Avgas Mogas in the winter

[6597]

So I was talking with one of my mechanics and we were discussing the use of avgas/mogas in colder weather. Somehow over the years I fell into the use of more avgas when it was colder and more mogas when it was warmer. I thought my colder weather use of avgas was based on easier starting and better long term storage.

So I read up on the properties of both fuels and find that avgas is 'less' volatile than mogas based on Reid Vapor Pressure. Less volitility means that the chance for vapor lock in certain fuel systems is reduced so that's a good thing (but in our birds probably not an issue). But somehow i thought a 'more' volitile fuel in the cylinder would be a good thing for colder weather starts.

So now that I"ve read several documents on avgas/mogas and their manufacture and properties I am starting to over think the issue.

My question to the gang is, for cold weather starting do you have a preference for avgas over mogas? Assume that you are still flying every few weeks in the winter time and that mogas storage is not an issue.

87 octane ethanol-free mogas is about a buck a gallon cheaper than avgas here in Vancouver WA

[tonycondon]

i've only run mogas or a mixture occasionally but with my stromberg carburetor and the lower specific gravity of mogas, the engine runs leaner on it, especially at low power. i've occasionally had to adjust the idle mixture or run with the carb heat on or primer open on the ground when running 50% or higher mixtures of mogas. I'd think this would be more likely in the winter with thicker air requiring a richer mixture anyway. I've never noticed a difference in starting.

[8224]

Just sold my 140 a month ago and during the time I owned it I never put anything in it except ethanol free mogas in it. Never had any trouble of any kind with it. I also have an Interstate Cadet with an 0-200 in it and The only time I put 100LL in it was my trip to Oshkosh. Couldn’t see any different except the price. I wish I could put mogas in my other two planes but they have high compression engines and that is a no-no.
Steve Hawley

[V529]

Somewhere I have an indepth article on the differences between Avgas and mogas, covering Reed vapor pressure, specific gravity, octane, etc and so on. One major difference between Mogas and Avgas is that Mogas has higher percentages of "lighter elements", (think "thanes" as in butane, methane, etc) these lighter elements are what allows you to leave an engine sit and idle for extended periods of time. Since Aircraft are designed to run at about 80% most of their life, Avgas has considerably less "thanes" in it than Mogas.

Coupled with "winter blend" when the thanes are increased even more (consequent loss of fuel milage to an extent is a byproduct), but it makes for easier starting and running in cold temps.

Bottom line, probably better to run mogas in the winter and Avgas in summer if so inclinded.

All that being said, due to the airport I'm based at, Avgas is typically my major fuel use and I've not had any problem over the years in cold starts or winter operation here in Michigan on it. Mogas here in Michigan is about 1/2 of Avgas.........

[6597]

Thanks for all the replies.

I noted that ethanol-free mogas 87octane here is actually $2 a gallon less than local avgas. That being said I think I"ll revisit the 'starting' issue and see if it's really a problem for me. Storage-wise with mogas I'll have to research how long it takes before it actually starts to degrade to any significant measure. In the meantime I'm still flying at least every other week, so far so good this winter. A big fan of pre-heating also!

[V529]

Thanks for all the replies.

I noted that ethanol-free mogas 87octane here is actually $2 a gallon less than local avgas. That being said I think I"ll revisit the 'starting' issue and see if it's really a problem for me. Storage-wise with mogas I'll have to research how long it takes before it actually starts to degrade to any significant measure. In the meantime I'm still flying at least every other week, so far so good this winter. A big fan of pre-heating also!

Great point. "storage". Gary Rice who owns one of our 120/140 engine STCs was a petroleum engineer for a large Oil company for 40 years. At the Tuscon convention in 2012? I spent about 2-3 hours with him discussing fuels, oils, usage, octane measurement etc and so on. I should have taken a recorder! I do recall him saying that Mogas is certifed to stay consistant for 60 days after it leaves the refinery. (since verfied with other people I know in the industry). AvGas can stay consistant.......assuming protected from, Oxygen, sunlight and any other contaminent) for almost 2 years.

I store all my lawn mowers, chainsaws, motorcycles, etc in the winter with Avgas in them. No carbureator issue for decades.......

[6183]

David, I've had the Avgas Mogas subject discussion with my brother several times in the past. He's a chemical engineer who made avgas/JP/Jet-A for Shell Oil for years before retiring. Several suggested points he really stressed if you are going to use non-ethanol in your airplane or boat are: (1) buy your Mogas/Non-Ethanol fuel from a reputable source that moves the product on a continuous basis. Try to stay away from small convenience stores that may have older fuel in their storage tanks. Also, stay away from stations that have one (1) dispensing hose that is utilized for both ethanol and non-ethanol sales. It is quite likely you will get a small amount of ethanol fuel in your containers due to a previous pumping of ethanol. Use stations that have a separate dispensing hose for non-ethanol only. (2) When using Mogas/Non-Ethanol carry it in dedicated cans to cut down on contamination, and when transferring fuel to the aircraft utilize a micron filter funnel system to remove trash and moisture (personally I use a Mr. Funnel that has the filter to put fuel in my lawn mower). (3) Don't store the fuel in containers for more than month before using, Better yet use newly purchased non-ethanol mogas right away if you're utilizing it in an aircraft. Mogas depending on what refinery made the gasoline will start to break down somewhat due to the chemical makeup of the components utilized in making the fuel.