Patent ID: 7451593

Claim:
An exhaust gas cleaning method in an exhaust gas cleaning system which performs NOx purification by an NOx occlusion/reduction type catalyst and PM purification by a continuous regeneration type DPF for exhaust gas from an internal combustion engine, and includes a temperature sensor which detects a temperature of the exhaust gas flowing into the DPF, a differential pressure sensor which detects a differential pressure across the DPF, first and second NOx concentration sensors which detect NOx concentrations across the NOx occlusion/reduction type catalyst, and a control unit which calculates a differential pressure increasing rate per unit time of the DPF and calculates an NOx purification rate from the NOx concentrations across the NOx occlusion/reduction type catalyst, the method comprising: reducing richness in a richness control performed in a catalyst regenerative control for regenerating the NOx occlusion/reduction type catalyst when: the temperature of the exhaust gas detected by the temperature sensor is not less than a self-burning temperature of PM, and the differential pressure increasing rate is not less than a specified determination value thereof; and increasing richness in a richness control performed in a catalyst regenerative control for regenerating the NOx occlusion/reduction type catalyst when: the temperature of the exhaust gas detected by the temperature sensor is not less than a self-burning temperature of PM, the differential pressure increasing rate is not more than a specified determination value thereof, and the NOx purification rate is less than a specified determination value thereof.