| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control system capable of achieving excellent NOx purification performance even under low-temperature exhaust gas conditions. <P>SOLUTION: This exhaust emission control system for an internal combustion engine mainly conducts lean combustion while an exhaust gas air-fuel ratio can be varied between rich and lean. An NOx trap catalyst is provided on an exhaust passage. As temperature of exhaust gas is 200°C or less at an inlet to the NOx trap catalyst, the exhaust gas air-fuel ratio is set to be rich in such a way that CO and O<SB>2</SB>exist in exhaust gas to satisfy an expression (CO/O<SB>2</SB>)≥5.0 (1). Using CO as reducer, NOx is desorbed and purified. When exhaust gas to satisfy relation of the expression (1) flows into the NOx trap catalyst, a ratio of inlet temperature to internal temperature of the NOx trap catalyst is 1.1 or more. <P>COPYRIGHT: (C)2004,JPO |
