Mogas - Technical Question

[Nipper2]

I tried this one on the LAA Forum but did not get any specific technical answers. I'm not interested in a debate on the merits of Mogas versus Avgas.

After a moderate period without use (six weeks), my VW engine was hard to start and ran roughly. Careful inspection of the carburettor revealed small, soft yellow/brown particles roughly the size of small sugar grains inside. There was also a yellow/brown residue on the ends of the needles.

The engine has been run on Mogas (with occasional Avgas).

In so far as I am aware I have not used fuel with alcohol in it, but is it possible that undetectably low levels of alcohol have caused the problem? The fuel system is all constructed from supposedly alcohol resistant materials but the tank is GRP. There does not appear to be any vissible sign of damage or decay in any of the fuel-system componenets.

On local expert tells me the yellow gunge is a corrosion product from zinc based cast carburettor-body and additives in the fuel. Is this possible or likely?

Can any of the fuel-experts on here provide a proper technical explanation of what is going on?

[magpienja]

Just wondering how often does you run your engine, I have seen results likes your with boat engines that are run infrequently.

Nick.

[Nipper2]

Normally runs about 50 hours per year, roughly one hour per week. On this occasion due to my working abroad it had been left for just under six weeks.

I too have seen similar problems with boat engines.

[flybymike]

Sounds typical of gum and varnish residue caused by evaporation of old fuel.
stabilising additives are available for long term fuel storage to help prevent the occurrence. (Typically used in lawnmowers , generators and other machinery infrequently used.) I wouldn't know whether it would advisable to use such additives for fuel to be used for aircraft though

[Pilot DAR]

Nipper,

I will not profess to be an expert in the chemical interaction between gasoline, alcohol, and GRP, but I know something about Mogas use. I have more than 3000 hours of trouble free flying on it.

Perhaps you have a compatability problem in the aircraft fuel system, and you are getting decomposed particles. To assure that your engine runs smoothly it such a case, a good fuel filter just before the carb will be your best defense. Yes, if disolved solids are reconstituting themselves in your carb, after the filter, that could be a problem, but frequent use will prevent this. Consider the possibility that the affect of the Mogas on the non-metallic components of the fuel system, might not be unique to Mogas, Avgas might have those effects too. If it is an older aircraft, it might struggle with either fuel.

Alcohol has potential for adverse affects too, but I will leave that for others to comment upon.

Presupposing that you are running a low compression engine, it, and the environment will be happy that you are using Mogas instaed of Avgas. A better understaning of the fuel delivery system is what you need. Having the gunk in an undesired place is one thing, why is the gunk not upstream where it is suppose to be is another. The long term airworthiness of your fuel system requires you understand what is happening. Some informal home made long term compatability tests with the materials, and your local gasoline supply could be informative. A good fuel filter, regular inspection and cleaning should keep you flying safely in the mean time.

I am a Mogas proponent, but do be sure that your use of it is approved from an STC point of view, if it is a type certificated aircraft. Some aircraft were not STC'd because they had fuel system problems. That's why the engine and airframe STC's for Mogas use were often sold separately.

Pilot DAR

Pilot DAR

[Rod1]

I understand you did not use the aircraft for 6 weeks, but how old is the fuel in the tank?

Rod1

[Nipper2]

Rod

Fuel has stood in good quality sealed 20 liter metal jerry cans for less than three weeks. Total time from pump to flight just over eight weeks. Standard unleaded fuel purchased from local garage.

[Lister Noble]

You could try asking the fuel company tech dept,I know oil companies are very helpful when asked,don't know about fuel companies though.
Lister

[mrpinks]

I have seen something similar in a gascolator which was fed by a GRP tank where the fuel had been standing for approximately a year (actually the contents of the gascolator looked like golden syrup, I scrapped the tank, the previous owner had quite happily flown the aircraft with this fuel and crashed it). One piece of advice I was given was if the materials from which the tank was made are still available ie you made it yourself and you have some of the resin left was to make up some test strips. Then dunk these strips in a jam jar of the mogas you are currently using and observe. Repeat frequently, Mogas formulation varies according to supplier and season

[Rod1]

If you go over to Flyer, MikeB flys a P300 on Mogas, but his day job is in the petroleum industry checking for contaminants in Avgas and Mogas. He is very knowledgeable and will probably be able to help.

Rod1

[SNS3Guppy]

Whereas auto fuel has no particular uniform standard and can vary in composition and quality from batch to batch, and whereas auto fuel tends to decompose more quickly, leave varnish and deposits in the fuel system, and can gum up fuel system components if left...sounds very much like you let it sit too long.

You don't want a discussion about the disadvantages of auto fuel in aircraft, but it's very hard to discuss your question without going there.

[Nipper2]

Thanks for the info so far.

I am an engineer by profession and have a very good understanding of the variability of Mogas. I have no doubt whatsoever that Avgas is a far preferable fuel and would use it if I could sensibly obtain supplies. Price is not the issue. When you tank capacity is just 34 litres useable and it consumes 12 litres to go and get some by air or requires 60 minutes round trip by road, things are not so simple......

What I'm trying to find out is what is the residue? Gum from evaporated fuel? Corosion product? Decay products from a slowly disolving fuel tank?

The tank is 1960s vintage so testing resin samples is not a realistic option. I am considering lining it with a modern sloshing compound, though there does not seem to be much good information on the suitability of these compounds for use with modern mogas.

[BackPacker]

What I'm trying to find out is what is the residue? Gum from evaporated fuel? Corosion product? Decay products from a slowly disolving fuel tank?

I would put that residu in fresh fuel, in a clear container, shake and see if it dissolves.

If it dissolves quickly, it's probably gum from evaporated fuel. If it doesn't dissolve, or doesn't dissolve quickly, it might be something more serious.

And to be honest, if you have a detectable amount of residue in your carbs and the origin is the fuel tank lining, then it is so severe that your fuel tank would have sprung a leak by now. And if your carb bowls are smooth, not pitted, if you clean them, it's probably not corrosion either.

Note that I'm not a fuel or fuel system engineer. Just trying to apply some common sense.

[Mariner9]

Just trying to apply some common sense

And so you did :ok:

One minor correction - gum can form in gasoline through oxidation without any evaporation whatsoever :=. (See MikeB's explanation of oxidation on Flyer forum - I invariably agree with him ;))

And finally one other point, the gum residue may or may not be soluble in fresh gasoline, especially if it is from a different source to the original gasoline.

[Oldlae]

As when laying up your lawnmower for the winter, and I don't know your configuration but if you know that the engine is not going to be run for a long period switch the fuel off when shutting down rather than using the ignition and let the engine run down until it stops, this will empty the carb and prevent gumming.