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An after-treatment system consisting of a lean NOx trap, diesel particulate filter (DPF), and diesel oxidation catalyst (DOC) was applied to a light-duty commercial vehicle engine. Extensive exhaust gas testing under the transient JE05 test cycle was carried out. Lean NOx trap characteristics and issues related to transient operation were clarified and areas for system improvement were suggested. When exhaust gas temperatures were low, the NOx conversion efficiency was low, then stored NOx remained in the catalyst during rich operation, without desorption and reduction. The first half of the JE05 test cycle has relatively low exhaust gas temperatures, while the latter half has more high speed operation with rapidly increasing and higher catalyst temperature. Because of this, NOx trapped in the catalyst during the first half was desorbed and expelled during the high temperature second half, resulting in a lower NOx conversion rate for the entire test cycle.

Since a NOx trap catalyst cyclically releases and reduces NOx with rich exhaust gas, generating of a rich spike becomes important for application to diesel engines, which always operate with overall lean combustion. In addition, a NOx trap catalyst is poisoned and degraded in performance by the presence of SO2 in the exhaust gas. When the NOx absorbing efficiency thus decreases, it is necessary to regenerate the catalyst by a sulfur purge (desulfation) process in order to remove SO2. It is apparent that there are many factors and effects to understand before one can apply this catalyst system to a diesel engine, therefore we have carried out an inquiry into a performance of the NOx catalyst used the model gas equipment with known gas mixtures. The rich spike was generated with diesel fuel (light oil), resulting in a transient equivalence ratio spike > 1, to simulate diesel exhaust gas.

Diesel emission control is being addressed worldwide to help preserve the global environment. In 1994, emission controls in the U.S. called for reduction of diesel particulate matter (PM) to 10 to 20% of 1986's initial limit. In the same year, we developed and marketed small and medium duty trucks which were equipped with PM reduction systems that oxidize soluble organic fraction (SOF) contained in the PM, in order to satisfy these new regulations. Prior to their marketing, a catalyst was selected from among several types of candidate catalysts. Durability tests were performed using a catalytic converter-equipped small duty truck to verify the durability of the chosen catalyst. The durability test course was set up combining urban areas and expressways in the southern part of California, U.S.A.. The cumulative total distance covered on the test course reached 200,000 km. During the durability test, the catalyst was evaluated by measurement of PM emission using a chassis dynamometer.