Method of purging lean nox trap

Abstract

A method of purging a NO,trap, located in the exhaust passage of a direct injection engine, is proposed that provides transition from a lean fuel mixture to a rich fuel mixture 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 in order to avoid any noticeable change in torque. The increased amount of fuel delivered during purge emits substantially more reducing species than in previous strategies and accordingly the required duty cycle may be substantially reduced while still obtaining the same reduction of NO,without any fuel economy penalty.