Diesel engines have attracted much attention due to their high efficiency, economy and low CO_2 emissions. Correspondingly, with the main emissions of NO_x and soot, the related problems of environmental pollution are serious day by day. In cylinder, there exist trade-off effects for NO_x and soot control. Therefore, soot and NO_x cannot be reduced efficiently by improving combustion alone. Simultaneous removal of soot and NO_x is the optimal diesel engine after-treatment technique, in which NO_x is r...