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A DPF system has three main components/processes: 1) the filter that collects or “traps” the PM, 2) a means for removing the PM from the filter or a “filter regeneration strategy” and 3) a mechanism for determining whether engine back pressure is increasing due to the build-up of ash and other constituents on the filter surfaces. A number of filter materials have been used in diesel particulate filters, including ceramic and silicon carbide materials, fiber wound cartridges, knitted silica fiber coils, wire mesh, and sintered metals. Filter materials can be designed for varying levels of PM control ranging from less than 60% to greater than 90%. Currently, the most prevalently used filter materials in both OE and retrofit applications are ceramic cordierite and silicon carbide which both maintain PM reductions of >90%.. Images of both ceramic and silicon-carbide wall-flow monoliths are shown in the figures located directly above. Cordierite filters are less expensive than silicon carbide filters, are easier to handle, but have a lower melting point (around 2000° Fahrenheit / 1100° Celsius). In extremely high temperature situations, which are normally a result of an overloaded filter, cordierite filters have melted during regeneration. Conversely, Silicon Carbide (SiC) is more expensive but does maintain its thermal structure in temperatures up to 4500 °F / 2500 °C. Both cordierite and silicon-carbide substrates are designed to allow the exhaust gases to pass through micro-porous walls. In passing through these walls, particles of diesel soot and other debris are removed from the gas stream via filtration. The following figure graphically illustrates a wall-flow filter. |
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Because the volume of diesel particulates collected by the system will eventually fill up and even plug the filter, a method for controlling trapped particulate matter and regenerating the filter is needed. The process of removing trapped particulates from a filter, thus allowing engine operation without an intolerable buildup of engine backpressure, is called “filter regeneration.” |
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D. Filter Regeneration |
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All standard DPFs take one of two basic approaches to regeneration: active or passive. A passive DPF allows for soot oxidation during normal vehicle/equipment operation without the assistance of external heating elements. An example of a passive DPF would be DET’s UltraTrap™, EnviTrap™, and OxyTrap™ filters which utilize a cordierite substrate that is coated with a proprietary catalyst coating. The coating enables the filter to passively regenerate the built up soot in the filter walls. The catalyst helps oxidize CO and HC that pass through the walls of the filter, and raises the local temperature at the surface of the catalyst. The catalytic active surface generates nitric dioxide (NO2), which oxidizes the accumulated particulate mass on the channel walls to carbon dioxide (CO2) and water (H2O). The system passively regenerates accumulated soot, provided that the engine duty cycle enables the inlet temperature of the system to reach a minimum operating exhaust gas temperature (EGT) of 260°C for 25% of the time. The advantage of passive systems is that they are technologically less complex, require far less servicing, and are less expensive than their active counterparts. |
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