Emissions: Oxides Of Nitrogen
by David Finlay (16 February 2000)
As mentioned in our emissions overview article, one of the three types of toxic substance that come out of a car's exhaust, and require attention if care of the environment is an issue, is the group of oxides of nitrogen (NOx).
About 80% of the Earth's atmosphere consists of nitrogen, which is an inert gas and therefore not particularly interesting in most circumstances. When you breathe in, for example, the small amount of oxygen in the air contributes to your well-being, but the nitrogen just sits there doing nothing until you breathe out again.
However, as temperature increases, nitrogen starts combining with any unburnt oxygen left over from a combustion process such as sparking off the fuel/air mixture in the cylinders of a car engine. This is where the bad news starts, because the result of combining nitrogen and oxygen is NOx.
NOx is particularly unfriendly stuff. On its own it can cause major respiratory damage if you breathe in too much of it. That's not a major issue in a car unless you have your mouth wrapped round the exhaust pipe, but NOx performs another nasty trick when it comes into contact with water in the atmosphere.
NOx plus water generally equals nitric acid, which when it occurs high in the atmosphere and falls to the ground is known as acid rain - the very stuff which is damaging Bavarian forests and killing the fish in Norwegian fjords.
As if that wasn't enough, NOx is also one element in the complex photochemical reaction which causes smog and makes life in places like Los Angeles and Mexico City such a misery.
In fact, smog-polluted cities get that way largely because climatic effects prevent NOx being blown away, and in any case cars produce very little of it. But it's so nasty that measures are constantly being taken to reduce output still further.
From January 1 this year the European petrol-engine maximum for NOx is 0.15 grams per kilometre (0.5 for diesels) travelled over a set test route (driven on a dynamometer rather than in the real world). In 2005 the limit goes down to 0.08g/km (0.25 for diesels), and most cars produced today can achieve that easily.
These figures are so tiny - less than a millionth of the total exhaust emission - that the NOx contained in the air going into the engine has to be carefully checked and discounted from the end result, which would otherwise be significantly affected by it.
How do you contain the amount of NOx produced by an engine? The easiest way is to combine it with water, but then we're back into the nitric acid scenario, and you definitely don't want that stuff shooting out of the tailpipe.
Catalytic convertors help to some extent by removing the oxygen from NOx and turning it back into plain old safe nitrogen. But it's even better to persuade the engine not to create so much NOx in the first place.
The most common method of doing this is by way of an Exhaust Gas Recirculating valve (EGR). Controlled both by vacuum and by temperature, this feeds exhaust gas from the manifold straight back into the engine, effectively damping down the force of the combustion by smothering it in smoke. The temperature of the explosion is therefore reduced, as is the tendency of the nitrogen and oxygen to combine.
As you can imagine, efficiency suffers quite badly and leads to worse performance and greater fuel consumption. The only good side to this is that modern ignition and fuel injection has made car engines hugely more efficient than they were ten years ago, so the overall effect is still positive compared with what it used to be.
Virtually all the nitrogen in a petrol engine which is converted (a relatively small percentage of the total) becomes nitric oxide (NO). The molecular arrangement of diesel fuel, on the other hand, means that it burns at a lower temperature than petrol, though the air temperature is much higher because of the greater compression ratio.
The upshot of this is that about a third of the nitrogen converted in a diesel engine becomes nitrogen dioxide (NO2), which is even more dangerous than NO. Fortunately the EGR method works just as well for both.
Direct injection petrol engines have an inherent problem when it comes to NOx because they use high compression ratios and, in some operating conditions, very lean fuel/air mixtures. Each of these factors increases temperature, which leads to a risk of high NOx output.
On the other hand, imagine the combustion temperatures involved in an aeroplane engine, or a power station. Then ask yourself exactly what it is that is causing such havoc to German trees and Scandinavian fish.
