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You must almost certainly have heard about the controversy over the car
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company Volkswagen and diesel engines and nitrogen oxides.
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Now I don't want to talk about the controversy but the chemistry is really
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interesting
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so I thought you ought to know about it. The first thing to say is that this
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involves diesel engines.
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There are two sorts of engines that you can have in cars. There're petrol engines
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and diesel engines and they use slightly different fuel and the main difference
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between the engines is that in the petrol engine
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there's a spark plug, which set fire to the mixture of petrol or gasoline as you
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stay in the states and air. And the flame spreads from the top of the
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cylinder downwards and pushes the piston. In a diesel engine
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there's no spark plug but the piston going upwards compresses the mixture of
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air and fuel which gets hot
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because you're compressing it and it spontaneously starts burning.
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The advantage of diesel engines is that you can build them very much bigger for cars,
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for trains or even for ships, but with a petrol engine
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there's a limit to the maximum size. The result of the different way of igniting,
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the setting fire to the fuel, is that the chemistry takes place inside slightly
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differently because the temperatures are different and the way the flame spreads.
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In both engines you have a mixture of air and fuel.
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Air contains oxygen which makes the fuel burn it also contains nitrogen gas (N2) and under the high
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temperature conditions, some of the nitrogen gas just a tiny bit can react
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with oxygen and it makes a mixture of oxides of nitrogen that are called NOx
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x is a number which is normally either one or two.
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The reason why we worry about NOx is because these molecules are really quite reactive.
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And the reason is that nitrogen has seven electrons and oxygen atom has eight.
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So if you have either NO or NO2
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It will have an odd number of electrons, it will have one extra electron that is
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not paired up. And this makes the molecules much more reactive.
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So if you want to preserve the quality of air in the city or beside the road,
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you want to minimize the amount of NOx coming out of the exhaust pipe.
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Diesel engines produce more NOx than petrol engines.
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So, in diesel engines you have to take special precautions or procedure to try
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and minimize the amount of NOx especially in the latest cars which are
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designed to meet stricter emission limits. The chemistry that is used is
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relatively simple.
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You have a catalyst which reacts the NOx with urea. Urea is a compound of
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ammonia and CO, or it's made from ammonia and CO.
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It has two NH2 groups bound to one CO group. Urea is made industrially
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but it is a product that you can find in urine for example. And there have been all
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sorts of jokes about people with diesel cars relieving themselves into the
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engine to make them better,
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but that's by the by. But the key point is that you have a reaction between urea
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and NOx which generates ammonia and nitrogen and CO2.
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So in the latest cars there is a tank which contains what I believe is a very
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concentrated solution of urea.
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I don't know the precise details because although I own a Volkswagen diesel,
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it's too old to have one of these. The idea is that a very small amount of this
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solution is injected into the exhaust stream
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there's a reaction and the NOx
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is removed. The scandal has been because the computer that controls the
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whole engine has been programmed so it only puts the urea in, when the engine is
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being tested for emissions. And I believe the reason for this is just to save the
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customers having to top up the tank of urea. Though some articles that I read
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suggest that it may affect the performance of the engine as well so you
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can't accelerate quite as quickly or whatever.
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It should be said that the catalytic converters in cars,
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whether they're gasoline cars or diesel cars, are really very clever because what
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is coming out of the exhaust pipe contains gases like carbon monoxide or
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perhaps even methane, which need to be burnt, reacted with oxygen, and there are
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other things like NOx that need to be reduced
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you need to remove the oxygen and therefore the computer that controls the
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engine, the so-called energy management system, has to adjust the amount of
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oxygen going into the engine very carefully and in fact one of the key
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components is a oxygen sensor that sits in the exhaust pipe so that it can work
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out oxygen levels and those of you who are watching in the UK who have long
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memories may remember that there was some cheap petrol that was sold a few
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years ago that managed to ruin the oxygen sensors in a whole series of cars
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I think 10,000 cars have been to those particular petrol stations.
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The reason that we have cleaner emissions from cars now is because the
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engines are computer controlled and we can adjust the chemical conditions
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inside them very precisely. And unfortunately it was realized, I don't
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know by whom,
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In Volkswagen that they could manipulate these things so that they
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could make
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the engine cleaner sometimes than others.
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So this is where we're going to get our CO2 from. A rather large cylinder, so on
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on the cylinder we have a device called a regulator, which you can see here.
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Well I wouldn't draw this direct line between the Volkswagen and Hitler.
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I mean there was, of course he was behind the will to subsidize it