Decoding DPF

The emission standards are becoming stricter and stricter every year. The pressure is on vehicle manufacturers, and we are now at that phase where diesel engines are slowly vanishing from vehicle manufacturers’ catalogues. I still remember when Maruti Suzuki discontinued all their diesel vehicles in India from April 1, 2020. Enthusiasts missed the Fiat-sourced iconic diesel engine in Maruti cars; critics stated it was a wrong move; however, Maruti Suzuki was spot on and aligned their strategy with the policy. As per many sources, there are companies like Mahindra that still have exponential sales of diesel vehicles compared to petrol. These companies are putting their best in terms of technology and inventions to keep the diesel engine alive and sustain the emission regulations. One of the systems which is there to tackle BS6 norms is the Diesel Particulate Filter (DPF) system.

Lets simplify the DPF System

Exhaust smoke mainly contains - CO, HC, NOx, and Soot (PM).

At the engine, when air (78% nitrogen + 21% oxygen) is at a temperature above 1300°C in the engine cylinder, it changes to harmful NOx (NO & NO2).

Stage 1 - At DOC (Diesel Oxidation Catalyst)

DOC converts CO → CO2 and HC → H2O.

DOC has a catalyst coating of precious metals like Platinum and Palladium, which help O2 react with CO and convert bad CO to less harmful CO2. In the same way, HC from exhaust gases reacts with O2 to form CO2 & H2O. This reaction increases the temperature and generates heat, which is used by the DPF to burn soot.

Stage 2 - At DPF (Diesel Particulate Filter)

It filters soot, the Particulate Matter (PM) particles, and the remaining gas passes away. But when too much soot is generated, the pressure rises, which is detected by the differential pressure sensor at the entry and exit of the DPF. When the pressure at the exit is too low compared to the entry, then the ECU initiates the regen process by increasing the temperature of the DPF chamber and burning the soot (Carbon (soot) + O2 → CO2). In many DPF chambers, fuel injectors are also present, where fuel is injected inside the DPF to burn the soot, especially in those cases where the DPF is not near the engine, due to which it is unable to achieve the temperature required to burn the soot.

Stage 3 - At SCR (Selective Catalytic Reduction)

SCR has a catalyst coating of materials like copper zeolite and iron zeolite, which help DEF/AdBlue (33% urea and 67% water), which is injected into the SCR, where water is evaporated due to heat and ammonia (NH3) reacts with NOx (bad gases - NO & NO2) to become N2 & H2O (water).

There is one nitrogen oxide sensor at the entry of the SCR and another at the exit. It calculates NOx and gives the information to the ECU; accordingly, the ECU adjusts the DEF dosing.

Note: Pure Ammonia is not used as it is toxic, corrosive, and difficult to store.

Stage 4 - At ASC (Ammonia Slip Catalyst)

ASC removes excess ammonia (strong smell, eye irritation, and respiratory irritation). During AdBlue dosing, if sometimes excess ammonia is injected, it slips to the ASC, where it reacts with a platinum-based catalyst, and ammonia (NH3) + O2 forms N2 & H2O.

The oxygen sensor before the DOC and after the ASC helps in optimizing the DEF dosing.

Now let’s take an example of the vehicle Mahindra Thar Roxx 2.2L 130 mHawk BS6 diesel engine. In the diagram, the turbo is connected to the DOC (Mahindra calls it CatCon), which is further connected to the sDPF (Selective Diesel Particulate Filter), which comprises DPF, SCR, and ASC in one complete unit, which they call sDPF.

Types of DPF Regeneration Process




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