Importance

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Abstract

An occlusion cycle and a rich reduction cycle are repeated alternately in a NOx occlusion reduction catalyst connected to an exhaust pipe of an engine. When a NOx occlusion rate decreases while the occlusion cycle and the rich reduction cycle are being repeated, a sulphur purge is performed. A NOx occlusion map, which indicates the NOx occlusion amount of the NOx occlusion reduction catalyst during the occlusion cycle, as affected by aging degradation, is prepared in advance, and an ideal NOx occlusion amount is determined based on the NOx occlusion map. In the meantime, an actual NOx occlusion amount during the occlusion cycle is calculated from a NOx sensor value. Degradation of the NOx occlusion reduction catalyst due to sulfur poisoning is distinguished from thermal degradation of the NOx occlusion reduction catalyst based on the difference between the ideal NOx occlusion amount and the actual NOx occlusion amount.

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First Claim