Emission Systems
DPF Technology and TroubleshootingThe growing popularity of diesel cars and light commercials over the last 4-5 years, means that a wave of vehicles are now moving out of their manufacturer’s warranty period, and DPF (Diesel Particulate Filter) replacements are starting to be required. In order to clean up some of the thick black particulate content found in diesel exhaust emissions, Diesel Particulate Filters (DPFs) were widely introduced on PSA Group vehicles from 2000 onwards, with the mainstream German marques following shortly afterwards.
So what’s the difference between the two main types and how do they fail? Both types of DPF are mounted in-line after the outlet manifold and before the first silencer box, and work by physically trapping soot particles in a ceramic matrix. The matrix looks a lot like a catalyst ceramic matrix, but does not change the state of the emissions passing through it on a continuous basis. It provides a simple labyrinth path for the exhaust gases to travel through and deposit the solid material within it. As the pathways slowly fill-up, a back-pressure increase is produced over time. Both types sense this, and then at a set-point, start an onboard ‘regeneration cycle’ to burn off the residue and start again. Failure modes include simple mechanical impacts from speed ramps, stones and other road debris. These impacts can damage the ceramic element, commonly resulting in the burn-off process not working properly, and the filters get too clogged to clean themselves and require replacement. PSA Group vehicles – Peugeot and Citroen – use a barometric pressure differential to detect soot build-up; this measures the pressure drop across the filter. The regeneration process works above 600 degrees Celsius, otherwise it stays in large particle format. Diesel exhaust emissions do not normally reach that temperature so a fuel burn catalyst is used to reduce the activation temperature of the burn cycle. Once activated, the regeneration process uses the fuel injection system to introduce a fuelborne catalyst from a small tank into the exhaust gas. The catalyst then starts the burn process throughout the ceramic filter substrate. Failure mode can happen if the reservoir is dry and the catalyst becomes blocked. The alternative, preferred by the German marques, works on a heat cycle, where the regeneration system is passive and relies on higher periods of engine revs to burn-off the soot for a short period. A vehicle’s ECU monitors the temperature and pressure within the exhaust system, and the engine management system will then periodically adjust the engine settings to raise the exhausts gas temperature to start the regeneration burn-off process. The downside is that if the car is kept at low speeds and low mileage, then the cycle won’t kick-in, and when there is too much back pressure, the car can’t overcome the restriction and it causes engine problems and loss of power, resulting in replacement being needed. Both designs of DPF look similar, but they are in no way interchangeable, so you’ll need exactly the right part number for each make and model of vehicle. Using a catalogue from a quality supplier such as Klarius will ensure that you won’t go far wrong. |
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