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