| 摘要 |
<p>PROBLEM TO BE SOLVED: To prevent deterioration in fuel consumption and driving feeling when a NOx adsorption catalyst is regenerated by providing the NOx adsorption catalyst for adsorbing NOx in an exhaust passage and lowering the NOx concentration at least near a catalyst layer in releasing the NOx adsorbed in the NOx adsorption catalyst. SOLUTION: In this control unit, it is judged whether operation with a lean air-fuel ratio is continuing for a predetermined period or not (S1). If it is continuing, control is executed so as to release NOx judging that adsorption of NOx to a NOx adsorption catalyst is progressing and the NOx adsorption function is deteriorating. When an ignition timing is delayed (S4) and NOx concentration in exhaust gas lowers, a balance between NO adsorbed by or adhered to the NOx adsorption catalyst and NO surrounding the catalyst is lost, thus allowing NOx to be released from the NOx adsorption catalyst. Then NOx released from the NOx adsorption catalyst is reduced by injection in an expansion stroke (S5). This transfer to injection in the expansion stroke suppresses NOx.</p> |
