| 摘要 |
<P>PROBLEM TO BE SOLVED: To avoid the drop phenomenon of a NOx reduction rate by suppressing the generation of excessive NO<SB>2</SB>by an oxidation catalyst arranged in the prior stage of a selective reduction type catalyst. <P>SOLUTION: The exhaust emission control device is equipped in the middle of an exhaust pipe 3 of an engine 1 with a selective reduction type catalyst 4 capable of selectively reacting NOx with ammonia even under oxygen coexistence, and equipped in its prior stage with each of oxidation catalysts 5A and 5B in a pair and in parallel. In an operating state in which the exhaust temperature is low, the distribution amount of exhaust gas 2 to each of the oxidation catalysts 5A and 5B is adjusted such that a ratio of NO/NO<SB>2</SB>in exhaust gas is set approximately to 1-1.5. Further, as a countermeasure against leakage ammonia which increases under an operating condition with a high exhaust gas temperature at which an influence on an NOx reduction rate of the NO/NO<SB>2</SB>ratio decreases, the exhaust gas 2 is flown reversely through the selective reduction catalyst 4 and each of the oxidation catalysts 5A and 5B by switching the flow passage, and ammonia passing the selective reduction type catalyst 4 with itself not reacted is oxidized by each of the oxidation catalysts 5A and 5B. <P>COPYRIGHT: (C)2006,JPO&NCIPI |
