EXHAUST EMISSION CONTROL DEVICE FOR INTERNAL COMBUSTION ENGINE

摘要

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of separately positively detecting a deterioration in a first stage catalyst and that in a second stage catalyst without inviting an increase in costs. SOLUTION: An exhaust passage between a first stage catalyst 20 and a second stage catalyst 30, and an exhaust passage at the downstream side of the first stage catalyst are provided with a first and a second oxygen concentration detecting means 32, 34 for detecting oxygen concentration in an exhaust gas. An oxygen concentration estimating means 40 is provided to estimate the oxygen concentration in the exhaust gas at the upperstream side of the first catalyst from the operating state of an internal combustion engine. A catalyst deterioration detecting means 40 detects a deterioration in the first catalyst based on the estimated value obtained from the oxygen concentration estimating means and an output from the first oxygen concentration detecting means when a bypass passage 22 is closed. A deterioration in the second stage catalyst is detected based on the second oxygen concentration detecting means when the bypass passage is opened.