Patent ID: 7228839

Claim:
Method of controlling a four-stroke internal combustion engine being operated in a controlled auto-ignition mode, said engine including a variable volume combustion chamber defined by a piston reciprocating within a cylinder between top-dead center and bottom-dead center points, intake and exhaust passages, a fuel deliver system and intake and exhaust valves controlled during repetitive, sequential exhaust, intake, compression and expansion strokes of said piston comprising: operating a variable valve system to control the intake and exhaust valves to effect in-cylinder conditions conducive to controlled auto-ignition; operating the fuel delivery system to effect an in-cylinder fuel charge at one of stoichiometric and lean of stoichiometric air-fuel ratios conducive to controlled auto-ignition; providing a measure of engine NOx emission index; and, when engine NOx emission index crosses a predetermined threshold, transitioning the in-cylinder fuel charge from the operative one of the stoichiometric and lean of stoichiometric air-fuel ratios to the inoperative one of the stoichiometric and lean of stoichiometric air-fuel ratios.