Air-fuel ratio control method for an internal combustion engine for vehicles in low load operating regions

摘要

A method for electronically controlling the air/fuel ratio of a mixture being supplied to an internal combustion engine for use in a vehicle, in response to operating conditions of the engine. A plurality of different operating regions of the engine are set beforehand, which are each defined by predetermined values of first and second parameters indicative of operating conditions of the engine. Detection is made of values of the above first and second parameters and the speed of the vehicle. At least one mixture-leaning operating region is selected from the above different predetermined operating regions, in dependence on a detected value of the vehicle speed. When it is determined from detected values of the first and second parameters that the engine is operating in the at least one mixture-leaning operating region selected, leaning of the mixture being supplied to the engine is effected. Preferably, the total range of the above at least one mixture-leaning operating region selected when the detected value of the vehicle speed is lower than a predetermined value is smaller than that selected when the detected value of the vehicle speed is higher than the same predetermined value. Further preferably, while the engine is operating in one of the at least one mixture-leaning operating region which is selected only when the detected value of the vehicle speed is higher than the above predetermined value, leaning of the mixture is effected to an extent different from that effected while the engine is operating in the other mixture-leaning operating region or regions.