Diesel particulate filter (DPF, French FAP – filtre à particules ) is an element of exhaust system with diesel engines cars, which catches particulates of soot or ash which are present in combustion gases.
DPF is necessary if the car is to be classified in the category of vehicles satisfying the standard of exhaust gas cleanliness EURO5.
Filter, which is a block of huge wall surface consists of little channels, part of which are blinded at the inlet and part at the outlet. Particles of soot or ash carried by exhaust gases settle down on filter walls while the gas permeates through the porous structure the filter is made of and cleaned from the solid fraction it reaches the further part of the exhaust system. The efficiency of properly working filter is between 85% to almost 100% which allows to reduce a major part of contamination and as an effect to obtain a clearer and more transparent air.
Diesel particle filters may be made of various materials:
Cordierite – ceramic filters made of material similar to that of catalytic converters. They are relatively cheap and they assure high performance efficiency. Its main disadvantage is its rather low melting point (approx. 1200°C), which may cause melting of the filter during regeneration and it depends on the level of its completion.
Silicon carbide – the filter made of this material has higher melting point (approx. 2700°C), however it is less thermally stable – it changes capacity as a result of temperature change. This fault was eliminated by putting SiC elements in clusters and then joining them into larger clusters by means of flexible glue which eliminates thermal expansion of clusters made of silicon carbide, which causes the creation of characteristic grid on the ceramic surface. Silicon carbide filters are characterized by a wide range of diameters of filtered particles 0.2 – 150μm, a high level of filtration up to approx. 95% as well as a high mechanical resistance.
Ceramic fibres – the filter is made of a variety of ceramic fibres which are selected in order to create a properly porous carrier which can be formed into any required shape. Thanks to the use of appropriate fibres, filters with a high level of cleaning can be obtained, as well as filters with a higher flow level, gradually decreasing the cleaning level. Another advantage of ceramic fibre filters is creating a very low back pressure.
Metal fibres – they are basically «woven» into the monolith. These fibres after connecting the power source heat up and allow to reach a filter operating temperature with very low ambient temperatures or with too low exhaust flow volume rate. Inserts with metals fibres and entirely ceramic are generally speaking incompatible, since the former require power connection.
Paper – special filtration paper is used is disposable filters, for instance in coalmines, where combustion gases are first cooled and once cooled they go to the filter; another application of disposable filters is a short lasting use of a vehicle inside the building, such as fork lift truck with a combustion engine. Such a filter is disposed of after its use and exchanged for another one due to their low cost.
Regardless applied DPF, it must be taken care of properly. Continual city drive and short distance drive lead to insufficient heat supply and inability of its «passive» regeneration. Unfortunately, the exhaust gases contain not only particles of soot which may be burnt but also ash and the remains of oil, which can effectively jam the filter. In such a case, the computer signals the user it is necessary to exchange or mechanically regenerate the filter outside the vehicle. In response to strict norms of exhaust gas emission and manufacture requirements, some car manufacturers started to use an addition of precious metals to DPF, which in connection with initial cleaning on the catalytic converter leads to higher oxidation of carbon oxide and hydrocarbons, and a result purer air.