Active diesel particulate filters rely on an external heat source for regeneration.
The major advantage of active regeneration is that it is independent of the exhaust gas temperature.
The disadvantage is that it is necessary to heat the filter element up to about 600°C before the diesel particulate matter starts burning. If the intention is to carry out regeneration whilst the exhaust gas is flowing through the filter, a huge amount of energy is required, typically about 50% of the power output of the engine. The only practicable way to provide this much energy is by injecting fuel into the exhaust, where it is either burned or catalytically combusted.
Alternative approaches are to actively regenerate the filter when the engine is stopped, or with the filter temporarily by-passed. In either case, the amount of energy required is reduced dramatically to the point where it can be provided from an electrical source. If the filter is to be by-passed during regeneration, the exhaust gas might be temporarily unfiltered, or directed through another filter. In a large installation, there are often 10 or more filter elements which can be individually by-passed, allowing them to be regenerated one at a time.
