Patent ID: 7191590

Claim:
An exhaust gas purifying method, using an exhaust gas purifying system for internal combustion engines providing with a NOx occlusion reduction type catalyst occluding NOx in the lean air-fuel ratio and discharging and reducing NOx in the rich air-fuel ratio, in which when it is judged that sulfur purging of said NOx occlusion reduction type catalyst is necessary, a sulfur purging control is performed by making the air-fuel ratio of the exhaust gas flowing in said NOx occlusion reduction type catalyst richer than that of the normal driving state of the internal combustion engines; wherein, during sulfur purging control and following the start of said sulfur purging control, judging that sulfur purging has started when an oxygen concentration detected by an oxygen concentration detection means disposed in the downstream of said NOx occlusion reduction type catalyst drops below a predetermined judgment value set to determine timing of transition from a first predetermined oxygen concentration indicating an NOx discharge condition to a second predetermined oxygen concentration indicating that the NOx discharge has ended and indicating that an SOx discharge has started.