Actually it’s completely relevant as some of the fuel specs for use in aircraft are in AKI, for example those specified by the EAA and Petersen STCs that allow use of auto fuel anywhere, worldwide.
Furthermore, the total confusion showed by e.g. Bristell and others who should know better is plenty of reason to discuss the issue completely for others who want to know more and better and not live in a bubble of spoon fed misinformation. The latter is indeed a European tendency, but one would hope that a more intelligent discussion would take place here.
Silvaire wrote:
ot live in a bubble of spoon fed misinformation. The latter is indeed a European tendency, but one would hope that a more intelligent discussion would take place here.
You are constantly reminding us that everything is better and more rational in the USA and that Europeans are deluded. We get the point. Can we move on?
@Airborne_Again, the problem with that POV is that if you want to have a conversation about aviation based on facts (as I do), with examples, and not a political conversation designed to prevent offense or pursue an irrational agenda, the implications of the facts become clear and the conclusions don’t change unless the facts change. That’s actually a good thing, unless you’re a politician. It has nothing to do with US versus Europe specifically.
Silvaire wrote:
based on facts
The fact is that MON was something Germany started to specify in the fuel standards when it was discovered that driving WOT on the Autobahn started to destroy engines in the 60s (when engines started to use higher and higher compression, more power). RON was historically the only specification from decades before. MON is similar to the aviation standard, but it’s not the same AFAIK. In a sense, European gasoline of any RON value, is standardized to run WOT on the German Autobahn without any adverse affect caused by the fuel. This has been the case since the 60s. The actual MON value is a redundant and uninteresting number, because it is in the standard. The RON value is not, because it is (or was) engine dependent, high/low compression engines. The AKI number on the other hand is very much useless, and obviously a political construct rather than an engineering one.
What you said was that in Europe the highest number was picked due to marketing reasons. While this is a plausible and a peculiar “fun fact”, it’s still just nonsense. The highest number was “picked” decades before, and stuck. MON is only in the fuel standards, and useless for everything else. It’s not something you need to know about, and completely useless for driving in most of Europe, at least driving legally. But it is good to know that running WOT on the Autobahn does not destroy the engine due to the fuel.
Similarly, while Bristel sounds like they don’t know what they are talking about, Rotax themselves has issued the same warning. You cannot run a high compression Rotax engine on full power (WOT) at all speeds with UL91. The same goes for mogas 95. Only 100LL or 98 octane mogas will do, as well as unleaded Hjelmco. It’s not that UL91 will destroy the engine, but it can if you don’t pay attention, especially with a constant speed prop. If this is due to low MON or low RON, I don’t know, and really don’t care. The only thing to understand is that UL91 is not a drop in replacement for 98 mogas.
The only thing to understand is that UL91 is not a drop in replacement for 98 mogas.
What is physics (or voodoo) behind this statement?
It’s like saying that 91 < 88? or saying 88 and 100 are ok as well as every mix in between are good while 91 is really bad?
If you look at SI1070 by Lycoming you will not find a single engine that take EN228/SP98 and can’t take UL91, same in Rotax, Conti or others, these are internal combustion engines not fusion engines 
SP98 has a MON of something like 87 (it’s written on the pump). I definitely don’t think it’s better than UL91 which has no ethanol.
The actual MON value is a redundant and uninteresting number, because it is in the standard.
If it’s actually redundant, then it looks like everyone should use RON everywhere just for simplification. Engines are not burning up anymore because manufacturers include actual MON in their testing and recommendation. Marketing uses RON, engine manufacturers une RON. So it’s really UL91 that’s at fault here. And it’s probably a big factor as to why people don’t use UL91 much. It’s better in almost every aspect than SP98: MON of 91 instead of 85-87, no ethanol, possibly even much lower tolerances in the fuel composition (since it’s aviation fuel). Judging by the confusion in my own club when talking about UL91 I think it was a very big marketing mistake.
I’ve seen some use the UL91/96 name, which is slightly better, but still confusing. UL96 would have been much better.
Ibra wrote:
What is physics (or voodoo) behind this statement?
RTFM ! Rotax have issued a SB or was it 2 or 3 about this (they are all on the Rotax web site, but cannot be bother to find them right now). They have tested with different fuels, and their conclusion is crystal clear. 98 mogas and 100LL (as well as Hjelmco I would think, but they haven’t explicitly tested it AFAIK) are the only fuels that allows you to run a high compression variant of the 912 series engine at any loads and at any speed without some amount of detonation.
The exact cause? Low RON, low MON or something else? Low AKI? I don’t know, and it’s not all that interesting either. Warter with their new AKI93 has made the “correct” fuel for Rotax by the looks of it. Only time and experience will show though. It doesn’t look that it’s better in any way than Hjelmco 91/96, and it’s RON97 instead of RON98.
Rotax have issued a SB or was it 2 or 3 about this (they are all on the Rotax web site, but cannot be bother to find them right now)
Without science behind, it’s superstition 
PS: I have not seen anything that says that the high compression super powerful turbo Rotax can’t use Avgas…
LeSving wrote:
The only thing to understand is that UL91 is not a drop in replacement for 98 mogas.
I did not say it was. But from the aircraft’s manual:
LeSving wrote:
They have tested with different fuels, and their conclusion is crystal clear. 98 mogas and 100LL are the only fuels that allows you to run a high compression variant of the 912
The engine in the above POH is a 100hp 912 S3, does it count as a “high compression variant” ?
The below line also mentions EN 228 which is also commercially known as “SP95” (without ethanol), and has definitely worse octane ratings (whatever the metric) than UL91.
EDIT: now I can’t find the reference to the RON of UL91… It’s not present anywhere in the norm.
It’s not something you need to know about, and completely useless for driving in most of Europe, at least driving legally. But it is good to know that running WOT on the Autobahn does not destroy the engine due to the fuel.
Saying that something is “completely useless” and then providing an example to the contrary is not helpful to making whatever point is trying to be made here.
MON is relevant for rating pump gasoline for use in motor engines because one needs a fuel that works for the full range of requirements for an engine in any possible phase of operation, considering the operating environment of an engine, not just a laboratory. That’s why the test was invented, and why it is relevant when you buy gasoline at the pump.
