Oxides of Nitrogen
The term 'oxides of nitrogen' (also known as Nox) refers to a group of compounds
each containing one or more atoms of oxygen and one of more atoms of nitrogen. They
are unique amongst exhaust gas pollutants in that they do not derive from the fuel
supplied to the engine. Instead they are formed as oxygen and nitrogen (which are
both present in the intake air) react with each other in the very high temperature
and pressure environment of the combustion chamber. For this reason formation is
greater under high-load conditions, and diesel engines are bigger producers than
spark-ignition engines because of the higher compression ratios they employ.
By
far the most common of the Nox group is nitric oxide (NO) which is fortunately one
of the least toxic. More of a concern is nitrogen dioxide (NO2) which affects the
respiratory tract and lungs, and can have especially pronounced effects on asthmatics.
As far as the wider environment is concerned, nitrogen dioxide reacts with hydrocarbons
in the presence of sunlight to create ozone and photochemical smog, and is also
involved in the formation of acid rain. Unfortunately nitric oxide can be converted
to nitrogen dioxide by some emission control systems and this is a problem which
needs to be taken into account when selecting a system.