Turbocharger cool down time

[rbp]

I do notice a reduction in  EGT during the the cooldown at idle. When it stabilizes, I shut down. 

[Rick7576]

I made the original post. I do a 3-5 min cool down. Ive only had the airplane for about 2 years (225hrs) no problems with airplane so far. except for the mag failure. 

[PeteMc]

When I first got my K everyone talked about making sure I did a 5 min cool down. 

It wasn't that long after going to various seminars and reading from some of the "biggies" on AVSIG that it became apparent that the "extra" cool down when I got back to the ramp was not doing any good and might actually be raising the temps ( a little bit) from the coolest point.

I think a lot of the theory for the extra cool down came from the Mooneys with the fixed wastegate.  With the fixed wastegate you were spinning up the turbo quite a bit just to taxi.  Especially if you had any kind of up hill taxi.  But with the turbos that have any kind of variable wastegate, the turbos start to spin down in the downwind and spin down even more when you pull the power back on short final.  So from that point on you're at the minimum speed on the turbo. 

Once on the ground there is a period where the turbo casing slowly continues to dissipate heat.  But basically after the taxi time at most mid to larger airport, you've lost as much heat as you're going to, and continuing to run the engine does nothing to further cool the turbo.

[Pinecone]

If I can taxi at less than 1200 RPM, I don't do any extra cool down/spin down.

If I need more power for an uphill or tight turn, I will give it a couple of minutes.

[PeteMc]

  On 1/8/2023 at 12:49 PM, Pinecone said:

If I can taxi at less than 1200 RPM

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The 1200 RPM sounds more like the old theory with the 231 fixed waste gate that I had when I first got the plane.  Are you sure your turbo in the 252 really spins up that much if you're at or sightly above 1200RPM.  And even if it does, by how much and how quickly does it spin back down? 

Maybe one of the engine gurus here can chime in or you can check with your mechanic.  Most everything I've heard is that if there's a momentary spins up (or even a couple), it's not going to make a difference to sit for 3-5 minutes.  The higher speed airflow with lower RPMs on final and the rollout are what gave you the most cooling.

 

[Steve Dawson]

Just a thought, the turbo is cooled by both the decrease in TIT, the air flowing around the turbo (not much) as well as oil supplied, so if your TIT cools down to as low as 850-900F look at your oil temp and see if it's cooled and stabilized. I don't think your turbo and the oil supplied is going to get any cooler than that. 

I have checked before but never timed it so I'll check this week and get back on how much time and the TIT, oil temperatures including the ambient temperature. 

[philiplane]

It's cooling down on approach and landing. Taxi in without using excess power and shut it down.

Idling while stationary is just heating it back up.

Edited January 8, 2023 by philiplane

[Pinecone]

  On 1/8/2023 at 3:39 PM, PeteMc said:

The 1200 RPM sounds more like the old theory with the 231 fixed waste gate that I had when I first got the plane.  Are you sure your turbo in the 252 really spins up that much if you're at or sightly above 1200RPM.  And even if it does, by how much and how quickly does it spin back down? 

Maybe one of the engine gurus here can chime in or you can check with your mechanic.  Most everything I've heard is that if there's a momentary spins up (or even a couple), it's not going to make a difference to sit for 3-5 minutes.  The higher speed airflow with lower RPMs on final and the rollout are what gave you the most cooling.

 

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It works as a marker for me.  And I only give it 1 - 2 minutes, not 5.

 

[PeteMc]

  On 1/8/2023 at 9:30 PM, Pinecone said:

It works as a marker for me. 

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But you don't have a fixed Wastegate that is boosting the turbo at near idle.  I do have a 231 and I tossed that concept as soon as I put in the Merlyn

What RPM does you mechanic say your turbo starts to spin up at?  (Or any other 252 drivers here that know?)

 

[carusoam]

This is one of the OWTs for turbos…

1) Cooling occurs continuously…

2) Heating is extremely variable… directly related to FF and TIT….

3) Following the POH is important… but engine management is only a small percentage of the 300 page POH…

4) Mooney owner experience gives a wide spectrum of solid real world answers…

5) Great mechanics keep track of machines from the inside… and share nicely with the owners… 

6) The guys at Gami have spread the word… How you cooled your turbo before landing… using the laws of physics…

 

Soooo…

7) Use the laws of physics to prove to yourself when your turbo is cooled…. No matter how the POH says it should be done…

8) If this was really important… a thermocouple would be mounted to the turbo…  why don’t we have one, and one for the oil leaving the turbo as well… they’re only a small addition to the existing engine monitor…???

9) similarly… exhaust valves have a tendency of coking oil just like a turbo… no thermocouple there either…

10) There are some odd cases of running an engine really hard, and not shutting down properly….

11) ordinary X-country flights… no extra cooling step is going to help… there just isn’t one to get it cooler than a low power descent, full rich, into the traffic pattern and landing…

In most cases… extra cooling steps just aren’t going to do very much…

If you have oil coked in either your valve guides, or turbo… get it cleaned out.  Coking interferes with the flow of the cooling medium… (oil)

There are many additional details to be aware of with turbo equipment… blade health, bearing slop, hours, OH….

PP thoughts only, all stuff I learned about around here…

Best regards,

-a-

[Pinecone]

  On 1/9/2023 at 12:39 AM, PeteMc said:

But you don't have a fixed Wastegate that is boosting the turbo at near idle.  I do have a 231 and I tossed that concept as soon as I put in the Merlyn

What RPM does you mechanic say your turbo starts to spin up at?  (Or any other 252 drivers here that know?)

 

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Like I said, it is just a marker point.  Most times, I can taxi at less than 1200.   Sometimes, due to an incline or tight turn, I have to use more power.  So I give it 1 - 2 minutes.

Maybe overkill, but surely in not doing harm.

[rbp]

after my flight yesterday, I decided to play around with this a little bit before shut down.

as usual, I set the throttle to idle (650RPM) and the EGT on the hottest cylinder stabilized at 925 +/- 20F.  so I bumped the power up, and as soon as the power came on, that EGT rose by 100F. So I set it back down to idle, and after about 15 seconds the EGT went back down. YMMV

[PeteMc]

  On 1/9/2023 at 6:29 PM, rbp said:

So I set it back down to idle, and after about 15 seconds the EGT went back down.

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Yes... but who's the metallurgist here who can tell us how long it takes for those gasses to actually raise the temp in the metal in the trubo.  And conversely, does it cool at an equal rate?

 

[PeteMc]

Interesting tid bit of data I just learned.... 

A friend that now has a 252 used to own a 231.  In his 231 setup he had a JPI Oil Temp prob on the Turbo OUT oil.  Said that the coolest oil temps would happen with low MP on a standard approach/landing.  No additional cool down needed as the GAMI report also concluded.

[rbp]

I am no physicist, so correct me if Im missing something. but it seems to me that if there is heat in the turbo charger that is being dissipated during cool down, wouldnt it show up in the oil temperature? so  once  the oil temperature has stabilized, doesn’t that mean that everything that the oil is meant to be cooling has already been cooled? 

[Pinecone]

It would if you had an oil temp probe on the return line from the turbo.

But the general engine oil temp is after the turbo return oil goes into the sump and after the oil cooler.

[PeteMc]

  On 1/10/2023 at 2:39 AM, Pinecone said:

But the general engine oil temp is after the turbo return oil goes into the sump and after the oil cooler.

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See my comment above.  The guy I know had a probe for the oil temp exiting the turbo.  His data indicated the same findings as GAMI's more scientific data collection.

 

[A64Pilot]

  On 1/5/2023 at 8:28 PM, RobertGary1 said:

I sometimes fly a t206H for an operator that insists on waiting for a certain tit before shutdown. Sitting there idle makes it warmer than landing. 

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Why do you say that? Are you measuring the temp of the hot section? If not then where are you getting your information?

The damage or coking anyway occurs AFTER shutdown and takes time, it occurs when the heat is transferred from the hot section to the center section, Center section heat doesn’t increase until after shutdown, when the cooling oil flow is stopped.

This kind of damage is cumulative over time, it’s like the guy that claims smoking isn’t bad for you, he’s smoked for 20 years and hasn’t contracted Cancer.

I keep posting about this because as good as these forums are in disseminating information, sometimes it’s incorrect information, and or partial info, and I hate seeing new owners led down the road to neglecting and or mistreating their equipment.

Don’t listen to ANY internet source, including me, call the manufacturers help desk, ask them. Anyone can and many often do a little Googling and reading and can really sound like an expert, and simply republish what they have read. 

If a Manufacturer is adamant about a procedure, especially if it’s not popular with pilots,  you can bet it’s not because they are stupid and enjoy punishing pilots, it’s most likely because they see every warranty item and a large portion of overhauls and tear down and inspect every one. Coking doesn’t take a scanning electron microscopes to see, it’s readily apparent to the naked eye once you break down the center section. With the exception of someone grossly over temping and burning out the hot side turbine, most every overhaul and or turbo failure is the center section bearing and or oil seal, both highly affected by blooming temps of the center section after shutdown.

Pick up a turbo, it’s not light, most of the weight is the hot end, the cold side pinwheel is likely titanium and the scroll case aluminum, but the hot side is steel and sometimes cast Iron or maybe high temp resistant steel as is the center section, that heavy mass retains high heat, just like a cast iron frying pan compared to an aluminum one.

For some reason the same people that recognize their engine isn’t warmed up instantly because of high EGT think the turbo cools instantly.

[Pinecone]

  On 1/10/2023 at 4:09 AM, PeteMc said:

See my comment above.  The guy I know had a probe for the oil temp exiting the turbo.  His data indicated the same findings as GAMI's more scientific data collection.

 

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Agreed.  That would be the best way to determine cool down requirements.

Although, the other issue is spin down time.  Not a big problem in aircraft due to pattern/landing/taxi time.  In cars, you can have the turbo spinning at 100,000 RPM amd shut off the engine.  The turbo takes a while to slow down.

[A64Pilot]

  On 1/10/2023 at 2:25 PM, Pinecone said:

Agreed.  That would be the best way to determine cool down requirements.

Although, the other issue is spin down time.  Not a big problem in aircraft due to pattern/landing/taxi time.  In cars, you can have the turbo spinning at 100,000 RPM amd shut off the engine.  The turbo takes a while to slow down.

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No, it wouldn’t, reason is it’s the heat of the hot section that’s the issue not the oil coming from the center section that’s the issue, The cool down periods is to allow the metal in the hot section to cool, so if you wanted to actually monitor something, monitor the temp of the metal in the hot section, and the center section temp, then if you really wanted to you could determine that with a hot section temp of X the center section will eventually increase to Y and oil begins to coke at Z.  Then with all that data you could determine that when the hot section case cools to some temp it’s safe to shut down and the center section never gets to Z temp.

Or you could just idle the thing at 900 RPM for five minutes.

The oil temp will of course be influenced by both its flow rate and its temp on entering the turbo, AND the heat from the hot section, so decent high flow combined with cooler oil will show cool oil even with a smoking hot hot section.

The turbo spools down pretty darn quick, matter of seconds when power is pulled, that’s why turbo’s have lag, because the thing spools down awfully quick. It’s unlikely that at idle it’s spinning much if any at all, most Diesels don’t and as they have no throttle valve they pump much more air at idle than spark ignition motors.

Only time I’ve seen spool down time being an issue is drag racing, there you could go from full boost to off quickly

[Pinecone]

I would have to pull out my turbo design books, but from my memory, it takes a while to spin down.  

Oil temp from the turbo is indicative of the heat in the turbo.  If you watch the oil temp just down stream from the turbo, when it stops dropping, the turbo is pretty cool.

[Schllc]

My poh says five min at low power settings. Period. 
I don’t think anyone here, so far, is arguing that a turbo doesn’t need to cool down before shutdown.  I think the discussion surrounds what constitutes a cool down. 
I cannot see a more gentle phase of engine power settings/temps than landing phase. 
High air flow, low power settings, followed by what would be typically a short taxi at near idle. 
If five minutes at idle is all that is required (per manuf) it would stand to reason that after a normal landing and taxi, a turbo is as cool as it’s going to get. 
Im not suggesting that manufacturers are stupid.  What I am saying is their recommendations are almost always predicated on the worst possible conditions, with a healthy safety margin. 
I also think if coking and turbo failures were that prevalent, there would a temp probe in the center section  

Simple thermodynamics, physics, common sense, and empirical experience are valid and relevant,  and while permutations do complicate absolute certainty, blindly following some generic guidance in a manual that doesn’t consider reality isn’t always the best path either.

 

 

[rbp]

  On 1/10/2023 at 8:53 PM, Schllc said:

Im not suggesting that manufacturers are stupid.  What I am saying is their recommendations are almost always predicated on the worst possible conditions, with a healthy safety margin. 

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I wonder if manufacturers use cool-down procedures based on bench test temps, rather than temps post-approach and landing

[LANCECASPER]

  On 1/10/2023 at 8:53 PM, Schllc said:

Simple thermodynamics, physics, common sense, and empirical experience are valid and relevant,  and while permutations do complicate absolute certainty, blinding following some generic guidance in a manual that doesn’t consider reality isn’t always the best path either.

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Exactly.

See how popular you are when you pull up to a busy FBO at a big airport after taxiing 5-10 minutes and then having the line guy standing there in front of the airplane waiting to chock your nose wheel while you are running it 5 minutes to cool down the turbo as he has business jets coming and going. They think single engine GA pilots are a little nuts to begin with and you've now removed all doubt. It's not necessary and definitely not practical.

[KSMooniac]

Re: the manufacturers not updating formal guidance after getting enlightened with real data, don't underestimate the bureaucratic inertia that must be overcome, along with a review from a legal department, if they're willing to change course after many decades.  That is reason enough for them to not alter past guidance, so long as it doesn't pose an acute safety risk.  I doubt the guy who owns Hartzell now would like to spend any money doing so, since he is apparently finally making money from those product lines.  Hopefully he has fixed the quality issues with many of those products that had been horrible for a very long time....that would be a far better use of investment dollars.  (FWIW, I saw the APS data in a live class as a chart of the screen, and have no reason to doubt it.  It makes perfect physical sense to this engineer.)

Another anecdote... the oil pressure limits in our vintage Mooney Lycomings have a green zone from 60-90 psi.  Consider a little thought experiment... Textron owns Lycoming and Cessna.  Cessna ceased production in 1986, but re-started in 1996.  IIRC, the new oil pressure green zone for essentially the same engines now goes up to 115 psi!  Why is that?  Could it be that they finally figured out that higher oil pressure lubricates and cools the top end more effectively, and leads to better longevity and fewer in-service issues?  Very likely so, since oil being squeezed through lifters and pushrods is the oil path to the rocker box, except for the Bravo engine and perhaps a limited number of other variants.  Why didn't Lycoming publish guidance for the rest of the fleet to crank up oil pressure, or pass along guidance to the airframers to publsih such guidance?  Likely because it would open a huge can of worms and liability exposure for engines that didn't make TBO due to sub-optimal oil flow & cooling.

A highly regulated industry with low volume and huge liability exposure makes change very, very difficult, even in light of better data.

Whenever I get around to adding a TN kit to my J, I will not sit there and idle for 5 minutes.  Everyone else, do what you want.