| 摘要 |
<p>PROBLEM TO BE SOLVED: To early recover the NOx storage capacity of a catalyst by reducing NOx stored in a lean Nox catalyst while avoiding a wasteful fuel consumption after recovering from a fuel cut. SOLUTION: When the lean operation of a direct injection engine 11 is performed continuously, the operation is switched temporarily to a rich operation to recover the NOx storage capacity of the lean NOx catalyst 39 before the NOx storage amount of the lean NOx catalyst 39 reaches a saturated amount. When a fuel is cut off during the lean operation, it takes a time for a while for the concentration of the oxygen in an exhaust pipe 37 to lower even when the operation is switched to the rich operation immediately after recovering from the fuel cut. Until that time, even if the rich operation is performed, NOx stored in the lean NOx catalyst 39 cannot be reduced, and fuel is consumed wastefully to deteriorate a fuel economy. To prevent this problem, the switching of the operation to the rich operation is prohibited until a specified time is passed after the recovery from the fuel cut (namely, until the concentration of the oxygen in the exhaust pipe 37 is lowered).</p> |
