| 摘要 |
<p>PROBLEM TO BE SOLVED: To prevent NOx from being released from a catalyst to the atmosphere as it is in unpurified state by setting the air-fuel ratio of exhaust gas to the state with air in excess of a theoretical air-fuel ratio (lean state) when the catalyst is in inert state where it cannot purify the absorbed NOx. SOLUTION: The exhaust gas from an internal combustion engine 1 is discharged through an exhaust passage 6, and an exhaust emission purifying catalyst 7 is interposed in the exhaust passage 6. The catalyst 7 has the characteristic of absorbing NOx in exhaust gas under the lean condition of the exhaust air-fuel ratio, releasing and reduction-purifying the absorbed NOx by the reduction component in the exhaust gas under the rich condition. When the catalyst temperature detected by a catalyst temperature sensor 16 is a prescribed value (for example, 300 deg.C) or lower, wherein the catalyst 7 is not in active state, the NOx absorbed in the previous stop of the engine might be released as it is unpurified. Therefore, to prevent the release of NOx from the catalyst 7, a lean combustion control is forcedly performed.</p> |
