114 ENGINE COMPONENT AND LOCATION Catalytic Filters W2033360 1. Diesel Oxidation Catalyst 2. Catalyzed Diesel Particulate Filter A catalytic filter (converter) is a porous form (like a sponge) that has been coated with a catalytic material. The flow of exhaust gas must pass through the pores on its way to the atmosphere. As a result, a high percentage of the PM is transformed. Diesel Oxidation Catalyst A catalytic filter, the diesel oxidation catalyst (DOC) reduces emissions in diesel exhaust by an average of approximately one third over a system without a DOC. Its honeycomb-like structure (called substrate) is coated with an active catalyst. The PM is changed to carbon dioxide and water.
The substrate can be made of a wide range of materials. Ceramic (cordierite) and sintered metal are common. Some PM usually remains unconsumed by the heat and collects on the filter. Eventually, the filter must be taken out to remove the accumulated residue by other means. There is a special machine to do this cleaning.