DIAGNOSTIC SYSTEM AND EXHAUST EMISSION CONTROL DEVICE FOR INTERNAL COMBUSTION ENGINE

To provide a diagnostic system capable of improving detection accuracy of abnormalities such as missing and deterioration of an NOselective reduction catalyst provided in an exhaust passage downstream of an NOocclusion catalyst, and an exhaust emission control device for an internal combustion engine.SOLUTION: A diagnostic system for diagnosing an abnormality of an NOselective reduction catalyst in an exhaust emission control device having an NOocclusion catalyst and the NOselective reduction catalyst provided in an exhaust passage of an internal combustion engine from the upstream side in this order includes: a downstream side ammonia concentration estimation section for calculating a downstream side ammonia concentration estimation value that is an estimation value of an ammonia concentration in the exhaust passage downstream of the NOselective reduction catalyst; a downstream side ammonia concentration detection section for acquiring a downstream side ammonia concentration detection value that is a detection value of an ammonia concentration in the exhaust passage downstream of the NOselective reduction catalyst on the basis of a sensor signal of an ammonia concentration sensor provided in the exhaust passage downstream of the NOselective reduction catalyst; and a determination section for determining an abnormality of the NOselective reduction catalyst on the basis of the downstream side ammonia concentration estimation value and the downstream side ammonia concentration detection value.SELECTED DRAWING: Figure 3 【課題】ＮＯX吸蔵触媒よりも下流側の排気通路に備えられたＮＯX選択還元触媒の欠落や劣化等の異常の検出精度を向上可能な診断装置及び内燃機関の排気浄化装置を提供する。【解決手段】内燃機関の排気通路に上流側から順にＮＯX吸蔵触媒とＮＯX選択還元触媒とを備えた排気浄化装置におけるＮＯX選択還元触媒の異常を診断する診断装置は、ＮＯX選択還元触媒よりも下流側の排気通路内のアンモニア濃度の推定値である下流側アンモニア濃度推定値を算出する下流側アンモニア濃度推定部と、ＮＯX選択還元触媒よりも下流側の排気通路に備えられたアンモニア濃度センサのセンサ信号に基づいてＮＯX選択還元触媒よりも下流側の排気通路内のアンモニア濃度の検出値である下流側アンモニア濃度検出値を取得する下流側アンモニア濃度検出部と、下流側アンモニア濃度推定値及び下流側アンモニア濃度検出値に基づいてＮＯX選択還元触媒の異常を判定する判定部と、を備える。【選択図】図３