Diesel particulate filter (DPF): what it is and how it works

How a DPF works

Inside the steel housing is a ceramic block made of thousands of small parallel channels. Alternate channels are plugged at the inlet and outlet ends, so exhaust gas entering an open channel cannot leave the same way. It is forced through the porous ceramic wall into a neighbouring channel and out of the filter. Soot particles are too large to pass through the wall and are left behind on its surface.

This “wall-flow” design is why a DPF is so effective, and also why it needs looking after: every particle it catches has to go somewhere. Soot is mostly carbon and can be burned off. Ash, which comes mainly from additives in engine oil, cannot.

Why DPFs are fitted

Particulate emissions from diesel engines are harmful to health, so emissions standards limit them. To meet the particulate limits introduced with Euro 5 in Europe, and with BS-VI in India from April 2020, diesel cars and most light commercial vehicles are fitted with a DPF as standard. Heavy vehicles, buses and many off-road machines use them too.

Regeneration: how the filter cleans itself

As soot builds up, the pressure difference across the filter rises. The engine control unit (ECU) monitors this with a differential pressure sensor and temperature sensors and, when the estimated soot load is high enough, raises exhaust temperature to burn the soot. This is DPF regeneration. It works best on longer drives at steady speed, which is why vehicles used for short trips are more likely to have DPF problems.

Ash, cleaning and replacement

Regeneration removes soot but not ash. Over time ash reduces the filter’s capacity, regenerations become more frequent and eventually the filter cannot be kept clear. At that point it can usually be restored by professional DPF cleaning, unless the ceramic is cracked or melted, in which case it needs replacing. See DPF cleaning vs replacement.

Catalysed and additive systems

Many DPFs carry a catalytic coating that helps soot burn at lower temperatures, often combined with a diesel oxidation catalyst (DOC) in the same exhaust line. Some vehicles, including many Peugeot and Citroën diesels, instead use a fuel-borne catalyst: an additive is dosed into the fuel from a separate tank to lower the soot burning temperature. On those vehicles the additive must be topped up at service intervals.