System for feedback control of air-fuel mixing ratio in intake system of internal combustion engine

A control system for feedback control of the mixing ratio of an air-fuel mixture in an induction passage for an internal combustion engine, the control system comprising:

• an electrically controllable fuel supply means for supplying fuel into the induction passage;
• a first oxygen-sensitive air/fuel ratio sensor which has an oxygen ion conductive solid electrolyte layer and two electrode layers formed on the solid electrolyte layer and is disposed in the induction passage in a section downstream of the fuel supply means;
• a second oxygen-sensitive air/fuel ratio sensor disposed in said section of the induction passage so as to be located close to said first air/fuel ratio sensor, said second sensor having an oxygen ion conductive solid electrolyte layer with two electrode layers formed thereon and exhibiting an on-off type output characteristic such that the magnitude of the output of said second sensor undergoes a sharp change between a maximally high level and a minimally low level when the air-fuel ratio of said mixture changes across a stoichiometric air/fuel ratio;
• a control means for providing a control signal to said fuel supply means based on a feedback signal produced by said first sensor to correct a deviation of the air-fuel mixing ratio indicated by the feedback signal from a predetermined mixing ratio, said control means including a discriminating means for interpreting information in said feedback signal with reference to the output of said second sensor;
• an ignition means for burning a fractional portion of the air-fuel mixture flowing in the induction passage such that said first and second sensors are exposed to a combustion gas produced by combustion of said fractional portion of the air-fuel mixture;
• and
• a power supply means for forcing a constant DC current of a predetermined intensity to flow through said solid electrolyte layer of said first sensor between said two electrode layers thereby selectively affording said first sensor with one of (a) first type slope output characteristic such that the magnitude of the output of said first sensor gradually varies as the air-fuel mixing ratio varies but remains above said stoichiometric air/fuel ratio, and (b) second type slope output characteristic such that the magnitude of the output of said first sensor gradually varies as the air-fuel mixing ratio varies but remains below said stoichiometric air/fuel ratio.