| 摘要 |
<p>PROBLEM TO BE SOLVED: To accurately estimate an NOx occulsion amount of an NOx occulsion/reduction catalyst. SOLUTION: Exhaust emission controlling catalysts (SC) 5a, 5b having O2 storage function are arranged on exhaust passages 2a, 2b of an internal combustion engine 1. An NOx occulsion and reduction catalyst 7 is arranged on a downstream side joined exhaust passage 2. A value of an NOx counter is increased by a constant rate during lean air-fuel ratio operation of the engine by means of an electronic control unit(ECU) 30. The value is decreased by a constant rate during rich air-fuel ratio driving of the engine. When the air-fuel ratio is shifted from the lean to the rich state, and the exhaust air-fuel ratio at an inlet of the occulsion/reduction catalyst is kept near a theoretical value due to the O2 storage function of the SCs 5a, 5b, the variation of the value of the NOx counter is prevented by ECU 30. It is thus possible to accurately reflect the value of the NOx upon the NOx occulsion amount.</p> |
