The Diesel Particulate Filter (DPF) is considered to be one of the most troublesome elements of modern diesel cars. It is also very expensive. How is a DPF filter built? How should a car with a DPF filter be operated to avoid breakdowns? How to protect the DPF filter from clogging?
The first diesel particulate filters appeared in cars with diesel engines as early as the 1980s. They allowed to eliminate the typical black smoke from exhaust pipes. DPF was used then sporadically, among others in Mercedes with three-liter engines.
No modern diesel engine has ever been able to fully mix and completely burn fuel and air during operation. And they are – the residual fuel and air are the cause of soot, i.e. solid particles. Carbon black particles are composed of carbon, hydrocarbons, sulfates, sulfur and metal oxides, and water.
In 2006, the Euro 4 emission standard entered into force in the EU. It drastically reduced the possibility of particulate matter emissions by diesel cars – from 0.05 g / km (Euro 3) to 0.009 g / km. And then car manufacturers went back to their familiar solution – diesel particulate filters. The filter has become an indispensable element of diesel equipment.
Technical solutions for permanent filters went in two directions. Most auto companies have started using standard DPF filters. French car manufacturers (Renault, Peugeot, Citroen) have introduced FAP filters, the so-called Wet.
DPF particulate filter – construction
The diesel particulate filter is part of the exhaust system. It is mounted after the catalytic converter. It also looks like a catalyst – it is placed in a round or flat metal can.
During many years of operation of a number of cars with diesel engines, it turned out that the closer the filter was to the engine, the slower it clogged. It is logical – the fumes that had a higher temperature went to it. The DPF particulate filter is controlled by the ECU. The computer collects information from pressure sensors (exhaust gas differential pressure sensors) mounted upstream and downstream of the DPF. There are also exhaust gas temperature sensors in front of and behind the filter. What’s inside a round or flattened metal case? Ceramic insert with a structure made of parallel channels. The structure is structurally very complicated. Each channel is blanked off – the inlet channels (on the engine side) are blanked off on the muffler side. In turn, the exhaust channels are clogged on the engine side. So how does the exhaust gas leave the diesel particulate filter? Between the channels there are walls (the so-called Wall Flow) with porous microchannels. Their size is selected so that they trap solid particles. What makes filters so expensive? Complicated construction and the fact that the inside of the channels is covered with platinum or other precious metal. Particulate filter – maintenance free? The particle filter was to be a maintenance-free element. The exhaust from the engine was supposed to go to the channels and the solid particles to be trapped on the walls between the channels. The solid particles were to be burnt during the typical operation of the car, when the temperature of the exhaust gases can range from 400 to 600 degrees C.
If the filter started to clog, the problem was to be solved by active regeneration, i.e. extra oil injection and burning of the soot. In practice, however, it doesn’t work that well. In addition, the diesel particulate filter requires a fully functional engine to function properly. Blows from the crankcase to the combustion chamber, engine oil consumption due to leaky valve seals, or driving with leaking injectors – all this causes accelerated clogging of the DPF filter. Some manufacturers wanted to eliminate the problem of DPF clogging, further complicating its construction. An example can be diesel engines from the D-CAT family, developed by Toyota, equipped with a complex DPF filter, which also combines the functions of SCR (Selective Catalytic Reduction) systems introduced many years later. The manufacturer placed a fifth Common Rail injector in the filter (the same as in the car’s engine), which was designed to directly inject fuel into the filter in order to burn off the soot in it. This solution allowed to significantly extend the life of the filter. However, the clogging was not completely eliminated, and the D-CAT filter was much more expensive than the standard filter, and the engines with this filter themselves suffered from other ailments. So is the DPF particle filter maintenance-free? On the one hand, yes, because you do not need to add anything to it (unless we will use an additive to the tank to lower the soot auto-ignition temperature). On the other hand, the filter requires keeping the engine in full working order, using special engine oils, as well as proper car operation.