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  • You must almost certainly have heard about the controversy over the car

  • company Volkswagen and diesel engines and nitrogen oxides.

  • Now I don't want to talk about the controversy but the chemistry is really

  • interesting

  • so I thought you ought to know about it. The first thing to say is that this

  • involves diesel engines.

  • There are two sorts of engines that you can have in cars. There're petrol engines

  • and diesel engines and they use slightly different fuel and the main difference

  • between the engines is that in the petrol engine

  • there's a spark plug, which set fire to the mixture of petrol or gasoline as you

  • stay in the states and air. And the flame spreads from the top of the

  • cylinder downwards and pushes the piston. In a diesel engine

  • there's no spark plug but the piston going upwards compresses the mixture of

  • air and fuel which gets hot

  • because you're compressing it and it spontaneously starts burning.

  • The advantage of diesel engines is that you can build them very much bigger for cars,

  • for trains or even for ships, but with a petrol engine

  • there's a limit to the maximum size. The result of the different way of igniting,

  • the setting fire to the fuel, is that the chemistry takes place inside slightly

  • differently because the temperatures are different and the way the flame spreads.

  • In both engines you have a mixture of air and fuel.

  • Air contains oxygen which makes the fuel burn it also contains nitrogen gas (N2) and under the high

  • temperature conditions, some of the nitrogen gas just a tiny bit can react

  • with oxygen and it makes a mixture of oxides of nitrogen that are called NOx

  • x is a number which is normally either one or two.

  • The reason why we worry about NOx is because these molecules are really quite reactive.

  • And the reason is that nitrogen has seven electrons and oxygen atom has eight.

  • So if you have either NO or NO2

  • It will have an odd number of electrons, it will have one extra electron that is

  • not paired up. And this makes the molecules much more reactive.

  • So if you want to preserve the quality of air in the city or beside the road,

  • you want to minimize the amount of NOx coming out of the exhaust pipe.

  • Diesel engines produce more NOx than petrol engines.

  • So, in diesel engines you have to take special precautions or procedure to try

  • and minimize the amount of NOx especially in the latest cars which are

  • designed to meet stricter emission limits. The chemistry that is used is

  • relatively simple.

  • You have a catalyst which reacts the NOx with urea. Urea is a compound of

  • ammonia and CO, or it's made from ammonia and CO.

  • It has two NH2 groups bound to one CO group. Urea is made industrially

  • but it is a product that you can find in urine for example. And there have been all

  • sorts of jokes about people with diesel cars relieving themselves into the

  • engine to make them better,

  • but that's by the by. But the key point is that you have a reaction between urea

  • and NOx which generates ammonia and nitrogen and CO2.

  • So in the latest cars there is a tank which contains what I believe is a very

  • concentrated solution of urea.

  • I don't know the precise details because although I own a Volkswagen diesel,

  • it's too old to have one of these. The idea is that a very small amount of this

  • solution is injected into the exhaust stream

  • there's a reaction and the NOx

  • is removed. The scandal has been because the computer that controls the

  • whole engine has been programmed so it only puts the urea in, when the engine is

  • being tested for emissions. And I believe the reason for this is just to save the

  • customers having to top up the tank of urea. Though some articles that I read

  • suggest that it may affect the performance of the engine as well so you

  • can't accelerate quite as quickly or whatever.

  • It should be said that the catalytic converters in cars,

  • whether they're gasoline cars or diesel cars, are really very clever because what

  • is coming out of the exhaust pipe contains gases like carbon monoxide or

  • perhaps even methane, which need to be burnt, reacted with oxygen, and there are

  • other things like NOx that need to be reduced

  • you need to remove the oxygen and therefore the computer that controls the

  • engine, the so-called energy management system, has to adjust the amount of

  • oxygen going into the engine very carefully and in fact one of the key

  • components is a oxygen sensor that sits in the exhaust pipe so that it can work

  • out oxygen levels and those of you who are watching in the UK who have long

  • memories may remember that there was some cheap petrol that was sold a few

  • years ago that managed to ruin the oxygen sensors in a whole series of cars

  • I think 10,000 cars have been to those particular petrol stations.

  • The reason that we have cleaner emissions from cars now is because the

  • engines are computer controlled and we can adjust the chemical conditions

  • inside them very precisely. And unfortunately it was realized, I don't

  • know by whom,

  • In Volkswagen that they could manipulate these things so that they

  • could make

  • the engine cleaner sometimes than others.

  • So this is where we're going to get our CO2 from. A rather large cylinder, so on

  • on the cylinder we have a device called a regulator, which you can see here.

  • Well I wouldn't draw this direct line between the Volkswagen and Hitler.

  • I mean there was, of course he was behind the will to subsidize it

You must almost certainly have heard about the controversy over the car

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B1 nox diesel petrol oxygen engine nitrogen

Diesel and Urea

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    林宜悉 posted on 2020/03/27
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