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Adhere to the power settings! Detonation damage in a Rotax 912S3

I’m not convinced about the detonation theory. Things doesn’t add up. Pre-ignition maybe, or the piston being worn out, or some damage due to foreign object that came during plug change (if it’s not broken, don’t fix it ). Do this engine have this “soft start” ignition fix? If not, pre-ignition during (a million) starts could do perhaps weaken the piston over time. But that one single cylinder should get it, while no sign on the others, if the cause was detonation due to max throttle and low rpm, a bit far fetched isn’t it?

It reminds me of a (trick) question the instructor at the ULPower factory asked all of us. When do the engine operate at max power? We all thought about HP, and rpm curves etc. He just said, forget about all of that. The engine operate at max power whenever the throttle body is at max opening. He was of course right. but perhaps not in the way we first thought, or in the way most people think about it. The engine having an ECU controlling spark and fuel mixture, will operate at max power whenever the throttle body is at max opening, regardless of rpm.

Last Edited by LeSving at 13 Nov 08:30
The elephant is the circulation
ENVA ENOP ENMO, Norway

Sorry for staying quit for so long, but life got in the way, but I can finally add some more info.

Regarding the amount of metal in the oil filter you are right, @dan, that this might look okay if it were a Lyco or similar. But for a Rotax nearing TBO and after one hour of oil use this was simply massive. You usually see way less chips at the standard 100h oil change. I have attached the oil analysis records for this engine, in case anyone might be interested:
Blackstone said they could not determine a definite cause for the damage from the oil data alone.

The engine was operated on Mogas EN228 RON98 for most of its life. Verey rarely someone flying away from the field had to put in a sip of 100LL, that is where the residual lead comes from.
Since you all seem interested in this case, I have some more photos for you. First, the cylinder head with the valves and the second spark plug still in place:

The plugs used are the genuine Rotax ones. No lawn mower involved ;) Of course everything is very oily in there, the cylinder had a direct connection to the crankcase the original designer had not planned for. But the valves themselves were closing properly, and the seats were undamaged. (No picture on this, sorry)

Regarding the piston damage I have these other pictures:


As the other cylinders still had normal compression, damage there cannot have been as severe as in #1. Because the culprit for the lack of compression and metal in the oil has been found, further dismantling of the engine was called off. Instead, it was removed from the plane as soon as possible to send it in as core in exchange for the new engine that had to be installed ASAP to prevent excessive accumulation of hangarage fees, and to get the primary trainer back into the air. So unfortunately I do not have access to the engine any more and cannot take any additional photos. That also means I cannot provide any additional infos on the second spot @Malibuflyer mentionned.

Regarding the ECU you are, in this engine, only partially correct @leSving. This is the bog standard carburetted version that does have electronic ignition with the great soft start feature, but mixture control is handled by the constant depression carbs all on their own. The pilot does not have any possibility to interfere with that apart from closing the fuel valve- Rotax engines do not have a red lever. The newer variants with electronic injection of course have electronic mixture control, a knock sensor and an internal data recorder. Sadly this engine did not have any of these, so some of the questions have to remain unanswered.

Detonation was the standard cause offered by the engine manufactureres representative. According to them they get this not often, but still regularly, with C/S prop installations. Since the engine had to come off anyway, we accepted that and took this as basis for an emphasis on engine handling procedures during inital and recurrent training.

EDXN, ETMN, Germany

CharlieRomeo wrote:

The drained, one-hour-old oil

Thanks for the comment, I think I had not read properly and realised the short run time. In this case the amount of chips would also be worrying for a bigger engine.
The additional pictures are impressive. I will use some in my next maintenance course to demonstrate the need to properly look at the piston crown whilst borescoping during an inspection. Again, thanks for sharing.

Dan
ain't the Destination, but the Journey
LSZF, Switzerland
What is obvious when looking at the piston pictures is that all ring gaps were NOT staggered but in line . This might add to the extra blowby from piston top down into crank case. That would heat up the piston at this place , more so with worn rings and wider gaps. Once you have a lot of blowby down it heats the piston to melting temps as can be clearly seen. So the last mechanic did not check ring gaps spread around the piston for even spacing – no great idea which can lead to more troubles like uneven wear on bores. Vic
vic
EDME

@aart wrotr

Here’s the chart to use for Rotax 91x operators with a C/S prop.

Acc. to the chart, accounting for an approximate loss of 1“ MAP per 1000ft (normally aspirated engine), above ~4000ft it shouldn’t matter at all which MAP (power lever) vs. RPM (prop rpm/pitch lever) combination is set, correct? E.g. even an „extreme case“ of 27“ MAP and 4400 RPM (ca. 2000 RPM at the prop) shouldn’t cause detonation?!

Also, the chart does not apply to the 912 80hp version.

So I’m wondering, is the 80hp version less or not susceptible to detonation because it was the original design (whereas the 100hp version is basically a maxed out version of the 80hp one with higher compr. ratio/bigger bore)?

Last Edited by Snoopy at 30 Apr 19:06
always learning
LO__, Austria

Acc. to the chart, accounting for an approximate loss of 1“ MAP per 1000ft (normally aspirated engine), above ~4000ft it shouldn’t matter at all which MAP (power lever) vs. RPM (prop rpm/pitch lever) combination is set, correct? E.g. even an „extreme case“ of 27“ MAP and 4400 RPM (ca. 2000 RPM at the prop) shouldn’t cause detonation?!

Correct and Rotax uses 3.500 ft as a reference value below which the chart applies.

Also, the chart does not apply to the 912 80hp version. So I’m wondering, is the 80hp version less or not susceptible to detonation because it was the original design (whereas the 100hp version is basically a maxed out version of the 80hp one with higher compr. ratio/bigger bore)?

I checked and found that it indeed does not apply for the 80 HP, see below, first NOTE. As you say it prob has to do with the lower compression ratio of that engine.

Private field, Mallorca, Spain

Probably unrelated, but there are more and more stories of “bad fuel” out there. The reason being (indirectly) ethanol, and it leads to detonation problems. Mogas 95 RON is a blend of gasoline and 5-10% ethanol. The ethanol is in principle only good for the engine, as it raises the octane level. The problem however is this:

Ethanol, being a light substance (more volatile) tends to evaporate more easily than heavier hydrocarbons. If left in a sealed container, no problems, but when left in a ventilated container, like the fuel tank of an aircraft, the ethanol will eventually evaporate. When the distributers mix the fuel, they use cheap and poor quality gasoline (low octane), then add ethanol to get the octane level up again. This will enhance the problem further, because it makes the difference between the volatility of gasoline and ethanol even larger. The original fuel without ethanol has a very low octane value. When left in the fuel tank for a couple of months, the fuel will have degraded. Exactly how much is dependent on several factors, but it is certainly not like the original RON 95 that was filed into the tank two months ago.

Now, to make the confusion complete, some producers, like Shell (at least around here) does NOT necessarily add ethanol in any of their fuel (could be 0.1%, could be 15%). The “E” content is usually something entirely different, and as a bi-product of pulp. It’s this. It’s basically gasoline made from wood. Even the 98 octane V-Power has up to 15% of this stuff, and it is a drop-in replacement for gasoline. What the octane of this 15% addition is, I don’t know, but since it essentially is gasoline, I would think the properties are mostly as gasoline (with a faint odor of Finnish pine forest ) V-Power has max 0.5% ethanol though, so it looks to be a “safe” fuel.

The elephant is the circulation
ENVA ENOP ENMO, Norway
27 Posts
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