Engine air/fuel ratio sensing device

Claim 1

An air/fuel ratio sensing device for an engine comprising an air/fuel sensor means having a gap portion for introducing an exhaust gas of said engine, a solid electrolyte oxygen pump cell for controlling an oxygen partial pressure within said gap portion, and a solid electrolyte oxygen sensor cell for producing an electromotive force corresponding to the difference between the oxygen partial pressure in the exhaust gas within said gap portion and the oxygen partial pressure in the exhaust gas outside said gap portion; and a control circuit means, coupled to said sensor means, having a current amplifier means for supplying to said oxygen pump cell a pump current necessary for keeping said oxygen partial pressure at a predetermined value, and a resistor (Rs) inserted in the electrical path through which said pump current flows for limiting said pump current to a predetermined value, said control circuit means further comprising: a voltage measuring means, connected to said resistor, for measuring and providing as outputs therefrom the terminal voltage across said resistor and the terminal voltage across said oxygen pump cell, a computing means, connected to said voltage measuring means, for calculating from said terminal voltage across said resistor and the resistance of said resistor a first value (Ip) corresponding to said pump current and for calculating from said first value and the terminal voltage across said oxygen pump cell a second value (R) corresponding to the internal resistance of said oxygen pump cell, and a calibrating means, connected to said computing means, for providing as an air/fuel signal therefrom a third value (Ipo) obtained by calibrating said first value in accordance with a predetermined relationship including said first value and said second value as a parameter.