Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985.
Asking about the storage life of 100LL and auto gas is like asking a doctor, “How old will I live to be?”
For example, a person’s life expectancy is affected by his or her genes. Likewise, a fuel’s storage life is dependent on how it is made and whether or not it is bottomed like 100LL. Our life expectancy is dependent on what we eat. A fuel’s storage life is dependent on what is put into it, such as detergents, additives, etc. A fuel’s life can also be affected by the type of container (a lined drum or the tank of an airplane stored outside), the temperature and temperature variation under which it is stored, the humidity of the air, and on and on.
And then there is alcohol. Ethanol in airplane fuel is a bit like alcohol in a pilot. It may not stop the engine immediately, but it certainly increases the risk factor.
But what limits the life of a fuel? One problem is evaporation of light ends in the fuel. This is a concern with small engines, especially 2-strokes, and may make it almost impossible to start. I have a 2-cycle dirt bike, and if it sits for a month or two, I have to put fresh gasoline in and drain the carb to get it to start.
But the major concern with aging fuels is gum formation. Over time, heavy ends and additives can start to form gums, which can plug up or, dare I say, “gum-up” a carburetor or fuel system. This can lead to bad fuel distribution or even an engine stall.
The point I am trying to make is that the storage life of fuels is just a guide. For 100LL, under normal conditions, it should be safe to use for at least a year. For Mogas, a normal life is about six months. But both of these can be shortened or lengthened by many factors. For the storage life of fuel with ethanol, there is no good answer. Under ideal conditions, it should last six months. But if there is any moisture, six months may be too long.
For automotive use, you can try a fuel stabilizer to help extend storage life. I use an additive marketed by Briggs & Stratton for my old tractors and it works well. But remember, these additives are not approved for aviation use.
Which brings up another question I get, which is why can you legally add alcohol as a fuel system anti-freeze, but ethanol is such a concern? It is true that the engine manufacturers do allow the addition of a certain percentage of Isopropyl Alcohol or IPA (usually from 1 to 3%). They do not allow any ethanol or methanol, so never use an automotive anti-freeze.
Not all alcohols are created equal and they each have different characteristics. For example, ethanol and methanol can drop out when they become saturated with water. This leaves a non-combustible mixture in your carburetor.
Always check the appropriate service instructions for your engine and use only the type and level recommended by your engine manufacturer.
You can contact Ben at Visser@GeneralAviationNews.com.