Electronic control unit for controlling air fuel ratio to reduce NOx occluded in NOx catalyst

摘要

An electronic control unit controls an air/fuel ratio of an engine having a lean NOx catalyst in its exhaust system in order to improve the cleaning of exhaust gas during rich process to reduce lean NOx catalyst. The electronic control unit includes an estimator for estimating NOx amount occluded in the lean NOx catalyst; a device for performing shift-to-rich process responsive to the NOx amount estimated by said estimator exceeding a predetermined value; a calculator for calculating an accumulated value of an exhaust flow amount; and a device for completing the shift-to-rich process responsive to the accumulated value of the exhaust flow amount exceeding a threshold value. Completing the shift-to-rich process using the accumulated value of the exhaust flow amount enables the NOx occluded in the lean NOx catalyst to be precisely reduced.