US Patent 7451593 Exhaust gas cleaning method and exhaust gas cleaning system

An exhaust gas cleaning method capable of maintaining an optimum NOx purification ratio in the continuously regenerative range of DPF in an exhaust gas cleaning system (1) having an NOx purification function and a PM purification function combined with each other and the exhaust gas cleaning system (1). In the exhaust gas cleaning system (1), NOx purification by an NOx occlusion/reduction type catalyst (42) and PM purification by a continuous regeneration type DPF (41) are preformed for exhaust gas from an internal combustion engine. When the temperature (Tent) of the exhaust gas flowing in the DPF (41b) exceeds the self-burning temperature (Tent0) of PM and the differential pressure increasing rate (dP) of a differential pressure across the DPF (41b) exceeds a specified determination value (dP0), at least either of a control to reduce the frequency of enriching the exhaust gas and a control to reduce the degree of richness is performed in a catalyst regenerative control for regenerating the NOx occlusion/reduction type catalyst (42).