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.”
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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