What is a DPF?

A DPF is a soot, or particulate matter filter for diesel engines. They are used to reduce the exhaust emissions as required by many governmental emission legislative bodies. DPFs are becoming more and more common on diesel engines.

How do they work?

Diesel Particulate filters (DPF) or ‘traps’ do just that, they catch bits of soot in the exhaust.

As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. For a DPF this process is called ‘regeneration’ – the accumulated soot is burnt off at high temperature to leave only a tiny ash residue. Regeneration may be either passive or active.

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

When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the journey’s a bit stop/start the regeneration may not complete and the warning light will illuminate to show that the DPF is partially blocked.

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

How is the DPF cleaned and what is DPF “regeneration”?

A DPF filter can hold a certain amount of soot, but not a huge quantity, so it needs to regularly go through a process of cleaning itself or “regeneration” in order to clear out the soot and allow the vehicle to operate properly. Regeneration occurs ONLY when the filter reaches a sufficiently high temperature allowing the soot to be converted to a much smaller and less harmful by-product, ash.

To allow the filter to automatically regenerate, the engine has to be used regularly at a sufficient engine speed, to ensure a high enough temperature of the exhaust gas. The engine must be under sufficient load to ensure that the exhaust gas temperature is naturally raised. Although it may vary from manufacturer to manufacturer, typically a vehicle must be driven at 80km/h or above for at least 20 minutes in order to automatically regenerate the filter. During the regeneration phase, high temperatures in the filter may cause a slight smell, especially during the first regeneration. Many people would have smelt and complained to the manufacturer about this.

What happens if it doesn’t regenerate?

If the vehicle is not driven in a way that automatically regenerates the particulate filter, it will build up an excessive amount of soot, which, if not resolved, will reduce the performance of the vehicle and damage the filter.

If the filter does build up too much soot, a vehicle warning light will appear to alert you. The problem can usually be solved by allowing the filter to automatically regenerate until the warning light goes out – i.e. by driving the vehicle at 80km/h or above for at least 20 minutes. Check the vehicle handbook for specific guidance on a particular vehicle.

If traffic conditions and speed limits do not allow the vehicle do be driven so that the filter regenerates, it will have to be returned to a dealer for a forced regeneration in order to clear the filter.

If the warning light is ignored and the vehicle is driven without regenerating the filter, it will cause damage to the vehicle, which may not be covered by the vehicle manufacturer’s warranty.

How the vehicle manufacturers get away with not offering warranty on their failures is most concerning to the writer! Seems like the only cover some owners are going to see from their manufacturers is when they cover their own ass!

In some active DPFs active cleaner is made by injecting diesel through an extra injector placed prior to the DPF. These active type DPFs run into their own issues. Problem has been identified whereby stop / start deliveries, for example, will never allow the system to complete the regeneration cycle. When the truck is started again the active DPF system re-institutes an active regeneration and if this happens again and again the catalytic convertor and or the DPF can pool the unburnt diesel within and can actually at times be seen coming out of the exhaust. Warning lights by this stage will come on and a very expensive DPF repair is on the cards. Not a cheap exercise.