Air/fuel ratio control system for an internal combustion engine

A method for switching between feedback control and open loop control in an air/fuel ratio control system for an internal combustion engine, in which the air/fuel ratio of an air-fuel mixture is selectively controlled in either a feedback or open loop mode based upon predetermined criteria and upon an exhaust gas component concentration, the method comprising:

• generating a reference signal and varying said reference signal in response to a first sensor signal from an exhaust gas sensor;
• comparing said first sensor signal and said reference signal to determine a rich or lean condition of said air-fuel mixture;
• measuring a time period starting from the time when said air-fuel mixture becomes rich;
• interrupting feedback control with said air-fuel mixture is rich for a predetermined time and clearing said measured period;
• interrupting feedback control when an engine coolant temperature is lower than a given temperature, or an engine load condition is not adapted to carry out feedback control or an engine speed is less than a given speed;
• carrying out open loop control in response to interrupting said feedback control.

An air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture based upon the concentration of an exhaust gas component of an internal combustion engine when a predetermined feedback condition is satisfied, and for performing open loop control of the air/fuel ratio based upon open loop parameters when the feedback condition is not satisfied, said system comprising:

• a first sensor for measuring the concentration of an exhaust gas component and producing a first sensor signal having a value proportional to the measured concentration;
• a second sensor for measuring engine speed and producing a second sensor signal having a value proportional to measured engine speed;
• a first means for producing a reference signal representative of a reference value which is determined based on said first sensor signal value;
• a second means for comparing said first sensor signal value with said reference signal value to discriminate between a rich and a lean air/fuel mixture and for producing a rich signal when the first sensor signal value is larger than said reference signal value and a lean signal when the first signal value is smaller than the reference signal value;
• a third means for measuring a period of time during which said second means continuously outputs said rich signal, said third means producing a first feedback disabling signal when the measured time period exceeds a predetermined value;
• a fourth means for producing a second feedback disabling signal when said feedback condition is not satisfied with respect to parameters defining said feedback condition other than rich/lean of the air/fuel mixture;
• and
• a fifth means for effecting feedback control and open loop control selectively, said fifth means being responsive to said first and second feedback disabling signals to switch the control mode from feedback control to open loop control and for resetting said third means to clear the measured time period in response to said first feedback disabling signal.