Exhaust gas cleaning method and exhaust gas cleaning system

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, comprising:

• 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,
• wherein the control unit reduces richness of the exhaust gas 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 the control unit increases richness of the exhaust gas 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.

An exhaust gas cleaning method for an internal combustion engine including an NOx occlusion/reduction type catalyst and a continuous regeneration type DPF to perform PM purification, comprising:

• detecting a temperature of exhaust gas flowing into the continuous regeneration type DPF;
• detecting a differential pressure across the DPR;
• calculating a differential pressure increasing rate per unit time of the DPF;
• and
• reducing or increasing richness in a richness control performed in a catalyst regenerative control for regenerating the NOx occlusion/reduction type catalyst based on whether the detected temperature is less than a self-burning temperature of PM and whether the calculated differential pressure increasing rate is less than or more than a specified determination value thereof.