| 摘要 |
The present invention provides an improved air-fuel ratio control system which eliminates a time lag of outputs of an outlet or second oxygen sensor at a high accuracy and adequately controls the air-fuel ratio, thus efficiently reducing an exhaust of harmful gases including HC, CO, and NOx and improving the fuel consumption. When an output voltage SOX of the second oxygen sensor is within a predetermined range between a first voltage E1 and a second voltage E2 including a reference voltage E0 corresponding to a stoichiometric air-fuel ratio, an update quantity DRSR of a rich skip amount RSR is equal to zero. When the output voltage SOX is in a range between a minimum output GSOXmin and the first voltage E1, the update quantity DRSR exponentially increases with the decrease in the voltage. When the output voltage SOX is in a range between the second voltage E2 and a maximum output GSOXmax, the update quantity DRSR exponentially decreases with the increase in the voltage. The rich skip amount RSR used in a main air-fuel ratio feed-back control is compensated with the update quantity DRSR thus determined. In the system of the invention, the air-fuel ratio is maintained in a desirable range to ensure a reduced exhaust of harmful gases when the output voltage SOX of the second oxygen sensor being within the predetermined range between E1 and E2, and rapidly approaches to a desirable target ratio for reduced emission of the exhaust gas when SOX is out of the predetermined range.
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