What is the MOGAS composition control and quality control like, compared to 91UL?
How much does a Lycontosaurus care? If it’s good for high tech car engines (the BMW i8 has a 3 cylinder 1500cc twin turbo engine with 250hp, that is impressive technology!), it is good enough for our stone age low tech engines. Ethanol content is easy to measure and airfields do that.
Mogas is not aviation fuel.Quote
Well, except in my C150 for the past 28 years, and a few ten thousand other GA aircraft in North America…
Any engine can be modified to run UL91. The only thing needed is to lower the CR. Worst case scenario is you lose 3-4% max power. The truth is, this is a bureaucratic problem (certifications, regulations) that has no cost effective bureaucratic solutionQuote
It is also necessary to assure that the airframe fuel system handles the new fuel too. There are certification requirements for both the engine and the airframe. Lowering the CR requires some effort, and could require changed pistons, which would require certification. A 3-4% power reduction makes all the aircraft performance charts invalid, and does horrible things to information, or even basic compliance for twins. A nice idea, but there’s still some effort required.
Mogas can be any hydrocarbon composition. The only specification is the RON number. Nothing about water content, how well it stores, ethanol content, or whatever. For this reason Mogas is a fresh delivery. A day old mogas can (and is) a different product than a month old mogas.
Avgas has specifications about RON and MON, and also content, storability, vapor pressure etc.
My view on this is: UL91 has the specs of avgas (100LL) with the lead removed. The RON is 91 and the MON is 96 (or the other way around)
UL91 is also mogas with the added specs for aviation use (storability, content, vapor pressure, RON/MON values)
And yet car fuel is totally fine for aero engines so you’re making a point to make a point, LeSving.
Is there any evidence – even apocryphal – coming from high-altitude countries (e.g. Bolivia) of car engines stalling due to Mogas vapor pressure?
No but there are the millions of cars not used in winter that wouldn’t start in spring because the terrible Mogas decomposes after one month!
Obviously the Lycontosaurus is a very demanding high-precision machine engineered to the ultra high fuel standards of 1950.
UL91 is also mogas with the added specs for aviation use (storability, content, vapor pressure, RON/MON values)Quote
These characteristics are also specified for Mogas, though they are different from Avgas. I can assure you that the demands for gasoline properties for today’s car engines are much more critical than for Lycomings or Continentals (LC’s).
Ethanol is a tiny to not at all consideration for most LC’s. I flew an O-200 equipped C 150 for years on pure ethanol, for a Government of Canada test program. It worked perfectly fine. Where ethanol challenges aircraft operation is the airframe fuel system. This is for non metallic materials compatibility, and for different corrosion characteristics. It was going to be difficult to approve in aircraft because of the lesser energy density (so greater fuel flow) and what that would do to the performance charts, particularly when mixed with gasoline.
Vapour pressure, and vapour lock are more issues, but again, very much more with the fuel system in the airframe, rather than the engine. If uncontaminated gasoline of adequate “octane” is reaching the engine at the required flow, the engine is going to run fine. It’s containing and getting the gasoline to the engine from the airframe, where the issues lie.
Yes, Mogas does “boil off” and reduce vapour pressure with time, temperature and ambient pressure. When it boils off, the vapour pressure of the Mogas will become similar to that of Avgas, and become less vulnerable to vapour lock in the airframe fuel system. Therefore when I buy winter Mogas, (1250 liters last week) and store it into the summer (when I don’t want the weight of the fuel delivery truck on my grass apron), it works fine. Yes, it boiled off, and I lost a bit of volume, but I have never had a problem. We measured the vapour pressure of the Mogas before flying with it for years, but eventually gave up, as we never found troubling properties.
I am a firm believer in Mogas use in approved aircraft/engine combinations, and well over 4500 hours of trouble free flying in various Cessnas backs that up….
Continental and Lycoming moving toward certifying on 91UL is simply the recognition that they have always promised that when the need and market demand were there, they’d approve their engines to run on it. Now they have – but don’t forget the airframe side of it!
Nothing about water content
That’s really useful… 
Of course most people will say they never go that high, but that is an entirely different argument. It just means that there will be few if any complaints from the currently MOGAS burning fleet, of which at least 99.9% never go above say 3000ft. If you look at it from the other end (airport fuel sales) then you can’t do much more with MOGAS than is currently done.
coming from high-altitude countries (e.g. Bolivia) of car engines stalling due to Mogas vapor pressure?
Probably not because all the car makers fly their cars down there (or “up” there…) for tests, to make sure they still run at 14000ft and +30C. But modern cars have ECUs with altitude compensation, and most of the time they run at about 20% of max HP so if you lose 50% nobody will notice anyway, and the fuel system is different anyway, so one cannot use this as a direct comparison.
Both my planes are certified to run on auto fuel, both ran in it in the past, but will neither will likely ever run on it again because of (not allowable) alcohol in the auto fuel I can actually buy. 91UL would be a nice solution for my planes, although I have no particular problem with 100LL. Using the right spark plugs has prevented any problems with lead fouling.
I wouldn’t use fuel with alcohol even if it were certified. I run 13 vehicles and obviously not all of them get used every day or even every week. Modern auto fuel with alcohol is terrible stuff for storage in my experience and 100LL is much, much better. 91UL would be the same, it’s the same fuel minus lead and I’d like to run that in ALL my vehicles.
Re fuel requirements from first principles, the higher the octane the lighter and simpler the engibe can be. Larger cylinder volume requires higher octane, as well as the other more discussed factors like compression ratio etc.
