Catalytic Converters
The catalytic converter is the longest-established and most widely used exhaust
gas aftertreatment device. It is located in the exhaust system so that hot exhaust
gases flow through it and come into close contact with catalytic material which
promotes various reactions between the constituents of the exhaust gas. These reactions
can be divided into two categories – 'oxidation' and 'reduction'.
Oxidation reactions
CO
(carbon monoxide) |
+ |
02
(oxygen) |
= |
CO2
(carbon dioxide) |
Hx Cx
(hydrocarbons) |
+ |
O2
(oxygen) |
= |
CO2
(oxygen) |
+ |
H2O
(water) |
Reduction reaction
NOx
(oxides of nitrogen) |
= |
N
(nitrogen) |
+ |
O2
(oxygen) |
It can be seen from the above that in the
oxidation reactions, pollutants which arise from partial combustion of the fuel
are oxidised to form non-toxic products of complete combustion. In effect, a second
combustion process has occurred which has compensated for any failures of the process
which occurred in the cylinders of the engine. In the reduction reaction, oxides
of nitrogen, which were the product of an unwanted reaction occurring between atmospheric
oxygen and nitrogen in the engine, are broken down into their constituent parts.
Catalytic converters are often described as 'two-way' or 'three-way', the difference
deriving from the type of catalytic material they are coated with. Two way catalytic
converters have coatings which only promote oxidation reactions, whilst the three-way
versions have coatings which give rise to both oxidation and reduction reactions.