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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.

The active regeneration method of a DPF system wherein fuel is supplied to a pre-catalyst with post-injection to cause filter heat-up, was described in previous reports. To succeed in the active regeneration, a pre-catalyst must have high fuel combustion performance at low temperature. Platinum (Pt) had been used as active species of the pre-catalyst. This conventional catalyst was improved by the addition of Palladium (Pd). Fundamental experiments for the catalyst have shown that Pd is very effective for fuel combustion and optimization of the ratio of Pd to Pt causes enhanced NO2 formation to contribute to lengthen the interval between active regenerations. The high performance and enough heat resistance of the newly developed pre-catalyst have been confirmed by engine test.

In order to meet increasingly strict PM legislation, diesel particulate filter systems (DPF) with a conversion rate of about 90% particulate matter (PM) are an essential after-treatment technology. Recently a filtering method using a catalyst has been proposed, which is called the “Continuously Regenerating DPF System[1],” and one can expect a significant degree of system simplification and cost reduction. In the previous report [2] basic characteristics about this continuously regenerating DPF were investigated. Results showed that in city mode driving, where exhaust temperatures are relatively low, continuous regeneration did not occur. Therefore, to “Continuously Regenerating DPF System,” active regeneration control system to oxidize and remove PM is necessary[3]. Then DPF system with higher reliability and active regeneration control method was proposed.