November 20, 2019
Diesel Particulate Filter

Diesel Particulate Filter: You have to know that

All diesel vehicles have long had particle filters. They make sure that diesels do not stink. We answer the most important questions about this cleaning system:  How does the filter ensure clean exhaust gases? When does he have to be exchanged? And can it be cleaned cheaply?

  • The soot particles must be burned in the filter at more than 550 degrees
  • A particle filter does not last forever. Cost of exchange: between 1000 and 2000 euros
  • Cleaning instead of replacement: The car manufacturers are skeptical

What does a particle filter do?

Particulate filters are capable of filtering out and burning more than 90 percent of the soot – including many fines suspected of causing cancer. 

The models installed as standard are usually so-called closed wall-flow filters. The diesel exhaust gases are passed 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 gas on the wall. These accumulate on the filter inside. 

As more and more particles accumulate in the filter over time, they are regularly burned off. For this, the exhaust gas temperature must be above the soot ignition temperature of 550 degrees Celsius. This process, referred to as “regeneration” , mainly produces carbon dioxide (CO 2 ) and water vapor. A small remainder remains as ash in the filter.

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That’s how regeneration works

The regeneration of the particulate filter depends on the system used, which varies from manufacturer to manufacturer. There is a distinction between active and passive regeneration. Mostly both possibilities are combined.

  • Passive regeneration: It takes place continuously, and always whenever the required exhaust gas temperature is reached by itself. This is especially the case with predominantly motorway or full load operation of the case.
  • Active regeneration: It is initiated by increasing the exhaust gas temperature to about 600 degrees Celsius and changing the number and timing of injections, sometimes by increasing the load (eg turning on large power consumers). This is necessary when the car is often operated at a lower load (eg 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 already from about 400 degrees. Disadvantage: The additives burn in the engine and ash remains in the filter.

The limits of regeneration

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

To avoid this, the particulate filter warning lamp in the dashboard lights up when soot loading reaches a certain limit. Then take a look at the manual and it is usually a so-called “regeneration drive” with increased load (eg higher speed on the highway) to perform according to the specifications of the manufacturer. There is also the possibility of an emergency regeneration in the workshop, when regeneration is no longer possible during normal driving.

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

Incidentally, even residues of engine oil provide residues in the particulate filter, which can not be removed by regeneration. Therefore, it is particularly important to pay attention to the oil quality prescribed by the manufacturer with so-called “low-ash properties” .

When does the particle filter have to be replaced?

Even with correct regeneration , the filter settles by  ash  and requires an exchange. The regeneration intervals become shorter until finally the indicator light indicates that regeneration is no longer possible and the engine goes into emergency mode.

If the particle filter is added by the ashes, the introduction of a filter regeneration by the workshop is no longer successful. In this case , the particle filter must be renewed . Its lifetime depends, among other things, on the driving style, the field of application, the mileage and the engine oil.

For some older models, there are specific intervals for replacing the particulate filter – usually between 120,000 km and 180,000 km. Meanwhile, it is more common to make the replacement of the particulate filter depending on its ash content. This is determined by the workshops during the inspection.

Cleaning instead of replacement: Does it work?

Can a filter be cleaned instead of replaced? Various companies offer exactly that. As a rule, these are mechanical or thermal processes or a combination thereof.

The exact sequence of cleaning betrayed the respective providers, but promise that the flow behavior of the filter should be compared to that of a new part. However, no statement is made by the companies on how long the catalytic coating will remain effective after that – no one mentions a “minimum running time”.

A particle 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 not be used for a few days.

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

Automakers do not recommend cleaning

At the request of the ADAC, no car manufacturer recommends the described cleaning methods or agents. The use of additives is not released by any car manufacturer .

If damage occurs that is attributable to filter cleaning or the use of additives, the manufacturer or seller will not be able to do so within the framework of vehicle warranty or material defect liability. This also applies to resulting consequential damages and irregularities.

What is the position of the ADAC? Unfortunately, it is not possible to check how good the cleaning process actually is. A representative consumer protection test by the ADAC is not possible because different cleaning methods are used (partial company secrecy), the pollution of the filter differs greatly and the design of the filter varies depending on the vehicle type (such as size, material, coating). A general statement would not be possible with a test.

Conclusion: 

For the reasons mentioned above, the ADAC experts can not make a fundamental recommendation for the particulate filter cleaning methods or cleaning agents available on the market.

Do not confuse the particulate filter with an SCR catalyst with Ad-Blue injection. The latter cleans the diesel exhaust gases from the currently much discussed nitrogen oxides. 

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