Diesel particulate filter: you need to know that

All diesel vehicles have long had particle filters. They ensure that diesel engines do not soot. We answer the most important questions about this cleaning system: How does the filter ensure clean exhaust gases? When does it need to be replaced? And can you also have it cleaned inexpensively?

  • The soot particles must be burned in the filter at more than 550 degrees
  • A particle filter doesn’t last forever. Exchange costs: between 1000 and 2000 euros
  • Cleaning instead of replacement: the car manufacturers are skeptical

What does a particle filter do?

Particle filters are able to  filter out and burn more than 90 percent of the soot  – including many fine particles that are suspected of causing cancer. 

The models installed as standard are usually so-called closed wall flow filters. The diesel exhaust gases are led through a  porous filter wall  made of ceramic or metal. The attraction of the molecules of different substances (adhesion) holds the soot particles in the exhaust on the wall. These accumulate on the inside of the filter. 

Because more and more particles accumulate in the filter over time, these are regularly burned off. For this, the exhaust gas temperature must be above the soot ignition temperature of 550 degrees Celsius. This  process, known as  “regeneration” , mainly produces carbon dioxide (CO2) and water vapor. A small remnant remains as ash in the filter.

This is how regeneration works

The regeneration of the particle filter depends on the system used, which differs from manufacturer to manufacturer. A distinction is made between active and passive regeneration. Usually both options are combined.

  • Passive regeneration: It takes place continuously and always when the required exhaust gas temperature is reached by itself. This is especially the case with predominant freeway or full load operation.
  • Active regeneration: It is initiated by increasing the exhaust gas temperature to around 600 degrees Celsius and by changing the number and time of the injections, sometimes also by a higher load (e.g. switching on large electricity consumers). This is necessary if the car is often operated with a lower load (e.g. in city traffic). 

There are also soot filters in which an additive in the diesel fuel lowers the soot ignition temperature and enables the combustion of the particles from around 400 degrees. Disadvantage: The additives burn in the engine and there is also ash left in the filter.

The limits of regeneration

Prolonged short-distance operation  can impair the regeneration of the particle filter because the exhaust gas temperature does not reach the required temperature. The result: The filter clogs or is damaged.

To avoid this, the particulate filter indicator lamp in the dashboard  lights up  when the soot load has reached a certain limit. Then a look at the operating instructions will help and it is usually a so-called “regeneration trip” with increased load (e.g. higher speed on the highway) according to the manufacturer’s instructions. There is also the option of emergency regeneration in the specialist workshop if regeneration is no longer possible during normal driving.

Once a certain ash load limit has been reached, the filter can no longer be regenerated in the workshop. The  particle filter must then be replaced.

Incidentally, residues of engine oil also cause residues in the particle filter that cannot be removed by regeneration. It is therefore particularly important to pay attention to the oil quality prescribed by the manufacturer  with so-called “low ash properties”  .

When must the particle filter be replaced?

Even with correct regeneration  , the filter clogs up with  ash  and requires replacement. The regeneration intervals become shorter until the control lamp finally indicates that regeneration is no longer possible and the engine goes into emergency operation. If the particle filter is clogged by the ashes, the initiation of a filter regeneration by the workshop is no longer successful. In this case  , the particle filter must be replaced . Its service life depends, among other things, on the driving style, the area of ​​application, the mileage and the engine oil.

Some older models have  specific intervals  for replacing the particle filter – usually between 120,000 km and 180,000 km. In the meantime, it has become more common to make the replacement of the particle filter dependent on its ash content. This is determined by the workshops during the inspection.

The installation of a new particle filter costs between 1000 and 2000 euros , depending on the model  . Some vehicle manufacturers also offer cheaper replacement filters as part of their own exchange programs.

Cleaning instead of replacement: does it work?

Cool perspective on a diesel particulate filter
The soot gets stuck on the porous wall made of ceramic or metal ∙ © ddp / Jens-Ulrich Koch

Can a filter also be cleaned instead of replaced? Various companies offer exactly that. As a rule, these are mechanical or thermal processes or a combination thereof. The  respective providers do not reveal the exact process of cleaning  , but promise that the flow behavior of the filter should be compared to that of a new part. However, the companies make no statement as to how long the catalytic coating will remain effective afterwards – nobody calls a “minimum term”.

Particulate filter cleaning costs around 400 to 500 euros. It should be noted that the particle filter must be removed and sent to the provider for cleaning  . The vehicle can therefore not be used for a few days.

In addition to the cleaning procedures mentioned above , chemical cleaning agents are  also available  . These are intended to remove the soot particles that have remained in the filter in the event of incomplete or interrupted regeneration. However, you cannot remove ash deposits. The same also applies to additives which are added to the fuel and which keep the loading of the particle filter low or are intended to clean the filter.

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