Diesel particulate filter systems explained

DPF stands for diesel particulate filter. These filters are used to minimise the number of hazardous particulates that escape the exhaust systems of diesel vehicles. The goal is to make the surrounding air cleaner and safer, especially in confined spaces. EHC Teknik produces DPF filters for various applications, and we are happy to answer any questions you have about this technology. Keep reading to learn more.

Euro 5 standards

Since 2009 when the ‘Euro 5’ standard came into effect, exhaust emissions standards for new diesel cars have effectively required fitment of a DPF to the exhaust system. In fact, many cars registered before 2009 will have a filter fitted as well.

And these measures are taken for good reason, as diesel particulate (soot) emissions cause serious health problems in humans. DPFs reduce diesel soot emissions by 80% but they’re not suitable for everyone. Even if your driving is not 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) catch soot particles to avoid them exiting the exhaust system. As any filter, they must be emptied regularly to maintain performance. For a DPF this process is called ‘regeneration’. In this process, 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 the motorway when the exhaust temperature is high. As many cars are not driven on the motorway very often, vehicle manufacturers have designed “active” regeneration. In this process the engine management computer (ECU) takes control of the process.

Active regeneration

When the filter is filled up with soot to 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 motor is turned off while the regeneration is in progress, it may not be completed, 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 increasing the temperature that way.

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 oil 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 will continue to build up until around 75% when you can expect to see additional dashboard warning lights come on. At this point driving at speed alone will not be enough to clear the filter 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 and soot levels in the DPF have increased to about 70%. If left the soot loading will continue. At this level a diagnostic tool must be used to force regeneration. At around 85% soot loading, regeneration can no longer be performed on the vehicle and you will need to remove the DPF 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 lit-up warning light or diagnostic trouble code logged in the engine management system may prevent active or catalyst regeneration
  • Low fuel level will prevent active regeneration from taking place. Generally ¼ tank is required.
  • Oil counter/service interval – exceeding the service interval may prevent regeneration
  • Additive tank low or empty

Would you like to learn more about DPFs? Read about different kinds of DPF systems here.