Abstract: In a horizontally opposed type engine, right and left exhaust pipes are provided for right and left cylinder banks, respectively and a collecting pipe is disposed to collect gases these two exhaust pipes. Further, in a two-stage catalytic converter system, two main catalysts are provided for the two exhaust pipes, respectively, and a subsidiary catalyst is provided in the collecting pipe. An oxygen sensors is provided in each of the exhaust pipes on the upstream side of it's main catalyst. The air-fuel ratios of both the banks are controlled simultaneously on the basis of the output of one of the sensors, and the air-fuel ratio of the other bank is corrected on the basis of a difference between the outputs of the two oxygen sensors. Accordingly, the air-fuel ratios of both right and left banks can be controlled appropriately, without the use of an auxiliary oxygen sensor. As a result, it is possible to improve the purification capability of the auxiliary catalyst in the collecting pipe.

Abstract: An air-fuel ratio control system for idling operation of an automotive engine has an O.sub.2 -sensor for detecting oxygen concentration of exhaust gas and for producing a feedback signal, a feedback control system responsive to the feedback signal for controlling air-fuel ratio to a desired air-fuel ratio. An idle switch is provided for detecting idling operation of the engine and for producing an idle signal. A lookup table stores a learning coefficient for controlling an actual injection pulse width during idling of the engine. In response to the idle signal the learning coefficient in the lookup table is updated with a new learning coefficient. When a predetermined learning operation is completed, the operation of the feedback control system is interrupted.