Having a bad day - Detonation

Stay away from old auto fuel and use the proper octane and if you change the needle clip position know what effects it has and remember cold OAT's cause even leaner running conditions.




Question:
Is there an easy way to just post a picture not attached to a URL?

Was it old gas or improper octane or both?

roger lee said:

...Question:
Is there an easy way to just post a picture not attached to a URL?

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There is a link to the picture posting instructions at the upper left of this page.

Insert Pics

It is necessary to host the photos somewhere. By the way I can't see the photo you linked to. Likely one needs to register on the site.

Hi Larry,

I saw that, but required some where to link to a url or another site. The admin should be able to enable picture posting direct with either jpegs or pdf's.

I think you guys are right about not being able to see the picture unless registered on that site. I'll look for another way to post it.

You can copy it to your computer then use the link provided above to host it and post it here.

Doug does not allow his forum to host pics due to the overhead required to host thousands of pics and files. I don't blame him when other places will do it for free.

Hi Gagarin737,

Thanks for posting the pictures.
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Typically detonation happens in one cylinder first. The circumstances are just right to happen in one, but usually doesn't happen in 2 or more at the same time. This looks like a typical detonation problem. I have seen several from people not following Rotax recommendations and doing their own thing. I don't think anyone knows yet what caused this, but I hope they report back and let us know. It is very costly.

Quote:
Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to auto-ignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.
Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.

Try a higher octane fuel. The octane rating of a given grade of gasoline is a measure of its detonation resistance. The higher the octane number, the better able the fuel is to resist detonation.

"Read" your spark plugs. The wrong heat range plug can cause detonation as well as pre-ignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application.

Check for engine overheating. A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by a low coolant level, a slipping fan clutch, too small a fan, too hot a thermostat, a bad water pump, or even a missing fan shroud. Poor heat conduction in the head and water jackets can be caused by a buildup of lime deposits or steam pockets (which can result from trapped air pockets).

Check for a lean fuel mixture. Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or the induction plumbing downstream of a fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter or a weak fuel pump.
If the fuel mixture becomes too lean, "lean misfire" may occur as the load on the engine increases. This can cause a hesitation, stumble and/or rough idle problem as well.
The air/fuel ratio can also be affected by changes in altitude. As you go up in elevation, the air becomes less dense.
A carburetor that's calibrated for high altitude flying will run too lean if driven at a lower elevation. Altitude changes are generally not a problem with engines that have electronic feedback carburetors or electronic fuel injection because the oxygen and barometric pressure sensors compensate for changes in air density and fuel ratios.

Change your flying habits. Instead of lugging the engine, try less prop pitch.

I hope and assume these are not your pistons Joeri after so few run hours ?

Gelukkig niet!

Just an educational item of interest

This piston isn't from any vans aircraft. To my knowledge a cause has not been determined as of yet.

It was just an FYI against wanting to alter or exceed Rotax recommendations that things can and do go terribly wrong before you have a chance to react. I have seen several of these over the years so I was hoping a preemptive FYI would help.

Roger, just a point of order, from somebody involved in (on the observation deck) the most intensive aviation detonation testing possibly in history. Or of recent times at least.

Check for a lean fuel mixture. Rich fuel mixtures resist detonation while lean ones do not.

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That is the opposite of the truth. A lean mixture is technically one of the lean side of peak EGT as peak is the reference point determining what is lean and what is rich.

A Rich mixture is one that is on the Rich side of peak EGT.

The most detonation prone mixtures are those about 35-50dF ROP and potentially out to 75dF, but almost impossible on the lesser side of 35 and even more so on the lean side of peak. And the drop off is dramatic.

High Inlet Air temperature, high CHT and low octane and perhaps even a bit of advanced timing are all good things for creating detonation.

A naturally aspirated conforming engine on conforming fuel will not detonate under normal conditions.

In addition colder air is even harder to cause detonation, you should see what we have to do with IAT, but that aside the bing carby is DA controlled until of course it reaches its end stop. So I do agree fiddling with the cir clip is not wise.

Does anyone have the real photos of the failure ?

I have to agree with David. LOP is less prone to detonation. The Voyager (around the world) primary engine ran a Cr in the region of 15:1 and ran some 200-300 F LOP (as I recall, maybe more in classified configuration, I did not need to know). 100LL.

RV10inOz said:

Roger, just a point of order, from somebody involved in (on the observation deck) the most intensive aviation detonation testing possibly in history. Or of recent times at least.

That is the opposite of the truth. A lean mixture is technically one of the lean side of peak EGT as peak is the reference point determining what is lean and what is rich.

A Rich mixture is one that is on the Rich side of peak EGT.

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I disagree... perhaps it just semantics but a "lean" mixture, as most of the aviation world views it, is something "leaner" than about 150 ROP.

Walt said:

I disagree... perhaps it just semantics but a "lean" mixture, as most of the aviation world views it, is something "leaner" than about 150 ROP.

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I agree with Walt (based on the training I have received).
It may be simply a geographical thing because it is all dependent on what the reference/base line being used is.
My training taught that the base line was the adjustment of mixture for max rated power at take-off or achieving max contentious power for cruise flight (depending on the circumstances being discussed)

I've been flying for a number of years and always thought running rich avoided detonation, but I have always wondered how I would know. Everybody has heard a car engine "diesel" at least once after shutdown, but what would detonation sound like in cruise?

rvbuilder2002 said:

I agree with Walt (based on the training I have received).
It may be simply a geographical thing because it is all dependent on what the reference/base line being used is.
My training taught that the base line was the adjustment of mixture for max rated power at take-off or achieving max contentious power for cruise flight (depending on the circumstances being discussed)

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Walt and 2002,

Disagree if you like, there are many things that people have been taught in the last 40 years in GA that have been plain wrong. As for a delineation at 150, that is a new one to me but I am sure it is documented somewhere as are many OWT's that have been around since the late 60's. Like everyone else I was subject to the same teaching.

2002, I do not see how you can have two reference points. By a scientific definition any think leaner than peak is a lean mixture, some that are slightly richer but still lean of peak, are still a lean mixture. Same for on the rich side, one is richer but both are rich mixtures.

Where the confusion has probably come from is historically LYC/TCM could only ever supply engines that were capable of running properly on the Rich side of the curves, thus they only ever talk about a RICH mixture, Some were richer or leaner than others relative to each other. But they were still rich mixtures by definition. Convention wisdom was born!

Back to the Rotax, to detonate a piston in a Rotax engine will take a fair bit of effort. I wonder if the truth will ever be revealed.

Bit of thread drift on fuels, I realise you can make most engines used in RV's run on Mogas, and even the lower grades, however I do not support people doing it as a general rule. One reason is olefins and gum buildup and I could well imagine this is a possible cause, along with some other issues.

Perhaps it was a piston with a lot of casting defects?..who knows but It would be nice to find out. They are a great little engine generally speaking.

rvbuilder2002 said:

My training taught that the base line was the adjustment of mixture for max rated power at take-off or achieving max contentious power for cruise flight (depending on the circumstances being discussed)

Click to expand...

RV10inOz said:

2002, I do not see how you can have two reference points. By a scientific definition any think leaner than peak is a lean mixture, some that are slightly richer but still lean of peak, are still a lean mixture.

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David,
I assume you mean any Thing leaner? I don't disagree with that statement, but as Walt already said, I think we are talking semantics here.

Please note that I wrote depending on the circumstances being discussed...
If two RV builders (or mechanics, or a mechanic and an owner, etc.) are discussing how to adjust the mixture for best power for a high altitude take-off, they don't use terminology describing what percentage of rich it is relative to peak EGT.... they describe leaning from the full rich condition. If in a different discussion they are talking about obtaining best power in cruise flight, then the terminology "degrees rich of peak is generally used".
So, if you are trying to use the scientific description for everything, that is likely the problem.
We lowly mechanic types may tend to speak and think in terms that are a bit simpler (and I see nothing wrong with that if it allows the two people communicating, to easily understand what is being described).

Lean = anything less than full rich.

Me: Your plugs are full of lead, do you lean on the ground and below 75% power?
Pilot: No I don't, should I be?

You get the idea... like 2002 says, we're just a bunch of knuckle draggers or cone heads, what do we know. I prefer to leave the scientific stuff to the smart guys

Hey?both of you, cut out this lowly mechanic rubbish would ya! I learn a lot from the gus who are handy with a spanner (wrench), and I am just a lowly mechanical engineer?..

Actually the areas of technical difference is born from even the FAA A&P exams. So it is not your fault. Pilots and A&P's (LAME's in Aus) are all spoon fed the same things, mostly all correct, but in some areas much of it wrong.

Find Mike Busch's sort about passing the A&P exam, now this guy is studious. He took the APS class three times?yes three! And when he did his A&P exam he studied hard, and deliberately studied the wrong answers in order to get the questions marked right.

Same thing down here, and with Commercial Pilot subjects, many a question must be answered wrong to pass.

So its not our fault. But If I can help straighten up some thinking, I am glad to help.

cheers!