Abstract: According to the device of the invention, measured values (V.sub.0 ', V') of an output V.sub.A of an opening sensor at two adjusted levels of throttle openings in both idling state and fast idling state and their ideal values (V.sub.0, V) are used. The value V.sub.0 ' in the idling state is subtracted from the output V.sub.A in a driving condition, (V.sub.A -V.sub.0 '), which is then multiplied by the ratio of ideal variation characteristics of the sensor output for a change of the throttle opening to those in the throttle valve product used, {(V-V.sub.0)/(V'-V.sub.0 ')}, to produce a correction signal T.sub.A matching the individual difference. Further, a reference value subtracted from the output V.sub.A in a driving condition is set as T.sub.AMIN which varies with the output V.sub.A, taking the difference of variation characteristics of the sensor output from ideal characteristics into account.

Abstract: An object of the invention is to detect an upper limit of a lean burn zone in an internal combustion engine in each ignition. The lean limit is detected on the basis of the characteristics of the ionic current in a cylinder of the engine immediately after ignition.

Abstract: This invention relates to a method of controlling the air-fuel ratio of an internal combustion engine having a main 0.sub.2 sensor and a subsidiary O.sub.2 sensor provided upstream and downstream, respectively, of a catalyst converter for detecting the oxygen concentration of the exhaust gas of the engine. The method comprises: feedback-controlling the air-fuel ratio of a mixture gas to be supplied to the engine to about a stoichiometric air-fuel ratio having the highest purification efficiency of the exhaust gas by adjusting the amount of fuel to be supplied by an injector in accordance with the output signal from the main O.sub.2 sensor; and also feedback-controlling more precisely the air-fuel ratio of the mixture gas to about the stoichiometric air-fuel ratio by adjusting the amount of fuel supply in accordance with the output signal of the subsidiary O.sub.2 sensor. The feedback control value set for adjusting the amount of fuel supply in accordance with the output signal of the subsidiary O.sub.