A Diesel oxidation catalytic converter ("DOC") is a type of 2-way catalytic converter particularly suited for use on a Diesel engine. The principal differences between a DOC and a 2-way catalytic converter used on a spark-ignition engine are as follows:
- a substrate with larger channels ("cells") is used, in order to reduce the likelihood of blocking by soot
- coating is designed to begin promoting oxidation reactions at the relatively low temperatures found in Diesel exhaust gas
- the coating is also designed to minimise the oxidation of sulphur dioxide to sulphate particulate matter
The extent to which a catalytic converter decreases pollutants in the exhaust gas is known as its 'conversion efficiency'. The main factors determining the conversion efficiency of a catalytic converter are as follows:
- surface chemistry of the coating
- temperature of the exhaust gas
- the length of time the exhaust gas spends in the catalytic converter (known as 'space velocity')
The following graph shows the conversion efficiency of a typical DOC for gaseous pollutants:
Effect of a Diesel oxidation catalytic converter on particulate matter
Catalytic converters are normally associated with the task of promoting reactions between gases and vapours, and as particulate matter is a solid, expectations should not be too high. However, if Diesel particulate matter is examined closely, it is found to have two distinct components: a core of carbon and an outer layer of hydrocarbons. This outer layer is known as the 'soluble organic fraction' or 'SOF' and makes up from about 5 to 80 percent of the total particulate matter. Particulate matter tends to contain a larger percentage of SOF when the engine is running under low load conditions and the percentage of SOF reduces as engine load increases. Diesel oxidation catalysts can oxidise up to about 75% of the SOF but have almost no effect on the carbon. For that task, a Diesel particulate filter is required.
