| 摘要 |
<p>PURPOSE: To provide reduction in NOx and torque shock by enriching air-fuel ratio at a prescribed enriching speed for returning to a theoretical air-fuel ratio if NOx adsorption quantity exceeds its allowance with a catalyst, and setting the enriching rate based on NOx desorbing rate. CONSTITUTION: The first catalyst for adsorbing NOx in the exhaust gas at the lean side atmosphere, and the second catalyst having an O2 stage function at the atmosphere near theoretical air-fuel ratio are placed in an exhaust gas passage. If a discriminating means 21 discriminates a lean driving region, air-fuel ratio is set at a lean side target value by a setting means 22, and NOx adsorption quantity by the catalyst is computer by a computation means 23. When switching to the driving at the theoretical air-fuel ratio, if a discriminating means 24 discriminates that NOx adsorption quantity exceeds its allowance, an air-fuel ratio control means 25 enriches the air-fuel ratio at a prescribed enriching speed to return it to the theoretical air-fuel ratio, and a setting means 26 sets the enriching rate based on NOx desorbing rate at that time.</p> |
