2.4. Diesel particulate filters
As with catalytic converter monolith, there are metallic and ceramic filter systems. So far, however, only ceramic filters have been used in passenger car applications. The method of installing and mounting ceramic particulate filters in the metal housing is the same as the process used for catalytic converters [1].
In the same way as the ceramic monolith for the catalytic converter, the ceramic particulate filter is made up of a large number of parallel channels. However, these channels are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material the filtration efficiency of these filters can attain up to 97%.
The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in two alternative processes.
2.4.1. Passive process
In the passive process, the soot is burnt off by a catalytic reaction. For this purpose, catalytically active iron and cerium containing additives in the diesel fuel reduces the flammability of the soot particles to normal exhaust-gas temperatures.
Other future passive regeneration options include catalytic coated filters or the Continuous Regeneration Trap process (CRT), which are still on the lab bench.
2.4.2. Active process
In the active process external measures are implemented to heat the filter to the temperature necessary burning off the soot. This rise in temperature can be achieved by a burner mounted upstream of the filter or by secondary injection initiated by the engine management and the use of a catalytic converter. The carbon containing compounds in the particulates can be oxidized at temperatures above approximately 600 °C into nontoxic CO2 using the excess oxygen present in the exhaust gas. (Fig. 7)