Abstract: 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.

Abstract: Even when a reaction delay time of an A/F ratio detection device changes, an A/F ratio controller prevents an A/F ratio feedback control from being disturbed. The controller is programmed to calculate an A/F ratio feedback coefficient by proportional term control and integration term control, and to set length of time for said integration term to be carried out in accordance with an operating state of the engine. It is programmed to calculate the integration term based on the set time and to set a proportional term for shifting said A/F ratio feedback coefficient. It is further programmed to detect a deviation between a first A/F ratio feedback coefficient before the integration term is carried out and a second A/F ratio feedback coefficient after the integration term is carried out and the coefficient is shifted by the proportional term set by said means for setting the proportional term. Based on the detected deviation, the length of time for the integration term to be carried out is corrected.

Abstract: The invention provides an electronic control unit for controlling 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 comprises: an estimator for estimating NOx amount occluded in the lean NOx catalyst; means 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 means 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, which is correlated with the amount of the reducing agent (HC, CO), enables the NOx occluded in the lean NOx catalyst to be precisely reduced, and the exhaust gas is cleaned at higher level.

Abstract: Even when a reaction delay time of an A/F ratio detection device changes, an A/F ratio controller prevents an A/F ratio feedback control from being disturbed. The controller is programmed to calculate an A/F ratio feedback coefficient by proportional term control and integration term control, and to set length of time for said integration term to be carried out in accordance with an operating state of the engine. It is programmed to calculate the integration term based on the set time and to set a proportional term for shifting said A/F ratio feedback coefficient. It is further programmed to detect a deviation between a first A/F ratio feedback coefficient before the integration term is carried out and a second A/F ratio feedback coefficient after the integration term is carried out and the coefficient is shifted by the proportional term set by said means for setting the proportional term. Based on the detected deviation, the length of time for the integration term to be carried out is corrected.