LDB

This may belong in one of the techie forums, but there’s a lot of talk about octane in the general sections, with some of it speculating about avgas as an octane source. So I’ll start out here and let the moderators move it if they wish.

The most common avgas I’ve seen discussed is 100/130LL, with the LL meaning low lead. The octane rating methods are different than those for street fuel and don’t translate very well unless you actually test the fuel. But the (R+M)/2 street rating of 100/130LL is typically in the 102-105 range. There’s the obvious problem of lead with 100/130LL, and even without the lead, it wouldn’t meet regulations for street use. But lead is the biggest issue, making it a total no-go with cats, and having the lead poisoning problem even without cats.

Recently, the first unleaded avgas was approved, called G100UL. While not widely available yet, it was mentioned in another thread as a possible future route to more octane without the usual avgas lead problem. I didn’t know much about it, so I looked into it. The big problem in making a fuel of that octane without lead is how to come up with the octane. Of the pure hydrocarbons (meaning carbon and hydrogen atoms only, with no oxygenates), only aromatics like toluene and xylene have octane ratings significantly over 100, and for various regulatory and performance issues, it’s nearly impossible to blend a reasonable fuel with more than about 40% aromatics. The best non-aromatics are isoparaffins, such as alkylate, but they are only about 94 octane. Thus a 60/40 blend of alkylate and aromatics is only going to make 97-99 octane, depending on the exact alkylate and aromatic composition. So where do they get the remaining octane? With low lead avgas, that’s easy. The lead.

With a 20 minute internet search, I found a draft MSDS for the unleaded G100UL fuel, and as I would have guessed, it said 40-60% alkylate, other isoparaffins totaling 5-25%, and various aromatics (mostly xylene) totaling 20-50%. It did not mention MMT or aniline, which are the only, at least semi-legal compounds that I know of which can boost octane several numbers by adding a small amount. So I was still scratching my head about where the octane came from. Then I noticed that one of the lines my eyes had originally read as toluene was actually 0-6% toluidine. Ooops, toluidine is not a refinery product like everything else mentioned on the MSDS. It is a chemical that is closely related to aniline, with just one more methyl group hanging on the side of the aniline molecule. So that’s undoubtedly where the octane boost is coming from. Trouble is, aniline is highly corrosive to fuel system components such as fuel pumps, injectors, and even the lines themselves. I do not know anything of substance about the corrosivity of toluidine, but with molecular structure so close to aniline, I’d certainly be worried about it if I were a pilot considering such a fuel. One would hope they have done their homework, and toluidine is either surprisingly and substantially less corrosive than aniline or they have some other corrosion mitigating approach. But I’d want to know the facts in that area with certainty before putting it in my car, or if I owned one, airplane.

John Harry

Thanks for the analysis. Your posts are always informative.

Factoid

I would leave this fuel to the C1 and C2 crew where a little lead in a high compression engine that doesn’t like ethanol but loves octane is a good thing (L84).

Maxpowers

Thanks for sharing. Have not heard about this. Interesting.

cheetah45

FYI: The more common aviation gasoline is 100LL(low lead)….not 100/130.
100 LL has a blue tint and reportedly has 6 times the amount of lead that 80/87 had. Over the years, 80/87 (red tint) is less common but much of the Cessna/Piper basic airplanes used it for the longest time. An airplane designed to run on 80/87 would need a lead scavenging agent to help digest the lead content of 100LL.
The 100/130 is tinted green and is not very common. I used it once when I landed in Tremonton Utah as they had no 100LL on the airfield.
There are a few unleaded aviation fuels that have been in development Over the last 20 years or so. Some airfields have the availability of unleaded auto fuel that certified to be alcohol free.

LDB

You are correct. Thinking about it, I don't believe there was ever a fuel called 100/130LL, it was just plain old 100/130. Both 100LL and 100/130 have the 100 lean, 130 rich octane ratings, but 100/130, which is the older spec and is generally unavailable today, had considerably more lead. I had forgotten, but your post reminded me that 100LL is more difficult for refineries to blend than the older 100/130. That’s because the higher lead in the old 100/130 spec gave more octane boost, meaning we didn’t need to be quite as careful about using only the very highest octane components in the old 100/130. The current 100LL spec won’t allow much, if anything lower in octane than alkylate, and even then, it needs the octane boost from its low but not zero lead. That’s why I was scratching my head, wondering where the octane was coming from in the lead-free G100UL. Anyway, post #1 should say 100LL everywhere that it incorrectly says 100/130LL. My bad.

Vetteman Jack

Good read - learned something new today.

dankhts

I used to use 110 av gas in my flat track motorcycle for years when I was ice racing during the winter months but used racing fuel during the regular dirt season... av gas was great on the ice 'cuz, never had freeze ups regardless of the air temp as it has a type anti freeze in it, required for higher altitude flights in air planes..

Ramets

Just the mention of Avgas brought back some great memories when I was in the USAF in the mid 60's. I was in POL(Refueling), and the Avgas back then was 115/145. We used to mix it with regular for our cars. You could always tell who was running it by their white tailpipe. I had a '66 Chevelle SS at the time, and it definitely added some pop to the car. You had to be careful though, you could easily screw up your valves if you ran too much of the 115/145.

Gearhead Jim

In the middle 1960's I was in a military flying club at Wheeler Field, in the middle of Oahu (Hawaii); the hangar walls still had bullet marks from the Japanese attack on Dec 7. We were flying Cessna 150's with the Continental O-200 engine, and I was told that we were using the military 115/145 gas because that was the only avgas available there. I recall talk about engine problems and additional maintenance, but not actual failures. I had no problems with it.

During WWII the US and Brits used some fuels with exotic octane boosters, allowing the Merlin engines to run up to 75" MP where is the normal max was 61"-65". Said to be nasty stuff if you got it on your skin, and tended to load up the plugs when running at normal cruise power. In fact, even the "normal" 115/145 loaded the plugs enough that for every hour in cruise, you were supposed to run at full power for 1 minute and at night you'd see a shower of sparks out the exhaust stacks as you did that. Still sorry that I missed those times.

One P-51 owner who knows a lot about the engines said that when he had to switch from the normal 100/130 octane to the then-new 100LL, his engine people told him to use only 55" of MP for takeoff instead of the usual 61". He did not comment on whether there were detonation problems or something different.