Diesel Particulate Filters

Diesel particulate filters (DPF)

DPFs reduce diesel soot emissions by 80% but they’re not suitable for everyone

The exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the ‘Euro 5′ standard came into force. In fact, many cars registered before 2009 will have had one fitted too in anticipation of the change in standards.

Standards aim to deliver an 80% reduction in diesel particulate (soot) emissions but the technology’s not without problems – AA patrols are regularly called to cars with the particulate filter warning light on indicating a partial blockage of the filter.

dpf warning light

Even if your driving isn’t mainly urban/stop-start, changes to driving style may be required to get maximum benefit from these systems.

If you’re buying a new car and plan to use it mainly for town-based, stop/start driving it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF) because of the possible hassle of incomplete ‘DPF regeneration’.

How do Diesel Particulate Filters 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 collected soot is burnt off at high temperature to leave only a tiny ash residue.

Regeneration is either passive or active

Passive regeneration
Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don’t get this sort of use car 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 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 10 minutes or so at speeds greater than 40mph.

If you ignore the warning 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 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 regeneration.

Expensive repairs

If you continue to ignore warnings and soot loading keeps increasing then the most likely outcome will be that you will have to get a new DPF costing around £1000.

DPF additives

The most commonly fitted type of DPF has an integrated oxidising catalytic converter and is located very close to the engine where exhaust gases will still be hot. This heat means that passive regeneration is possible.

There’s not always space close to the engine so on some models, across a wide range of manufacturers, a different type of DPF has been used which can be located further from the engine. These rely on a fuel additive to lower the ignition temperature of the soot particles.  Many newer models are managing without the fluid because the DPF has been relocated nearer to the engine.

The additive is stored in a separate tank and is automatically mixed with the fuel whenever you fill up. Only very small quantities are used though so a litre of additive should treat around 2800 litres of fuel – enough to cover 25,000 miles at 40mpg.  No, lasts about 72000 miles and is replenished during a service – at extra cost

Don’t be tempted to ignore a warning light showing that the additive tanks need refilling.  It’s absolutely essential this tank is refilled as without it regeneration is unlikely to be successful and a new DPF may be needed – at significant cost.  Fuel consumption can increase as a result of failed regenerations too.

We’re seeing some evidence of DPF systems failing to regenerate even on cars used mainly on motorways.

On cars with a very high sixth gear the engine revs may be too low to generate sufficient exhaust temperature for regeneration. Occasional harder driving in lower gears should be sufficient to burn off the soot in such cases.

With this type of DPF regeneration will be initiated by the ECU every 300 miles or so depending on vehicle use and will take 5 to 10 minutes to complete. You shouldn’t notice anything other than perhaps a puff of white smoke from the exhaust when the process is completed.

There’s no evidence that the problem’s going away – newer car models seem just as likely to suffer DPF problems if not driven ‘correctly’ as those built when DPF’s were introduced.