Air-fuel ratio control system for engine

摘要

In an engine having first (#2,#3) and second (#1,#4) cylinder groups, a first sensor (12) senses an air-fuel ratio of an exhaust gas mixture into a first catalytic converter (7) for the first cylinder group, a second sensor (13) senses an air-fuel ratio of an exhaust gas mixture into a second catalytic converter (8) for the second cylinder group. A controller (11) normally controls the air fuel ratios of the first and second cylinder groups independently by using first and second air-fuel ratio feedback correction coefficients. When a diagnosis for the catalytic converters is required, the controller measures a rich time and a lean time in the air-fuel ratio variation of the second cylinder group in accordance with an output of the second sensor to determine a second cylinder group's rich/lean ratio between the rich time and the lean time, calculates a correction quantity to bring the second cylinder group's ratio closer to a target ratio, and determines a modified coefficient by modifying the first air-fuel ratio feedback correction coefficient with the correction quantity feedback-controls the air-fuel ratio of the second cylinder group with the modified coefficient as the second air-fuel ratio feedback correction coefficient.