Detonation caused by lean mixture

[CharlieDeltaUK]

I've been studying for my aircraft tech ground exam and reading about the hazards of over-leaning the mixture. It brings to mind a time when I did a solo flight and managed to taxi to the holding point and do power checks with lean mixture, right up to the point where the checklist told me to check 'Mixture Rich'. I had noticed it seemed to be rough running, and of course increasing to full rich sorted that out. Now I've read about the theory, I'm wondering if I did damage to the engine. How would one know?

Shows the power of using checklists, but I wouldn't like to think I've damaged the engine.

[IO540]

You cannot damage the engine by over-leaning, at the relatively low power settings involved in ground ops.

It is normal practice among advanced pilots to lean during taxi, to the maximum degree possible. It keeps the plugs clean, and you cannot takeoff with it because the engine would just cough and splutter if you went to full throttle like that.

In general, to cause detonation, you need the wrong mixture, a high power setting (a high combustion chamber pressure) and a very high CHT. Or the wrong grade of fuel (unsufficient octane rating, etc).

[englishal]

I don't think you can damage the engine anyway, according to some, if you lean beyond optimum mixture during cruise. The reason is that the temps drop off again with a lean mixture which is why LOP operations work nicely in some aeroplanes (mainly where the fuel is metered correctly - i.e. fuel injected).

[bravobravo74]

I'm no expert on the subject but it seems that the likelihood of detonation is over-estimated in the texts (and possibly instruction) that PPL students generally come across. I'm sure that were I to delve into my Trevor Thom books I'd convince myself that moving the mixture lever away from the full rich position at any time would promptly blow-up the engine.

[Aerozepplin]

Do a search by threat subject for "LOP" (lean of peak), and "lean". There are many threads with some very valuable information. Its an area of knowledge greatly lacking in many of the references and flight training organisations. Even ground-leaning for example is a great idea, but many new instructors (and some experienced) look at me like I'm going to make the engine explode.

Just the fact that a lean mixture doesn't magically cause detonation on its own blew my mind when I was a new CPL. There's far to much to write about in one post, but my advice is always to keep researching the material. Don't trust things that refer to "over-lean" or "leaning too agressively", because these statements are only half true and in my opinion indicate a reliance on bar-talk myth. Many of the operating practices that people use such as lean of peak, and wide open throttle likely aren't possible in a carby training aircraft, but the principles behind them are valuable in understanding what happens inside an engine at different power and mixture settings.

Or read the bible:
Pelican's Perch - More Stories (http://www.avweb.com/news/pelican/list.html)

There's one heck of a lot in there though. I spent many months reading, understanding, not understanding, re-reading, and pondering the information in Mr Deakin's work. Now I preach about him like its a cult.

[Big Pistons Forever]

It is very difficult to get a non turbocharged carburated engine to detonate. Assuming proper fuel about the only way I would think detonation could possibly occur would be a prolonged low airspeed climb with the carb heat full on and the engine leaned to 50 rich of peak. This will result in very high CHT's due to the heated induction air and hottest flame from a "best power" mixture. Under any normal operating procedures the risk of detonation is virtually zero. This is doubly so for the run of the mill 4 bangers because they were all designed for 80/87 0r 90/96 fuel but usually operated on the far more common 100 Octane 100LL fuel, which will dramatically increase the detonation margins.

It is a different story however for some of the high output turbocharged engines. These engines can, even in "normal" operation have very small detonation margins. There was a spate of early inflight engine failures caused by detonation in early Piper Malibu's for instance. These engines must be operated with care to keep the CHT's down and respect the EGT and TIT limits.

[Fuji Abound]

I suspect you will do more good than harm by taxing lean.

The check is there for exactly that reason, if the taxi was always full rich there would be no need to check it was rich before tafe off (albeit a little tongue in cheek being as it is a critical check).

Of course there may even be times you dont want to depart fully rich, but that is another story.

[Captain Smithy]

I've been studying for my aircraft tech ground exam and reading about the hazards of over-leaning the mixture. It brings to mind a time when I did a solo flight and managed to taxi to the holding point and do power checks with lean mixture, right up to the point where the checklist told me to check 'Mixture Rich'. I had noticed it seemed to be rough running, and of course increasing to full rich sorted that out. Now I've read about the theory, I'm wondering if I did damage to the engine. How would one know?

As has been said running the engine lean at idle/low RPM while taxying will do the engine more good than harm. But before power checks or advancing the throttle you must put the mixture back to fully rich, otherwise the engine will start to pink, as you found... you probably didn't damage the engine, I think repeated abuse would cause a failure, but you won't be able to tell until the donk one day suddenly goes bang.

That said there are signs of repeated running on too-lean a mixture, valve seats getting burned etc.

Amazed how few people are taught how to use the big scary red lever.

Smithy

[FlyingStone]

But before power checks or advancing the throttle you must put the mixture back to fully rich, otherwise the engine will start to pink, as you found... you probably didn't damage the engine, I think repeated abuse would cause a failure, but you won't be able to tell until the donk one day suddenly goes bang.

That said there are signs of repeated running on too-lean a mixture, valve seats getting burned etc.

One can do "power check" at 1700 rpm or whatever is prescribed by the engine manufacturer with mixture leaned. It's just a matter of how much is it leaned. If it's too much, you may not get 1700 rpm at all - in this case, just move the lever towards the rich side until you do. I doubt you can ruin the engine by doing a power check with mixture leaned to max RPM/peak EGT - if you can do the same method (best power/best economy) in the cruise at much higher power settings, why would it hurt on the ground (provided CHTs are observed)?

Burned valve seats are usually signs of consistently extremely high CHTs (450°F and higher) and you will usually see this temperatures in prolonged climb at Vy or slower (depends on the engine and the engine cowling) or perhaps in poorly cowled engines in cruise with mixture set to best power (50° - 100°F ROP). If the engine is capable of (better said: if air/fuel mixture is consistent in all cylinders) running 50°F lean of peak, you will get very low CHTs than 50°F rich of peak - with the same EGT. Lycoming itself supports leaning techniques and if you read their Flyer publication, you will see that you can set whatever mixture you like (provided the engine isn't running rough) with power settings below 75%, but usually in terms of SFC, leaning too much on the lean side isn't very productive, since power starts to decrease quite rapidly after let's say 50°F LOP.

[Captain Smithy]

FlyingStone

You are correct, I don't think you can ruin an engine at power check RPM but depending on how much you've leaned the mixture it will start pinking. If it's a long taxi (not usually at my home base, but at my old airport it was a very long taxi) then I'll have the mixture back pretty far.

Interesting info on ROP/LOP, quite (most) often the problem is that the mixture isn't the same for all cylinders, so you end up having to compromise and often that leaves you running ROP, however if its possible to run 50 LOP it's a good idea. Also cockpit instrumentation is a problem - the most many of us have to go on is an EGT, but more often than not there isn't even that, I have to lean using just the tacho because in the aircraft I fly that's all there is, so its ROP for me. But if you've got the instrumentation...

[hingey]

Rough running with the mixture pulled back is (in the case of injected engines) uneven distribution of fuel into the cylinders. This will increase vibration, generally not good for the engine, but not itself a risk of detonation.

h

[Fuji Abound]

.. .. .. which will be evident by the EGT and CHT readings for each cylinder. I am not sure I am that much of a fan of LOP without being able to monitor each cylinder, rather run richer, accept the extra fuel burn in the knowledge that you are not damaging a cylinder that has a blocked injector or some other issue.

[dirkdj]

It is far safer to have a partially blocked injector when at peak or LOP than ROP. Having a partial blockage when ROP could put this one cylinder in the 'Red Box' and you might not detect it. When at peak or LOP having a partial blockage might cause some roughness but would immediately be noticed.

[FlyingStone]

You are correct, I don't think you can ruin an engine at power check RPM but depending on how much you've leaned the mixture it will start pinking. If it's a long taxi (not usually at my home base, but at my old airport it was a very long taxi) then I'll have the mixture back pretty far.

I highly doubt it. In order for detonation to occur, you need high cylinder temperatures and very high peak internal pressures, none of which you have available at run-up power setting. Have you ever checked where in the EGT curve are you exactly when you do ground leaning for taxi operations? It takes a lot of mixture lever movement to get to best power, let alone peak EGT. By "not leaning too much" on the ground, you're probably on a good path to kill yourself one day. If you lean the engine on the ground, do it brutally - right to the last point when the engine is still running smooth. By doing that, you have ensured that the engine is getting the minimum fuel required for combustion at idle power and - most importantly - if you forget the mixture before takeoff (I have and I'm probably not the only one), the engine will just cough and may even quit if you don't retard the throttle or increasing the mixture. However, if you don't lean the mixture enough on the ground and try to take off, the engine just might develop more than rated power, since there won't be any excessive fuel - and that is when detonation would develop quite rapidly, right to the point of engine failure if not recognized very soon. If you don't understand the concept of ground leaning, just leave the mixture full rich - it's cheaper and safer in the long term.

Here are couple of quotes from Lycoming Flyer Key Operations:

First, we must know that cruise power for Lycoming normally aspirated engines is generally considered to be 55% to 75% of the maximum power for which the engine is rated. At these power settings, the engine may be leaned at any altitude.

It is also important to understand that leaning to roughness at the engine manufacturer’s recommended cruise power is not an indication of detonation, but indicates normal characteristics of distribution to the individual cylinders. The roughness indicates that the leanest cylinder has become so lean, it is beginning to miss. This is typical of an engine with a float-type carburetor. Damage, to an engine from leaning does not occur at the manufacturer’s recommended cruise power, but takes place at higher than cruise power.

On the other hand, with any direct-drive normally aspirated Lycoming engine, the pilot can and should lean the mixture at any altitude as long as the aircraft is in cruise configuration at 75% power or less.

Again, we repeat that maximum leaning (peak EGT) does not damage an engine at the engine manufacturer’s recommended cruise power. Damage is caused by maximum leaning at higher than recommended cruise power where the manuals do not spell it out or allow it, and when the aircraft does not have a complete set of reliable engine instruments to protect the power plants. Excessive leaning under the latter high power conditions can cause detonation and/or preignition and possible engine failure.

[hingey]

I remember somewhere in Deakin's columns he states that about 50f ROP is the place most likely to do damage, which is exactly where many schools teach to run the mixture (pull it back until it gets rough, then push it in a bit). Any thoughts?

We never operate our normally aspirated IO360 above 75%, but do lean quite aggressively. If Deakin's statement is true, would we be better running AT LOP or are we going to risk damage to the exhaust?

h

[Fuji Abound]

We never operate our normally aspirated IO360 above 75%, but do lean quite aggressively. If Deakin's statement is true, would we be better running AT LOP or are we going to risk damage to the exhaust?


As I said earlier I would only run at LOP if I could monitor every cylinder. If I could I definitely would. If I couldnt then I would run rich. I cant understand the logic in risking the engine when an overhaul is going to cost you way more than the extra fuel - unless of course it is not your engine - and then I would be very upset that you treated my engine with so little respect, but understand many do.

A CHT and EGT monitor on each cyliner is not a costly piece of kit and the risks in MO of running LOP with a miss behaving cylinder and not knowing are too great.

[hingey]

My mistake, meant to sat at peak EGT. Wouldn't dream of running LOP without decent Gamijectors and a monitor!

h

[Tinstaafl]

You don't necessarily need Gami injectors to run LOP. It depends on how well matched the current injectors are. I manage a Panther Navajo & run it LOP in cruise as standard. It runs fine at 50 deg LOP although fairly quickly becomes rough once more than ~60-70deg LOP (not that it's worth going that lean. Too far off the efficiency curve)

Its manual approves 50 deg using just the factory single point probe although I had the owner fit an EDM 760 to it before doing so, other than a single demonstration of fuel savings & lower temps to show him the difference. I'm just not comfortable operating close to margins without being sure that each cylinder is OK. Interestingly, the EDM probes showed different cylinders were more limiting for EGT & CHT than the factory fitted probes. Not that they were outside limits but certainly closer. With the EDM I can make sure every cylinder is comfortably cool.

[yann1123]

Below 40%NRP and above 75%NRP detonations and preignition are most likely to happen. Still, as stated above, it's extremely rare. But during a long taxi pay close attention to your mixture setting because running to lean with an air-cooled engine at less than 40% will cause a nice rise in CHT.
:ok:

Yann

Edit: forgot to mention that when running to rich carbon deposits form in the cylinders and can cause detonation, so leaning is good practice during taxi.:)

[avionimc]

Here is an interesting read about leaning, EGT, CHT & TIT from INSIGHT's GEM Pilot's Guide (http://www.insightavionics.com/pdf%20files/GEM%20GUIDE.pdf) . I used a GEM for the past twenty years. Not only a GEM allows to lean exactly to the optimum, but also helps recognize magneto troubles and many other snags way before they become problems.