The Regeneration Process

Regeneration is the term used to describe cleaning of the DPF. During this process the ‘wet and sooty’ particulate matter is subjected to further combustion inside the DPF itself. Surface temperatures inside the DPF can exceed 600 Degrees Celsius, at which point the soot turns into a hot white ash. Some of this ash is blown out of the exhaust whilst some is retained inside the core of the DPF. There are 3 methods diesel particulate filters can be regenerated: Passively, actively or forced.

Passive regeneration

This type of regeneration takes place automatically but only if the vehicle is used often and for medium to long journeys. What also helps is if the vehicle is driven hard where the DPF benefits from plenty of hot exhaust gas flow. Brisk driving helps generate higher exhaust gas temperatures with the added benefit of forcing some of the ash waste out of the exhaust system.

Active regeneration

If the engine management system detects that the vehicle isn’t being used in a manner that allows passive regeneration or it detects a higher than normal DPF exhaust back-pressure reading, it should initiate active regeneration. This function occurs automatically but only when certain conditions are met.

During active regeneration diesel is injected into the engine on the exhaust stroke which allows it to pass unburned through the open exhaust valves and into the DPF, where it combusts using the excess abundance of unused oxygen. During this forced regeneration temperatures in the core of the DPF can exceed 600 Degrees Celsius. Active regeneration normally occurs whilst the vehicle is being driven at steady motorway cruising speeds and may last around 30 minutes before the cycle is complete. The problem however, is how the engine management system can ever know how long the vehicle is going to be driven at steady motorway cruising speeds (and of course it can’t), which can lead to difficulties in initiating active regeneration.

It is not unusual see excessive smoke being emitted from the tailpipe of a diesel vehicle that is being driven at speed during regeneration or for the owner to observe a dramatic decrease in the instantaneous MPG during this process. Sometimes, if a vehicle is stopped and re-started during regeneration, interrupting the DPF regeneration process, excess smoke can be seen coming from the tailpipe. Some, but not all, vehicles will alert the driver with a dash warning light if the DPF regeneration cycle is taking place or, more importantly, has not been completed for whatever reason. Should this occur the driver must refer to the owner’s manual.

Forced Regeneration

If both passive and active regeneration attempts have failed and, as a result exhaust back-pressure has become excessive, the vehicle will soon go into ‘limp mode’, lose most of its engine power and  be almost impossible to drive. When this happens the only option left is a forced regeneration. This process involves using a Main Dealer level diagnostic Scan Tool, connected to the vehicles 16 pin OBD (on board diagnostic) socket, which instructs the vehicle’s engine management system to regenerate the DPF. This procedure is sometimes programmed by the vehicle manufacturer to ideally take place while the vehicle is stationary but, more often than not, during a long high-speed drive cycle, which in cold conditions makes ideal regeneration temperatures difficult to achieve. On many occasions this forced method of re-generation is disabled for safety reasons, even with the Dealer level scan tool, and when this happens it is normal practice to have the DPF replaced.

Alternatively, some common rail diesel specialists, such as CRD Performance, may be able to initiate a forced regeneration using methods not commonly used (or known); one of which involves using a Rolling Road. Sometimes regeneration fails to recover a blocked DPF, either because the material inside the unit has melted or disintegrated or it’s just too badly blocked.

DPF Cleaning

Where DPF regeneration has failed or not been possible due to excessive soot contamination there is a further option of a chemical clean to be carried out. This process involves using several expensive chemicals and the procedure must be carried out exactly in accordance with the fluid manufacturer’s instructions and takes about 3 to 4 hours. Following this cleaning process a full regeneration still has to be carried out to completely remove and burn off any chemicals from inside the DPF. The downside to this is that it is not always successful and is a very expensive process of which the customer will have to pay for.

Successful Regeneration

If regeneration has worked there are now two options to consider; either to leave the DPF in-place or have it removed. The correct decision must take into account the vehicle’s intended use and driving style. If the vehicle is to be used for short, stop-start journeys, with few frequent, long distance runs, then removal is probably the best option. If it is likely that the vehicle will be driven hard, with frequent, long journeys then it may be better to leave the DPF in-place, to save the cost of removal. It may be that you’ve recently purchased the vehicle and the previous owners driving style was responsible for blocking the filter, while yours might not.