Significant problems with Rotax engines?

Anyway, after the previous ones, I have very little faith in the technical veracity of BRM‘s service publications. The really seem to be treading in the dark.

I’ve had the same impression, and found it disconcerting. This time I expected it.

Sure, the current empirics might lead to this conclusion for now.

Correlation is not the same thing as causation.

Correlation is not the same thing as causation.

That was the first thing that popped up in my mind as well. It’s puzzling on many fronts.

boscomantico wrote:

I have very little faith in the technical veracity of BRM‘s service publications

I have the same impression, I know some B23-912 with powerloss on take-off running on Mogas, with no access to UL91.

That UL91 has too low octane is nothing new. This has been reported before. When I had the owner/maintenance course at ULPower in 2019, the lead engineer warned against using UL91 due to low octane values. The problem being that the (real) octane values were lower than the specification would lead you to believe.

What the root of the problem actually is, simply poor quality fuel or some other odd reason remains to be seen.

IMO, there are only 2 usable fuels. 100LL or Mogas 98 (free of ethanol). Probably also Hjelmco is OK? I have heard of no problems in Sweden (this doesn’t mean there aren’t any of course). It’s the same spec octane numbers I believe.

LeSving wrote:

That UL91 has too low octane is nothing new. This has been reported before.

Where? Official source please.

europaxs wrote:

Where? Official source please.

No official sources this time This is what I have heard from several places, and I have mentioned it before. ULPower tested several fuels and compared them regarding knock tendencies in their engines. Could be explained with bad batches, their engines have some “oddities” or whatever. But, these B23 episodes is yet another instance (which looking at those isolated can be explained by lots of other things I’m sure).

When it quacks like a duck and so on. It may be that the true octane level (anti knock resistance in real engines is probably more correct), is right there on the borderline. Theoretically, in the laboratory it’s probably OK, but not OK in every circumstances in a practical application. This simply isn’t good enough when Mogas 98 and 100LL have none of these problems, and this UL91 is supposed to be a drop in replacement for both.

It will be interesting to see where this goes UL91 is, according to spec, 91 MON and 95 RON. This is in theory substantially better than Mogas 95 which is 95 RON and MON in the 80s.

The other reason might be vapor lock. But if UL91 theoretically is better than Mogas 95 regarding knock, it should be even better regarding vapor lock in comparison.

Isolated the B23 episodes might just be coincidence. 2 is not a large sample by any means But, you still have to explain why it happens with UL91 and not with Mogas. It makes no sense at all, unless there’s something fishy with that UL91.

LeSving wrote:

Probably also Hjelmco is OK? I have heard of no problems in Sweden (this doesn’t mean there aren’t any of course). It’s the same spec octane numbers I believe.

I haven’t heard about any problems either. I know of several clubs that run Rotax 912 engines on 91/96UL.

LeSving wrote:

But, you still have to explain why it happens with UL91 and not with Mogas.

Sorry @LeSving but there’s absolutely nothing new here. You still have no official sources for your claims. UL 91 is Avgas with certain specifications like 100LL. If you had followed the thread you’d have noticed, that there were indeed issues with power loss in B23s with no access to UL91…

Hard to draw conclusions from incomplete reports of engine “failures” with Rotaxes and mostly while on takeoff runs. I´d like to know statistics about engine troubles on high wing or low wing types. I cannot tell much about ultralight aircraft which may mostly have fuel tanks in the fuselage at about carburettor levels ? With tanks in real high wings I´d not expect problems from vapour lock as all fuel is gravity fed into carbs, no pumps required here. This is quite unlike with wing tanks in low wing aircraft where you are advised to switch on electric pumps for take off runs. Don´t ask why I know. You may be lucky with avgas or 40 years ago with super gasoline like it was then. But today you better have a feed pump on a low wing tank close to the tank for serving fuel to the engine driven pump – or you will see cavitation in the standard pump at max power. It is exactly there in the pump you get that “vapour lock” cavitation with modern fuel, alcohol or not , not the question. It is all other contents in the fuel like acetone, butane, toluol and all , very different to old gasoline. As to octane rating, on car engines they have knock sensors that control ignition timing, seems these are sort of microphones in engine blocks listening for unusual noise and retarding ignition accordingly. I wonder if one could have sensors for own tests ? Anyway, even with heavy knocking an engine will not stop running , it may hole a piston after a while but accident reports here appear to tell dramatic loss of power , unspecified so far. So I suspect fuel supply troubles as mentioned above. Vic