The first thing to say about diesel engines is that they always run ‘lean’.
One outcome of this is that the catalytic converter can only operate in a ‘two-way’ mode which means that it can oxidise carbon monoxide and hydrocarbons but will not be able to reduce Nox.
For this reason, a catalytic converter used on a diesel engine is often known as a ‘diesel oxidation catalytic converter’.
Another important characteristic of diesel engines is that the exhaust gas is cooler than that of spark-ignition engines, in some cases falling to a temperature of barely 100 C at idle.
Since catalytic reactions are temperature dependant, it is a good idea to find out the exhaust gas temperature for a particular application at an early stage.
Coatings on diesel catalytic converters can be varied to give ‘light-off’ at different temperatures, as illustrated on the following graph:
Users and manufacturers of diesel engines are often keen to reduce particulate emissions, and although catalytic converters are not designed for this purpose, they do in fact have a small beneficial impact for the following reason.
Diesel particulate matter largely comprises carbon (soot) but around the outside of the carbon particles there is a layer of hydrocarbons which add to the mass of particulate matter. Since a diesel oxidation catalytic converter can oxidise hydrocarbons, it can reduce diesel particulate matter to a certain extent, but usually only 10-20%.
Achieving greater reductions than this will necessitate reducing the carbon component and this requires the fitment of a filter.
As described above, the main benefits of a diesel oxidation catalytic converter are its ability to oxidise toxic carbon monoxide and hydrocarbons into non-toxic carbon dioxide and water. Unfortunately they also tend to oxidise the slightly toxic nitrogen oxide into rather more toxic nitrogen dioxide, especially at higher temperatures.
The more active a catalyst is at achieving the beneficial reactions, the more it will produce this undesirable side effect. A common approach to this problem is to reduce nitrogen oxide emissions at source through a technology such as exhaust gas recirculation, but if this is not practicable then it will be necessary to find the coating which gives the best trade off for a given application.
Another consideration when specifying catalytic converters for diesel engines is the sulphur content of the fuel.
Sulphur disagrees with catalytic converters for two reasons – firstly the catalytic converter will oxidise it into a solid which adds to the diesel particulate matter, and secondly the sulphur will gradually deactivate the catalytic coating.
As catalytic converters have become ubiquitous on road vehicles, the sulphur content of forecourt diesel has been progressively reduced. In order to allow exhaust gas after treatment technology to be fitted to off-highway engines, the sulphur content of off-highway fuels such as gas oil or red diesel has also been cut in markets such as the European Union and USA.
Outside these regions, or if there is any doubt, it is essential to check that the sulphur content of fuel used in conjunction with catalytic converters is no more than 50 parts per million.