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Autogas aging

Ex Bonanza Bucko

Well Known Member
I read that auto gas with ethanol will lose octane after about three weeks because the ethanol is used to boost octane and it then combines with moisture in the air and thus reduces the octane. The lower octane than can produce detonation in the Rotax engine.

If this has happened to you please tell us. Also please tell us how long you have, or can, store auto gas before it does this.

EBB
 
I am not a chemist, but MTBE was used to replace tetraethyl lead in autogas as an octane booster / anti-knock compound. My understanding is that MTBE has a limited shelf life of about 30 days before it starts losing oomph (technical term).
 
Not in three weeks it won't. Many don't worry under 6 weeks. That number can have some individual preferences depending on what school of thought you subscribe to. Water absorption in fuel out of air is way over stated in our case.
Think about your fuel system. The fuel in the tank is not fully exposed to air. First you have a tiny vent in your fuel system somewhere to prevent pulling a vacuum during use. That may be around 1/8"+/- for different aircraft. Fuel is subjected to OAT and above -45F +/- it produces a vapor pressure. Since we tend to have fuel at 40F-90F depending on the time of year and where you live it off gases and produces vapor pressure. If you don't believe that put 5 gal. of fuel in a plastic fuel can with the tops on tight and watch it swell in warm weather. Crack the fuel can vent and it hisses out fuel vapor. With the fuel off-gassing out the tiny vent how in the world is that tiny vent hole sucking in so much moisture to contaminate a tank of fuel. Fuel contamination from moisture through your tiny fuel vent hole gets over stated. Even if a slight amount of moisture could get in then its volume would be insignificant within your total volume of fuel.

Our overall goal is to not let enough octane degrade to a point where we could have such a low octane to have detonation with our high compression engine. You may get away with a lower octane at times, but if conditions are just right it happens so fast your done before you even know it happened. I have numerous pictures from Rotax cylinder detonations and it is an expensive fix.

If you are going to leave a plane sit for really long periods with auto fuel either add some 100LL or drain it. It is important not to forget the carb bowls. The super tiny hole in the idle jet can clog easily with dried fuel.


If you don't need to take chances with your engine why roll the dice.
 
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