Abstract: An air-fuel ratio control system for an internal combustion engine includes two air-fuel ratio sensors, one upstream of a catalytic converter and the other downstream of the catalytic converter. When an output of the downstream air-fuel ratio sensor has been inverted between rich and lean, a target air-fuel ratio is corrected by a skip amount in a direction opposite to that of the inversion. On the other hand, at no inversion, the target air-fuel ratio is corrected by an integral amount in a direction opposite to deviation of the output of the downstream air-fuel ratio sensor. A fuel injection amount is derived based on a differential between the target air-fuel ratio and an output of the upstream air-fuel ratio sensor. An engine operating condition, the output of the downstream air-fuel ratio sensor or a deterioration degree of the catalytic converter may be used to change the skip amount and the integral amount.

Abstract: For stable air-fuel ratio control without hunting, an output condition of an air-fuel ratio sensor is monitored to decide whether or not a modern or advanced control using a dynamic model may be performed. The appropriateness of using the dynamic model may be determined by monitoring whether the output value of an air-fuel ratio sensor exists in a predetermined range or whether the air-fuel ratio sensor is maintained stably in an air-fuel ratio detectable state. When the sensor output condition is out of the predetermined range suitable for the modern control, an optimum feedback gain for performing the modern control is switched over to a lowered gain or the modern control is switched over into normal feedback control by proportional-and-integral control. By this control mode switch-over, most appropriate air-fuel ratio feedback control may be performed.

Abstract: An ignition timing control system for an internal combustion engine which can perform fast catalyst warm-up operation and prevent stalling at the restart of the internal combustion engine at a low temperature is disclosed. This control system judges whether the engine is in the start state or not, whether the cooling water temperature is lower than the specified temperature or not, and whether the intake air temperature is higher than the preset temperature or not. When these judgments are all affirmative, the system further judges whether the time after the start has reached the control starting time or not, and then whether the engine is in the idle operation state or not. When these judgments are both affirmative, the system sets the target ignition timing retard amount and the ignition timing gradual change time according to the intake air temperature, and then actually performs the ignition timing control according to the target retard amount and gradual retard time.

Abstract: An engine temperature is measured an opening of a door is detected so an electrical conduction is made to a heater for heating a sensor element by setting a target element temperature to a relatively high level if such engine temperature (cooling water temperature Thw) is relatively low or, on the other hand, by setting the target element temperature to a relatively low level if the cooling water temperature is relatively high. Then the element temperature is detected and the heater is turned off when the element temperature has reached the target temperature.