Silicone in Unleaded

[QDMQDMQDM]

We don't have fancy oxygen sensors to go wrong, but following the recent problems with unleaded fuel contamination, are we more worried about sticking this stuff into our aero engines? What implications does this have?

QDM

[Grummanaa5]

With massive hikes in AVGAS duty due and many areas of GA looking at MOGAS as a cheaper alternative (especially as petrol octane has increased to 98 and AVGAS reduced to 100) you can see the attraction.

I wonder how people will feel after this sorry episode. If I had a microlite now full of petrol from Tesco, I don't think I would want to go fly it.

[tangovictor]

from what I understand of the problem, its the sensors, in the exhaust systems of "more" modern type cars, that's giving the problem, the sensors telling the engine to shut down, due to, the exhaust emissions, correct me if i'm wrong, I doubt any mogas running airplane, would have such sensors or catalytic converters to give such a problem,

[airborne_artist]

QDM - FWIW the substance found in the contaminated petrol was silicon.

Silicon has the atomic number 14, and is a tetravalent metalloid. Silicone is a rather different substance, formerly used to enhance ladies' assets, and still used to seal gaps in bathrooms.

I'd hope that as a doctor you'd know the difference between sand and a 34D.

[Rod1]

If you had an aircraft which ran on mogas you would not have used any of the contaminated fuel. If you had you would not have followed the procedures which are imposed to keep us safe. Anyone, like myself, with a Rotax 912 in a Group A aircraft would be very ill advised to flout the regulations. I run on Mogas all the time.

Rod1

[bookworm]

QDM - FWIW the substance found in the contaminated petrol was silicon.

Silicon has the atomic number 14, and is a tetravalent metalloid. Silicone is a rather different substance, formerly used to enhance ladies' assets, and still used to seal gaps in bathrooms.

I think it's much more likely that the substance found was indeed silicone. Silicone, in the form of polydimethylsiloxane, is used as an anti-foaming agent in fuel. It reduces surface tension and therefore bubbles. There's no reason for silicon to be anywhere near fuel.

[SkyHawk-N]

Not having any knowledge of chemistry whatsoever, isn't Silicone made from mixing (I'm sure there is a proper word for this) Silicon and Oxygen? Burn off the oxygen and you are left with Silicon. Or am I looking at this too simplistically?

[bookworm]

Or am I looking at this too simplistically?

Yes, I'm afraid so.

Conventional polymers are made from long carbon chains. Silicone (http://en.wikipedia.org/wiki/Silicone) is the name given to a family of polymers (plastics) that contain the element silicon as part of that chain instead of or as well as carbon.

When analyses are performed on petrol, these are most likely looking at constituent elements. So when the results show the presence of "silicon", that may mean any compound of silicon, including silicones, just as when the doctor talks about the level of sodium in your blood he's not implying that you have the highly flamable metallic element sodium floating around in your veins in chunks. :)

[SkyHawk-N]

he's not implying that you have the highly flamable metallic element sodium floating around in your veins in chunks

You mean I've been sitting here worrying about that for years for no good reason? :*

[QDMQDMQDM]

If you had an aircraft which ran on mogas you would not have used any of the contaminated fuel. If you had you would not have followed the procedures which are imposed to keep us safe.

What procedure which you follow would have detected this contaminant?

[Zulu Alpha]

I would think that aircraft engines are unaffected.
The catalytic convertors in cars require a certain level of oxygen to clean up the engine exhaust ie convert carbon monoxide to carbon dioxide. The oxygen sensor feeds back the actual level of oxygen in the exhaust and the engine management system varies the fuel to get the amount of oxygen correct. The contaminant is a silicon compound which breaks down in the hot exhaust and coats the sensor with Silicon and stops it working. The engine management then either tries to change the fuel air to get the ratio right or it puts the engine into limp home mode.

I don't think aircraft engines have catalytic convertors or oxygen sensors so should not be affected.

[GOLF_BRAVO_ZULU]

I tried to provoke some debate on this at http://www.pprune.org/forums/showthread.php?t=266124 , but failed! That was before the presence of silicone had been detected, though.

Well, I'm still concerned. In the same way that silicone wax can craze GRP, silicone oils can migrate into certain plastics and provoke brittleness and cracking. Just one insight to this is, http://www.springerlink.com/content/n5g72817632813n8/ . I have an ex 3 Cdo 1/2 Ton Rover Truck that spent most of its Service life in Noggyland. The grease in the steering column had been replaced with silicone grease; you could tell from the feel. The bloke I bought it off had just replaced the Indcator switch assy. About 3 years later the unit fell part. The ABS moulded body had more cracks than a Bangkok brothel! I replaced it with the correct "arm and a leg" Lucas unit and, lo and behold, that too fell apart after just 4 years. I'd used MS33 grease in the past, for what it was intended for, and already noticed that the plasic cap on the tube never seemed to hold together for much over 2 years.

I know that my aeroplane has some reinforced PVC fuel lines and the exhaust vents directly into a GRP propellor. Maybe I'm paranoid but I'm glad that it's got AVGAS 80 in the tanks. As an aside, it does annoy me that CIVGAS/ULGAS (the supporting DEFSTANs have been cancelled, incidentally) is now adulterated with ethanol. I'm not normally inclined to fly at 6,000 + feet but I don't like having to worry about feed line vapour locks.

[Rod1]

QDM

The statement was;

“If I had a microlite now full of petrol from Tesco, I don't think I would want to go fly it.”

All of the fuel, which was contaminated with Silicone, also contained 5% ethanol (all Tesco petrol does). You HAVE to test for the presence of ethanol so not possible to have a “microlite” with a full tank of contaminated fuel without having failed to test it!

Rod1

[QDMQDMQDM]

OK, that happens to be so in this instance, but it isn't a given that silicone contamination (or other contamination) will always have a proxy marker of ethanol content.