System for feedback control of air/fuel ratio in IC engine having means for supplying controlled current to oxygen sensor

A system for feedback control of the air/fuel ratio of an air-fuel mixture supplied to an internal combustion engine, the control system comprising:

• an electrically controllable fuel supply means provided in the intake system of the engine;
• an air/fuel ratio detector disposed in an exhaust passage for the engine and having an oxygen-sensitive element of a concentration cell type comprising a substrate, a microscopically porous reference electrode layer formed on the substrate, a microscopically porous layer of an oxygen ion conductive solid electrolyte formed on the substrate so as to cover the reference electrode layer substantially entirely and a microscopically porous measurement electrode layer formed on the solid electrolyte layer;
• operating condition detecting means for detecting at least one of (i) an exceedingly high air/fuel ratio condition where the air/fuel ratio is above an upper boundary of an expected range of fluctuations of air/fuel ratio under feedback control and (ii) an exceedingly low exhaust temperature condition where the temperature of the exhaust gas in the exhaust passage is below a lower boundary of a temperature range for effective function of the oxygen-sensitive element, and for generating a command signal indicative of the existence of the detected condition;
• and
• control means for providing a control signal to the fuel supplying means to control the rate of fuel feed to the engine so as to maintain a predetermined air/fuel ratio by utilizing the output of the air/fuel ratio detector as a feedback signal, the control means including a circuit for providing a DC current of a predetermined intensity to flow through the solid electrolyte layer of the oxygen-sensitive element to thereby establish a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer, the control means further comprising a current-intensity altering means for decreasing the intensity of said DC current from said predetermined intensity while the operating condition detecting means continues to generate said command signal whereby an undesirable rise of said reference oxygen partial pressure is precluded.