The functioning of the diesel particulate filter
99.9%
A particulate filter is a device on diesel engines to separate particles from the engine exhaust gases. With a particulate filter in the exhaust stream, up to 99.9% of particles can be removed.
Particulate matter is defined as anything that can accumulate on a special filter paper through which diesel exhaust passes during a defined test procedure. Diesel particulate matter consists of hydrocarbons from the fuel and the lubricating oil, carbon and inorganic substances such as sulphur and ash.
A particulate filter must be regenerated regularly. This can be done passively where the filter is regenerated during normal operation or actively where the temperature in the filter is increased in different ways. For this to work better, the oxidation temperature of soot can be reduced by using additives in the fuel or by increasing the proportion of nitrogen dioxide in the exhaust gases. Nitrogen dioxide oxidizes soot at a lower temperature than oxygen and can be formed by oxidation of nitric oxide over a catalyst before the filter. The particulate filter can also be coated with catalytic material that contributes to the oxidation of nitrogen oxide.
In active regeneration, the temperature in the filter is increased to accelerate the oxidation of soot. This can be done by additional injection of fuel, either into the cylinder to increase the exhaust gas temperature or through an oxidation catalyst mounted before the filter in the exhaust gas stream. The oxidation of the fuel over the catalyst leads to an increase in temperature which accelerates soot combustion. For fuel to be oxidized in the catalyst, the catalyst temperature must exceed about 250 °C, which can be a problem in some applications where the exhaust gas temperature is low for a long time. Other ways to regenerate the filter are by using a burner before the filter or by electrical heating, either of the filter or the exhaust gas.
Most modern diesel passenger cars have active control of the particulate filter with pressure drop measurement and temperature measurement (before and after the filter). Advanced control algorithms determine when regeneration is required and monitor the condition of the filter. Often a combination of passive and active regeneration is used.
Today, retrofit CCT systems (active particulate filters) are available that can be applied to older machines that were not originally equipped with filters. These systems normally operate with a separate fuel injector in the exhaust pipe. Earlier retrofittable particulate filters were often completely passive, resulting in frequent regeneration problems. Some systems operate with a separate regeneration system with electrical heating of the filter, but these usually require the filter to be dismantled or the machine to be shut down for an extended period of time.
There are different types of particle filters: ceramic, sintered metal filters, and also so-called partial filters, which, however, have a low level of particle purification.
