Piston engine momentary detonation , consequences?
Posted by whatdoesthismean1111@reddit | flying | View on Reddit | 26 comments
I’ve recently started flying a 1967 Piper PA-28R-180 Arrow, powered by a 180-horsepower, 4-cylinder Lycoming IO-360-B1E engine. It’s a fuel-injected engine. I have conflicting directions on leaning from the previous owner and my flight instructor. Previous owner said to keep it at 1200 EGT (single probe egt/cht) they instructor says I should be leaning to 1300 to 1400 EGT. Recent flying experience has shown me when leaning to rough and then backing it off. I can get no more than about 1330 EGT. On a flight yesterday when trying to aggressively lean at 4000 feet in cruise , the engine momentarily started detonating at which point I enriched the mixture and it stopped. I would say it detonated about three or four times. My question is; do you think any damage was done to the engine? I’ve never experienced this before in any aircraft. It was loud and violent. The flight continued normally for another 45 minutes with no issues indicated. All my relevant temperatures were in the green before during and after. Thanks in advance!
Purgent@reddit
Detonation would cause CHT to spike and is only going to happen under very high load. You are not describing detonation here.
Sounds like you leaned until the mixture became too light on fuel and it was misfiring.
Did it sound like gunshots or was it just loud vibration?
whatdoesthismean1111@reddit (OP)
Definitely like gunshots thank you for the reply!
greaseorbounce@reddit
I don't mean any disrespect here, but are you sure it was truly detonation and not a backfire?
Backfires can be very loud, and generally don't cause a lot of damage.
True detonation can cause tons of damage, but is usually BARELY audible in an airplane.
dopexile@reddit
I wonder if they are getting uneven fuel distribution to the cylinders... maybe one cylinder is getting too much fuel and the other not enough.
greaseorbounce@reddit
I suspect that is absolutely what's happening.
whatdoesthismean1111@reddit (OP)
Thank you for the insight!
Lost_Obligation2453@reddit
It doesn't matter what egt is other than it is not close to 0. It matters what egt does when you lean. It either increases or decreases which tells you where you are on the power curve. Anyone who says "egt should be this" doesnt understand engines.
240 cht is really cold. It's an issue if you can't get your engine hotter than that. Lead won't come out of the jugs until you hit like 350. That lead will build up and wreck the engine compression which screws the cooling and cracks the jugs. Your valve faces are likely covered in lead.
Detonation is very bad. Check your spark plugs, the first thing to happen with detonation is the ceramics of your spark plugs crack. Next the interior of your cylinders will look like someone smacked them with a ball-peen hammer over and over.
I doubt you experienced detonation though. That's pretty rare, it's not easy to do. If you didn't have to change your pants, that was just the leftover fuel burning in the exhaust.
arnoha@reddit
I was going to write roughly the same thing, but this is accurate. The actual EGT value is irrelevant. Exactly where the probe is placed matters greatly as to the temperature reported and it frankly does not matter what the number is. It can vary a couple hundred degrees on placement and style. You'll be leaning after discovering peak value, which itself will change from flight to flight depending on weather, altitude, and power settings. The difference from this is all that matters.
240°F cylinder head temperature? You sure? Something is very wrong. 340°F would be a fantastic number. 240 is near impossible as a steady state value in cruise, so either your memory is off or the sensor is off. Piggy back probes can read cooler than reality, if that's what you have, but not that cold.
Good luck with figuring it out!
Lost_Obligation2453@reddit
Maybe his cht is metric?
I suppose it's possible it is. With a single cht he might have over temped the other jugs and had some detonation.
Wasn't there a big one they put litres instead of gallons or something? Could just be that again.
dodexahedron@reddit
If metric, 240⁰ is very bad territory for aluminum.
arnoha@reddit
Thought about that, but I'm going to call it unlikely. That's 465°F. While technically under red line, it would be really hard to get up that high. And if the meter is Celsius, it would be scaled for that and obvious that something is very wrong.
I'm actually betting on the number being 340°F. But we'll need the OP to let us know the truth.
dopexile@reddit
What is the prop setup? Is it fixed or constant speed? What RPM are you running the engine at?
Lowering the RPM can slow the engine down and provide more time for complete ignition so that nothing gets wasted out the exhaust for a backfire to occur.
whatdoesthismean1111@reddit (OP)
I have a constant speed prop and I was running 24 squared at that time thank you!
dopexile@reddit
I haven't looked at the operating manual for that engine. I am more familiar with the O-360, but that sounds really high. 2400 RPM is generally not a good trade-off; you go a few knots faster, might save 3 minutes on your flight but might burn 20-30% more fuel.
You should consider operating at something like 2250 or 2300 RPM with full throttle. It will cause less wear and tear on your engine and you'll get better fuel economy. With the engine turning slower there will be more time for it to combust and extract additional energy so you'll be less likely to have the types of problems you mentioned.
aftcg@reddit
Probably just had a lean die out of one of the cylinders that got remedied when you enriched the mix to a stoichiomentric ratio that allowed for combustion. One cylinder not firing at 24" and 2400 rpm (no such power setting is called "squared") will cause quite an imbalance. Sauce: 100's of hours behind Lyc IO-360A1B6s.
whatdoesthismean1111@reddit (OP)
Thank you for all this great advice. I had not considered a backfire. Actually is a great relief because the sound was pretty loud! This is all good information and I’m definitely gonna look into that low cylinder head temperature possibly a bad probe and doesn’t seem to be consistent with my EGT’s.
bhalter80@reddit
Which instrument are you using for EGT? Does it show you actual values or is it a scale with a * at the top.
The reason it's important is because the Alcolor ones (the ones with a scale and a * but no numbers) are calibrated in flight and so that at your peak EGT in cruise the needle is on the *. There's also some seasonal variation to this because as you can imagine when it's 100F degrees out the exhaust gas doesn't cool as fast as when it's -20F.
Now if you have a JPI or something that puts a number on it it could be a bad probe or again it could be cold out 1300-1400 as a peak doesn't sound that off in New England winter.
The point isn't the temp it's the peak and whether you're on the rich or lean side of it which is why the Alcolor ones are fine
whatdoesthismean1111@reddit (OP)
I do have the JPI single channel with numbers, thank you!
improvedmorale@reddit
Detonation only happens in a small area of the mixture/power setting graph. If you pulled the mixture far enough that the engine was beginning to run rough, you were not getting detonation. You say you fly a fuel-injected engine — enrichment loops are a part of carburetors, which you do not have.
dopexile@reddit
On my O-360, I usually get a backfire when running too rich. There is extra fuel going out of the combustion chamber valves, and it ignites in the exhaust, making a loud bang.
x4457@reddit
1330 EGT and 240 CHT says something is fucked there. There's something wrong with that engine.
Cant_Work_On_Reddit@reddit
Hey now, 240* is a perfectly acceptable cht.
20 min after shutdown.
ReadyplayerParzival1@reddit
In cruise it should be around 1400 something and in the mid 300’s for the cht. I don’t think combustion is happening there
x4457@reddit
The CHTs will vary (anything over 310ish wouldn't have caused alarm for me), but that is a properly leaned mixture value, yeah.
No-Relationship-2169@reddit
Going from nothing to 3 or 4 that were “loud and violent” doesn’t seem consistent with typical pre mature detonation. It’s possible that what happened was that you leaned the fuel mixture such that it didn’t ignite in the cylinder and later ignited when it hit the hot exhaust manifold. If this is the case, I’d say there’s little risk you damaged the engine internals but it’s possible that the exhaust manifold is damaged, although unlikely.
Speaking from a general technical background of internal combustion engines, not aviation specific experiences.
rFlyingTower@reddit
This is a copy of the original post body for posterity:
I’ve recently started flying a 1967 Piper PA-28R-180 Arrow, powered by a 180-horsepower, 4-cylinder Lycoming IO-360-B1E engine. It’s a fuel-injected engine. I have conflicting directions on leaning from the previous owner and my flight instructor. Previous owner said to keep it at 1200 EGT (single probe egt/cht) they instructor says I should be leaning to 1300 to 1400 EGT. Recent flying experience has shown me when leaning to rough and then backing it off. I can get no more than about 1330 EGT. On a flight yesterday when trying to aggressively lean at 4000 feet in cruise , the engine momentarily started detonating at which point I enriched the mixture and it stopped. I would say it detonated about three or four times. My question is; do you think any damage was done to the engine? I’ve never experienced this before in any aircraft. It was loud and violent. The flight continued normally for another 45 minutes with no issues indicated. All my relevant temperatures were in the green before during and after. Thanks in advance!
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