Detonation - cylinder destroyed

[Shadrach]

3 hours ago, philiplane said:

A melted piston only comes from pre-ignition. Pre-ignition will come from a spark plug that has lost its insulator, which turns it into a glow plug. Now you have an out-of-time ignition source, which lights the fuel that appears the moment the intake valve opens. The piston is now trying to compress the expanding gases, rather than being pushed down by them. The heat rise is rapid and off the scale. Melting occurs in a matter of minutes.

Mixture settings, no matter how improper for the power setting, cannot melt the piston. 

Regarding the shop's though of crossfire: There is no way for plug wires to misfire into one cylinder. It would affect two or more at the same time. And this thought is a relic from the old days of cheap automotive plug wires, which have never been used on airplanes.

While they may be a competent overhaul shop, they don't understand the principles of pre-ignition. Pre-ignition rarely comes from the ignition system, it's usually from a hot spot in the cylinder. There are a few good books on engine damage, and what causes it. The best one is:

https://herringbonebooks.indielite.org/book/9780768008852

About 20 years ago, I melted a piston on a Triumph inline twin. I was in my 20s and ignorant of what was happing to my engine. I had just entered the Golden Gate Bridge from the Marin County side when the engine began running abnormally.  The noise from the TT pipes and my over ear helmet likely masked the initial pinging. I tried to nurse it across the bridge because I was uncomfortable stopping on the bridge in traffic, at night.  I started over the bridge with a twin but had become a single before reaching the other side.  My mechanic's analysis was that the event initiated from a head gasket failure causing the failed cylinder to draw in unmetered air and detonate. I'm not sure if that was the actual mode of failure or not.  I am sure that pre ignition came into play in the final moments of the piston's life. There was so much debris and damage inside that some of that debris had to be hot enough to ignite the intake charge. It was hot enough to melt the piston crown leaving a hole you could drop a quarter through on a piston that was less than 3" in diameter. The connecting rod underneath had bits of aluminum debris fused to it. Quite a learning experience for me. It triggered my curiosity in combustion tuning and trouble shooting.

Edited May 20, 2022 by Shadrach

[A64Pilot]

13 minutes ago, Shadrach said:

Agreed, but detonation from whatever cause can damage the cylinder and plugs in such a way that causes pre ignition then contributing to the uncontrolled combustion shitshow.  I would not think a flame front propagating from a damaged, overheated spark plug components, a sliver of metal from damaged spark plug threads/ helicoil  or the like would be considered normal flame propagation.

 

Read the lycoming article. Normal means propagates at the normal speed. Pre-ignition just means the charge is ignited early. Detonation is essentially Dieseling, Diesels operate by timing the introduction of fuel, the moment fuel becomes present, ignition begins in a normal operating Diesel, a cold Diesel may be delayed a little and that’s why they knock and rattle normally when cold. If we had direct timed injection like a Diesel, then we couldn’t detonate.

A spark ignition engine differs by having the entire fuel charge present in the combustion chamber prior to ignition. Spark plug ignites the mixture and the flame front proceeds across the chamber and pressures of course build at a controlled rate, controlled by ignition timing.

Detonation is the sudden nearly instant explosion of the entire charge and pressures and heat goes through the roof. The way to stop it is by removing the excess heat, reducing power and going rich removes heat, and of course just pulling power removes the heat because you remove the fuel, so either add or remove fuel

A real problem in understanding is people from somewhere were taught that a spark ignition engines runs off of explosions in the combustion chamber, this is incorrect, they run off of the expansion of gas from heating, there is a burn, no explosion.

Improper as in too lean or not lean enough will cause detonation, it will cause it from an excess amount of heat, get a combustion chamber hot enough and the mixture will essentially spontaneously explode.

[A64Pilot]

Pre ignition isn’t always that damaging, pre ignition can cause detonation from the excess heat from pre ignition, it’s the sudden uncontrolled rapid rise in pressure, pressure so great that it can blow off heads, crack cylinders, bend connecting rods, hammer rod bearings out etc. That can from either, but it’s my opinion that detonation damage is more common because a pilot can cause that, pre-ignition is often beyond our control

So pre-ignition can lead to Detonation, some will argue the reverse is true too, but it’s almost arguing semantics, the bullet didn’t kill you, the loss of blood is why you died.

But like most things Detonation isn’t always a killer, mild detonation exists, it’s still bad and should be avoided but if mild it doesn’t kill right away.

It’s sort of like arguing the fall didn’t kill him, the sudden stop at the bottom did. The rest of the story is if not for the fall, the sudden stop wouldn’t have happened.

As operators all that really matters is we need to either understand the “red box” which I truthfully don’t as I’ve done very little reading on it as I view it as simply putting a name on something that’s always existed, but bottom line be real rich at high power, richer than best power, we actually lose power taking off at full rich, but that excess fuel keeps things cool

However if you operate IAW the POH, runaway cylinder temps shouldn’t happen, and if they do from some reason, know how to recognize it and know immediately what to do.

Your Triumph certainly could have been pushed into detonation from too lean as is close to peak a mixture from an intake leak if operated at moderately high power (highway speed). Some of the older Brit bikes were actually pretty high performance motors, and our car fuel isn’t always as good as we would like. 

on edit, detonation that occurs at or after TDC often isn’t harmful, it’s the rattling you used to hear in 1970’s cars, but detonation that occurs well before TDC is a killer.

pre-ignition should be very rare, detonation though can be pilot induced

Edited May 20, 2022 by A64Pilot

[Shadrach]

2 hours ago, A64Pilot said:

Read the lycoming article. Normal means propagates at the normal speed. Pre-ignition just means the charge is ignited early. Detonation is essentially Dieseling, Diesels operate by timing the introduction of fuel, the moment fuel becomes present, ignition begins in a normal operating Diesel, a cold Diesel may be delayed a little and that’s why they knock and rattle normally when cold. If we had direct timed injection like a Diesel, then we couldn’t detonate.

A spark ignition engine differs by having the entire fuel charge present in the combustion chamber prior to ignition. Spark plug ignites the mixture and the flame front proceeds across the chamber and pressures of course build at a controlled rate, controlled by ignition timing.

Detonation is the sudden nearly instant explosion of the entire charge and pressures and heat goes through the roof. The way to stop it is by removing the excess heat, reducing power and going rich removes heat, and of course just pulling power removes the heat because you remove the fuel, so either add or remove fuel

A real problem in understanding is people from somewhere were taught that a spark ignition engines runs off of explosions in the combustion chamber, this is incorrect, they run off of the expansion of gas from heating, there is a burn, no explosion.

Improper as in too lean or not lean enough will cause detonation, it will cause it from an excess amount of heat, get a combustion chamber hot enough and the mixture will essentially spontaneously explode.

Just reread it and I can't find a definition of pre-ignition, only detonation.  There is an allusion to pre-ignition where they say "We’ve also seen cases where cracked or otherwise damaged spark plugs create a “hot spot” in the engine and detonation occurs.  This is why it’s never a good idea to use a plug that’s been dropped on a hard floor or otherwise damaged." but they do not call it pre-ignition by name.

The reason I bring it up is because my thinking has always been that the difference between the two was that detonation was an irregular combustion event that occurred at the time of, or after a normal ignition event and that pre-ignition was an irregular combustion event initiated prior to and from something other than a normal ignition event.  It would seem to me that the type of pressures generated by most preignition events would also result in detonation (as Lycoming alludes in the quote above) of the intake charge, so not normal by definition.  At the end of the day it's semantics, but it seems odd to define pre-ignition as a "normal" combustion event. The pressure, heat and rate of combustion are very different from normal ops.

 

Edited May 20, 2022 by Shadrach

[EricJ]

2 hours ago, Shadrach said:

Just reread it and I can't find a definition of pre-ignition, only detonation.  There is an allusion to pre-ignition where they say "We’ve also seen cases where cracked or otherwise damaged spark plugs create a “hot spot” in the engine and detonation occurs.  This is why it’s never a good idea to use a plug that’s been dropped on a hard floor or otherwise damaged." but they do not call it pre-ignition by name.

The reason I bring it up is because my thinking has always been that the difference between the two was that detonation was irregular combustion event that occurred at the time of, or after a normal ignition event and that pre-ignition was an irregular combustion event initiated prior to and from something other than a normal ignition event.  It would seem to me that the type of pressures generated by most preignition events would also result in detonation (as Lycoming alludes in the quoted above) of the intake charge, so not normal by definition.  At the end of the day it's semantics, but it seems odd to define Pre ignition as a "normal" combustion event. The pressure, heat and rate of combustion are very different from normal ops.

 

The FAA agrees with you in FAA-H-8083-32A, Pages 10-30, 31.    Detonation occurs after ignition, as the flame front increases pressure the remaining charge ignites spontaneously due to the excess pressure increase.   Pre-ignition occurs prior to the ignition spark event (hence the name), due to something inside the cylinder becoming incandescent (e.g., a broken plug, chunk of overheated deposit, etc.), often heated to incandescence due to detonation.

Caution:  92M download:

https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/FAA-H-8083-32-AMT-Powerplant-Vol-2.pdf

 

In the OPs case I suspect there was detonation due to the usual culprits of injector blockage, intake leak, whatever, that may have eventually led to pre-ignition.   

Edited May 20, 2022 by EricJ

[A64Pilot]

I typed a whole response and it disappeared and I won’t try to repeat most of it. But detonation is often operator induced, even if you get mis fueled and the line guy gives you a load of jet, it mixes with Avgas, seriously degrading the octane and you get detonation. I’ve seen detonation blow cylinders off of a Radial, you can induce detonation by being too lean at high power or by over boosting, but I don’t know how you can induce pre-ignition as an operator. You have to essentially have a glow plug to cause pre-ignition, how do you get that hot to start with?

I’ve seen detonation damage several times, but I’ve never seen pre-ignition occur, one of the signatures for detonation is it breaking the insulator on spark plugs and even melting electrodes, I can see how the extreme heat that comes from detonation leading to pre-ignition, that’s logical, but we are back to discussing if someone died from a gunshot or the loss of blood it caused.

 

[Fly Boomer]

45 minutes ago, A64Pilot said:

You have to essentially have a glow plug to cause pre-ignition

I read that a Heli-Coil tang protruding too deep will do it.

[A64Pilot]

2 hours ago, Fly Boomer said:

I read that a Heli-Coil tang protruding too deep will do it.

I could see that, especially if an end was bent down, but that’s nothing I’ve ever seen

[carusoam]

JP,

There is probably a lot of really good info resident in your JPI….

1) See if you can get it downloaded from the JPI….

2) upload it Savvy…

3) Click the share button…

4) Copy the link here…

5) People can review your flights data…

6) Sooooo many smart people around here…. With sooooo much experience…

7) Delivering the data minimizes the guesses, and does a great job of ferreting out what happened to your engine, when….

8) Some things are really obvious…. A single EGT rising out of control…. Is often a partially clogged fuel injector… it could happen over time, or all at once… depending on what came down the fuel line…

Or a mag timing change will show clearly in the run-up data…. Unless you zip through the run-up old style…

9) If a fuel injector got fouled…. Cleaning it, AND catching what is in there is important…

10) Crossfire… sounds like a guess.   There will be plenty of data, and other melted/smoky looking parts to go with that…

11) getting mis-fueled is a common cause for destroying all of the pistons within minutes…

12) there are many things that are normal MS experiences…. They show up in the data… melting a piston is not very common… ping or pre-ignition is a good cause…. Cracked spark plugs, and helicoils can be the cause or after the the event…

 

Work on posting the data… and of course any pics you can get…

Good luck with the next steps…

This would make a great post in the engine monitor thread…. 

PP thoughts only, Not a mechanic…

Best regards,

-a-

[Pinecone]

On 5/20/2022 at 7:16 AM, A64Pilot said:

Yes but that was for much higher energy ignitions and much poorer shielded leads, if nothing else the outer shield on aircraft leads should short any leakage to ground. Don’t over think this running them together has worked for 100 years.

If we ever get actual high energy ignitions we may need better wires, or maybe do it right and put a coil pack on each wire or maybe at least one per cyl with two short leads.

You didn't read what I quoted.  It was in response to a post about twisting auto ignition leads together.

Twisting does not reduce cross fire.  It reduces radio interference.

 

[Pinecone]

On 5/20/2022 at 1:19 PM, Shadrach said:

Just reread it and I can't find a definition of pre-ignition, only detonation.  There is an allusion to pre-ignition where they say "We’ve also seen cases where cracked or otherwise damaged spark plugs create a “hot spot” in the engine and detonation occurs.  This is why it’s never a good idea to use a plug that’s been dropped on a hard floor or otherwise damaged." but they do not call it pre-ignition by name.

The reason I bring it up is because my thinking has always been that the difference between the two was that detonation was an irregular combustion event that occurred at the time of, or after a normal ignition event and that pre-ignition was an irregular combustion event initiated prior to and from something other than a normal ignition event.  It would seem to me that the type of pressures generated by most preignition events would also result in detonation (as Lycoming alludes in the quote above) of the intake charge, so not normal by definition.  At the end of the day it's semantics, but it seems odd to define pre-ignition as a "normal" combustion event. The pressure, heat and rate of combustion are very different from normal ops.

 

People often mix them up and mix the terms.

[Jungle Pilot]

Great discussion gentlemen; I appreciate it. Here’s the link to the JPI data on the Savvy Aviation website. In summary, it shows #6 cylinder temperatures starting to climb right at takeoff. You’ll see that as detonation continues to overheat the cylinder, the #6 EGT remains pretty normal, indicating to me, that there’s no clogged injector, and the overhaul shop confirmed #6 injector was fine. 
https://apps.savvyaviation.com/my-flights/19086/18be2bd1-bb4a-4f2f-8d9c-73b073858174
 

#6 CHT finally drops precipitously at first power reduction from cruise.  
i agree with comment about crossfire between ignition wires could only occur if the outer shield is worn through by chafing or other means. 
 

[EricJ]

2 hours ago, Jungle Pilot said:

Great discussion gentlemen; I appreciate it. Here’s the link to the JPI data on the Savvy Aviation website. In summary, it shows #6 cylinder temperatures starting to climb right at takeoff. You’ll see that as detonation continues to overheat the cylinder, the #6 EGT remains pretty normal, indicating to me, that there’s no clogged injector, and the overhaul shop confirmed #6 injector was fine. 
https://apps.savvyaviation.com/my-flights/19086/18be2bd1-bb4a-4f2f-8d9c-73b073858174
 

#6 CHT finally drops precipitously at first power reduction from cruise.  
i agree with comment about crossfire between ignition wires could only occur if the outer shield is worn through by chafing or other means. 
 

Intake leak?

[Shadrach]

On 5/22/2022 at 12:44 PM, EricJ said:

Intake leak?

It’s a turbo, wouldn’t that mean lower thAN upper pressure at the cylinder’s intake, meaning a richer mixture?
EGT is in normal range. Certainly looks like something other than mixture drove the detonation event.

[Cruiser]

6 hours ago, Jungle Pilot said:

Great discussion gentlemen; I appreciate it. Here’s the link to the JPI data on the Savvy Aviation website. In summary, it shows #6 cylinder temperatures starting to climb right at takeoff. You’ll see that as detonation continues to overheat the cylinder, the #6 EGT remains pretty normal, indicating to me, that there’s no clogged injector, and the overhaul shop confirmed #6 injector was fine. 
https://apps.savvyaviation.com/my-flights/19086/18be2bd1-bb4a-4f2f-8d9c-73b073858174
 

#6 CHT finally drops precipitously at first power reduction from cruise.  
i agree with comment about crossfire between ignition wires could only occur if the outer shield is worn through by chafing or other means. 
 

so we are back to cracked spark plugs 

[EricJ]

26 minutes ago, Cruiser said:

so we are back to cracked spark plugs 

That's a cause-and-effect mystery as well.   Often the thing that cracks the nose insulation is detonation or overheating, so there's still a chicken-and-egg question.

Another confounding issue is that the sequence leading to the failure may have been from different events.   A plug insulator may crack in one event and then fail at some other time which influences another event.

This one doesn't sound straightforward at all.

[Shadrach]

Definitely looks ignition related to me. There’s an odd 30° EGT drop on #6 just preceding the runaway CHT. EGT then increases 30° as CHT sky rockets.  None of the other EGTS do this.

[jaylw314]

12 hours ago, Jungle Pilot said:

Great discussion gentlemen; I appreciate it. Here’s the link to the JPI data on the Savvy Aviation website. In summary, it shows #6 cylinder temperatures starting to climb right at takeoff. You’ll see that as detonation continues to overheat the cylinder, the #6 EGT remains pretty normal, indicating to me, that there’s no clogged injector, and the overhaul shop confirmed #6 injector was fine. 
https://apps.savvyaviation.com/my-flights/19086/18be2bd1-bb4a-4f2f-8d9c-73b073858174
 

#6 CHT finally drops precipitously at first power reduction from cruise.  
i agree with comment about crossfire between ignition wires could only occur if the outer shield is worn through by chafing or other means. 
 

IIRC, the 711 doesn't keep the CHT alarm on if you exceed limits, and then it falls below (which it eventually did).  So it may simply have been that the operator at the time didn't notice the alarm, then at the power reduction when the CHT suddenly cooled below the alarm limit (presumably when the cylinder was destroyed), there was no more alarm to see.

[jetdriven]

I had a friend who had a magneto go bad on a trip. The mechanic replaced both magnetos with brand new ones. After that one of his cylinders ran really really hot all the time. And after messing with intake leaks and contriming valve lift and Borescoping it and all kinds of everything else, they finally sent the engine for overhaul and when they put it all back together that cylinder still ran hot. As it turned out, there was something wrong with one of the magnetos making it fire way too early on that cylinder alone. 

[A64Pilot]

11 hours ago, Shadrach said:

Definitely looks ignition related to me. There’s an odd 30° EGT drop on #6 just preceding the runaway CHT. EGT then increases 30° as CHT sky rockets.  None of the other EGTS do this.

EGT often actually drops with denotation. I’ve not looked at the charts, but in aircraft mixture is almost always the cause of detonation, in other vehicles Octane and or compression ratio or too advanced timing often is, but those should not occur with aircraft, so that pretty much leaves not rich enough at high power as the cause, one cylinder will always detonate first because there are differences in compression, mixture distribution etc. that’s why even if fuel is perfectly distributed EGTs will differ somewhat, from I believe slightly different compression ratios, low compression interestingly is the higher EGT.

 

Edited May 23, 2022 by A64Pilot

[PT20J]

Here’s a related Beechtalk thread,
https://www.beechtalk.com/forums/viewtopic.php?f=37&t=193567

[Shadrach]

Looking at the engine data, EGT was smooth and consistent 1259° where it is the coolest of the 6 EGTs bust still within 65° of the hottest. At 17:26 it starts to decline while the other 5 cyl remain constant. At 17:48 it's 1228° and begins a steady climb with in a minute it becomes the hottest EGT and CHTs have exceeded 500°. EGT does not track CHT at all during the 15 mins that CHT varied between 500° and 634°.  This very looks like an "uncontrolled" or pre ignition event. Would be helpful to see FF as well.

Edited May 23, 2022 by Shadrach

[Shadrach]

7 minutes ago, PT20J said:

Here’s a related Beechtalk thread,
https://www.beechtalk.com/forums/viewtopic.php?f=37&t=193567

Started off as an interesting thread. Sad that there was no follow though and leaves the reader with more questions than answers. Very kind of George B to offer a post mortem on the cylinder after removal. 

[A64Pilot]

25 minutes ago, Shadrach said:

Looking at the engine data, EGT was smooth and consistent 1259° where it is the coolest of the 6 EGTs bust still within 65° of the hottest. At 17:26 it starts to decline while the other 5 cyl remain constant. At 17:48 it's 1228° and begins a steady climb with in a minute it becomes the hottest EGT and CHTs have exceeded 500°. EGT does not track CHT at all during the 15 mins that CHT varied between 500° and 634°.  This very looks like an "uncontrolled" or pre ignition event. 

uncontrolled and or pre-ignition is detonation.

On edit we can argue the difference all day long but it doesn’t really matter, VERY often the cause of pre-ignition in a well maintained aircraft engine is detonation, detonation drives internal temps so high the fuel charge is ignited on some hot something, carbon maybe.

Detonation is of course the self ignition of the fuel charge, heat is what often causes it, the hotter the charge is, the smaller the margin between detonation and spark ignition is. I’m not saying some bizarre ignition issue didn’t cause it, because we are all just guessing, we don’t have the aircraft to run any diagnostics on.

‘Detonation initially drops EGT, one theory is the heat goes into the head and the ignition event is completely over prior to exhaust valve opening, but the head can only hold so much heat and as detonation persists EGT will rise.

So what your saying is consistent with detonation

‘Maybe not the best explanation, but seems pretty good to me

https://www.experimentalaircraft.info/articles/aircraft-engines-detonation.php

 

I wish I had a quarter for every racing two stroke that I’ve detonated to death, two strokes run their best just prior to detonation, very often seizing, but sometimes just melting piston material around the ring so compression is lost. But it’s as common as a flat tire back when we raced two strokes.

Edited May 23, 2022 by A64Pilot

[Shadrach]

6 minutes ago, A64Pilot said:

uncontrolled and or pre-ignition is detonation.

Detonation is of course the self ignition of the fuel charge, heat is what often causes it, the hotter the charge is, the smaller the margin between detonation and spark ignition is. I’m not saying some bizarre ignition issue didn’t cause it, because we are all just guessing, we don’t have the aircraft to run any diagnostics on.

‘Detonation initially drops EGT, one theory is the heat goes into the head and the ignition event is completely over prior to exhaust valve opening, but the head can only hold so much heat and as detonation persists EGT will rise.

So what your saying is consistent with detonation

‘Maybe not the best explanation, but seems pretty good to me

https://www.experimentalaircraft.info/articles/aircraft-engines-detonation.php

Certainly detonation occurred. Can't say what caused it but it is not clearly mixture related.  It looks like it was driven by an inconsistent ignition source as the CHT climbs to 611° and then drops when the pilot makes a change to either MP, mixture or RPM (EGT change across all cylinders). CHT drops to 520° then starts a gentle rise all the way to a high of 634° until the power is pulled back for approach. CHT and EGT the drop to way below normal levels (150° and 410°)