Mogas and Ethanol

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Re: Mogas and Ethanol

Post by hum » Mon Jan 11, 2010 12:45 pm

I fly an aircraft with 2 tanks and usually have mogas one side and avgas the other. Normally I cannot tell the difference between Mo or Avgas; however, I have had one instance a few summers ago where the engine would not run on Mo, whereas it ran quite happily on Av... Thinking it was a tank vent snag I landed, checked the plumbing and went off again to discover exactly the same symptoms when passing through the same altitude; mind you it was a little higher than one is supposed to go, but only by 1000 ft or so... Everything (EGTs CHTs RPM, power output etc) seemed normal below 5000 ft. on both Mo and Avgas. I can only assume vapour in the line with Mo... unusual as its a carburetted engine, but nevertheless, it happened. I took samples from the container afterwards and tested for ethanol but negative, however, did not have the more accurate 'airworld' kit bluebeard advocates... have ordered it and will use from now on...

Last week flying an aircraft with an accurate carburettor temperature gauge in the cold weather I was interested to note that while using Mogas, even with full carb heat selected, the carb temp would not rise above the 'yellow' zone where icing is possible... When I switched to Avgas the temperature in the carburettor rose by approx 10 Deg C as the Avgas flow took over; now the temp could easily be brought above +5 deg C... apparently it was taking more heat to vaprorise the Mogas....

There are definitely issues to be taken into account when running an aircraft engine on Mogas - even if it is designed to do so. Subtle symptoms due to the factors associated with MOgas are often not noticable to the GA pilot. There are those that say 5% ethanol will not affect performance; I don't know, but I would be prepared to bet it certainly won't improve matters!

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