Among the myths that one occasionally hears regarding autogas (aka mogas) is that it is of poor quality compared to avgas. We asked Todd L. Petersen, owner of more than 100 auto gas STCs, to comment on the changes he’s seen in gasoline quality in the three decades since the FAA approved the first autogas STC in 1982:
“Today, some people criticize the auto fuel STCs obtained in the 1980s, claiming that the fuel we have today isn’t the same as it was when we conducted our testing. When I hear that I always agree with them. It’s not the same. It’s better today than it ever was. Gasoline today is cleaner, and the RVP is lower than ever, making it more like avgas than ever before.”
RVP is Reid Vapor Pressure, a measure of a fuel’s volatility. The higher the RVP, the quicker the fuel evaporates, or ignites. In the winter, fuels are blended to have a higher RVP to allow better cold start. In the summer RVP is lowered. Higher RVPs are a problem when an airplane climbs. At some altitude, the fuel will evaporate, causing vapor lock and the engine will cough or quit. Due to advances in auto engines, modern fuel has lower RVP than in the past, nearly as low as in leaded avgas.
“Another issue are the detergents and additives that some companies add to their fuel,” he continued. “These have no effect on octane or on how it burns in an engine and are largely irrelevant. Since they are usually added downstream of the terminal, if airports obtain the pure, ethanol and additive-free fuel there, they can be assured of having a high-quality aviation fuel that rivals all others.
“Remember, too, that when testing an aircraft/engine combination for an autogas STC, we used 89.5 AKI, although the STC specifies a minimum of 91 AKI. We also used a winter blend with its higher RVP in our hot fuel tests, to duplicate a worst-case scenario of a pilot using a winter blend on a hot summer day. So I would agree with those who claim that our fuel today is not the same as that used in our testing — it is far better!
He adds: “The only caveat is ethanol, which is not approved for small airplanes and most likely never will be. We recommend against using gasoline containing ethanol.”
Thanks, Todd!
The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., a pilot, homebuilder and expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.
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Ethanol is bad for airplanes. The problem is one never knows if the car gas we buy has ethanol in it or not without testing each and every time. Relying on what the pump says without testing is like playing Russian roulette.
In fact, up to 10% ethanol is approved in many Rotax engines, and 5% in many Jabiru engines.  Auto engine conversions that are approved for E10 will also be fine, as long as the entire fuel system, from the tank to the cylinders, is designed to handle the problems of ethanol. Aske any Rotax mechanic though and they will tell you that the best fuel for the engine is 91+AKI ethanol-free autogas. Ethanol in aircraft engines is nothing new – tests were already made in the 1930s in the U.S. and several engines have been run on 100% ethanol in ND, LA and in Brazil.  But they come with many requirements, such as a much higher fuel consumption and draining the entire fuel system at night. You can make an engine run on just about any combustible fuel, but in the end, it is pretty hard to beat good old gasoline. Biofuels would not exist without massive government subsidies and mandates, not a secure future for any fuel in my mind. I would be amazed if we still saw large-scale ethanol use in the U.S. in five years, especially with the new shale oil fields coming on line that will cause unprecedented reductions in the cost of petroleum-based fuels.
Dream on brother!
 “The only caveat is ethanol, which is not approved for small airplanes and most likely never will be.”
Yep, with that attitude it will likely never be. Sounds a lot like the predictions that piston engine aircraft would always need 100LL. It’s the same attitude that the Wright brothers fought until they finally flew.Â
Dennis, Do you think a change in attitude is all that is needed to make ethanol a good fuel? My fuel lines in my ’86 Harly were rotted from using gas with 10% ethanol.The dissolved rubber clogged the carb.People with boats have had a huge problem with ethanol attracting water and corroding carburetors, so much so that many boat ramps now have ethanol free fuel.I purchase ethanol free 91 octane racing fuel from a local jobber in 55 gal drums to run in my ’77 c150. I save about $1 a gal.