A Diesel Particulate Filter, sometimes called a DPF, is used to remove Diesel Particulate Matter (including soot) from the exhaust gas of a diesel engine. Unlike a catalytic converter which is a flow-through device, a DPF cleans exhaust gas by forcing the gas to flow through the wall of the filter. DPFs are made of several materials, the most common being cordierite (a ceramic material that is also used as catalytic converter supports (= cores)) and silicon carbide. The devices look like catalytic converter cores that have had alternate channels plugged - the plugs force the exhaust gas to flow through the wall and the particulate collects on the inlet face.
The DPF has several channels in parallel with the ends plugged, in a checkerboard pattern. A DPF is usually followed by a lean NOx catalyst in a proprietary catalytic converter for diesel engines. By trapping the particulate matter, soot, the DPF allows the lean NOx catalyst to work without being clogged by the soot. Hence the effectiveness of the lean NOx catalyst is not reduced by soot covering the active sites of the catalytic converter.
These particle filters are a new technology: high cost catalytic pollution control module added to diesel engined cars to eliminate the characteristic black smoke trail. It was first offered as standard by the French manufacturer PSA Peugeot Citroën in early 2005. Slow adoption by the German car industry sparked local protests in March 2005. Despite costs, the device will be mandatory for all newly built vehicles in the EU starting in 2007 (projected).
The filter getting clogged
In most forms the exhaust gas is forced to flow through a material with extremely tiny pores, achieveing very high filtration rates even on nano-particles; however, this type of filter may become blocked by soot if there is not a mechanism to oxidize the accumulation, or if that mechanism fails. In any case, engine oil ash will build up on the face of the filter, needing periodic cleaning (unlike a Diesel Oxidation Catalyst which can often last the life of the engine without maintenance).
Several forms called "leaky filters" have also been devised, which are supposed to be incapable of being plugged, however their filtration efficiency seldom rises above 60%, and they have a tendency to "blow off" accumulated particulates after some hours of operation.
Regeneration is the process of removing the accumulated soot from the filter. This is done either:-
- Passively (by additing a catalyst to the filter).
- Actively. Active filter management can use a variety of strategies such as using engine management to increase exhaust temperature or using microwave energy or resistive heating coils to heat the filter core. In this soot burn-off, the DPF is heated to a temperature that fully burns the soot. This lets the DPF be used continuously; however, it uses extra fuel, which is burnt to heat the DPF. A sensor that measures back pressure decides when the DPF needs to be heated. A properly designed filter will have little effect on fuel usage, and an improperly designed filter can be catastrophic.
Back-pressure should be avoided in a large diesel engine's exhaust system, as the exhaust gas should be allowed to cool down as much as possible by drop in pressure as it leaves the engine's cylinders, else the exhaust valve may get damaged by overheating.
DPF's are in common use in Europe where Peugeot introduced them as standard fit on passenger cars. The Peugeot system uses a combination of engine management, and a fuel borne catalyst to regenerate the filters.
In North America filters are used heavily on diesel-powered underground mining machines, and also on transit buses, and the filters are installed as "retrofit" devices (i.e. not factory fitted).
In Japan, the Prefecture of Tokyo passed a law banning trucks without filters from entering the city limits.
Increasingly stringent regulations mean that filter technology will soon be fitted as standard to engines worldwide.