Partial Diesel Particulate Filters

Although wall flow diesel particulate filters   have very high particulate trapping efficiency, but their regeneration over the entire life span extending up to 496,000 kms for heavy duty vehicles is a challenging problem. In Europe, many diesel vehicles meeting Euro 4 standards are fitted with ‘wall flow’ particulate filters as original equipment. Metal supported flow- through diesel filters employing CRT operational principle have  also been developed recently  These filters have been developed to provide 50 to 70 percent reduction in PM emissions and therefore, are called as ‘Partial Particulate Filters’.   Like CRT, upstream in the first section an oxidation catalyst is installed where NO is oxidized to NO₂.  In the second section, which consists of flow-through type filter element collection of soot and its combustion processes occur.
.A schematic cut-away section and working principle of the filter is shown on Fig.  6.19. The metal PM filter consists of flat and corrugated foils in a flow-through monolithic configuration. The corrugated foils are stamped to produce blades like structure to direct the flow towards the flat foil. The flat foil is made of porous sintered metal fleece (wool) packed in the form of a sheet and compressed between metal foils.  Part of the exhaust gas is directed by the blades in the corrugated foil towards the porous metal wool that traps the particulate matter. The soot trapped by the metal wool is oxidized by NO₂ generated on the catalyst in the upstream first section. The design of this  diesel particulate filter has open channels  and it does not get clogged due to excessive accumulation of soot as happens in the ‘wall flow’ filters on failure of regeneration. All the exhaust is able to flow through the open channels if the metal fleece is choked. However, in such a situation removal of PM from exhaust does not take place. Typical cell density of these filters is 200 cpsi. With use of these particulate filters reduction in PM emissions ranging from 30 to over 70 % have been obtained.