What's the SHELF LIFE of Avgas?

Discussion in 'Maintenance Bay' started by Mister Mystery Man, Mar 15, 2012.

  1. Mister Mystery Man

    Mister Mystery Man Filing Flight Plan

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    Yeah I know the experts say one year...

    http://www.eaa.org/autofuel/autogas/articles/1What is the shelf life of avgas_.pdf

    That's the official report. But I've seen aircraft that had set in their hangars for more than five years and they started up and ran fine when taken out of storage.

    In winter, I always leave Avgas in my lawn mowers. I run it exclusively in my 2-cycle engines and they never have a gum residue problem. One chainsaw sat for 4 years with no fuel problems using Avgas.

    My argument is that all of the tar and resin is refined off so there's nothing to precipitate. It's just super-clean gasoline.

    On the other hand the nasty gas (MoGas) is only partially refined, leaving a lot of tar and resin suspended in it. (The reason it stinks so bad)

    I've had the overflow on my Stinson to drip onto my suit on my way to a business meeting and I thought, "DANG! Now I'll smell like gasoline!" Not to worry, when the Avgas evaporated there was no odor at all... kinda like when isopropyl alcohol evaporates... it leaves no odor.

    So I say that Avgas ought to be good for at least 8-10 years if it doesn't become contaminated with too much moisture or rust. I've never heard of any shelf life problems with the fuel. (?)
     
    Last edited: Mar 15, 2012
  2. pericynthion

    pericynthion Pre-takeoff checklist

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    I'm not sure about precipitation of solids, but it makes sense that over a long time in a non-hermetically-sealed tank, the lighter fractions will evaporate in preference to the heavier fractions, changing the composition somewhat.
     
  3. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    Ummm. No.

    The residue comes from oxidation of the hydrocarbons in the fuel, not left over "stuff".

    The smell comes from the aromatics used to boost the octane rating in the absence of lead. Relatively small compact molecules with benzene rings.
     
  4. Mister Mystery Man

    Mister Mystery Man Filing Flight Plan

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    I'd think that Avgas would also have hydrocarbons too, so it ought to 'go bad' as quickly as MoGas unless there's less "stuff" in it to oxidize.


    There wasn't any absence of lead in the Super Shell that I pumped into my '59 Ford. And when I spilled some of it on my shoes, I had to leave them out in the garage for a week or the whole house would smell like nastygas.

    MoGas stank (stunk?) long before the unleaded revolution. Even the old 80 octane tractor gas on the farm was stinky. Once we ran out of gas in the Super C Farmall but we found a glass jug half full of old gas laying in the weeds beside the overhead tank (which was locked).

    The gas had accumulated a stringy looking stuff in it like vinegar does when it sets for a long time. We took an old rag and strained the solids out of the stuff and it stank 10 times worse than fresh gasoline with a kind of sweet smell.

    So we did what kids would logically do and we poured it into the tractor. On the way back to the house the exhasut stack wafted that awful stench back where we had to breathe it and it was almost enough to make us puke.

    Naw, I'm convinced there's something more in the way of 'stuff' in that MoGas than there is in Avgas.
     
  5. Cap'n Jack

    Cap'n Jack Final Approach

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    All gasoline is composed of "hydrocarbons". The gum is from partial oxidation/polymerization of alkenes, of which there are a greater proportion than in 100LL. Store the auto gas in an air-tight container and keep the container full (less air=less oxygen) and the gasoline will keep longer. Keeping the fuel cool helps too.

    This reference notes the "olefins" (what I termed alkenes) are the most reactive, and the presence of oxygen: http://pubs.acs.org/doi/abs/10.1021/ie50239a026


    This reference shows what that bottle of gas smelled so bad; storing the gas in a bottle in the light hastened the reaction: http://pubs.acs.org/doi/abs/10.1021/ie50296a020

    Those alkenes also tend to give auto fuel a different smell than aviation gas. This MSDS lists alkenes as forming part of the fuel: http://www.carsonoil.com/pdfs/ULgasolineMSDS.pdf
     
  6. peter-h

    peter-h Line Up and Wait

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    What would be the degradation mechanism in a sealed container?

    It cannot be selective evaporation because the vapour has nowhere to go. Once the vapour goes saturated (for each gas constituent) the evaporation must stop.

    Would the fuel need stirring up, to be sure?
     
  7. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    With limited oxygen, you would get limited degradation.
     
  8. Cap'n Jack

    Cap'n Jack Final Approach

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    What he said below. It shouldn't need stirring up as it's all dissolved with nothing coming out of solution.

     
  9. Mister Mystery Man

    Mister Mystery Man Filing Flight Plan

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    I just got back from buying 100 LL at Crossville Memorial airport. Price? $5.25 per gallon. Looking at some net information, seems like a good price, comparatively speaking.

    http://www.100ll.com/

    I also got some to put into a glass jug just to see what will happen and how long it might take. Personally, I don't expect anything to happen for at least 5 years.
     
  10. Cap'n Jack

    Cap'n Jack Final Approach

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    So? If nothing happens, it will only prove that avgas and autogas have differing compositions (quantity of alkenes) as I mentioned in post #5 of this thread. Make sure you store it outside as you mentioned in your post (#4). It doesn't need to be "in the weeds".