Introduction:
A diesel particulate filter (DPF) is an essential component of the exhaust aftertreatment systems of modern diesel engines. Its primary function is to filter soot particles (particulate matter) from the exhaust gases and thereby reduce the emission of harmful substances. Soot particles are formed during the combustion process, especially in the case of incomplete combustion, and can be harmful to both health and the environment. Since the introduction of stricter emission standards, diesel particulate filters have become standard on new diesel vehicles to comply with the Euro 5 and Euro 6 standards.
By trapping the soot particles in the filter and regenerating it regularly, particulate emissions are drastically reduced. This makes diesel particulate filters indispensable in the pursuit of cleaner air and compliance with environmental legislation.
Formation of soot particles:
Soot particles are always formed during the combustion process of a diesel engine. Soot is a sign of incomplete combustion. Soot particles are small carbon spheres with a core of pure carbon, with a diameter of approximately 0.05 micrometre. Certain harmful substances, including hydrocarbons, sulphur and metal oxides, adhere to the core of this carbon. Some hydrocarbons can be harmful to health.
The formation and quantity of soot particles depend mainly on usage, air supply, injection and the combustion process of the engine. In particular, with too little oxygen the soot emissions increase, because the mixture is too rich and combustion is therefore incomplete. Chiptuning can also be a cause of excessive soot emissions.
The size of the soot particles is almost the same for both indirectly injected diesel engines with swirl chambers and directly injected, common-rail and unit injector engines.
Operation of a diesel particulate filter:
The diesel particulate filter consists of a honeycomb-shaped ceramic body made of silicon carbide housed in a metal casing. The ceramic body consists of many parallel, microscopically small channels, which are alternately closed at one end.
When the exhaust gases flow through the diesel particulate filter, the particles are trapped in the inlet channels, while the other gaseous components of the exhaust gas pass through the porous walls of the filter.
Regeneration of the diesel particulate filter:
Because, after a while, many soot particles remain trapped in the self-regenerating diesel particulate filter, it would become clogged without any measures. To prevent this, the filter must be regenerated regularly (every 200–1000 km). This is called the “regeneration process”. This means that the accumulated soot particles are burned off. During combustion the particles are converted into the harmless substances carbon dioxide and water. The combustion temperature of the soot particles in diesel particulate filters without a catalytic coating is at least 600ºC, but the exhaust gases are often not hot enough for this. To still burn off the soot particles, the following methods can be used:
Diesel particulate filter with catalytic coating:
A diesel particulate filter with catalytic coating is designed to remove soot particles (particulate matter) from the exhaust gases of diesel engines. Thanks to the catalytic layer, the accumulated soot particles can burn off at a relatively low temperature of around 250°C. To limit heat loss, this type of diesel particulate filter is placed as close as possible to the engine, usually directly after the exhaust manifolds.Oxidation catalyst:
An oxidation catalyst is often placed before the diesel particulate filter. This catalyst converts part of the nitrogen monoxide (NO) in the exhaust gases into nitrogen dioxide (NO₂). NO₂ is a strongly oxidizing gas and reacts more readily with soot, allowing the soot to burn at a lower temperature. In this way, regeneration of the diesel particulate filter is supported. In some systems, two oxidation catalysts are even used to reinforce this effect.External heating:
When the exhaust gas temperature is insufficient to initiate regeneration, external heating can be used. This can be done with a glow plug or a fuel injector in the exhaust system, which generate additional heat. The engine control unit (ECU) continuously monitors the signals from the differential pressure sensors before and after the diesel particulate filter. When these sensors indicate that the back pressure is rising — which indicates a filling filter — the external heating is activated to start regeneration.Additional fuel injection:
Another method to raise the temperature in the exhaust system is additional fuel injection during the exhaust stroke. The unburned fuel burns further on in the exhaust system, causing the exhaust gas temperature to rise. This ignites the soot in the filter and allows the filter to regenerate.
Retrofit diesel particulate filter:
There are diesel particulate filters for vehicles that were not originally equipped with a diesel particulate filter from the factory. They are therefore installed afterwards (retrofit). Nowadays, they are becoming less and less common, because all modern diesel cars are equipped as standard with a fully closed diesel particulate filter.
This type of diesel particulate filter has a partially open structure, allowing the exhaust gases to flow relatively freely through the filter, which minimizes back pressure in the exhaust system. Soot particles are deposited on the walls of the filter, where a catalytic coating (usually platinum or cerium oxide) enables the combustion of soot at 250–300°C. The coating converts nitrogen monoxide (NO) into nitrogen dioxide (NO₂), which allows the soot to burn more easily, even at the lower temperatures reached during normal operation. As a result, continuous regeneration takes place as long as the engine is sufficiently loaded and the exhaust gases are at the correct temperature.
An important advantage is that the filter rarely becomes clogged, provided the vehicle is regularly driven until it is properly warmed up.
Although under ideal conditions the filter can reduce soot emissions by 50–60%, in less optimal conditions (for example city driving) its effectiveness is often only between 20–40%, depending on the driving profile and engine temperature.
Exhaust gases of a diesel engine:
In the image alongside and in the explanation below you can see what the exhaust gases of a diesel engine consist of.
- 67% Nitrogen (N2)
- 12% Carbon dioxide (CO2)
- 11% Water (H2O)
- 10% Oxygen
- 0.3% other substances, including soot particles (PM), Hydrocarbons (HC), Nitrogen oxides (NOx), Carbon monoxide (CO).
To reduce NOx emissions, diesel engines are increasingly using an SCR catalyst with an AdBlue dosing system.
