In the face of increasingly stringent exhaust emission regulations, manufacturers needed a method of capturing and oxidising collected soot particles. Enter the DPF (Diesel Particulate Filter), which is now standard equipment on most modern diesel vehicles.
While advanced engine designs and ultra-high fuel injection pressures have made diesel engines cleaner and more efficient than ever before, none of these advances has the ability to control or prevent the formation of solid particulate matter, also known as “soot”, in diesel exhaust. Generally speaking, soot consists of conglomerations of unburned or partially burned hydrocarbons, and their formation is a direct result of the diesel combustion process.
In terms of their construction, DPF's somewhat resemble catalytic converters and silencers, but are fitted to the terminal ends of exhaust systems. Although DPF's differ in both size and physical appearance, they all contain a core through which the exhaust gas flows. The core is designed so that the total surface area of the core is much greater than the inner dimensions of the canister to maximise the contact area between the core surface and the exhaust gas that flows over/through it.
As the soot-laden exhaust gas flows through the core, the soot particles stick to the core material, where it collects until the ECU determines that the ability of the core to collect more soot has fallen below a minimum allowable threshold. To do this, the ECU receives input data from two exhaust gas pressure sensors; one located downstream from the DPF, and another that is located upstream of the DPF.
By monitoring both sensors in real time, the ECU uses the progressively increasing pressure differential across the DPF to calculate an efficiency value, and when this value falls below a minimum allowable limit, the ECU initiates a process to burn off, or oxidise the soot in order to regenerate the DPF’s ability to collect more soot.
Regeneration can take place in one of several ways. One way is to make adaptations to the fuel injection timing to allow some combustion to occur in the exhaust manifold, which raises the temperature of the exhaust gas to the point where the regeneration process is initiated and sustained until regeneration is complete.
Another way involves the injection of precisely metered amounts of a liquid reductant that consists of a mixture of 32.5% high-purity urea, and 67.5% deionised and demineralised water. This mixture is commonly known as AdBlue®, and is stored in a dedicated storage tank on the vehicle. Nonetheless, the injection of the mixture initiates a chemical reaction that raises the temperature of the DPF core to the point where regeneration can be sustained until the process is complete.
Both of these processes occur automatically, without any action required from the driver. While the first process is generally reasonably reliable, the second process is less so, which has direct negative consequences for the maintenance costs of diesel vehicles that use chemically induced DPF generation processes.
Although a failure to initiate DPF regeneration can happen on any diesel vehicle, this is more common on vehicles that use the temperature of the exhaust gas to initiate the process, and especially on vehicles that are used only for short trips like everyday city driving.
Under these conditions, the exhaust gas can never reach the temperature required to initiate the regeneration process, with the result that the DPF can become clogged with soot. In some cases a professional technician may be able to regenerate the DPF by performing a forced regeneration with the aid of a suitable scan tool or diagnostic computer. However, in severe cases, this might not be sufficient to regenerate the DPF successfully, in which case, the only effective remedy is replacement of the hugely expensive DPF with a new unit.
While most of the issues and their associated costs can be avoided by having your vehicle professionally inspected on a regular basis, there is one particular issue on many diesel vehicles that affect the health and operation of DPF devices more severely than almost any other, this issue being excessive oil consumption which you can learn more about here.