Turbocharger cool down time

[Will.iam]

18 hours ago, 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.

 

I would be very curious what your friend’s oil temp behind the turbo was in a prolonged sustained climb to altitude. Just curious what max temp he saw in what phase of flight. In our jets the oil is cooled with fuel heat exchanger so a long steady climb is not the hottest. It’s when you start the descent and the thrust levers go to idle there is very little fuel to cool the oil that is still wicking away the heat from the engine so it’s highest at top of descent but starts coming back down as the engine is also producing very little power. 

[PeteMc]

9 hours ago, Pinecone said:

Although, the other issue is spin down time. 

Spin down is also one of things possibly related to the old FIXED wastegate on the 231s.  Any of the more modern turbos or the 231s modified with the Merlyn are not "spun up" the way they used to be.  All the science shows the cool down time is an old wives tail.

 

[PeteMc]

3 hours ago, Schllc said:

My poh says five min at low power settings. Period. 

And you don't think Pattern or an Approach followed by landing, rollout and then taxiing constitutes a "low power setting" for your engine???? 

It does not say 5 minute idle on the ramp does it?  I think they had issues with the K and the fact that the turbo was really SPUN UP even just taxiing.  So that became the belief of all turbos.  Not true as proven my your POH and the numbers GAMI showed (and someone I know).  It just say low power setting....  Nothing about after the plane has come to a stop on the ramp.

 

Edited January 11, 2023 by PeteMc

[EricJ]

2 hours ago, KSMooniac said:

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.

Sometimes the upper hydraulic pressure limits are set by seal or hose or accessory limitations or expectations, i.e., pressure seals become more likely to give out above a certain pressure, or something like that.   Sometimes a max adequate pressure is set and the seals are designed with some safety margin, and that sets the max.   So it could just be that the seal technologies got better during that time, or they designed in an oil cooler with a better max pressure rating if that was the limitation, or a hose or something.   I wouldn't assume it then retroactively applied to every other airplane in the fleet.    This is why the airframe manufacturers set the operating limits instead of the engine manufacturers.   

[jlunseth]

All I can tell you is that I wasted some time doing the 5 minute cooldown 13 years ago when I first got the plane, and the temps were no cooler than if I just taxied in and shut down. I had the turbo OHd right after I got the plane and that is now roughly 1700 hrs ago. The plane had 600 and some hours on it and it now has 2380. As the engine went over TBO I started to worry about the turbo, and about once a year I forget to put the mixture full in for takeoff and the temps get up there before I catch it (the JPI tells me I have screwed up). All of that, so I have had my A&P borescope the 1700 hour turbo the last couple of years during the annual and he says he can’t see any reason to OH it. Works for me I guess.

Bear in mind that for probably the last 1000 hours my cruise setting most of the time has been 34”/2450/11.1 GPH lean of peak. That means that the turbo is working harder than if I were at 30 or 31” for a ROP setting. So I was always concerned I would see premature turbo wear but that has just not been the case.

[Pinecone]

2 hours ago, PeteMc said:

Spin down is also one of things possibly related to the old FIXED wastegate on the 231s.  Any of the more modern turbos or the 231s modified with the Merlyn are not "spun up" the way they used to be.  All the science shows the cool down time is an old wives tail.

Actually a fixed waste gate would spin up the turbo less.  It is a fixed opening.   An automatic wastegate will be fully closed at lower power settings, so all the exhaust goes through the turbo.

My 85 Dodge Daytona Turbo Z was one of the first water cooled turbo center housings.  It used a thermal siphon to cool the turbo after the engine was shut down.   My 2012 Fiat Abarth has a pump that comes on and runs about 8 minutes after engine shutdown.

Other  80s turbo car engines also had spin down/cool down recommendations.  And ate turbos on a regular basis.

You can make the argument that with the approach, landing, and taxi, it is already done, but it DOES exist.

[PeteMc]

7 minutes ago, Pinecone said:

Actually a fixed waste gate would spin up the turbo less.  It is a fixed opening.   An automatic wastegate will be fully closed at lower power settings, so all the exhaust goes through the turbo.

You want to rethink that one.... 

The old fixed wastegate bolt would increase the the turbo spin as it WAS NOT FULLY OPEN and therefore forced more pressure/exhaust through the turbo.  With a variable wastegate (or Merlyn upper deck equivalent) fully open it would allow the exhaust gasses to bypass the turbo.  Therefore it spins less. 

Your statement is why the old theory that you have to idle when you get to the ramp is TOTALLY WRONG.  And again, I'm no expert, but I've done enough research to believe in the data I'm relaying.  If you have HARD PROOF that says something different, I'm open to looking at it.  But if your just going on old defunct theories, take a look at the real data....  (Just say'n)

[jlunseth]

We don’t have a “turbometer,” for example an RPM gauge on the turbo, or any exact measure of the work output by the turbo given that some is wasted through the wastegate. But what I can tell you from observation is that the turbo is not working much if at all during start up or taxi, which for me is about 1100-1200 RPMs and a leaned out fuel flow. One way of knowing this is that the last two cylinders in the induction system barely register after start up and sometimes don’t at all, until I get to the line and do a run up. The JPI does not show  temp on cylinders that are less than 240 dF in “Normal” mode. So that tells you that those last cylinders are not getting much air, the turbo is not working very hard. The other way of knowing is that during the takeoff run, when I put in takeoff MP, there is turbo lag and then a distinct power surge of several inches of MP, and that’s when the turbo begins to work. So we don’t really know if the Merlin is causing the turbo to work more or less hard at near idle than the fixed wastegate, but it isn’t working very hard. The fuel flow is only about 3.3 GPH at that point, compared to 22.5-24 on takeoff or 11 GPH in my cruise setting. 

Remember that how hard the turbo is working is driven in the first place by the pressure in the exhaust system, and there is just not much of that at near idle, so it is to me, sort of academic what the Merlin might or might not be doing at that point.

Edited January 11, 2023 by jlunseth

[Pinecone]

11 hours ago, PeteMc said:

You want to rethink that one.... 

The old fixed wastegate bolt would increase the the turbo spin as it WAS NOT FULLY OPEN and therefore forced more pressure/exhaust through the turbo.  With a variable wastegate (or Merlyn upper deck equivalent) fully open it would allow the exhaust gasses to bypass the turbo.  Therefore it spins less. 

Your statement is why the old theory that you have to idle when you get to the ramp is TOTALLY WRONG.  And again, I'm no expert, but I've done enough research to believe in the data I'm relaying.  If you have HARD PROOF that says something different, I'm open to looking at it.  But if your just going on old defunct theories, take a look at the real data....  (Just say'n)

As I have said several times, I agree with idling during landing and taxi being enough.

Unless you have to use excess power to taxi for some reason.  The question is, how much power is too much power.  

MY technique is if I go over 1200 RPM within a short time before I am parked, I let it idle 1 - 2 minutes.   This is probably very conservative, but I am fine with that.  If it happens and I have a couple more minutes of taxiing, I don't bother with any idle time, as it has already had it.

Wastegate is fully closed at low power settings, so ALL the exhaust gases go through the turbo.  Keeping it at the highest RPM possible for faster response.  Once the boost pressure gets near the maximum, the wastegate opens to reduce the amount of exhaust to control the maximum boost pressure.   As you climb, the waste gate closes to increase the turbo output to maintain the manifold pressure.

[A64Pilot]

12 hours ago, PeteMc said:

You want to rethink that one.... 

The old fixed wastegate bolt would increase the the turbo spin as it WAS NOT FULLY OPEN and therefore forced more pressure/exhaust through the turbo.  With a variable wastegate (or Merlyn upper deck equivalent) fully open it would allow the exhaust gasses to bypass the turbo.  Therefore it spins less. 

Your statement is why the old theory that you have to idle when you get to the ramp is TOTALLY WRONG.  And again, I'm no expert, but I've done enough research to believe in the data I'm relaying.  If you have HARD PROOF that says something different, I'm open to looking at it.  But if your just going on old defunct theories, take a look at the real data....  (Just say'n)

He’s right, just isn’t the popular opinion here. He obviously has significant experience with turbos

Where is this proof you talk about?

For instance borescoping a turbo doesn’t tell you anything at all about its bearings than bore scoping and engine will tell you about its bearings, it will only tell you if the turbine is rubbing or has over temp damage. It’s a viable inspection, but not for the damage that’s being discussed here.

What research have you done? Curious is all, how many turbos have you been into? Not aircraft turbos, but my turbo experience came from building turbo drag bikes in the late 80’s, and we held over 50% of all world records for turbos at the time.  I had to rebuild center sections usually after every race weekend, and I’ve blueprinted more turbos than I can count.

Nothing new with these engines and their parts, no new technology, no new exotic metals etc. 

Oh, and a fixed wastegate will have the turbo spinning less at low power because it’s fixed, meaning there is bypass at all times, where an automatic is usually fully closed, meaning all the exhaust is flowing through the turbo until it hits set point

[Will.iam]

13 hours ago, jlunseth said:

 

Bear in mind that for probably the last 1000 hours my cruise setting most of the time has been 34”/2450/11.1 GPH lean of peak. That means that the turbo is working harder than if I were at 30 or 31” for a ROP setting. So I was always concerned I would see premature turbo wear but that has just not been the case.

What is your TIT temp at that setting?

[jlunseth]

I make my max TIT 1600. I modify the fuel flow downward slightly if TIT starts to exceed 1600. I usually don’t have to go lower than 10.8 or .9 and most of the time a couple of tenths higher works fine. I don’t obsess about the 1600. If it is sitting at, let’s say, 1606 and is stable there I will leave it, but not if it keeps drifting up. TIT varies quite a bit with OAT where I am. Right now, with temps well below freezing, I couldn’t get TIT to 1600 if I tried hard. Temps are more like 1585. Middle of the summer and higher altitudes is a different deal.

[RobertGary1]

On 1/10/2023 at 5:45 AM, A64Pilot said:

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.

Do you have a reference to the source of your data? In the T206H example neither Cessna nor Lycoming recommend this cool down procedure you're stating.

Lycoming says...

Engine Shut-Down Procedure. (1) Set propeller at minimum blade angle (high RPM). (2) Idle until there is a decided decrease in cylinder head temperature. (3) Move mixture control to Idle Cut-Off. (4) When engine stops, turn ignition switch off

https://www.lycoming.com/sites/default/files/60297-23A.pdf

-Robert

[T. Peterson]

23 hours ago, jlunseth said:

I make my max TIT 1600. I modify the fuel flow downward slightly if TIT starts to exceed 1600. I usually don’t have to go lower than 10.8 or .9 and most of the time a couple of tenths higher works fine. I don’t obsess about the 1600. If it is sitting at, let’s say, 1606 and is stable there I will leave it, but not if it keeps drifting up. TIT varies quite a bit with OAT where I am. Right now, with temps well below freezing, I couldn’t get TIT to 1600 if I tried hard. Temps are more like 1585. Middle of the summer and higher altitudes is a different deal.

I’m very envious of your engine! You have it dialed in. 
I don’t have Gami’s or an intercooler. I tried duplicating your numbers and I can’t get close. The engine would melt. When I have to overhaul I will add in all those other goodies provided the wallet can stand it.

In the meantime I have to run 29 inches and 13 gallons to maintain 165 kts true. I can go slower, but then I should have just got an E model. I can also run LOP but then I might as well ride a bicycle and burn nothing.

 I could run the TIT a little hotter, but I don’t like it exceeding 1580. 

[philiplane]

Don't worry about turbo spool up during taxi.... unless you're using at least low cruise power to taxi. In which case you'll be airborne and have other things to worry about.

[jlunseth]

@T. Peterson 

I am putting a new engine in this winter, the old one will be 21 years old and 600 hours over TBO. I have my fingers crossed the new one will work as well. I will certainly be doing some experimenting.

[Will.iam]

On 1/11/2023 at 6:59 PM, jlunseth said:

I make my max TIT 1600. I modify the fuel flow downward slightly if TIT starts to exceed 1600. I usually don’t have to go lower than 10.8 or .9 and most of the time a couple of tenths higher works fine. I don’t obsess about the 1600. If it is sitting at, let’s say, 1606 and is stable there I will leave it, but not if it keeps drifting up. TIT varies quite a bit with OAT where I am. Right now, with temps well below freezing, I couldn’t get TIT to 1600 if I tried hard. Temps are more like 1585. Middle of the summer and higher altitudes is a different deal.

I just wished my jpi warning light was set for 1600 instead of 1550 as it keeps flashing a warning when I’m above 1551. It will go red if you get to 1650 which is fine. Just wish the caution zone was smaller. 

[carusoam]

5 hours ago, Will.iam said:

I just wished my jpi warning light was set for 1600 instead of 1550 as it keeps flashing a warning when I’m above 1551. It will go red if you get to 1650 which is fine. Just wish the caution zone was smaller. 

JPI900?

Stuck with the goofy precision issue like when rpm hits 2701 during the T/O roll…

A case where we can have too much precision…

Passengers love the Christmas tree affect of multiple warnings flashing during early phases of flight…  

 

APS was a great resource for training Mooniacs how to best manage their engines… especially if you had a Continental engine.  I only got the online class… planned to go with @Cris around 2012 for the in person class… he had a Screamin’ Eagle…

Kind of a way to get the most out of what the POH half covers…

Get a copy of the user manual for your engine from the manufacturer… you can see where Mooney made edits and left out some details…

The engine manufacturer probably has a 50page POH that covers all kinds of extra details…

+1 for instrumenting the turbo… or the oil down stream from the oil….  The downstream OilT is really important…. If you do end up with an oil coking challenge… this OilT sensor will be showing the issue, before the failure that will occur…

+1 for a CHT type probe for the turbo…

+1 for following the modern POHs…

+1 for taking the APS class to best understand the whys behind the POH…

 

What is the real issue we are solving by cooling the turbo…?

1) Get the turbo cool enough so it doesn’t coke the oil after the flow stops…post shut-down

2) While the oil is flowing… it might not give a good feel for the remaining turbo heat… still to be cooled…

3) there isn’t any measurable turbo wear if the oil is still flowing…. Keep the oil flowing.

4) There isn’t any turbo wear unless the TIT gets above 1650°F… above this temp we start seeing the affects of blade stretching… the high rpm/centripetal forces run the blade tips into the turbo case…. Causing a high wear rate of the blade tips… turbo efficiency depends on the blade gap staying nice and uniform…and small.

Kinda makes you want to select one TIT to use in cruise all of the time…the lower the better for turbo blade health…

5) Some people use a magnet on a stick method of checking turbo health…a push pull method to check the turbo’s shaft movement… a bearing health measurement…. Similar to pushing and pulling on the prop’s shaft to check its slop…

6) A metallurgist would be nice… MS has a good one!

But, a heat transport guru would also be helpful to have on board… Most engineers have suffered through a class called transport phenomena… covering time, heat capacity, temperatures, and those thin films at the boundary layers…   

Remember to be nice to ALL the other MSers…. Everyone can make a mistake, because we are still all human…

Its how we bring the mistake, or a misunderstanding, to light that counts… 

PP thoughts only… not a speech therapist… or language artist…

Best regards,

-a-

[Pinecone]

6 hours ago, carusoam said:

Get a copy of the user manual for your engine from the manufacturer… you can see where Mooney made edits and left out some details…

The engine manufacturer probably has a 50page POH that covers all kinds of extra details…

 

Interesting information. 

1)  The Continental Operators Manual for the TSIO -360-MB and -SM:

There is no mention of priming for starting.

Does not mention leaning for ground operators.

Recommends climbing at 165 HP (about 75% power). 

And for shutdown, very interesting procedure.  Shown below.  Notice, they state you should do another mag check, at 1700 RPM, then immediately shut the engine down.   

However the POH ) excerpt below) states to operate at idle (below 1200 RPM) for 5 minutes "to allow the TURBOCHARGER TO COOL."   It then states that "taxi time after (emphasis mine) landing" maybe part of that 5 minutes.

So I am going to stick with my, not over 1200, not to worry, if over 1200 in taxi, give it a 1 - 2 additional minutes.

 

[jlunseth]

That manual page is interesting. Do you turn the boost on for landing in the Encore? The POH for the 231 advises that the boost should only be on for emergencies, that it can cause an over rich condition in normal operations, so it is part of my checklist to see that both high and low boost are off for landing. Also, I do a brief mag check by having the engine at low idle, say 1100, and then quickly turn the ignition switch full off and then full on. You can hear the engine stop making power. The switch should be in the off position for only about a second or you will get a build-up and fuel and maybe a backfire when switched back to on. Works good. I did find a bad P-lead once, have the FBO flag the prop so the line guys would not move it and got it fixed right away. Good thing to do. I do my run up before takeoff but don’t see a reason to do one at shutdown.  To much run up is how a blade gets chewed up.

[carusoam]

Taken to the next level…

1) What temperature do the bearing surfaces see while operating…?

while the flow rates are at their maximum… (max coolant flow, highest sustained TIT, good airflow)

 

2) What temp do the bearing surfaces see while taxiing…?

while the flow rates are cut by lower rpm…  (less coolant flow, low TIT, less airflow)

 

3) Now check the oil manufacturer’s recommendations….

knowing that oil’s molecular weight gets cleaved with elevated temps and residence times…

sensitive to temps over 250°F, at higher risk when temps exceed 300°F

 

4) Then add the complexity of flow distribution in the complex geometry of the turbo’s various channels and rotating parts….

knowing that the oil’s velocity at the walls ideally (math term) is very close to zero… (not a very ideal situation)

while the flow at the wall is incredibly low, its residence time is really high…

5) With measured temps in the turbo running from anywhere near 1750°F (TIT redline)

and oil temps near 220°F at the oil cooler’s OilT sensor…

 

6) a few things that are amazing…

It is Amazing that the oil’s molecular weight actually survives so many hours between oil changes…

It is also amazing that the oil continues to lubricate as its mw is broken down…

7) Turbos are not alone…

exhaust valves and their lubrication are also at risk for the same reasons… same incredibly high temps, same temp sensitive cooling fluid….

everything is good until the flow gets inhibited…

flow really gets inhibited when coking occurs leaving carbon deposits in the flow channels…

where slight blockages increases residence time locally to increase…

once coking begins, it is somewhat auto-catalytic, causing more coking, and more blockage, and more residence time…

8) completely understandable why people settle in for a procedure that works…. And not change anything until they have to.

9) Our engines really don’t have much instrumentation to measure oil flow, or temps anywhere near these really hot spots…

 

How many hours did the wright brothers get on their engine before TBO?

 

Modern oil is amazing stuff!   

Modern private pilot knowledge is also quite amazing!

PP thoughts only, not a mechanic… or heat transfer guru…

Best regards,

-a-

[jlunseth]

13 hours ago, Will.iam said:

I just wished my jpi warning light was set for 1600 instead of 1550 as it keeps flashing a warning when I’m above 1551. It will go red if you get to 1650 which is fine. Just wish the caution zone was smaller. 

JPI sets some alerts at arbitrary levels and I have noticed over time that they have removed some from my instrument. For example, there is no low fuel pressure redline for the 231, but when I first got my JPI they had set one anyway. Every time I taxied, which takes next to no fuel, the warning would come on. I learned to ignore it, but some of the instructors I flew with would ask what that was and I would have to explain. Years into the first JPI I sent it in to make it work with CiES senders and when it came back that warning was gone. So I am sure it could be changed but it would mean sending it to JPI to have it done I am pretty sure. My JPI does not alert at 1550 or 1600, there is an alert level but I don’t know what it is, I have never been brave enough to push the turbo that much, and on the one or two occasions when I have gotten an alert I have done something about it immediately. 

[Will.iam]

24 minutes ago, jlunseth said:

. Also, I do a brief mag check by having the engine at low idle, say 1100, and then quickly turn the ignition switch full off and then full on. You can hear the engine stop making power. The switch should be in the off position for only about a second or you will get a build-up and fuel and maybe a backfire when switched back to on. Works good. I did find a bad P-lead once, have the FBO flag the prop so the line guys would not move it and got it fixed right away. 

I don’t understand why checking each mag individually doesn’t verify if a p-lead is broken or not since if one was broken you would not get an RPM drop. Even my surefly as good as it is over my mag still has a 10 rpm drop or leaned aggressively has a 50 RPM drop when the mag has like a 150 drop when lean. 

[Fly Boomer]

1 hour ago, Pinecone said:

Interesting information. 

1)  The Continental Operators Manual for the TSIO -360-MB and -SM:

There is no mention of priming for starting.

Does not mention leaning for ground operators.

Recommends climbing at 165 HP (about 75% power). 

And for shutdown, very interesting procedure.  Shown below.  Notice, they state you should do another mag check, at 1700 RPM, then immediately shut the engine down.   

When I was young, I thought that the Oxford English Dictionary was the definition of the English language -- any word or definition not in there was (by definition) not part of the language.  Many years later, I learned that dictionaries (including the OED) are changed to match current usage.  I was crushed.  As you point out, there is no engine "bible".  Unfortunately, we have to combine ancient texts, more recent knowledge, and personal experience.  I long for the engine bible -- do what it says, and nothing will ever go wrong.

[kortopates]

Way to complicated to do a full mag check!

The much simpler way to check p-leads for a hot p-lead is while at idle rpm, move the key to off momentarily just long enough to tell that the engine is dying but go back to both before it dies and then kill the engine with mixture.

If the engine doesn't show its dying in the off position you have a hot p-lead and exercise caution.

It won't cause backfire done at low rpm and is quick and easy.

I see now John (@jlunseth ) describes the same method above.

I'll add vast majority of primary students learn this technique these days which wasn't widely taught when I learned 30+ years ago.

Edited January 13, 2023 by kortopates