Silicone in Petrol

We discuss why cars have been failing all over the UK recently, and what was in their petrol to cause the problem.
11 March 2007

Interview with 

Richard Van Noorden from Chemistry World


Chris - Earlier this week there were a whole number of retailers who admitted that stocks of their petrol had been contaminated with something that had caused hundreds of cars to grind to a halt, but what was that substance and how did it get into the petrol in the first place? To tell us what he's flushed out in the fuel line about this story, Richard Van Noorden is here from Cambridge-based Chemistry World. Richard what have you found out about this story?

Richard - Well Chris, the culprit was silicone in the petrol causing the cars to judder to a halt. Slightly confusingly though it probably wasn't the element silicon as the silvery-grey semi-metal that you find in computer chips; that wouldn't have got in to the petrol as its not soluble in petrol. What actually happened, was that people who were testing for silicon found the presence of silicon atoms in the petrol and the way you do that - its a bit like your old school flame test - you heat up the petrol and each atom gives off a characteristic wavelength of light and you get that colour that's characteristic to silicon.

Chris - Because this is the way that people work out what is in the sun, if you look at the sun's spectrum you can work out what elements there must be in the sun and in roughly what quantity, because you get light of a very characteristic wavelength coming off.

Richard - Exactly, so sodium street lamps give off that characteristic orangey sort of colour.

Chris - Why was the silicone in the petrol? Do we know?

Richard - We don't actually know why the silicone contaminated the petrol, it shouldn't be in the petrol. As silicones, which is when the element silicon combines with oxygen to form long SI-O -SI-O silicon-oxygen chains with Carbon and Hydrogen hanging off the end of the silicon, these kind of polymers, called silicones, can be gel-like, rubbery

Chris - Its bath sealant isn't it?

Richard - Yes, used also as breast implants, as lubricants as greases so its very likely that silicone which is soluble in petrol caused cars to judder to a halt, because when the silicones burnt when the petrol was combusted, the hydrocarbon bits (the carbon and hydrogen atoms) were burnt off leaving silicon and oxygen - silica which is of course sand, glass.

Chris - So you made sand in your engine?

Richard - You made sand, its going to come out as a kind of whitish deposit and that will clog up the oxygen sensors in the engine. The oxygen sensors just gave up, they said "look we're controlling the flow of fuel and we're being clogged up so we're going to give up and your car is going to judder to a halt."

Chris - So presumably only some cars which would have had high-tech oxygen sensors like this would have been affected?

Richard - Yeah, it looks like older cars actually carried on ok, despite the fact they had an engine full of sand.

Chris - Are there any fuels in which you would normally place silicones?

Richard - Yeah, you would put silicones in diesel where they're anti-foaming agents so they stop the diesel literally foaming up as you're pouring it in. You should never have it in petrol.

Chris - So why don't they form sand in a diesel engine then?

Richard - Diesel is not combusted by sparks, in the same way as petrol is, in diesel you use pressure it also ignites whereas in petrol you're using a spark to burn off the carbon and hydrogen.

Chris - But that shouldn't make a difference to whether there's a heat there that can burn silicone and react with oxygen surely?

Richard - I'm not exactly sure of the answer to that question Chris, but I know there isn't that problem on diesel.

Chris - Maybe its just because diesel engines are so robust that they'll burn anything, what do you think Dave?

Dave - I guess if a diesel engine doesn't have an oxygen sensor its just like an old-fashioned petrol engine so it would just keep on going, there's nothing to gum up.

Chris - Sure, we've now got to the bottom of it - we think its silicone, it got into the petrol it shouldn't have done. Is there any way that we can prevent this happening again?

Richard - One of the problems with finding out that it was silicone was that the standard test for petrol doesn't include a test for silicone because it shouldn't be in there. They test for all the additives in petrol - lead, copper and so on. So harvest energy who are one of the suppliers to the supermarkets like Tesco, that the faulty petrol came from; they say that they are going to now include a standard test for silicone in the petrol whenever they check it. So there's obviously a quality-control, 'how should we check our petrol' issue there.

Chris - Has it ever happened before Richard?

Richard - As far as we know, not in the UK but intriguingly according to the American oil company Chevron's website used toluene which is a solvent, from a manufacturing process containing soluble silicon, has apparently found its way into gasoline as the Americans call petrol and again fouled up oxygen sensors. Now that's on our website and when I checked this with a specialist from the American petroleum institute he couldn't remember details of that so I'm just throwing this out, nonetheless I have had people say this to me; that it has happened in America and somehow re-cycled toluene with silicon in caused the problems and got in to the petrol. Now of course I don't know if that's happened here and no doubt in the next few weeks we'll find out exactly what has happened.

Chris - Thank you Richard, from Chemistry World of the Royal Society of Chemistry, you can see the article Richard has written about this on their website:


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