Detonation

ScottM

ScottM

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I caught myself over leaning again. My indication was a strange thumping in the motor sound. When I enrichened the mixture it went away. In the short term, short time I did this is there any harm done to the engine? Should I have anything special looked at? The detonation was occurring for less than 5 minutes and is only the 2nd time I had done that since getting the top redone a little over 200 hours ago. The top was redone because of an AD not anything that I had been doing.
 
The "thumping" could just have been roughness from uneven fuel mixtures cylinder-to-cylinder. Carbureted engines can't be balanced as precisely as injected engines; and uneven mixtures become more apparent as you get leaner. That's why carbureted engines usually can't be run lean of peak.

How much power were you pulling and how high were you?
Did you happen to notice cylinder head temperature (and do you have a four-probe CHT)?
 
***Minor thread hijack*** Jeff, what CHT do you recommend with a 0-360? Our P28A has a 4 probe system and I get mixed signals as to where to run the bird at middle altitudes.
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Pilawt said:
The "thumping" could just have been roughness from uneven fuel mixtures cylinder-to-cylinder. Carbureted engines can't be balanced as precisely as injected engines; and uneven mixtures become more apparent as you get leaner. That's why carbureted engines usually can't be run lean of peak.

How much power were you pulling and how high were you?
Did you happen to notice cylinder head temperature (and do you have a four-probe CHT)?
Click to expand...

Single probe, 374F, 70% power, 4500MSL.
 
smigaldi said:
Good article. I had been taught that too lean =thumping = detonation. But I think you are right it was probably one cylinder running a littler leaner than the others.
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Almost certainly. Those who've experienced it say that detonation (and preignition) are impossible to detect from the cockpit without instruments. Deakin has published more recent information about detonation here. For Kevin's benefit that article also has a detailed discussion of CHT's.

Regards,
Joe
 
smigaldi said:
I caught myself over leaning again. My indication was a strange thumping in the motor sound. When I enrichened the mixture it went away. In the short term, short time I did this is there any harm done to the engine? Should I have anything special looked at? The detonation was occurring for less than 5 minutes and is only the 2nd time I had done that since getting the top redone a little over 200 hours ago. The top was redone because of an AD not anything that I had been doing.
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Scott, there's just about no possible way you could have had a detonation event nor could you hear it if you did (until something came apart). It's normal (and not particularly harmful) for an engine to shudder/shake/rumble (which could sound like thumping) when you lean far enough that it can't maintain normal combustion in one or more cylinders and that's what I'm sure you encountered. I assume your engine is carbureted which means it's unlikely to be able to run LOP smoothly although some folks have had success with closing the throttle slightly (from wide open) and/or pulling on some carb heat.
 
Depending on which engine monitor you have, you might also want to look at the manual to get a better idea of what a detonation event would look like. There's a lot of places online to get good information on the subject now (as opposed to 2 years ago), and at least the JPI manuals have 2 pages on detonation / preignition / etc. detection that are worth printing out as a reference card.

As a point of interest, there's data that shows that detonation for 5 minutes, and even for extended periods of time, can do very little or no damage to aviation piston engines. At least one case comes to mind where an engine (520, I think) didn't show any damage after more than one hour of detonation in the lab. Of course, that doesn't mean it's a good idea :)

-Felix
 
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kevin47881 said:
***Minor thread hijack*** Jeff, what CHT do you recommend with a 0-360? Our P28A has a 4 probe system and I get mixed signals as to where to run the bird at middle altitudes.
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First, the Operator's Manual on the O-360-A engine (which is what you probably have) says the max CHT at the bayonet location is 500F in all flight phases. However, the note to that says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation."

Second, the Lycoming Flyer Reprints book (1996 edition) says "the engines have benefitted during continuous operation by keeping CHT below 400F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F."

Other interesting points from the Reprints book include the following: "If an engine has bayonet probes in all cylinders it is not unusual to see variations in CHT readings...as much as 150F on engines with float-type carburetors [due to] the kind of distribution of fuel and air to the individual cylinders." Our Cheetah had a JPI EDM-700 with four probes, and I regularly saw 75-100F difference between the front and back cylinders of the O-320-E2C with STC'd high compression pistons. However, our current Tiger (also with a Lycoming O-360-A engine) has a very good baffling system (better than the Cheetah's) and I usually see only a 30F spread from coolest (#1) to hottest (#3).
 
Ron Levy said:
First, the Operator's Manual on the O-360-A engine (which is what you probably have) says the max CHT at the bayonet location is 500F in all flight phases. However, the note to that says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation."

Second, the Lycoming Flyer Reprints book (1996 edition) says "the engines have benefitted during continuous operation by keeping CHT below 400F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F."

Other interesting points from the Reprints book include the following: "If an engine has bayonet probes in all cylinders it is not unusual to see variations in CHT readings...as much as 150F on engines with float-type carburetors [due to] the kind of distribution of fuel and air to the individual cylinders." Our Cheetah had a JPI EDM-700 with four probes, and I regularly saw 75-100F difference between the front and back cylinders of the O-320-E2C with STC'd high compression pistons. However, our current Tiger (also with a Lycoming O-360-A engine) has a very good baffling system (better than the Cheetah's) and I usually see only a 30F spread from coolest (#1) to hottest (#3).
Click to expand...

I have an EDM-900. I see a spread of as much as 50-70 degrees EGT between the 6 cylinders on MY O-540 - especially if it's leaned. I lean to 50 degrees rich EGT on the leanest cylinder.

I do keep my CHTs well under 400 degrees, in fact I think I have an alarm set at 400. I think like Ron, my CHT spread never goes much above 25 degrees.
 
Ron Levy said:
First, the Operator's Manual on the O-360-A engine (which is what you probably have) says the max CHT at the bayonet location is 500F in all flight phases. However, the note to that says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation."

Second, the Lycoming Flyer Reprints book (1996 edition) says "the engines have benefitted during continuous operation by keeping CHT below 400F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F."

Other interesting points from the Reprints book include the following: "If an engine has bayonet probes in all cylinders it is not unusual to see variations in CHT readings...as much as 150F on engines with float-type carburetors [due to] the kind of distribution of fuel and air to the individual cylinders." Our Cheetah had a JPI EDM-700 with four probes, and I regularly saw 75-100F difference between the front and back cylinders of the O-320-E2C with STC'd high compression pistons. However, our current Tiger (also with a Lycoming O-360-A engine) has a very good baffling system (better than the Cheetah's) and I usually see only a 30F spread from coolest (#1) to hottest (#3).
Click to expand...

Thank you for the info Ron. That helps.
 
mikea said:
I have an EDM-900. I see a spread of as much as 50-70 degrees EGT between the 6 cylinders on MY O-540 - especially if it's leaned. I lean to 50 degrees rich EGT on the leanest cylinder.
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We were discussing CHT, not EGT, but FWIW, your EGT spread is fairly typical. Also, it sounds like you're leaning pretty close to best power, which will give pretty close to peak CHT. If your CHT's are still OK at that point, you've got quite satisfactory cooling.
 
I'm not an engine expert, but I have spent time at a Holiday Inn Express. As others have said, I think your fears are unwarranted. Your engine is fine. Ron's data should help you. Your engine monitor is your best protection against engine damage. From what I've read (and those Lycoming Flyer reprints are REALLY informative) as long as you keep the CHTs below 400 (425, but I believe in keeping a comfortable margin) and EGTs 100F below peak you'll do fine. Obviously, LOP ops with injected engines will have different requirements, but for a carb'd Lyc 0360, it's hard to argue about throwing some fuel at the cylinders for longer life.

ALLLLLLTHOUGGHHHH, most of that wisdom was concocted in the days of $1.50 100LL. Now that it's upwards of $4.50, perhaps EGT at Peak ops makes sense, since x gallons times high prices may be more than the cost of new cylinders. IMHO YMMV SDBM *(So Don't Blame Me)
 
I would point out that Lycoming as well as Continental are about 15 years behind what is generally known about engines now. In this case, for example, this sentence

"In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F"

could be considered to be misinformed at best and crazy at worst. You most certainly don't want to see anything close to 430dF in cruise or climb, ever. If that happens for more than a few minutes, you are doing damage to your cylinders. There's some good data about the effects of CHTs on engine longetivity and they all show that 380-390dF is a good red line for continuous operation. It's ok to get up to 420 or 430dF for a few minutes once in while, but aiming at CHTs below 400 will significantly impact your engine's ability to make TBO and beyond. The other important factor that Lyc or Continental rarely talk about is ICPs. You'll want to keep those down as well; there are a few ways of doing it, running LOP is one, not running above 75% power ROP is another.

-Felix
 
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sba55 said:
I would point out that Lycoming as well as Continental are about 15 years behind what is generally known about engines now.
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It also seems that far too many pilots are about 15 years behind as well and refuse to accept anything else.
 
i always try to keep the engine on the superduper cub under 400 ont he CHT when towing. anything else is just damn hot. mixture throttle and speed usually manage to get it done. of course the best thing that helps is when the stupid glider lets go
 
sba55 said:
You most certainly don't want to see anything close to 430dF in cruise or climb, ever. If that happens for more than a few minutes, you are doing damage to your cylinders.
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Can you point to some research or technical studies which support these statements? So far, my engine's condition monitoring suggests that's simply not true. Despite running well up into the CHT range you say will cause damage (in particular, the #3 cylinder runs up near 430F in climb and a tad over 400F in cruise), SB388C checks show my engine is well inside crucial wear tolerances, oil consumption is within normal limits, and borescope inspection during the SB388C checks shows no signs of trouble.
 
Ron Levy said:
Can you point to some research or technical studies which support these statements? So far, my engine's condition monitoring suggests that's simply not true. Despite running well up into the CHT range you say will cause damage (in particular, the #3 cylinder runs up near 430F in climb and a tad over 400F in cruise), SB388C checks show my engine is well inside crucial wear tolerances, oil consumption is within normal limits, and borescope inspection during the SB388C checks shows no signs of trouble.
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Turbocharged TO-360 engine here. My #4 CHT (the one by the turbocharger) runs between 395 and 405 in climb and cruise. Others are in the 390-400 range. My experience is about the same as Ron's over about 900 hours on the engine.
 
Ron Levy said:
Can you point to some research or technical studies which support these statements? So far, my engine's condition monitoring suggests that's simply not true. Despite running well up into the CHT range you say will cause damage (in particular, the #3 cylinder runs up near 430F in climb and a tad over 400F in cruise), SB388C checks show my engine is well inside crucial wear tolerances, oil consumption is within normal limits, and borescope inspection during the SB388C checks shows no signs of trouble.
Click to expand...

That's interesting Ron. Our O-360 has the same situation with #3. We checked baffling, probe installation and location to no avail. We determined it is "characteristic" for this engine.
 
kevin47881 said:
That's interesting Ron. Our O-360 has the same situation with #3. We checked baffling, probe installation and location to no avail. We determined it is "characteristic" for this engine.
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In a Tiger? Absolutely -- the Tiger's speed is primarily due to drag reduction, in part from the tight LoPresti-designed cowl. In a PA-28? I wouldn't think so -- much looser cowling equals better cooling. I'd expect 400F tops in the climb in a PA-28-180, C-172Q/C-172-180STC, BE23, etc.
 
Ron Levy said:
In a Tiger? Absolutely -- the Tiger's speed is primarily due to drag reduction, in part from the tight LoPresti-designed cowl. In a PA-28? I wouldn't think so -- much looser cowling equals better cooling. I'd expect 400F tops in the climb in a PA-28-180, C-172Q/C-172-180STC, BE23, etc.
Click to expand...

Ah heck, back to the drawing board for some additional testing with the PA28.
 
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