How long can fuel be safely stored?

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.

Comments

  1. Hi Ben
    It would be great if you could email me. I am an Australian (Aussie) pilot and operate a Beech 58 Baron with the bigger engines. I would like to ask you some questions. hey if you ever come to Australia let me know
    fellow aviator
    Sandra Mcrae Cabot

  2. I think it is not the engines that are the problem but rather alcohol’s (irespective which kind)affinity to water that is the problem. But I feel that problem will sort itself out when the world will realice that growing plats to make fuel is a bad idea and will return to pure gas and let the GREENS drink their Alcohol. This way they will be much happier!

  3. Dan Colburn says:

    I’m certainly not a “Greenie”, I vote to go with Ben Visser. You can’t beat years of experience.

  4. Dennis Reiley says:

    While aviation engines and automotive engines differ in their specific designs. Both are internal combustion engines and share a basic functional design. Automotive engines unlike aviation engines have been modified to use fuel containing ethanol. That does not mean that ethanol in fuel is ideal, it simply means an automotive engine functions with ethanol quite well and without long term problems from ethanol. Ethanol also provides de-icing because it absorbs water and itself is absorbed by fuel so fuel system moisture is suspended in the fuel until it enters the combustion chamber and is burnt.

    I agree Ben Visser is far more knowledgeable then I when it comes to fuels. But his statements beg the question of why is he so against aviation engines being modified to accept MoGas including with ethanol? Aviation engines that exist now have a problem with ethanol it is true. But why not have the aviation industry do what the automotive industry did a quarter century ago and modify their products to use MoGas with ethanol. It can not be argued that MoGas has a low temperature or high temperature problem. Automotive engines operate in Alaska and northern Canada, not to mention northern Europe, Asia and Antarctica. And who has not heard of vehicle races in hot arid climates.

    It seems to me the aviation industry and its “experts” keep using a circular argument. That is aviation engines are not built to handle MoGas with Ethanol, therefore they should not be required to use it. It’s as if they feel being in aviation that “their” engines should not be modified to use more ecological friendly fuels as is done in the automotive industry and everyone including the government should accept that.

    It is most certainly an invalid argument that aviation engines can not be modified to accept MoGas with ethanol, what the automotive industry has done the aviation industry can do. But if there is a particular argument having to do with altitude or some other reason why it can not be done; then let’s hear it. But so far all I’ve heard is that circular argument that it can’t be done because it isn’t done.

    There are limited benefits, and some harmful effects, that can be attributed to high octane fuels, especially the harm caused by leaded fuels, especially to health. In automotive engines spark plug life used to be limited to between 12,000 and 24,000 miles. When tetraethyl lead was removed from gasoline spark plug life went to 50,000 mile intervals. With other fuel efficient modifications it is now in the 80 to 100k range.

    The aviation industry needs to get rid of that broken record and start dealing in facts instead of decades old arguments.

    • Michael J. O'Neill says:

      Please see and read the information at the following WEB site: http://www.avweb.com/news/maint/187232-1.html?redirected=1

      I see from most of your comments that you may not have much knowledge about aviation engines and the requirements necessary for the fuel used at high altitudes and low pressures you typically see at normal flight levels. Automotive fuels DO NOT perform in the same manner as Avgas. It also is not regulated like Avgas and thus has great variability and can not be counted on to work at altitude and temperatures that are typical when flying.

      The bottom line here is the safety of the passenger and pilots flying the aircraft not to mention those who are on the ground if the aircraft should crash due to a fuel problem. My suggestion would be that you should only fly in aircraft using automotive fuel. I’ll keep using Avgas until the correct solution is found rather that the sea level gas you use. Oh, by the way, gas you buy in Denver is not the same as gas you buy on the California coast. Why? Altitude. One type does not work well in both locations for automotive fuels.

      Enjoy your flight.

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