What is DPF?

This is a diesel particulate filter system

DPFs reduce diesel soot emissions by 80% but they’re not suitable for everyone.
What is a DPF

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 a filter fitted too.

Standards aim to deliver a 95% reduction in diesel particulate (soot) emissions but the technologies are not without problems. Even if your driving isn’t mainly urban/stop-start, changes to driving style may be required to keep these systems working properly.

How do diesel particulate filters work?

Diesel Particulate Filters (DPF), they catch soot in the exhaust. As with any filter 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 ash.

Regeneration is either passive or active

Passive regeneration
Passive regeneration takes place automatically on motorway when the exhaust temperature is high. Because many cars don’t get this sort of use vehicle manufacturers have had 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 vehicle’s ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is too short while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked.
It should be possible to complete a regeneration cycle and clear the warning light by driving and increase the temperature.

Symptoms of active regeneration

During active regeneration you may notice the following symptoms:
• Cooling fans running
• Increased idle speed
• Deactivation of automatic Stop/Start
• A slight increase in fuel consumption
• A hot, acrid smell from the exhaust.
• Engine note change

If the regeneration is unsuccessful due to an insufficient driving cycle the extra fuel injected into the cylinders will not burn and will drain into the sump. As a result, oil quality will deteriorate and the level will rise. Most DPF equipped engines will have an oil quality/viscosity sensor but it is important that you check that the oil level does not increase above the maximum level on the dipstick as diesel engines can run on their own oil if the level is excessive – often to the point of destruction.

If you ignore the diesel particulate filter 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 need to take the car to a dealer for ‘forced’ regeneration.

Forced regeneration

Forced regeneration is required where `Active` regeneration criteria have not been met or where soot levels have increased in the DPF, around 70% soot loading. If left the soot loading will keep rising. At this level of soot loading a diagnostic tool must be used to force regeneration. Around 85% soot loading regeneration can no longer be performed on the vehicle and the DPF will need removing to be cleaned or replaced.

What can prevent normal regeneration taking place?
• Frequent short journeys where the engine does not reach normal operating temperature
• Wrong oil type – diesel particulate filter equipped cars require low ash, low Sulphur engine oils
• A problem with the inlet, fuel or Exhaust Gas Recirculation (EGR) system causing incomplete combustion will increase soot loading.
• A warning light on or diagnostic trouble code logged in the engine management system may prevent active or catalyst regeneration
• Low fuel level will prevent active regeneration taking place. As a general rule ¼ tank is required
• Oil counter/service interval – exceeding the service interval may prevent regeneration
• Additive tank low or empty