Development of DPF System for Commercial Vehicle - Basic Characteristic and Active Regenerating Performance - 2003-01-3182

Diesel Particulate Filters (DPFs) having an effectiveness of around 90% reduction of particulate matter (PM) are an essential after-treatment technique in order to meet upcoming PM regulations (Japan2005, Euro4, US07), which are all increasingly stringent. The continuous-regenerating DPF system [1] has been drawing particular attention, because it is possible to significantly simplify the system and reduce costs. The study presented herein investigated the application of a continuous-regenerating DPF system to commercial vehicles.

Since exhaust temperatures that are encountered during a significant portion of engine operation are too low to initiate oxidation of PM, a continuously regenerating DPF must employ an oxidation catalyst. However, when the basic characteristics were investigated, an adequate PM oxidation rate was not obtained during city mode operation, during which the exhaust temperature was notably low. Consequently, continuous regeneration did not occur, and it was necessary to utilize active regeneration [2] to forcibly oxidize and remove the PM. Moreover, during high-load operating conditions, when more PM was accumulated on the filter, rapid PM oxidation inside the filter generated an extremely high temperature, resulting in filter breakage. Recently PM combustion phenomena under high temperature in filter is going to be analyzed by using of visual method [3]. In this work, a robust SiC filter [4][5] was used to withstand the high temperatures and prevent black smoke leakage.

Next, using SiC filter, an active regeneration method carried out during driving was investigated. In this method, the fuel injection process was modified such that an additional late injection into the cylinder (post-injection) took place. This supplied unburned fuel to the pre-catalyst, and the PM accumulated on the filter was combusted by the heat of oxidation. Therefore, by first achieving controlled catalyst heat-up, which brought the pre-catalyst up to activation temperature (1st stage), followed by controlled filter heat-up by the post-injection process (2nd stage), the temperature became sufficiently high so that the PM was oxidized on the filter, and then regenerated actively. A catalyst temperature feedback method was devised to control the post-injection process, in order to control filter temperature, to suppress abnormal heat-up during active regeneration, and to obtain an acceptable regeneration efficiency. With this control, when active regeneration was initiated during vehicle operation, the resulting filter temperature was very stable, and a good regeneration efficiency is obtained.

Based on the above results, a DPF system was suggested for commercial vehicles frequently operated in a city mode, and a high-reliability, active regeneration system was devised. For practical use in the future, there are still significant tasks to be solved, for example, how to remove the ash build-up on the filter. However, a DPF system that offers reduced impact on the environment would hopefully be realized soon.