Rotax Piston Meltdown

[AndrewR]

Originally Posted by DanH

detonation typically takes a while to progress to serious damage

That might be true for a Lycoming, is it also true of a Rotax?

One of the documents I have read on detonation says
"An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes."

A Lycoming is about 0.5 HP/in3, a Rotax is 1.2 I think.

[RV10inOz]

Originally Posted by rv6ejguy

Looks like pre-ignition damage, not detonation. I'd also be interested in seeing the other spark plug electrode.

rv6ejguy = ON THE MONEY

That might be true for a Lycoming, is it also true of a Rotax?

Applies to all spark ignition gasoline engines. ALL of them.

APS/ GAMI many years ago contracted Air Data research to compile all the engine SDR's and crash reports into a massive pile.....yes massive, measured in feet. George Braly painstakingly went trough them all weeding out what was allegedly reported was detonation and what was not, what actually was preignition. The conclusion to this research was best summed as as;

All conforming engines on conforming fuel, only a mismanaged Turbocharged engine could detonate, a NA engine would not.

Preignition however will destroy a Rotax a Lycoming or a Stihl chainsaw. The most common cause is from a cracked ceramic, probably around 90+% with a few %, say 5% from cross firing mags/harness. maybe 1% from helical tangs.

DanH is onto it with a plug failure, being the ceramic as the most likely almost for certain cause., perhaps a poor fit and this not heatsinking.

The fuel was stated as being 91ULP, is that MON, RON, (R+M)/2 ? I am assuming this is the (R+M)/2 or AKI number, which is around 95 RON or 85MON (avgas language)

The likelihood of detonation causing a ceramic failure is low, but a dropped spark plug is very high indeed.

[BillL]

Originally Posted by PilotjohnS

You said you thought it was piston meltdown, but could it be debris inside the cylinder? Just thinking out of the box.

Don't think so John. The debris was an intermediate result of the root cause. The top edge of the piston is clearly melted on the surface. DanH has been asking for a week about the top plug for a reason. Ross has lots of experience with small bore, high power engines. While still no clear root cause it is in the preignition/detonation mix. Could be as simple as a broken electrode insulator.

The static RPM, and cooling shroud are red herring items as Scott has pointed out.

Just my opinion.

[DanH]

Originally Posted by AndrewR

The barrel temperature is not normally measured on Rotax or Lycoming.

Can't speak for Rotax. True at Lycoming; I asked some years ago.

So I don't think the cooling duct is likely to have contributed. (My opinion only, I am certainly not an expert!)

Nor do I. Nor am I. But don't cut yourself short. You post good stuff.

Re .."takes a while to progress..";

One of the documents I have read on detonation says
"An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes." A Lycoming is about 0.5 HP/in3, a Rotax is 1.2 I think.

Note "within minutes". I too hesitate to assign a hard time period, just like the above author, as it depends on severity. In this case, let's see about the time hack units. The RPM plot seems to suggest a very short period at WOT.

All conforming engines on conforming fuel, only a mismanaged Turbocharged engine could detonate, a NA engine would not.

I have repeatedly posted detonation plots from Lycoming (IO-360) and the FAA (IO-540K). A normally aspirated Lycoming will detonate given the necessary conditions. Let's not go there....subject is a Rotax failure.

[rvbuilder2002]

Originally Posted by DanH

In this case, let's see about the time hack units. The RPM plot seems to suggest a very short period at WOT.

I agree. The time period at max. RPM and at 4000 for the run up / ign. check are about the same. It would be unusual for an ign chech to take more than 30 seconds.... and 15 seconds would probably be more typical.

[rv6ejguy]

As someone who's built over 150 turbocharged engines, driven, raced and flown them for the last 40 years and done thousands of dyno pulls, here is what I have observed:

Detonation almost always breaks the 2nd ring land first, 3rd ring land next, almost always with no major crown damage and usually leaving the plug ground electrode intact. The ceramic on the plug may be cracked in some cases. Depending on severity and piston design and material, this damage can take place in 1 second to many hours.

Pre-ignition will melt the piston crown in as little as 5 to 15 seconds and no piston can withstand continuous pre-ignition for even 1 minute, no matter how strong, what coatings are applied etc. Pre-ignition, again depending on severity and duration, most often removes the ground electrode closest to where the event is occurring also.

The higher the specific output, the higher the heat flux on the piston crown. As pointed out, the Rotax has much higher specific output than a Lycoming. This in itself is not a problem on a well designed engine but can be expected to shorten the time it takes for pre-ignition to take out a piston.

Pre-ignition is most likely to occur with high IATs and head/ chamber temps and as Dan pointed out, plug heat range and/or heat transfer through the plug can also initiate it.

[Aggie78]

Put up by Mike Busch:

https://blog.aopa.org/aopa/2015/04/1...y-your-engine/

If you go to the article, good pictures of engine damage and flight recorder screenshots to go along with Mike?s writing.

Worth a read...

[rv6ejguy]

Three different piston failure modes.

Here is what a typical detonation failure looks like on a cast piston.



Note broken ring lands but totally intact crown.

Below. Detonation induced pre-ignition failure on a forged piston with central plug location. Ring lands on the forged piston were able to take the detonation without breaking but it was no match for the pre-ignition that followed. Spark plug ground electrode melted off.



Below, 2 bottom photos. Crown and land distortion below caused by excessive heat flux for the piston design. Aluminum gets to plastic state and combustion pressure reshapes the piston which then loses gas seal, hot gas flow past the piston then quickly melts the aluminum.





These would all give you a bad day but detonation failures typically are more benign and besides a big loss of compression and tons of blowby, the engine continues to run in some fashion. Not so much on the others.

[waterboy2110]

Thanks everyone for your input and wanting to help solve this issue. I will get the head and plug pics posted.

The x axis on my charts is data points I believe. I can change it to time.


While I agree that the 575 RV12’s flying speaks to the success of this plane I think the number is being thrown around and cannot ever tell the whole story. Vans nor anyone else has actual data on how people pitch the prop, what fuel they use, what MAP they see on WOT. Rotax published the SL because failures were reported.

Again thanks. Let’s keep looking. I’ll post pics this week.

[aerofurb]

There seemed to be some intimation earlier in the thread that the RV-12 uses a non-standard air filter set up (dual K&N filters taking air from inside the cowling).

In my experience, this is standard on non-certified 912 installations, with certified installations (Tecnam P2002JF and P2006T) using a single airbox with muffler fed hot air for carb heat.

Seeing as Rotax supply the filters, surely they don't consider it a problem?