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Definition
Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent their release to the atmosphere. Diesel particulate filter materials have been developed that show impressive filtration efficiencies, in excess of 90%, as well as good mechanical and thermal durability. Diesel particulate filters have become the most effective technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.
Due to the particle deposition mechanisms in these devices, filters are most effective in controlling the solid fraction of diesel particulates, including elemental carbon (soot) and the related black smoke emission. Filters may have limited effectiveness, or be totally ineffective, in controlling non-solid fractions of PM emissions. To control total PM emissions, DPF systems are likely to incorporate additional functional components targeting gas emissions, typically oxidation catalysts, while ultra-low sulphur fuels may be required to control sulphate particulates.
The term “diesel particulate trap” is sometimes used as a synonym for “diesel particulate filter”, especially in older literature. The term “trap” covers a wider class of particle separation devices. Several particle deposition mechanisms other than filtration are commonly employed in industrial dust separation equipment. Examples include gravity settling, centrifugal separation, or electrostatic trapping. None of these techniques could be adopted to control diesel PM emissions, due to the small particle size and low density of diesel soot.

It may be noted that diesel oxidation catalysts (DOC) can also capture diesel particulates, but provide a much lower overall efficiency than diesel particulate filters.