Technology/Strategy
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Particulate Filters - Partial or Flow-Through Filter
Diesel particulate filters remove particulate matter found in diesel exhaust by filtering exhaust from the engine. Diesel particulate filters (DPFs) come in a variety of types depending on the level of filtration required. The simplest form of particulate removal can be achieved using a diesel oxidation catalyst (DOC). DPFs can be either partial, flow-through devices or wall-flow designs which achieve the highest filtration efficiency.
Partial or Flow-Through Filters: The first level of filtration can be achieved using a partial or flow-through particulate filter. In this type of device, the filter element can be made up of a variety of materials and designs such as, sintered metal, metal mesh or wire, or a reticulated metal or ceramic foam structure. In this type of device, the exhaust gases and PM follow a tortuous path through a relatively open network. The partial filtration occurs as particles impinge on the rough surface of the mesh or wire network of the filter element. Partial filters can be catalyzed or uncatalyzed and are less prone to plugging than the more commonly used wall-flow filters.
Particulate Filters - High-Efficiency Wall-Flow Filter
Diesel particulate filters remove particulate matter found in diesel exhaust by filtering exhaust from the engine. Diesel particulate filters (DPFs) come in a variety of types depending on the level of filtration required. The simplest form of particulate removal can be achieved using a diesel oxidation catalyst (DOC). DPFs can be either partial, flow-through devices or wall-flow designs which achieve the highest filtration efficiency.
High-Efficiency Wall-Flow Filters: In order to meet the stringent particulate emissions that are required for diesel heavy-duty vehicles starting with the 2007 model year, the highest efficiency particulate filter is required. These are commonly made from ceramic materials such as cordierite, aluminum titanate, mullite, or silicon carbide. The basis for the design of wall-flow filters is a honeycomb structure with alternate channels plugged at opposite ends. As the gases pass into the open end of a channel, the plug at the opposite end forces the gases through the porous wall of the honeycomb channel and out through the neighboring channel. The ultrafine porous structure of the channel walls results in greater than 85 percent collection efficiencies of these filters. Wall-flow filters capture particulate matter by interception and impaction of the solid particles across the porous wall. The exhaust gas is allowed to pass through in order to maintain low pressure drop.
Since a filter can fill up over time by developing a layer of retained particles on the inside surface of the porous wall, engineers that design engines and filter systems must provide a means of burning off or removing accumulated particulate matter and thus regenerating the filter. A convenient means of disposing of accumulated particulate matter is to burn or oxidize it on the filter when exhaust temperatures are adequate. By burning off trapped material, the filter is cleaned or "regenerated" to its original state. The frequency of regeneration is determined by the amount of soot build-up resulting in an increase in back pressure. To facilitate decomposition of the soot, a catalyst is used either in the form of a coating on the filter or a catalyst added to the fuel. Filters that regenerate in this so-called "passive" fashion cannot be used in all situations. The experience with catalyzed filters indicates that there is a virtually complete reduction in odor and in the soluble organic fraction of the particulate. Despite the high efficiency of the catalyst, a layer of ash may build up on the filter requiring replacement or servicing. The ash is made up of inorganic oxides from the fuel or lubricants used in the engine and will not decompose during the regular soot regeneration process.
In some applications or operating cycles, the exhaust never achieves a high enough temperature to completely oxidize the soot even in the presence of a catalyst. In these instances, an "active" regeneration system must be employed. Active regeneration utilizes a fuel burner or a resistively heated electric element to heat the filter and oxidize the soot. Active regeneration can be employed either in-place on the vehicle or externally. During external regeneration, the filter is removed from the vehicle and heated in a controlled chamber.
