What the Term “DPF” Means and how it Applies to you.
Especially invented for the Diesel engine, the DPF is the most commonly used exhaust filtering device in compression combustion engines. They are retrofitted or installed ex works inside the system by the original car manufacturer. Single-use filters can be made out of paper or several other materials and are especially used inside the retrofit devices. Once they are saturated with particulate matter they have to be dismounted, in a technically correct manner discarded and replaced. The filtering process with re-usable filters can be obtained by two different methods. The so-called wall flow (filtration efficiency of 99.9%) and flow-through filters (filtration efficiency of 40 – 60% but cannot be blocked) can strain out particles with less than 100 nm in diameter, depending on the actual technique. Honeycomb-like structures conduct the exhaust gas through porous walls or channels, thus the solid particles will be separated from the gaseous flow. The wall flow filters are alternately plugged at the output side (chessboard look) and the exhaust is forced to pass the inlet walls of the filter material (e.g. ceramic, silicon carbide or synthetic cordierite). To regenerate these systems and avoid damaging excess pressure, due to clogged pathways, a certain temperature is needed to activate the catalytic surface of the monolith structures (known as coated DPF or CSF – catalyzed soot filter). The particles get burned at relative low temperatures (350 – 500 °C / 660 – 930 °F), regarding to the temperatures needed without the help of a catalytic compound (more than 600 °C / 1110 °F). The pores are relieved of the soot and particulate matter, carbon dioxide and nitrogen monoxide leave the exhaust pipe. This happens permanently and is known as ‘Continuously Regenerating Trap’ concept (CRT).
Image: DPF “chessbord look” device
The on-board computer of the car controls the sensor based measurements and tells the driver of the car if an extra DPF-generating drive is needed. To activate the CRT process, a certain velocity resp. exhaust temperature is needed. City traffic is not sufficient to gain these physical characteristics. “Only” 200 °C / 400 °F can be reached during stop-and-go journeys. If the driver avoids these recommendations persistently, the DPF will get damaged and a replacement is inevitable. Newer systems can operate this extra burning process whilst the car is not in motion and the revs are accelerated up to 1400 revolts per minute. Alternatively, heating elements (electrically) can support these operation or an extra injection of diesel fuel into the exhaust gas system can heat up the polluted gases, due to, in fact, incomplete combustion (fuel economy is reduced). Additive based systems can lower the regenerating temperature (450 – 550 °C / 840 – 1020 °F) by blending in an additive to the diesel fuel. The impregnated particles deposit on the surface of the DPF filter walls and chemically lower the needed thermal energy. In that way the regeneration of the DPF during city traffic is possible. Though in total more ash is formed with the help of an additional substance inside the diesel fuel.
In selective catalyst reduction (SCR) systems the AdBlue resp. Diesel Exhaust Fluid (DEF, 32.5 Vol.-% urea-solution) is used to chemically reduce the NOx molecules. Several oxidation and hydrolysis catalysts complete the setup. The fluid is brought into the exhaust flow via injection nozzle and is not inside the fuel like in additive based catalysts. The consumption of DEF equals approx. 5% of the fuel amount. The fluid has to be carried in an extra tank and needs to be refilled about every 20th refuelling process.