Particulate filters or Wall-flow Particulate filters remove particulate matter from the exhaust by filtration. They have a honeycomb structure like a catalyst substrate but with the channels blocked at alternate ends, which forces the exhaust gas to flow through the walls between the channels, filtering over 90% of the particulate matter out. The filters are self-cleaning, by one of a number of alternative methods which periodically modify the exhaust gas concentration in order to burn off the trapped particles oxidising them to form CO2 and water.

Particulate filters become clogged by Sulphur, which is present as an impurity in Diesel fuel. It is therefore essential that “sulphur-free” (<10 ppm) Diesel fuel is made widely available to enable this key technology to be fully successful.