Diesel particulate filters.

Diesel particulate filters (DPF) also sometimes known as particulate emissions filters (PEF)are designed to remove particles from exhaust gases before they are expelled into the atmosphere. Even the most refined, modern diesel car produces exhaust gases with high levels of soot particles. On average a diesel engine produces several billion per kilometre. Soot particles are between 1/100 to 1/600 times the width of a human hair.

The DPF works by forcing the exhaust gasses through a honeycomb structure with porous walls. Unlike a Catalytic converter the DPF is not a flow-through device, the gasses move across the honeycomb cell walls while the particulate matter is trapped. A well designed DPF can reduce soot particulate emissions by up to 85% and drastically reduces hydrocarbon and carbon monoxide emissions.

The particulate removed from the gases is deposited on the cell walls and over time the soot builds up in the DPF. To prevent frequent replacement, DPF’s are designed to operate a regeneration process when they reach 45-50% capacity to keep them operational. Regeneration normally occurs every 300-500miles depending on engine and driving style. During regeneration the soot is burned off using very high temperatures in the exhaust. The process can be coupled with a short expulsion of dark looking smoke from the exhaust which is perfectly normal.

Regeneration can be triggered in two ways, Active regeneration when the ECU recognises the DPF’s recommended capacity is reached. At which time the ECU will adjust the engines running to induce a high temperature in the exhaust which will then burn off the particulate matter trapped in the DPF. Passive regeneration requires the temperature of the exhaust to naturally reach the optimum level (in excess of 600 degrees C) usually during sustained high speed driving (IE longer dual carriageway or motorway travel at a steady speed above 50mph) at which point the soot will start to “burn off”. Some systems (Peugeot and Citroen for example) will use an additive that bonds with the particulate soot and will reduce the temperature required for the soot to burn off and therefore make regular regenerations more likely.

A car taken on  motorway journeys as part of its regular routine will regenerate when required and the driver may never know that the DPF exists.  If a car is only used on short journeys or stop start town/city driving then the exhaust temperature is very unlikely to exceed 200 degrees C .

Smaller cars are often bought solely for the school or shopping run for which they are ideal so long as the consumers are aware of the necessity to facilitate regeneration, approximately,  every 500 miles .A well managed DPF can last many regenerations before needing replacement .  If it doesn’t have an active system to initiate burn-off at lower temperatures and is not taken on longer journeys, problems tend to occur; the cell walls can become blocked, affecting the exhaust back pressure and impacting on the overall performance and efficiency of the engine. Once this occurs, if a regeneration cycle can not be started it is necessary to replace the DPF prematurely.