Rotax auto fuel

[dick seiders]

I believe this may have been discussed already, but can't find it, sorry.
Madison Airport (52A) only 19 mi. from my home base (9A1) has installed a tank and is going to start selling car gas that is rated for aircraft. It is (guarenteed) 90 octane with NO ethanol, and is called Recreational Aviation Fuel. The airport is run by the county so will add only 15 to 20 cts. more than 91-93 sold at gas stations that does include ethanol of course. My question: is the 90 octane rating sufficient for the Rotax 912 ULS? All the info I have seen usually calls for 91-93 oct. Don't know if this is due to fact that is the state (Fed?) gas station rating for cars, or is it because it is the min. for the 912 when referred to for our engines? I'd like to be certain as I don't want to use an octane that may detonate in the engine.
This issue becomes more important down the road as I hear that our omnipotent Gov/EPA is planning on boosting the ethanol from 10 to 15%, or maybe even 20% in the not too distant future. Interested in comments please, and thanks.
Dick Seiders

[Jetguy]

The minimum Octane is 91 per the Rotax Manual. Anything less could cause problems. The information you have received from the airport management does not seem accurate. Here in Texas in smaller counties where non-ethanoll fuel can be found there is no octane rating of 90. Flying to OSH twice in the last 2 years I have never seen mogas at an airport with a 90 octane rating. Only 87 or 91 for non-ethanol fuel. Maybe some one with more experience in Mogas can speck to this matter.

[bhassel]

I've seen some mo-gas locally that is only rated at 90% octane and some at 91% octane. That's with ethanol of course. Thankfully you can run mo-gas with ethanol (up to 15%) in the Rotax.

Bob

[fmiddleton]

Given that it's billed as an aviation fuel, I'd be curious if it's 90 MON or AKI. 90 MON is closer to 94 AKI (which is what you get at the regular gas pump).

[dick seiders]

Have done much reading in research on web about recreational fuel for LSA's. More confused than ever. There are a lot of data out there, but no guarantee that the info is supported by scientific fact as near as I can determine. Some are referred to as 92 to 97 octane E0 fuel, some are stated to be 91 octane E0 fuel, some are 90 octane E0. Seems to me a lot of data is available as a wish list, but can find no hard evidence as to why all the spread in octane on the ethanol free Rec. fuel. At this point I choose not to experiment with my fine running and expensive Rotax engine. I will conduct further inquiries of airport manager at 52A as to why the new fuel is 90 instead of 91 to try to find a fact based answer. Failing that will make an effort to contact the producer/supplier to seek an answer.
Until I am totally satisfied I will continue to go to the gas station for my 91-93 premium motor fuel. If and when I learn anything of substance I will post the findings.
Dick Seiders

[N1593Y]

If the fuel is being accurately identified as "Recreation Fuel" it is undoubtedly a product of Marathon Oil that markets 90 AKI Recreation Fuel for marine and off road use in many states in the face of the ethanol onslaught. Here is an announcement about it on the Petersen Aviation web site. Marathon Oil does not want its ethanol free fuel used in aircraft and they state that emphatically on their web site. It turns out that 90 AKI mogas satisfies practically all marine and off road uses, but does NOT meet the needs of the Petersen high compression STC's nor the Rotax 912. It will be perfectly legal for the EAA and Petersen low compression STCs.

It is unfortunate that any airport would select this product for sale to the aviation community, because it does not meet every pilot's needs and because of the attitude of the producer towards aviation. If I was a pilot in the vicinity of this airport, I would urge them to find a different distributor that will deliver ethanol free 91 AKI gasoline. I wouldn't be surprised that if Marathon finds out about this they will lean on the distributor to cease delivery to an airport, even though it would be illegal restraint of trade. I am only aware of one airport that ever stood up to an oil company and their distributor and put in mogas over a distributors objection.

[NASA515]

Without trying to muddy the waters (Oh! why not?), I went to Repairman's School with Jim Scott in Arlington, Wash. Jim is a VERY experienced Rotax user and mechanic.

We had quite a discussion about fuels - Auto vs Avgas. I'm paraphrasing his comments and hope I do this accurately - but I wrote it all down clearly and reconfirmed.

What he said was that - on teardown - engines that had run MoGas were CLEAN; engines that had run 100LL were DIRTY, and engines that had run about 50/50 MoGas/AvGas were EXCEPTIONALLY CLEAN, i.e. CLEANER than those which had run just MoGas. He ascribed this to the higher octane in the 100LL allowing for a hotter, better fuel burn than possible with just the MoGas, while the MoGas reduced the overall lead content to a level below which the lead was contaminating the engine with deposits.

Based upon that, I do not fear running 100LL as much as when I started down this road. Actually, I am seriously considering running the 50/50 blend as the normal and preferred fuel load.

Bob Bogash
N737G

[N1593Y]

Dick -

There are many "octanes" in the fuel world, Iso, MON, RON and a couple of others. But when it comes to auto gasoline, octane is not used, Anti-Knock Index is used, AKI. AKI is the average of the MON and RON spec for the particular gasoline. That is why you see a label on the side of every gas pump in the U.S. which must list the minimum AKI and under it it will say something like (RON + MON) / 2.

There is a difference between "octane" and AKI. Avgas specs refer to MON octane only, 100 LL has a 100 MON octane lean rating, and an unexpressed 115 octane rich rating, hence the old 100/115 rating years ago, when we also had 80/87 and 91/96 and 115/145, etc. Today we only use the lean octane rating when designating avgas, hence we have 100LL, 82UL, 87UL, 91UL and 94UL today, although none of the UL versions of avgas are in production in the U.S.

When you see an "octane" spread on auto gasoline it is usually referring to the RON octane which is always higher than the MON octane rating for gasoline, so it looks better. Auto gasoline in most of the rest of the world is reported as RON.

All aviation STC's list the minimum AKI necessary and the LSA engines do also for mogas operation in the U.S.

[dick seiders]

Thanks all for the feedback thus far. I am interested ion the 50 - 50 mix of mogas and 100ll. Sounds logical. Raises a question: If burning half 100ll and half mogas I suspect the 90 octane E0 Rec Fuel would be ok to use in that the overall Octane (and AKI) would be sufficiently raised to meet the Rotax minimums?? Question 2: How about half&half with 87 mogas? Does anyone out there have the answers? Sure would like to know. Has anyone with a rotax in a 12 tried the 50-50 mix yet, and if so let's hear from you. Of course mixing the fuel is even more of a hassle than going to gas station and hand pumping from portable bottles in that after filling 1/2 tank one would have to take the bottles (or the 12) to the 100ll pump, etc etc. Then there is the increase in fuel costs as well. Very interesting idea tho.
I plan to call the City Mgr at 52A and ask why they aren't insisting on 91 octane Rec Fuel. I suspect they aren't aware of the difference. I hope to find out if they are interested or even care a whit. We'll see.
Dick Seiders

[MartySantic]

Dick,

There is a document on the ROAN (www.rotax-owner.com) website entitled "Avgas and Autogas Mixing".

I'll e-mail you a copy.