Patent ID: 8186148

Claim:
An exhaust gas purification method for an exhaust gas purification system that includes a NOx purification catalyst for purifying NOx when an air/fuel ratio of exhaust gas is in a lean condition and for recovering a NOx purifying ability when the air/fuel ratio is in a rich condition, and a catalyst regeneration controller for performing regeneration control to recover the NOx purifying ability of the NOx purification catalyst, and uses air-intake system control for decreasing an air-intake amount and fuel system control for increasing a fuel injection amount into a cylinder in combination, thereby controlling the rich condition for the regeneration control, the method comprising the steps of: measuring an oxygen concentration in an exhaust passage of the system; calculating a time-dependent combustion air/fuel ratio in the cylinder based directly on the oxygen concentration measured in the exhaust passage; calculating an instant injection timing based on the time-dependent combustion air/fuel ratio in the cylinder; and changing an injection timing of the fuel injection into the cylinder to bring the injection timing to the instant injection timing during switching intervals from the lean condition to the rich condition and from the rich condition to the lean condition in the regeneration control of the NOx purification catalyst, wherein, the angle of the injection timing of the fuel injection into the cylinder is advanced to bring the injection timing to the instant injection timing during the switching intervals from the lean condition to the rich condition at a beginning of the regeneration control, at a start time of a transition to the rich condition, decreasing an air-intake amount and increasing a fuel amount, and then advancing in angle the injection timing from a lean fuel injection timing to a target fuel injection timing for rich combustion in response to a change in combustion air/fuel ratio, which is a relatively slow change during the transition, at a time of continuing the regeneration control after the transition to the rich condition, decreasing the air-intake amount and increasing the fuel amount, and making the injection timing stay in a condition of the target fuel injection timing, and the angle of the injection timing of the fuel injection into the cylinder is delayed to bring the injection timing to the instant injection timing during the switching intervals from the rich condition to the lean condition at an end of the regeneration control, at a start time of a transition to the lean condition, decreasing the air-intake amount and increasing the fuel amount, and then gradually delaying in angle from the target fuel injection timing to the lean fuel injection timing in response to the relatively slow change in combustion air/fuel ratio.