Purging of NOx trap in exhaust passage of direct injection engine initiating purge portion of trap cycle when amount of NOx stored in trap exceeds a set amount by abruptly changing from lean to rich air-fuel ratio

摘要

The method involves determining the accumulated amount of NOx stored in the trap and then initiating a purge operation when the amount of stored NOx exceeds a predetermined amount. The trap is purged by changing from a lean air/fuel ratio to a rich air/fuel ratio for a set time, with the transition achieved in a single engine cycle without substantial change in the engine air flow. This is accomplished by substantially increasing the fuel flow rate, while retarding the spark. The mixture is set to be as rich as possible.