ENGINE EMISSION CONTROL SYSTEM

摘要

PROBLEM TO BE SOLVED: To accurately detect NOx adsorption amount and oxygen storage capacity of an NOx adsorbent by judging an NOx adsorption amount while considering an oxygen storage amount based on results of oxygen concentration detected in an exhaust gas downstream of the NOx adsorbent when air-fuel ratio changing means changes an air-fuel ratio to stoichiometric or rich condition. SOLUTION: Whether an output Vo of a second air-fuel ratio sensor exceeds threshold value VS2 representing rich condition is judged (step 202). If not exceeding, then whether Vo exceeds threshold value VS1 representing lean condition is judged (step 203). Cumulative value SQa of the air flow amount Qa and cumulative number counter CQa are renewed (step 205). If Vo exceeds VS2 in the step 202, an NOx purge requirement flag is cleared (step 206). Then, an average air flow amount Qave (=SQa/CQa) during NOx emission is calculated (step 207), and NOx adsorption amount detection value TNOD is calculated (step 208). Since timings of oxygen emission and NOx emission are detected separately, an NOx adsorption amount and oxygen storage amount can be detected with high accuracy.