Catalysed Diesel Particulate Filters (cDPF)
Since about 2005, particulate filters have been generally used with modern diesel engines to remove diesel particulate mass emissions (PM) while significantly reducing the emitted particulate number (PN).
The most effective type is the so called wall-flow filter. Particulate matter is removed from the exhaust by physical filtration using a honeycomb structure similar to an emissions catalyst substrate but with the channels blocked at alternate ends. The exhaust gas is thus forced to flow through the walls between the channels and the particulate matter is deposited as a soot cake on the walls. Such filters are made of ceramic honeycomb materials (cordierite, silicon carbide or aluminium titanate).
Ceramic wall flow filters almost completely remove the carbon particulates over a wide range of engine operating conditions. Since the continuous flow of soot into the filter would eventually block it, it is necessary to 'regenerate' the filtration properties of the filter by burning off the collected particulate on a regular basis. Therefore, the exhaust temperature in Diesel passenger car applications is frequently increased until collected soot is burned off. In order to reduce the burn-off temperature, the filter is usually coated with an oxidation function.
This catalytic coating also contributes to the reduction of CO and HC due to oxidation and in addition it can form NO2 to support the functionality of an SCR catalyst.