Diesel particulate filters

What is a particulate filter?

Particulate filters are one of the main elements that vehicle manufacturers rely on to ensure that vehicles meet the limits set by emission standards. Despite advances in engine efficiency, even today it is not uncommon to see vehicles with black clouds coming out of their exhaust pipes especially when accelerating sharply. This smoke is a sign of inefficient combustion, which leaves a lot of residues.

Polluting gases, such as carbon monoxides or hydrocarbons are treated by catalytic converters, but what about solid micro soot particles? They are also a serious health threat, as they are easily spread. Due to their size, they easily enter the lungs and are linked to respiratory, cardiovascular and cancerogenic health problems. Particulate filters are used to eliminate them.

What is the purpose of particulate filters?

Like catalytic converters, particulate filters are exhaust emission control devices.  Their purpose, however, is to retain the solid soot particles resulting from the operation of the combustion engine and to subsequently disintegrate them. 

As mentioned above, they were introduced in diesel vehicles as early as 2000 and are currently used in practically all of these. Although the concept is the same, the technology is still in development in petrol models, as they typically work better at higher temperatures and do not produce as much particulate matter.

It is common for the exhaust line to include a precatalytic converter positioned at the front, or even inside the metal casing itself. As we will see below, particulate filters require high temperatures to work correctly, so they are usually located near the engine, where the exhaust gas is hotter. Externally they vary in size and shape, depending on their location in the vehicle. 

Resembling a catalytic converter or a silencer in appearance, they are also often provided with thermal protection. Internally they consist of a ceramic substrate with alternately arranged non-through channels held in place by a thermal protective band between the ceramic and the steel casing. 

How do particulate filters work?

The incoming exhaust gases pass through the porous ceramic walls and exit the other end heading toward the exhaust pipe. The soot particles, on the other hand, are retained and gradually accumulate. When a certain level of accumulation is reached, a process called regeneration must take place. Similar to pyrolysis, regeneration is a temperature rise of the exhaust gases to 700-800ºC, which disintegrates the stored particles so that they can pass through the ceramic and out of the filter. This regeneration can be achieved either passively (by high-speed driving) or actively (with engine management intervention, via post-injections to increase the temperature). Some vehicles (particularly PSA group models) are equipped with a system that mixes a special additive with the fuel to ensure that the particles disintegrate at a lower temperature. 

The main problem encountered with this component is excessive soot build-up, which could block the exhaust line and prevent the vehicle from operating.

There are different strategies to monitor the degree of accumulation (by the time since the last generation, by means of a differential pressure sensor between the inlet and the outlet of the filter…) to ensure that the filter never becomes blocked. 

If a certain threshold is exceeded, the dreaded particulate filter warning light on the instrument panel comes on. One way to clean the particulate filter is to drive for about 15 minutes at continuous high speed in a lower gear than usual so that the temperature rises, the regeneration cycle is completed, and the filter is ready to continue performing its function. However, if this warning is ignored and soot build-up increases, it is possible that a certain level may be exceeded which prevents regeneration and the only solution is to replace the particulate filter.  

At Kat Europe we have a wide range of particulate filters to cover the needs of over 90% of the European car fleet, offering the best prices in the market and fast and efficient shipping. If you have any questions concerning particulate filter performance or how to find the right one for your vehicle, please do not hesitate to contact us or to visit our help centre.