Emissions 101

Wednesday 23rd March, 2011
With the Chancellor, George Osbourne, about to announce his 2nd budget today; there's going to be a lot of hot air. Sadly, when it comes to car exhausts, that isn't all we get. For those that would like to understand a little more about vehicle emissions, I'll try to explain in a little more detail.

Fuels like petrol and diesel are composed mainly of hydrocarbons - that is, molecules containing just Hydrogen(H) and Carbon(C) atoms. Diesel, for example, is often assumed to be C12H22 - 12 carbon atoms and 22 hydrogen.

If you burn pure hydrocarbons, in an atmosphere or sufficient pure oxygen, all you will have at the end is CO2, from the oxidised Carbon atoms, and H2O(water) from the oxidised Hydrogen. In our example, C12H22, 1g of fuel will emit 3.2g of CO2 when the 2 heavier Oxygen atoms are bound to the Carbon. The water generated comes out as steam and can easily be seen coming out of the exhaust on a cold morning.

Of course, fuels like petrol and diesel aren't pure hydrocarbons; there are other things in the mix too. Also, we don't burn in anything like pure oxygen - we burn in atmospheric air which is only around 20% Oxygen.

So, on each intake cycle, the engine must take in a measure of fuel and also sufficient air to provide enough oxygen to combust it. On cold days, the air will contain a higher density of oxygen, so fuelling has to be carefully measured to match up to it - why a car uses more fuel but is also a bit more nippy on those cold mornings. Needless to say, even with the use of lamda/O2 (oxygen) sensors in modern engines, it isn't always a perfect science and we don't always get the perfect combustion we want.

If any carbon from the fuel isn't fully oxidised, instead of relatively harmless CO2, you get poisonous Carbon Monoxide (CO). This was choking our towns for years and resulted in the introduction of the catalytic converter - a magic box on the tail pipe that takes in CO and, when hot, gives out CO2. The clever process they use didn't work with lead in the fuel, so you may remember that unleaded fuel was introduced to our pumps at around the same time as cats were fitted to new cars.

So, armed with a cat, we could easily oxidise the poisonous CO to CO2; but it also offered other benefits. If any fuel came out of the engine that hadn't been burned, quite common when slamming the throttle closed at high revs, the catalytic converter can also oxidise this to water and CO2 - like should have happened in the engine.

Any Carbon Monoxide that isn't treated by the cat appears in those CO figures on the VCA document. Likewise, any remaining hydrocarbons appear in the THC or THC+NOx count.

The carbon in the fuel can only come out in 3 forms. The "best case scenario" CO2, the poisonous CO or, finally, completely uncombusted Carbon in the form of soot. On older inefficient engines, the smoke particles were very large and you basically saw smoke out of the exhaust. Or a modern "lean burn" injection engine, soot is kept to a minimum and hard to see but still very much there. The scientists call these particulates and we count those larger than a certain size as pollutants. The number in the name PM10 refers to the size of those soot particles and can be seen in the particulate count on the VCA document.

At the time the cat was introduced, emissions began to change. You could easily recognise a new car with a cat by the characteristic "stink bomb" smell. This was an unwanted side effect as sulphur in the fuel reacted on the cat to form hydrogen sulphide (H2S) gas. This being pretty undesiderable, you will have noticed a recent move to ultra low sulphur petrol and diesel (ULSP/ULSD) which has significantly reduced this unwanted emissions. Also, low sulphur diesel was required to make modern Diesel Particulate Filters (DPF) more efficient. Switching to ULSD allowed vehicle manufactuers to fit new diesels with a DPF and reduce those particulate emissions by catching the soot on the way out and oxidising it CO or, hopefully, CO2.

Sadly, however, Hydrogen Sulphide isn't the only side effect of installing a cat. The air we use to burn the fuel is around 78% Nitrogen. Whilst normally pretty unreactive, in the high exhaust temperatures found in a modern engine, it can easily be undesirably oxidised by the cat into compounds like NO2 which cause ill health and can generate other side effects like acid rain.

The better and more efficient the cat, the more NOx will come out. No cat at all, NOx is at a minimum, but then our friends CO and THC come back to haunt us. Finding a balance is difficult but, given the recent focus on CO2, vehicle manufacturers have a primary target of getting CO2 down which has clearly generated a rise in other emissions. The future introduction of Euro 6 legislation requires these high NOx emissions to be addressed, and companies like Mercedes are making extensive use of urea based additives like Adblue to treat NOx output and convert it back to more healthy compounds like N2O.

I think CO2 taxation was wrong in many ways. I completely agree that we need to reduce our consumption of Carbon based fuels, indeed they may even run out one day, but measuring CO2 wasn't a healthy way of doing it. If you want to ultimately reduce CO2 coming out, you need to put less fuel in. Less in, less out - it's not rocket science. We already heavily tax people on the fuel going in, so we're easily addressing the worst consumers without any need to measure CO2 emissions at all. We know if we put in 1g of diesel, we'll get 3.2g of CO2 out if all is working as intended.

What we need to do is address those who AREN'T emitting CO2. The emissions like Carbon Monoxide and particulates need to be addressed. Unburned hydrocarbons are a waste of fuel and there are measures to reduce NOx albeit at a cost. Carbon usage is taxed at the pump so, in future, I hope to see vehicles being taxed on those more important emissions and the dirtiest engines being taxed the highest.

Get the Vehicles Manufacturers to convert as much of their exhaust as possible to CO2 and then simply tax the fuel to reduce carbon emissions. Job done.

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