Air-fuel ratio controller for multiple cylinder bank engine

An air-fuel ratio controller for an engine provided with a plurality of cylinder banks, an intake manifold provided for each cylinder bank to provide a fuel mixture to each cylinder in the bank, an exhaust manifold provided for each cylinder bank for collecting exhaust gas from each cylinder of the bank, an exhaust pipe combining the gas flow from each of said exhaust manifolds, and a three-way catalyst interposed in said exhaust pipe, said controller comprising;

• an air-fuel ratio sensor installed in each of said exhaust manifolds, said sensor sensing an air-fuel ratio of the fuel mixture provided to the cylinder bank,
• means for performing feedback control of the air-fuel ratio of all the cylinder banks based on the air-fuel ratio detected in a specific bank such that the air-fuel ratio of said specific bank varies within a predetermined amplitude about the theoretical air-fuel ratio as center value,
• means for measuring a rich time during which the air-fuel ratio detected in a cylinder bank is greater than the theoretical air-fuel ratio and a lean time during which the air-fuel ratio detected in this cylinder bank is smaller than the theoretical air-fuel ratio, said measuring means being provided for each of cylinder banks,
• means for computing a weighted average of proportions of the rich time and lean time measured in a plurality of air-fuel ratio varying cycles, said computation being based on a predetermined weighting coefficient, and
• means for correcting said feedback control for each cylinder bank such that the proportion of the rich time and the proportion of the lean time computed by said computing means are identical for any cylinder bank.