Diesel particulate filters (DPF) or FAP (filtre a particules) "catch" and stop harmful particles by means of narrow canals. The filter reminds a honey plaster in structure.
Diesel particulate filters (DPF) or FAP (filtre a particules) “catch” and stop harmful particles by means of narrow canals. The filter reminds a honey plaster in structure. An average diesel engine emits about 50mg of particles per every kilometer. You can easily notice that after covering 100000km the filter keeps as much as 5 kg of soot if it’s not cleaned. Without methods of regeneration which clean these filters they would have to be removed every few dozen of kilometers or be enormous in size. Regeneration means burning out the particles of soot. This process leads to emission of less harmful gases. Unfortunately, a filter can’t be fully regenerated through burning out. This process leaves a small amount of ash forever.
As filter gets dirty the computer which controls the engine starts regeneration mode. How does it know when to do it? There are pressure sensors behind and in front of the filter which measure the exhaust fumes pressure. If the difference between the pressures is too big, regeneration starts. The parameters change, the additional fuel is injected. During normal exploitation conditions the temperature of gases is too low to burn out the soot in the filter. In order to do that there needs to be temperature of 550-600◦C while normal temperature is about 150 ◦C The difference is enormous and during usual driving conditions it’s impossible to reach the temperature mentioned earlier. There are several solutions to deal with the problem.
Most frequently the additional fuel injection is provided while decompression stroke and/or the exhaust. The fuel reaches exhaust system and raises the temperature or it burns earlier and the flame raises the temperature. Sometimes the catalytic converters are used. They are placed before DPF and they help oxidize the soot. The filters are built in right behind the turbo compressor so that the temperature could be high.
Another popular method, used particularly by PSA group is quite the opposite, that is, to lower the temperature of burning the soot of about 200 ◦C by means of chemical substances. Usually a special liquid is used which lowers the temperature. Once in a while it must be added to a special tank next to the fuel tank. Suitable proportions of the liquid are dosed to the fuel. The method described above makes it possible to switch the engine to the regeneration mode, however, lower temperature allows for less aggressive engine regeneration.
Yet another solution is placing an additional fuel injector inside the filter. When the filter is full, the injector doses the fuel necessary for burning out the soot. An attempt is made to burn out the filter. If the conditions are met there’s no problem. Otherwise, the engine will try several times to burn out the soot and if it fails and the filter gets full, the suitable indicator will flash on the dashboard. Then, one has to visit a service station or an engine test stand. The latter allows for conducting the process of burning the soot out while the vehicle is not in motion. At the service station usually the engine is forced to work at full capacity for several minutes. Unfortunately, multiple attempts to burn the soot out can result in undesired situation. The injectors provide huge amounts of fuel, however, under the bad conditions if flows down the cylinder to smearing system. Small amounts pose no threat but huge amounts of fuel contribute to dilatation of oil which makes the oil unusable. Extreme situations (like multiple unfinished regeneration process) lead to the turbo compressor or the engine failure. Also, it happens that the engine ‘runs’ because there’s too much fuel in oil. And that is by no means a pleasant situation to sit in a car with engine working at full capacity which can’t be stopped even without keys in the ignition.
The cheapest way of dealing with filter problems is FAP/DPF removal. However, it’s not sufficient to just remove the filter because the engine ECU turns the engine into safe mode as it sees the lack of filter. In that case it’s absolutely necessary to modify the ECU software.