Particulate Filters Regeneration: A comprehensive guide for instructors

Particulate filter types

For the purpose of reducing the emission of harmful soot, closed particulate filters are the most effective option, capturing more than 90% of soot particles. Open particulate filters, in contrast, only capture 60% at best.

However, the soot trapped in the filters needs to be eliminated through regeneration, a process that burns soot and converts it into carbon dioxide.

Closed particulate filters can present different regeneration capacities depending on their material.

• Silicon carbide filters have a high melting point temperature of 2,700°C, making them more adequate for regeneration and a more durable solution. They also offer high soot storage and the highest filtration efficiency—up to 99%.

• Cordierite filters, commonly found in the aftermarket, offer less efficiency (90%) and frequently melt during regeneration due to their relatively low melting point of 1,200°C. They are prone to clogging and present a higher fuel consumption.

Types of particulate filters regeneration

DPFs can eliminate soot through two different regeneration processes: active and passive.

Active regeneration

For the active regeneration to occur, the filter must be actively heated above 550°C so that the soot can react with oxygen. This temperature can only be achieved if ‘active’ regeneration strategies are applied, such as:

• The injection of additional fuel in the exhaust stroke or directly into the exhaust;
• the electrical heating of the particulate filter.

Active regeneration by additional injection

During the active regeneration, the exhaust gas temperature can be increased by burning additional fuel in an engine cylinder or the exhaust system (upstream of the filter).

• In the first instance, extra fuel enters the exhaust through post-injection. Existing diesel injectors insert fuel into the cylinder during the exhaust stroke.
• In the second method, fuel is added and burned by an injector placed in the exhaust system.

In the exhaust system, fuel can be burned to increase the exhaust gas temperature using three methods: catalytic combustion, flame burning, and combined flame and catalytic combustion.

• Catalytic combustion: The fuel is introduced through an injector in the exhaust, vaporised, mixed with exhaust gas, and oxidised by an oxidation catalyst.
• Flame burning: The fuel is burned in a fuel burner, usually with a special supply of compressed air, with the flame entering the exhaust system.
• Combined flame and catalytic combustion: A combination of the above methods, where a fuel burner is followed by a catalytic combustion system.

The latter two systems are rarely used in the passenger car industry. 

The advantage of a system with an additional injector in the exhaust is that it can also be used on engines where post-injection is impossible. For example, pump injector systems and systems that still use a distributor injection pump.

Passive regeneration

In the passive regeneration process, the soot in the filter is burned at normal exhaust temperatures, between 250°C and 400°C, requiring nitrogen dioxide.

Filters with precious metals in their ‘wall’ benefit from a catalytic action, auto-igniting the soot from a temperature of 250°C. This temperature should be regularly reached so that regeneration occurs spontaneously. Thanks to its capacity to regenerate at lower temperatures, the ‘Catalytic Wall Flow’ filter is a suitable product for retrofitting diesel engines.

In addition, the combustion temperature of soot particles can be reduced by 100°C with the help of fuel additives.

Passive regeneration with fuel additives

A fuel additive (Eolys) can be mixed with diesel to reduce the combustion temperature of the soot particles. The additive enters the particulate filter along with the soot and helps to ignite it from a temperature of 450°C. By using this product, the vehicle is capable of passive regeneration.

However, the additive (Eolys), which is used mainly by PSA, also has drawbacks. Being metal-based (mostly cerium), it remains in the filter after regeneration, like all other metals. This increases the ash content in the DPF, results in a higher pressure drop across the filter, and shortens the necessary cleaning interval. Therefore, the replacement interval of a particulate filter where an additive is applied is shorter.

The additive is filled into a separate tank during maintenance, and this procedure must be carried out with precision and following all safety instructions.

Ash cleaning

Unfortunately, soot is not the only thing that ‘fills’ the filter. Substances such as calcium, phosphorus and fluorine do not regenerate. They remain in the filter in the form of ash. These are substances that come mainly from the engine oil, and that should be blown out periodically.

In addition to preventive maintenance, using the appropriate engine oil is essential to avoid ashes fill-up. The wrong oil creates additional ash and shortens the life of the diesel particulate filter. 

Specialised companies can clean the ashes, but the filter will eventually need a replacement. The regular replacement interval of a particulate filter ranges from 120,000 to 300,000 kilometres.

Some manufacturers present a fixed interval for replacing the DPF, whereas other brands use information from the ECU, tracking factors such as the number of engine starts, the pressure loss across the filter, and its capacity to fully regenerate.

Why does particulate filter regeneration and cleaning matter?

In conclusion, maintaining and cleaning DPFs is essential to ensure optimal engine performance and prolong the lifespan of particulate filters. As an automotive instructor, you can use our guide to help students understand the various types of DPFs and the regeneration process.