Air/fuel ratio control system for an internal combustion engine with a function for preventing the blackening phenomenon of oxygen concentration sensor

An air/fuel ratio control system for an internal combustion engine, comprising:

• an air supply passage connected to an air intake system of said internal combustion engine, for varying the air/fuel ratio of the mixture to be supplied to the engine;
• an open/close valve disposed in said air supply passage for controlling the air flow in said air supply passage;
• an oxygen concentration sensing unit disposed in an exhaust gas of the internal combustion engine, said sensing unit including an oxygen pump element and a sensor cell element which define a diffusion restricted region therebetween, each of said elements including a solid electrolyte member having oxygen ion permeability and a pair of electrodes sandwiching said electrolyte member therebetween;
• current supply means for supplying a pump current across the electrodes of said oxygen pump element so as to keep constant voltage generated across the electrodes of said sensor cell element thereby causing said sensing unit to monitor a magnitude of said pump current which is substantially in proportion to the oxygen concentration in the exhaust gas;
• valve open time period control circuit means connected to said oxygen concentration detection device, for determining the oxygen concentration of the mixture supplied to the engine by means of the magnitude of said pump current and calculating the open time period of said open/close valve in each of a plurality of cyclic periods in response to the result of said determination every cyclic period;
• valve drive means connected to said valve open time period control circuit means for opening said open/close valve for said calculated valve open time period in each of said cycles;
• and
• control means responsive to output signals of said current supply means and said valve open time period control circuit means for stopping the supply of said pump current across the electrodes of said oxygen pump element when the valve open time period calculated by said valve open time period control circuit means exceeds a predetermined upper limit of valve open time period.