Diesel Particulate Filter DPF

A diesel particulate filter sometimes referred to as a DPF removes the diesel particulate matter (or soot) from the exhaust gas of a diesel vehicle, therefore reducing particulate emissions.

Why Do Cars Have DPF's?

With changes to the car emissions legislation, the 'Euro 5' standards will make diesel particulate filters a commonplace in diesel car exhausts as catalytic converters are in petrol cars.

How Does a Diesel Particulate Work or DPF Work?

Unlike a Catalytic Converter a DPF is not a flow through device, and works by forcing the gasses to flow through the filter. As the channels of the filter are blocked at alternate ends, the gasses are forced to flow through the cell walls in order to exit the filter. As the cell walls are porous, the gasses are allowed to pass through, but the particulate matter is deposited on the cell walls. This ensures that only the clean exhaust gasses can exit, and the particulate matter is trapped in the filter.

Passive Regeneration

Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Many cars don't get this sort of use though so manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Active Regeneration

Active Regeneration occurs when the level of soot in the filter reaches around 45%. The ECU makes small adjustments to the fuel injection timing and increases the exhaust gas temperature. This increases the exhaust temperature which then initiates the regeneration process, burning away the soot trapped in the DPF. If the journey is a bit stop/start the regeneration may not complete and the warning light will come on to show that the filter is partially blocked.

It should be possible to start a complete regeneration and clear the warning light by driving for 20 minutes or so at speeds greater than 40mph.

If you ignore the warning light and keep driving in a relatively slow, stop/start pattern the soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights come on too. At this point driving at speed alone will not be enough and you will have to take the car to a dealer for a forced regeneration.

If you continue to ignore warnings and the soot loading keeps increasing then the most likely outcome will be that you will have to get a the DPF replaced.