Air/fuel ratio feedback control system for lean combustion engine

Abstract

The invention relates to a control systems for feedback control of the air/fuel ratio in an internal combustion engine, e.g., an automotive engine, which uses a three-way catalyst to purify the exhaust gas, by using an exhaust sensor to detect actual values of air/fuel ratio in the engine. The control system has the function of varying the target value of air/fuel ratio according to operating conditions of the engine. The target value becomes super-stoichiometric during steady-state operation of the engine and changes to a lower value optimum for the activities of the three-way catalyst, such as the stoichiometric value, under predetermined transient conditions of the engine. At the start of such a change in the target value, the control system functions so as to intentionally deviate the air/fuel ratio from the value optimum for the three-way catalyst in a direction away from the target value immediately before the change. By doing so NOx is effectively removed by the three-way catalyst with little delay from the change in the target value of air/fuel ratio accompanying the shift to a transient operating condition.