Patent ID: 7673444

Claim:
An exhaust gas purification apparatus, comprising: a reduction catalytic converter disposed in an exhaust passage of an engine to reductively purify nitrogen oxide in exhaust gas using ammonia or its precursor as a reducing agent; a first concentration sensor configured to detect an ammonia concentration in an exhaust gas moving on a downstream side of the reduction catalytic converter; a control unit incorporating therein a computer; an oxidation catalytic converter disposed on the exhaust downstream side of the reduction catalytic converter to oxidize the ammonia in the exhaust gas; and a second concentration sensor configured to detect an ammonia concentration in an exhaust gas moving on the downstream side of the oxidation catalytic converter, wherein the control unit conducts calculation to estimate the ammonia concentration in the exhaust gas moving on the downstream side of the reduction catalytic converter, and when a difference between the ammonia concentration detected by the first concentration sensor and the estimated and operated ammonia concentration becomes larger than a first predetermined value, the control unit determines that the reduction catalytic converter is deteriorated, and wherein the control unit conducts calculation to estimate the ammonia concentration in the exhaust gas moving on the downstream side of the oxidation catalytic converter, and when a difference between the ammonia concentration detected by the second concentration sensor and the estimated ammonia concentration by the calculation becomes larger than a second predetermined value, the control unit determines that the oxidation catalytic converter is deteriorated.