Any filter has a finite capacity. Diesel particulate filters must be cleaned out, intermittently or continuously, if they are not to block. This is most important, since an overfilled filter can damage the engine through excessive exhaust back pressure and can itself be damaged or destroyed.
The material trapped in the filter is in most part carbon particles (C) with some absorbed hydrocarbons. There are two principle techniques for removing the particles:
1. combustion with oxygen (O2)
[C] + O2 → CO2
2. combustion with nitrogen dioxide (NO2)
[C] + NO2 → CO2 + NO
The merits of the two different techniques can be summarised as follows:
NO2-based systems are greatly favoured because the reaction takes place at temperatures seen in most diesel exhausts. The leading DPF system around the world is the CRT (Continuously Regenerating Trap) technology from Johnson Matthey, which uses an oxidation catalyst in front of the filter to generate the NO2 required to keep it clean.
Passive and active regeneration
DPF systems that are able to regenerate themselves using only the exhaust gas stream, without additional energy inputs, are known as passive systems. The CRT system is able to function in a wider range of conditions than any other passive system and this is the basis of its success. Its advantage is most clearly seen in applications with low exhaust temperatures, an advantage further enhanced in the CCRT system.
Passive systems are favoured, particularly for retrofit applications, because they require no integration with the engine, no source of energy other than the exhaust gases themselves, and no complicated control systems.
Applications with variable or unreliable duty cycles, or with exhaust conditions not suitable for passive systems, require active regeneration.