Air-fuel ratio control system for internal combustion engine

An air-fuel ratio control system for am internal combustion engine, comprising:

• a pair of cylinder banks;
• a pair of exhaust passages connected to said cylinder banks, respectively;
• a common exhaust pipe where said exhaust passages join each other at their downstream ends;
• a pair of catalytic converters provided in said exhaust passages, respectively;
• a pair of main air-fuel ratio sensors provided in said exhaust passages upstream of said catalytic converters, respectively;
• a pair of auxiliary air-fuel ratio sensors provided in said exhaust passages downstream of said catalytic converters, respectively;
• a catalytic converter provided in said common exhaust pipe;
• deriving means for deriving first and second air-fuel ratio feedback control correction values for each of said cylinder banks based on outputs of said auxiliary air-fuel ratio sensors, said deriving means deriving said first and second air-fuel ratio feedback control correction values so as to control said outputs of said auxiliary air-fuel ratio sensors to be in antiphase with each other when the outputs of said auxiliary air-fuel ratio sensors are in phase with each other, said first air-fuel ratio control correction value for each of said cylinder banks to be used for controlling an air-fuel ratio of an air-fuel mixture for the corresponding cylinder bank to be leaner while said second air-fuel ratio control correction value for each of said cylinder banks is used for controlling the air-fuel ratio to be richer;
• and
• feedback control means for feedback controlling the air-fuel ratio of the air-fuel mixture for each of said cylinder banks based on an output of said main air-fuel ratio sensor for said cylinder bank and one of said first and second air-fuel ratio feedback control correction values for said cylinder bank.