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What causes exhaust valve failure


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Looking to define the reason(s) exhaust valves fail.

reading the LOP thread, there is a lot of generalization about operations - such as 

damage the engine

internal pressure - high ICP

burn up your engine

detonation

there are continual reports of top end replacements at 1000 hours or less, yet some operators can go to TBO without issues. 

How can we define these differences that make sense relating to exhaust valve failures. Mechanical issues are well documented, alignment and tolerancing is obviously important but it is more than that, even with the mis-alignment it seems some engines survive but others do not. It is certainly linked to operational methods. 

Let's try to develop specific, measurable factors in operations.

Some things that seem important - CHT, EGT, FF, ROP, LOP, POWER, RPM etc. 

 

 

Edited by Cruiser
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I was focused on burned exhaust but the other failures can be discussed. 

It may be too technical to get into the weeds on this but the idea is to see if we can set some loose parameters on things like excessive heat. Are we concerned with 380°F CHT ? what about 400°F ? is there a dwell time involve? i.e. can I go over 400° in climb out if the cruise is below that ? 

 

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My understanding of a common failure mode is that the valve seat gets an irregularity due to either a deposit (carbon, whatever) or pitting/erosion.   This can create a leak around that spot in the valve.    If the valve is rotating normally and maintains integrity it may not create an issue, but if the valve also has or develops and irregularity at the seat face (maybe because of the irregularity in the seat), then every time the valve rotates them into alignment the valve burns a little bit more there.    Eventually this continues to aggravate until the valve becomes "burned" (and turns green at that spot), which weakens the valve.    An alternate fault that can lead to a burn is if the guide gets sticky/worn/damaged/whatever so that the valve stops turning, any existing irregularities or leaks can then turn into burns.

Once the valve is burned badly enough to fail (in any of a number of possible failure modes, apparently), the disintegrating pieces will further damage the valve, piston, etc., and can get reverted up into the intake and take out adjacent cylinders as well.

Lapping the valve in place is an attempt at smoothing out the irregularities between the seat and the valve to eliminate or minimize any leaks that could lead to burning the valve.

I had to replace a head on the V6 in my 20-year-old Ford Ranger a while back because it had a "burned valve" that killed the compression in that cylinder.   In that motor a "burned valve" doesn't affect the valve itself, but the seat burns away and leaves a gap when the valve is "closed" and reduces compression.    This is a very soft failure, in that engine performance gradually decays and the low-speed operation becomes rougher than usual.   It still starts and runs and is usable, just with some degraded performance.   I suspect it fails that way because it was engineered to fail that way, which seems to me to be a lot better than the way they fail in our aircraft engines.

 

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2 minutes ago, EricJ said:

My understanding of a common failure mode is that the valve seat gets an irregularity due to either a deposit (carbon, whatever) or pitting/erosion.   This can create a leak around that spot in the valve.    If the valve is rotating normally and maintains integrity it may not create an issue, but if the valve also has or develops and irregularity at the seat face (maybe because of the irregularity in the seat), then every time the valve rotates them into alignment the valve burns a little bit more there.    Eventually this continues to aggravate until the valve becomes "burned" (and turns green at that spot), which weakens the valve.    An alternate fault that can lead to a burn is if the guide gets sticky/worn/damaged/whatever so that the valve stops turning, any existing irregularities or leaks can then turn into burns.

Once the valve is burned badly enough to fail (in any of a number of possible failure modes, apparently), the disintegrating pieces will further damage the valve, piston, etc., and can get reverted up into the intake and take out adjacent cylinders as well.

Lapping the valve in place is an attempt at smoothing out the irregularities between the seat and the valve to eliminate or minimize any leaks that could lead to burning the valve.

I had to replace a head on the V6 in my 20-year-old Ford Ranger a while back because it had a "burned valve" that killed the compression in that cylinder.   In that motor a "burned valve" doesn't affect the valve itself, but the seat burns away and leaves a gap when the valve is "closed" and reduces compression.    This is a very soft failure, in that engine performance gradually decays and the low-speed operation becomes rougher than usual.   It still starts and runs and is usable, just with some degraded performance.   I suspect it fails that way because it was engineered to fail that way, which seems to me to be a lot better than the way they fail in our aircraft engines.

 

I used to think that way too, but with a little more digging the consensus is the warn valve guide is what leads to uneven seating, that leads to burned valves.

Kind of hard to find any real data about this. Most of what you find are anecdotal opinions.

The people who could gather data about this are cylinder rebuilders, but they just chuck the parts of interest and put new ones in. It would be hard to convince them to clean and dimensionally inspect the parts they are throwing away. Besides, cylinder rebuilders are a dying breed, most folks these days just buy new cylinders.

But all of this gets us away from the OPs question. What effect does heat have on cylinders? I've never seen a cylinder that I could point my finger at and say "That cylinder failed because it was run too hot" The book says you can run it at 475 all day long.

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Just now, N201MKTurbo said:

I used to think that way too, but with a little more digging the consensus is the warn valve guide is what leads to uneven seating, that leads to burned valves.

Kind of hard to find any real data about this. Most of what you find are anecdotal opinions.

Yeah, there are probably multiple failure modes.   I think these old engines are kinda fragile in some aspects.

 

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2 minutes ago, EricJ said:

Yeah, there are probably multiple failure modes.   I think these old engines are kinda fragile in some aspects.

 

I think quite the opposite. People think of them as old designs, but they have never stopped tweaking and updating them. For what they do, I think they are very robust. People like to compare them to their trouble free auto engines that lollygag along at 15% power for most of their lives, and the super high horsepower engines are just running at lower %power for daily driving.

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1 hour ago, Yetti said:

Excess heat not properly delt with.

I believe it’s pretty much it, people want it to be something real complex and sinister, but sometimes it’s not.

I’m talking exh valve temp, not cylinder head, although high cyl head temp will to some extent raise valve temps because a lot of cooling comes from contact with the valve guides  and valve seats, how much oil flow there is in the heads is another contributor.

One big reason Lycomings don’t seem to have as much of an issue if they have sodium filled exh valves, which to a great extent properly deals with the heat.

But surely there must be more to it than just fitting sodium filled valves, or someone would have gotten an STC long ago and made money.

Edited by A64Pilot
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What causes exhaust valve failure? Inattention.  This stuff doesn’t pop up overnight, so building a library of borescope images at 50 or 100 hour intervals will show when something is going bad.  It’s not hard, even on the TN with the intercoolers blocking access to the top plugs.

-dan

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FWIW Mike Busch's take on the topic linked below. He doesn't favor a single cause. According to him, primary etiologies can be imperfectly machined valve guides, suboptimally ground contact area between valve and seat, or deposits on the valve seat.  Wobble in a bad guide can create a hot spot on the valve face. But a deposit on the seat can also create a hot spot on the valve face that additionally lets hot gases during combustion stroke leak to trash the guide. The wobbly guide then sustains the feed forward loop even if it wasn't the primary cause.  The biggest preventative measure to take is to lean aggressively on the ground - i.e. avoid rich low temp ground operations that are too cool for lead scavenging, making deposits most likely.

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2010-07_dont-fail-me-now-exh-valve.pdf

A couple of editorial points by me:

1.  Interestingly, Mr. Busch doesn't bring up poor cylinder head cooling at all during the discussion, though he's an evangelist for CHTs under 400.  In that regard, he appears to be primarily concerned with high CHT as a surrogate for high ICP at an abusive power/mixture combination (red box/red fin concepts).  C model owners with crappy cooling like me sometimes deal with high CHTs even at non-abusive power/mixture settings.  As a correlative anecdote, I've had to pull my #1, #2, and #3 cylinders for burnt exhaust valves in the range of 600-1200 hours,  but my #4, which is the real problem child in terms of CHTs in climb, outlasted them all.  I don't think high normal CHTs (i.e. low 400s) are a major primary cause of burnt exhaust valves.

2. Mike Busch mentions that nickel typically appears in oil analysis as an indicator of a degrading valve guide before a hot spot is visible by borescope.  In my experience, this does seem to be true.  But now that I borescope myself every 75 hours or so, I think the appearance of the valve gives plenty of advance warning every time, and so I find no value in the oil analysis - I'd never pull a jug based on the rising nickel reading alone.

Edited by DXB
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Interesting question. 

It seems obvious that that the exhaust valve leads a pretty tough life bathed in extremely hot exhaust gasses. It's not fair to compare aircraft engines with automobile engines since aircraft engines run continuously at high power during cruise and cars cruise at very low powers (maybe 20 hp or so). Being heat engines, higher powers necessarily generate higher temperatures. The exhaust valves are designed for this operation, but their cooling mechanisms are critical and any mechanical defect can cause them to overheat and "burn." Continental uses solid valves and the heat is rejected primarily through the valve seat to the head. Lycoming favors sodium filled valves and much of the heat is rejected through the valve guide to the head. Mechanical defects that can disrupt the cooling are several:

Failure of the valve rotator to rotate the valve and keep the seat/valve interface clean (seems to be fairly common in Continentals)

Misalignment of valve and seat during manufacturing or reconditioning (Continental apparently had an issue with this in the past)

Worn valve guides (more of an issue in Lycomings)

Deposits on the valve guides causing sticking (more prevalent in Lycomings)

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10 minutes ago, PT20J said:

It seems obvious that that exhaust valve leads a pretty tough life bathed in extremely hot exhaust gasses. It's not fair to compare aircraft engines with automobile engines since aircraft engines run continuously at high power during cruise and cars cruise at very low powers (maybe 20 hp or so).

People keep making that comparison, but for people who race regularly, including racing junkyard cars in endurance races, it doesn't seem like a supportable argument.   Most aircraft engines don't make 100% power for extended periods, and often cruise for long periods at medium power in thermally and mechanically stable conditions.    Come flog random old, janky auto engines on repeating 100%/0% cycles including frequent trips to redline rpm and hop curbs around a road course for long periods or just regularly, especially with custom tunes to make sure you're really getting that 100% or more, and you get a better feel for where the reliabilities really are.   And usually this is done with engines that are >10 and up to 30+ years old.

Usually if a near-stock auto engine fails in a race it is because of a very few common causes:  a) it got overheated for a significant time period, b) it got significantly over-revved,  c) it lost sufficient oil pressure (sometimes because of (a), sometimes because of repeated loss of scavenging on hard corners).    If you keep those things under control, the likelihood of reliability under conditions of long periods or frequent aggressive use is quite high.   Bad tunes can kill engines, too, but that's not the fault of the engine design, imho.

I can't think of the last time I saw a valve fail in a race car that wasn't the result of a piston hitting it due to a bottom end or timing chain/belt failure.

We get engines that fail from time to time, but usually because of the a-c reasons or because it was highly built with some highly stressed parts or had a bad tune.   I have some friends making >1000 hp out of an early Ford Taurus SHO (with the stock Yamaha block and crank) that got a ton of use of out it and set some records before an aftermarket rod failed last year.   That's more than 4.5x the stock output.    It's not apples-apples, but modern (i.e., less than 30 years old) automotive engines I find to be basically far more reliable and better engineered than our ancient air-beaters under stressful use.

https://www.dragzine.com/features/blue-turd-zach-wrights-one-of-a-kind-1000-hp-ford-taurus-sho/

There have been some metallurgy/material changes in our aircraft engines for the better (e.g., lifters), but for the most part they're still ancient designs and we still see engines getting failures in normal use cycles, which really aren't that stressful, imho.   If Lycoming/Continental had been able to invest and adapt our engines with the sorts of economy scales that automotive engines have enjoyed, I think there'd have been a lot of important and useful improvements made over what we actually have.   I think it's notable that when they need to adapt a new aviation application, they make an entirely different engine that looks nothing like what we're using:

https://www.lycoming.com/engines/del-120

Just my dos centavos.    Opinions clearly vary on this topic.

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My point was only that an engine propelling a car down a highway generates significantly less heat than an aircraft engine cruising at 65% -75% power. Now racing is another matter. Usually valves don't fail in airplane engines until after several hundred hours of service. I wonder how many hundred hours some of these old engines run at high power in race cars? 

BeechTalk has lots of threads about valve destress, failed rotators, and lapping in place. It seems that Lycomings eat camshafts and Continentals eat valves. Lycoming made improvements for the camshaft problems (DLC and roller lifters). No idea what Continental is doing about valves.

I think the best we can do as pilots is operate at a conservative power, ground lean, and keep the CHTs within a reasonable range. LOP creates fewer deposits that ROP, so if you can operate LOP and achieve your mission goals, that's an additional step you can take.

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Exhaust valves fail....

1) This is where the agreement stops....   :)

2) We have a few different types of exhaust valve failures...

3) So..... why they fail isn’t nearly as important as detecting their failure early....

4) Have JPI... recognize valves not working properly....  saw tooth graphs...  recognize sticky valves

5) Have dental camera... recognize nice pizza images...

6) Have none of these tools, low ownership time, new 2U airplane.... have exhaust valve stick open and crash into piston crown...   a tough way to start...

7) My issue was carbon build up in the valve guide...  the cooling oil wasn’t flowing, low lubrication, and bam.... The fun part  of transition training came abruptly to an end.... first cross country flight in my M20C with a CFI...

8) Recognize stuck valve...  one step worse than sticky valves....

9) sticky valves don’t rotate... this causes cooling challenges.... that reshapes the valve....

10) Stuck valves don’t go up and down...

11) There is a rope trick for cleaning out the valve guides...

12) If you are inspecting your oil filter and you find bits of carbon that are the shape a valve guide....  this would be a good time to look further down the guides....

PP thoughts about valve issue recognition... not a mechanic...

Best regards,

-a-

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15 hours ago, A64Pilot said:

I believe it’s pretty much it, people want it to be something real complex and sinister, but sometimes it’s not.

I’m talking exh valve temp, not cylinder head, although high cyl head temp will to some extent raise valve temps because a lot of cooling comes from contact with the valve guides  and valve seats, how much oil flow there is in the heads is another contributor.

One big reason Lycomings don’t seem to have as much of an issue if they have sodium filled exh valves, which to a great extent properly deals with the heat.

But surely there must be more to it than just fitting sodium filled valves, or someone would have gotten an STC long ago and made money.

For awhile there seemed to be a bunch of Cirrisus parachuting from the sky with engine issues.   I noticed in the Grumpy IA shop that Cirus have heat tape on the inside of the cowl.   And it was burning off.   So I am going with there are some unresolved heat issues that are taking the engine out.

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On 6/9/2021 at 5:37 PM, DXB said:

FWIW Mike Busch's take on the topic linked below. He doesn't favor a single cause. According to him, primary etiologies can be imperfectly machined valve guides, suboptimally ground contact area between valve and seat, or deposits on the valve seat.  Wobble in a bad guide can create a hot spot on the valve face. But a deposit on the seat can also create a hot spot on the valve face that additionally lets hot gases during combustion stroke leak to trash the guide. The wobbly guide then sustains the feed forward loop even if it wasn't the primary cause.  The biggest preventative measure to take is to lean aggressively on the ground - i.e. avoid rich low temp ground operations that are too cool for lead scavenging, making deposits most likely.

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2010-07_dont-fail-me-now-exh-valve.pdf

A couple of editorial points by me:

1.  Interestingly, Mr. Busch doesn't bring up poor cylinder head cooling at all during the discussion, though he's an evangelist for CHTs under 400.  In that regard, he appears to be primarily concerned with high CHT as a surrogate for high ICP at an abusive power/mixture combination (red box/red fin concepts).  C model owners with crappy cooling like me sometimes deal with high CHTs even at non-abusive power/mixture settings.  As a correlative anecdote, I've had to pull my #1, #2, and #3 cylinders for burnt exhaust valves in the range of 600-1200 hours,  but my #4, which is the real problem child in terms of CHTs in climb, outlasted them all.  I don't think high normal CHTs (i.e. low 400s) are a major primary cause of burnt exhaust valves.

2. Mike Busch mentions that nickel typically appears in oil analysis as an indicator of a degrading valve guide before a hot spot is visible by borescope.  In my experience, this does seem to be true.  But now that I borescope myself every 75 hours or so, I think the appearance of the valve gives plenty of advance warning every time, and so I find no value in the oil analysis - I'd never pull a jug based on the rising nickel reading alone.

My impression of MB's take on CHT's is that he mainly tries to keep the temps below 400 not because of valve life but to reduce the risk of cylinder fatigue, and presumably, cracks.  He provides no data to support that conclusion other than the general reduction in strength of aluminum alloy around that temp.  The other reason is that high CHT's are a contributing factor to detonation and pre-ignition, but I don't think we'd see that at 400 (at least for the IO-360 crowd)

IIRC, he did also cite at least one example of someone's motor that swallowed an exhaust valve despite barely anything noticeable on borescope the prior annual.  This is, of course, an n=1 and one could argue that he was using the example to sell his SavvyAnalysis product.  In fact, I think that example was in one of those marketing e-mails they send me all the time.

Edited by jaylw314
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Pilots apparently need something engine-related to worry about. 30 years ago it was shock cooling. Today it’s red boxes. Shock cooling was supposed to be the cause of cracked cylinder heads (I believe the real cause was reconditioned cylinders that reused head castings for who knows how many TBO runs. The problem seems to have evaporated when the factories lowered their prices for new cylinders and most overhaulers started providing them as standard with an overhaul). As someone noted, it’s not entirely clear exactly what calamity operating in the red box is supposed to precipitate. 

Here’s something to ponder:

Cessna 172s don’t come equipped with CHT gauges. They don’t have cowl flaps. There is a flight school in my area with a fleet of old but exceptionally well maintained C-172s, mostly C-172Ns. Recently two were upgraded to include a G3X with EIS. For the first time in 45 years, everyone could see that the CHT on some cylinders well exceeds 400F during a climb. Now CFIs are scrambling to avoid this. Red box! Red box! But for many years, this operator has been regularly exceeding TBO on it’s O-320 engines powering these same Skyhawks with everyone blissfully ignorant of the CHT in a climb.

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I’m going to get people upset again, so here goes.

Its mostly marketing, Common truth is to run LOP you must have someone’s after market injectors and an engine monitor, Of course you have to have a monitor, how else could you determine your “Gami spread” plus you need a recording one so that you can post the recordings.

Ugly truth is many CB’s and some who were after extended range have been running LOP since before the transistor, they didn’t call it LOP just called it “leaned out”, Gami spread is completely irrelevant, all that matters is if it runs smoothly or not.  If it doesn’t then a monitor has value determining which cylinder, or cylinders are the problems, they have diagnostic value.

It’s simple, be at a power that you can’t hurt the motor, often quoted as 75% or less for a Lycoming which happens I believe to also be the max recommended continuous power setting, and pull the red knob until you get a very noticeable loss of power and leave it there if engine runs smooth, or slowly enrichen until it’s smooth, your done.

Pretty quickly you will determine norms, for me down lower it’s 22 squared and 7 GPH, so all I do is reduce power to 20”, then reduce RPM to 2200, which brings manifold pressure back to 22, and pull the red knob back to 7 GPH, that’s it, I’m done, no lean find, no fiddling etc. That gives me 135 kts true which for me is fast enough, due to the loss of power from being LOP, I’m likely in the 50ish percent power. I’ve not yet determined norms for higher altitude, but 22 squared and 7 GPH doesn’t work for some reason, it takes more fuel, but I get a higher true airspeed too. I’ve not flown enough long legs to determine those norms yet.

‘I don’t have a engine monitor and for some reason my motor will run 100C or so LOP smoothly, but produces so little power that its just not enough so I don’t run as lean as it can.

‘However not all engine designs are so tolerant, the bigger Conti’s supposedly are not, but the IO-520 in our C-210 ran just as well with stock injectors as it did with Gami’s

I was an early adopter of the injectors back around 2003 or so when they first came out, I put engine monitors and Gami injectors in both the C-210 and in my IO-540 Maule.

In both engines on both aircraft fine wire plugs made more difference than the injectors did, which wasn’t huge, but it was measurable thanks to the UBG-16’s and the UBG-16’s did give more accurate fuel flow and were accurate devices, but really weren’t necessary, they were if you really wanted to run the richest cylinder exactly 25 LOP, but not to just run LOP.

The red box exists due to detonation and should be avoided, but again different engines are well different, some are very difficult to detonate, others not so much. I don’t think you could detonate my C-85 on 100LL for instance, maybe cheap car gas you could but not Avgas.

However detonation can be an immediate death, but usually not, over temp operation damage is cumulative, meaning that it may not result in a failure for years as many of us only fly 100 or so hours a year. Over temp doesn’t guarantee detonation, but detonation will  result in over temp.

You can choose to believe me or not, but if you want along life out of the motor, reducing power and temps as well as flying often will get you there, Running hard and hot may not.

The flight school engines are an enigma, just about everything your not supposed to do for long life is done to them, snd yet they last. short warm-ups, who wants to pay the hourly rate to warm uo an engine? Repeated wide open throttle, followed by idle, repeat excessively, and prolonged rich operation, who leans in the pattern?

A lot of why they last in my opinion is that they are relatively low output smaller motors and rarely are run at red-line RPM, fixed pitched props shouldn’t allow redline intake offs and in climbs, and most air work isn’t done at high RPM either. but it’s been over 30 years for me so maybe things are different now.

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24 minutes ago, PT20J said:

Seems like everyone remembers the TBO in hours but we forget that Lycoming also has a 12 year TBO limit also. 

httpswww.lycoming.comsitesdefaultfilesSI1009BE20TBO20Schedule_pdf.thumb.png.cdc9c38b325847a24c41ad78978182da.png

But as Part 91 operators, we can blissfully ignore it.

Here in the southwest we don't do corrosion, the southeast people and other places (you know who you are) might want to pay attention to it.

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20 hours ago, jaylw314 said:

IIRC, he did also cite at least one example of someone's motor that swallowed an exhaust valve despite barely anything noticeable on borescope the prior annual.  This is, of course, an n=1 and one could argue that he was using the example to sell his SavvyAnalysis product.  In fact, I think that example was in one of those marketing e-mails they send me all the time.

I'm pretty skeptical that an exhaust valve can reach the point of failure without showing clear visual evidence of a hot spot at a rim well in advance. That cannot be said of the SAVVY FEVA signature on the EGT trace, which is often a very late event when failure is imminent [also I believe they check for it for free when you upload data to their site, though they don't publicize this.]

I suppose the hours you fly between annuals makes a big difference - if it's a lot, getting your own borescope and checking every 50-75 hours is a quick, easy, cheap do-it-yourself option that most any owner can avail.

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