First, a general understanding of DPFs is needed. Catalyzed DPF systems (CDPF) remove >95% of particulate matter from the exhaust. CDPF and other DPF systems fill with soot (primarily elemental carbon) and will plug unless the soot is burned off in some fashion - this is called regeneration. There are a couple of ways to accomplish regeneration. One is to oxidize the soot using excess oxygen in the exhaust, which requires temperatures of about 600C or higher. Exhaust temperatures that high are very difficult to achieve with a diesel engine and typically only occur at sustained operation near peak torque conditions. Typically, some sort of additional burner system consuming diesel fuel or (in production applications) the use of an additional, late or post-combustion fuel injection event is needed to achieve such high temperatures. The use of post-combustion fuel (either within an exhaust burner or via late injection) is referred to as active regeneration. There is also passive regeneration. Soot will react and oxidize with NO2 at much lower exhaust temperatures (~250C). A fraction of the NO emissions from the engine can be oxidized to NO2 over Pt or Pt-Pd oxidation catalysts located upstream of the CDPF and catalyzed directly to the surface of the CDPF in order to greatly extend the range of conditions under which regeneration can occur passively. The NO2 reaction with soot reduces the NO2 to NO. Care must be taken regarding how much NO2 is generated - excess NO2 production in systems lacking a downstream catalytic NOx control system (e.g., urea-SCR in typical North American truck applications) is undesirable due to the toxicity of NO2. Most CDPF systems rely on both passive and active regeneration. Passive regeneration is favored due to reduced fuel consumption. Active regeneration is kept as a reserve function to prevent trap plugging in the event that significant light-load, low temperature operation results in insufficient passive regeneration and soot begins to build up within the CDPF in a manner that could lead to filter plugging. This function places considerable thermal stress on the CDPF and its catalyst coatings and so efforts are made to minimize active regeneration in most system designs.
There are two major obstacles to implementing DPF retrofits in India. First are the fuel sulfur levels. Diesel fuel with 15 ppm sulfur or less is required to achieve reliable passive regeneration. Oxidation of SO2 competes with oxidation of NO2 and thus impedes passive regeneration. Oxidation of SO2 to sulfate also creates sulfate particulate matter composed primarily of sulfuric acid aerosol - an very undesirable result. Even at diesel fuel sulfur levels of only 50 ppm, CDPF systems would oxidize enough SO2 to sulfate PM to nearly replace the PM emissions originally removed by the system. So virtually sulfur-free fuel should be the first goal. Second, designing retrofit CDPF systems are a greater engineering challenge than engineering the system as part of an overall solution together with the entire powertrain. There are a number of degrees of freedom impacting exhaust temperature, soot loading, passive regeneration conditions (especially catalyst llight-off control) and active regeneration conditions that are far easier to implement in an OEM CDPF system integrated into a powertrain than would be possible for a retrofit of such a system onto an existing platform.
Retrofits can be done successfully, but tremendous care must be taken with respect ot application engineering. For example, it is particularly important to understand the operational conditions, exspecially exhaust temperature, that a retrofit application will be applied to. A long-haul truck operating on steep grades with consistent high loads might be a good appliction. A lightly loaded delivery truck at moderate ambient temperatures and with few grades and only occaisional high load operation might be a poor application. Both trucks might even use identical engines.
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