The chemical reactions which occur during the oxidation of pollutants are usually ‘exothermic’, which means that they give off heat.
In most applications, the goal is to minimise the emissions of pollutants and the heat produced is an unwanted by-product.
However, sometimes the heat can be put to good use, for example to raise the temperature of exhaust gas so that it can help in the regeneration of a diesel particulate filter.
The temperature of a catalytic converter derives from two sources:
- the temperature of the exhaust gas passing through it,
- the chemical energy which is released when pollutants or fuel are oxidised.
In some cases, this second component can be responsible for an increase in temperature of several hundred degrees centigrade.
One useful application of this ability is where a diesel engine is deliberately over-fuelled and a catalytic converter is used to extract some of the chemical energy in this fuel to raise the exhaust gas temperature upstream of a diesel particulate filter.
The excess fuel required may be delivered via the standard fuel injectors, if the injection system is the ‘common-rail’ type, or alternatively an additional injector with its own pump can be installed into the exhaust system for this purpose.
Where an exothermic catalytic converter is installed on a spark ignition engine it is also possible to obtain very high surface temperatures, in excess of 1000 ℃, with the key requirements being that the exhaust gas contains some unburnt fuel and enough oxygen to burn it.