the turbo slow down before you kill the engine and correspondingly it's oil feed.

Running it for a while to let it cool may be a good idea but I haven't seen anyone do it in about ten years. Oil and Metallurgy have come a long way which should help.

[wyldman]

The main thing is temperature,not turbo speed.

If your running it hard,the turbo can get quite hot,and without oil flow to the bearings,the oil begins to coke and damage occurs.If you just putting around,it doesn't get as hot.

Having an EGT gauge helps,as the EGT readings will show you just how hot the turbo is.You can then decide if you need to let it idle to cool down.

Using a good synthetic oil helps too,as it doesn't break down as easily at higher temps.

On my Cummins,it can take 5-6 minutes to get the turbo cooled down after a hard pull on a hot day.Shutting the truck off would do some serious damage.

[HP]

What kind of damage, to be more precise, technically?

[LeeG]

If the turbo is hot enough the oil in the turbo bearing when its shut off will boil.  Boil it a bit and it gets thicker like tar.  Boil it a lot and it gets hard, like carbon.  Get enough tar or carbon in the turbo bearings and next time you start up some damage could be done to the turbo bearings.  

The problem of excess heat on shutdown isnt confined to turbo charged engines.  Any engine with a hot spot can damage itself after shutdown when the flow of cooling fluids stops.  I have seen lots of cars that have been run hard and shut off suddenly cook off the radiator.  If you have a really hot head for example, when the engine stops the coolant stops and a hot pocket in the head can flash to steam and overpressure the rad cap.

Just thinkin about it brings back fond memories of my 'bump and pass' racing and demolition derby days.  I must be a redneck!

[hillfolk'r]

iknow at work we let egt get to below 300f before shutting down an engine that was dyno'ed,,,i believe the "cool down" cycle does a few things,,it lets oilcirculate thru turbo,and carry away the heat,,so  bearings dont coke up,etc,,,,and also the turbois spinning slower so when you shutdown,,it isnt spiining fast w/no oil pressure,,not good,, dry bearings,,u guys with open straight pipes  listen to the turbo stillwhisle after you shut it down,,i hear mine for 3-4 seconds,,,thats the turbostillspinning w/ no oil pressure,,,,sometimes it helps too,,in the cooldown cycle,to hold the idle up a few hundred rpm above idle,,this lets more air go thru turbo to coolit off,and the previously mentioned increased oil circulation tocarry away the heat,itlldrop the egt down a little quicker,,,,,,,then let it dropto idle before shutting down,,,,hell on hot summer days i feel guilty aftter beatin onit,,,and ill pop the hood too,,why trap all that heat,and let it cook hoses+wiring???years ago,,my dad had a saab 900 turbo,,never replaced a turbo,,and he drove it hard,but let it cool down,,a guy at his work,never let it(he had a saab too) cool down,he did 2 turbos in by 100kmiles

[fspGTD]

Here are a couple examples of some turbos I've recently had apart of the coking usually present at in high-mileage turbos, which generally starts accumulating around the turbine end sealing ring:


Due to religious use of synthetic oil and cooling the engine off before shutting it down, the second example had less coke build-up than the first, but both showed some build-up.

The coke build up is indeed super-hard and abraisive.  It will not "wipe" off - but needs something more like a razor blade to chip it off.  Scotch brite and solvent works well for removing small amounts.

I doubt the soft turbine end journal bearing would last long if the coking were to spread next to it.

The job of the turbine end sealing ring is very important for preventing coking build-up.  It keeps the hot, sooty, and pressurized pre-turbine exhaust gasses out of the oil-lubricated turbo bearing housing.  Even synthetic oil is not going to stand a chance of staying liquid if a lot of 1500 deg. F pre-turbine exhaust gas gets blow past the turbine sealing rings and into turbine the turbo bearing area!

Therefore, if you want your turbocharger to last very long, I recommend:
1. using synthetic oil, which can withstand significant higher temps before "coking" than petroleum-based oil

2. let your motor cool down before shutting it off when it's hot (use a turbo timer so you don't need to sit there to do it, and/or have an EGT gauge to be able to monitor the temps.)  This will allow things to cool off while the oil is still circulating.

3.  replace old, worn out sealing rings and bearings with fresh ones every 100k miles or so via a routine maintenance "do-it-yourself" turbo rebuild.  Also, upgrade the turbine sealing rings to gapless designs when available to further enhance the seal (I generally include gapless turbine ring seal upgrades them my rebuild kits when they are available).

The pic below shows a noticeable concentric wear groove present in the old "gapped" turbine sealing ring (on the right) from my 1.6lTD Garrett turbocharger (and compares it with a new, Dieselicious gapless upgraded version I am replacing it with on the left):

[HP]

OK, and for those who don't have an EGT gauge, how long should we wait until shuting off the engine?
Let's say... after driving on the highway for long minutes @ 120-140Km/h, for instance? How much time to let the turbo cool? Just to have an idea..

[fspGTD]

I'd expect on a flat road with those continuous speeds, your EGTs would probably by 600 deg. F or higher - so you definitely would not want to just shut it down if you were to abruptly pull into let's say a rest stop parking lot right off the freeway.  You'd probably be OK to let it idle in that situation for about 2 minutes before switching off.

Going over mountain passes and long hills at high speeds can also really drive up the EGTs.

I use my turbo timer (which runs for about 1 minute, and is usually long enough) whenever my EGTs are still above 200 deg. F when I've arrived at my destination.  But often I find that if I drive the car gingerly before arriving at my destination, no cool down is necessary.

You could probably safely get by with less cool-down time if you used synthetic oil, and would be well advised to add more cool down time if you're using petroleum oil.