| 摘要 |
PROBLEM TO BE SOLVED: To dispose of exhaust emission at high efficiency over a long period of time by combining a NOx decomposition stage with a catalyst regenerating stage in a suitable time pattern. SOLUTION: An active metal-containing catalyst (catalyst) 7 decomposes NOx when an engine is operated under the lean condition of an air-fuel ratio, so that NOx is contained in the exhaust emission, and the catalyst 7 is gradually oxidized to form an oxide film on the surface of the catalyst 7, so that the NOx decomposition activity of the catalyst 7 is gradually decreased. Such activity decrease is detected by a NOx sensor NS2 installed at an outlet side of the catalyst 7, and when it is known from the NOx concentration that the catalyst activity is unsuitably low or lowered, the air-fuel ratio is put in the rich state. In this state, the oxide film on the surface of the catalyst 7 is removed by reaction and the catalyst 7 is regenerated. The regeneration of the catalyst 7 is determined by monitoring the HC concentration by hydrocarbon sensors CS1, CS2 installed upper-and downstream. When the catalyst is once regenerated the air-fuel ratio is returned to the lean state. |
