EXHAUST PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE

摘要

Disclosed is an exhaust purification device whereby a hydrocarbon supply valve (15) and an exhaust purification catalyst (13) are disposed in an engine exhaust passage. When the engine is running, either a first NOx purification method or a second NOx purification method is selected, depending on the degree of sulfur poisoning in the exhaust purification catalyst (13). The first NOx purification method purifies NOx contained in the exhaust gas by injecting a hydrocarbon from the hydrocarbon supply valve (15) at predetermined supply intervals, while the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (13) is kept lean. The second NOx purification method purifies the NOx by switching the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (13) from lean to rich, using longer intervals than the abovementioned predetermined supply intervals.