Rotax Piston Meltdown

[DanH]

Gents, the futility of diagnosis by long distance is generally due to lack of complete information.

In this case, at two weeks we continue to have incomplete information. Let's back it down until we see it, or the the thread dies.

[rv6ejguy]

Originally Posted by AndrewR

That is very different to detonation sustained for a minute or 2 at maximum power. There is detonation that breaks things within seconds. There is detonation that can continue for a long time without damage. Presumably, somewhere in the middle is detonation that does damage over 1-2 minutes. What does that damage look like?



Everything I have seen on detonation says it can shock the boundary layer that protects the piston and head from the heat of combustion, with the result that they overheat. That is not consistent with "no heat damage".

Another thing that destroys the protective boundary layer is hot gas escaping through a small gap. So if you damage the ring seal and the blow by is severe enough i.e. you maintain high power, the combustion gases act like a blow torch on that area of metal.



Detonation is short enough that cylinder volume change and crank angle are irrelevant.

Your point about pre-ignition is important though, it occurs before normal ignition so you have a cylinder full of unburnt mixture. The heat and pressure are applied to the whole combustion chamber so you would expect to see generalized heat damage - not just one area. With pre-ignition I would expect to see some evidence of melting on e.g. other areas of the piston. I do not understand how pre-ignition damage would be so localized.

Detonation on the other hand is likely to happen in the areas furthest from the plug and last to burn. The damage could quite reasonably be localized to those areas, and that is what I see here.

I'm not talking theory here. I've seen the real world end results of both phenomenons for decades and I invite you to look at piston damage photos from other forums if you want another view from mine. I think you'll find the detonation failures will all look pretty much like the photos I've already posted with simple fracture of the 2nd and 3rd ring lands. You won't find heat damage to the crown unless pre-ignition follows the detonation like in the second photo I published. I was driving that engine when the wastegate failed and know the sequence of events that followed.

Heavy detonation usually ends with the failure of the ring lands because of the massive compression loss which lowers the cylinder pressure below the point where detonation can occur. I've measured the static compression pressure below 40 psi on several engines after ring land failure. You cannot maintain high power with broken ring lands as you assert. Light detonation, even sustained, typically does little or no damage for quite sometime, depending on piston strength of course.

In the case at hand, the OP says one ground electrode was melted off. This never happens with detonation in my experience, almost always happens with pre-ignition.

Pre-ignition always shows localized damage as in the photos I published. It has to start somewhere which has an overheated component and since it is a normal combustion event started prematurely, I don't see why you'd expect to see otherwise? Your conclusion doesn't make sense.

Your statement about the duration of a detonation event being irrelevant to heat damage is also illogical. Things like pistons have thermal mass, it takes a finite amount of time to heat them up enough to melt them. A short event cannot do this no matter how intense. Detonation events are typically way less than 1 millisecond in duration. Typical detonation shockwaves travel at around 6800 fps while normal combustion is at around 40-75 fps.

Frame 3 shows a normal combustion pressure trace, frame 23 shows one with detonation. Notice the amplitude and duration differences.



Your conclusions are supported neither by experience in this field nor by the science and photographic evidence available.

[waterboy2110]

Originally Posted by rvbuilder2002

I am not at all clear how if no one at Van's asked you about your engine and that you have spoken (or communicated via text message) with seven people there, that it implies there could be other failures that are not known about.
You mentioned you were calling because of an engine failure, correct?

I also don't understand what is unreasonable about saying there has only been one other failure if that is all of the information that is publicly available.

Nothing in the world is a certainty.
I admit it is possible the other one I mentioned isn't the only one, assuming someone had a failure and told no one at Van's, told no one else that took the time to mention it on line (until now like you have, which would be extremely rare) and didn't make any attempt to file a warranty claim with one of the Rotax Dealers.
I think most people would agree that was rather unlikely, but yes, not impossible.

If the one you mention is real, can you provide some details? A link to a for sale ad of an airplane with a broken engine perhaps?


Your post seems to have a desire of inducing doubt that there are only two (for what reason I am not entirely sure).
Maybe there is one there in CA for sale with a failed engine, but without knowing what the cause was it is not relevant to implying a problem with RV-12's......... an engine failure could be caused by something as simple as an improperly tightened oil hose coming loose in flight causing a loss of oil and failure of the engine. Surely you can agree that though unfortunate, that would be entirely the fault of the builder/maintainer

I think it would be of more value if there was emphasis on discovering what the cause of your engine failure was.
I think there has been requests for more info/photos that might provide some answers, but I haven't seen that info presented yet.

Scott,
I?d like to go on record by saying that I personally appreciate and value your participation and contributions to this forum. I?m sure many others feel the same as I do. You insight to the products help us all to understand the complexities of creating some great airplanes.

It?s perfectly reasonable and I?d go as far as to say expected that you defend Vans products and the decisions made while developing those products. Anything less would be unacceptable in my option.

I don?t intend to go head to head with you in this forum as it serves to no ones benefit. I?m simply looking to those who are interested to help solve this failure and hopefully help others to avoid the same fate as my little engine and quite possibly save their investment or perhaps their life.

I expect bias on your part but as I pointed out in the original post I?d like to remain open and objective.

Thank you for the value you bring to all of us who have taken the Vans path with our time and money.

[Sam Buchanan]

I don't have a dog in this hunt....but I've detected "bias" on more than one side of this discussion (and a previous fuel thread).

I'm enjoying the exchange of info, my personal understanding (bias...?) has been expanded as a result of it. Looking forward to a resolution of the engine failure.

[DonFromTX]

I totally agree Sam, I learned a lot from just reading the responses.
Valuable information I feel.

Originally Posted by Sam Buchanan

I don't have a dog in this hunt....but I've detected "bias" on more than one side of this discussion (and a previous fuel thread).

I'm enjoying the exchange of info, my personal understanding (bias...?) has been expanded as a result of it. Looking forward to a resolution of the engine failure.

[C-GRVT]

I don't have an RV-12 and do not fly a Rotax, but am following this thread to learn something about engine detonation and pre-ignition. Responses by those with demonstrable extensive experience with "alternative" engines and a long history of contribution to this list are valuable and contribute to the knowledge base.
The public suggestion that rvbuilder 2002 has shown "bias" in his responses is unjustified by anything that was said and unfair. It is of great benefit to this list to have the attention of a Van's employee of long experience. The general high level of knowledge, assistance, and respectful discussion on VansAirforce is a refreshing change from some of what is out there. No need to undermine it.
Bill

[bret]

On post 102 can you explain the photos, trying to learn something here, if photo 3 is normal combustion at 40 FPS and photo 23 is detonation at 6,800 FPS, are we to compare the ripple length or amplitude, if length, are they on the same time scale, because the first ripple is not 1,700 times as long, more like 5? Not at all questioning your experience, just trying to comprehend the above photos. Thanks.

[RV10inOz]

Originally Posted by bret

On post 102 can you explain the photos, trying to learn something here, if photo 3 is normal combustion at 40 FPS and photo 23 is detonation at 6,800 FPS, are we to compare the ripple length or amplitude, if length, are they on the same time scale, because the first ripple is not 1,700 times as long, more like 5? Not at all questioning your experience, just trying to comprehend the above photos. Thanks.

I think you will find the time scale (x axis) is the same, the amplitude is what matters. The first big spike is the event followed by the pressure waves bouncing around the chamber.

I have a graph somewhere that shows the reflections, being measured by two very high speed probes at opposite sides of the head (two plugs) and you can see it clearly. This was data collected on the Carl Goulet memorial test facility at TAT and GAMI.

I think it is important to note that cylinders exposed to continual detonation testing (like done at GAMI) last a very long time. Hundreds of hours. To destroy a piston alone with detonation takes a very big pressure peak and high temps, and then do it long enough. I seriously doubt the average Rotax owner can do that, running on reasonable fuel and normal flying ops.

A damaged plug and the resulting preignition events can do it in minutes (and not many).

[waterboy2110]

Pulled the engine out of the box today and took some pics.

Plugs look fine
Number 2 top is pretty fouled but the insulator looks intact.
Number 2 Bottom has ground pushed into the electrode otherwise intact. Most likely due to being struck FOD. Expect this is the cause of the failed ignition check post failure.
Plugs from number 4 look fine.

O-Rings on intake manifold at heads and carb are fine - always were. This is the conclusion Rotax came to once the found out the fuel met spec.

Piston, cylinder and head for your enjoyment.

Thanks to everyone that found this thread interesting and useful. Special thanks to Neil who put me in contact with Bill.

[rv6ejguy]

Thanks for posting the detailed photos of the plugs and piston. Plugs look totally fine and heat range is 8 which is stock for this engine. Insulator color looks good too so I don't see evidence of an overly lean condition.

I would say from these better photos and the fact that the ground electrodes are intact and the insulators don't show any cracking, heat distress or aluminum micro spheres on them, this was probably not a pre-ignition event and certainly not a detonation event.

Photo 12 of the piston shows an overheated crown failure very much like the 3rd photo I put up in Post #58 which is of a Subaru EZ36 piston and a typical failure mode on these engines. On the 912, this is hard to explain since they are very well proven at high continuous power levels. I have a customer who's been doing flight training on these engines for over 15 years and has over 4000 hours on them with no issues. His last engine had EFI on it and went right to TBO with no work inside.

One unusual thing I see is that a large portion of the #1 and a smaller portion of the #2 rings seems to be missing. As you can see in photos 3 and 4 in post #58, the rings are intact, even though in photo 4, they are completely unsupported. I've never seen rings melt or break in this sort of failure or even from pre-ignition as the melting points of the rings are around double that of the aluminum. Did you find any large ring bits in the debris? It looks from some of the impressions in the piston crowns that hard, sharp edged objects made some of these marks.

Does anyone know the ring material used on these engines? Are the pistons cast or forged?

Is it possible to get a clear photo of the edge section of what is remaining on the 2 top rings?

Any screws or other bits missing from the carbs?

This is a very unusual failure mode but there has to be a reason for it.