Air-fuel ratio control system for internal combustion engine

Claim 2

An air-fuel ratio control system for an 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 an air-fuel ratio feedback control correction value for each of said cylinder banks based on outputs of said auxiliary air-fuel ratio sensors, said deriving means deriving said 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 said outputs of said auxiliary air-fuel ratio sensors are in phase with each other; and feedback control means for feedback controlling an air-fuel ratio of an 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 said air-fuel ratio feedback control correction value for said cylinder bank.

Claim 3

The air-fuel ratio control system as set forth in claim 1, wherein said deriving means largely changes said air-fuel ratio feedback control correction value for at least one of said cylinder banks so as to control the output of said auxiliary air-fuel ratio sensor for said at least one of said cylinder banks to be in antiphase with the output of said auxiliary air-fuel ratio sensor for the other of said cylinder banks when the outputs of said auxiliary air-fuel ratio sensors are in phase with each other.

Claim 4

The air-fuel ratio control system as set forth in claim 1, wherein said deriving means includes means for determining whether the outputs of said auxiliary air-fuel ratio sensors are in phase with each other when at least one of the outputs of said auxiliary air-fuel ratio sensor is inverted between rich and lean sides with respect to a given reference value.

Claim 5

The air-fuel ratio control system as set forth in claim 3, wherein, when the outputs of said auxiliary air-fuel ratio sensors are in phase with each other, said deriving means changes with a first correction value said air-fuel ratio feedback control correction value for the cylinder bank where the output of said auxiliary air-fuel ratio sensor is non-inverted, while said deriving means changes with a second correction value said air-fuel ratio feedback control correction value for the cylinder bank where the output of said auxiliary air-fuel ratio sensor is inverted, said first correction value being set greater than said second correction value.

Claim 6

The air-fuel ratio control system as set forth in claim 1, wherein said deriving means includes determining means for determining whether the air-fuel ratios as monitored in said exhaust passages downstream of said catalytic converters are rich or lean by comparing the outputs of said auxiliary air-fuel ratio sensors with corresponding given reference values, respectively, and wherein said determining means sets one of said reference values to be greater than a value corresponding to a stoichiometric air-fuel ratio and the other of said reference values to be smaller than said value corresponding to the stoichiometric air-fuel ratio.

Claim 7

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.

Claim 8

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.

Claim 9

The air-fuel ratio control system as set forth in claim 6, wherein said deriving means increases said first air-fuel ratio feedback control correction value when the output of the corresponding auxiliary air-fuel ratio sensor is on a rich side with respect to a given reference value, and wherein said deriving means increases said second air-fuel ratio feedback control correction value when the output of the corresponding auxiliary air-fuel ratio sensor is on a lean side with respect to the given reference value.

Claim 10

The air-fuel ratio control system as set forth in claim 7, wherein said first and second air-fuel ratio feedback control correction values have such a relationship that said first and second air-fuel ratio feedback control correction values change in opposite directions from each other.

Claim 11

The air-fuel ratio control system as set forth in claim 8, wherein a sum of said first and second air-fuel ratio feedback control correction values is a fixed value.

Claim 12

The air-fuel ratio control system as set forth in claim 9, wherein said deriving means largely increases said first air-fuel ratio feedback control correction value for at least one of said cylinder banks so as to control the output of said auxiliary air-fuel ratio sensor for said at least one of said cylinder banks to be in antiphase with the output of said auxiliary air-fuel ratio sensor for the other of said cylinder banks when the outputs of said auxiliary air-fuel ratio sensors are in phase with each other and on the rich side, and wherein said deriving means largely decreases said first air-fuel ratio feedback control correction value for at, least one of said cylinder banks so as to control the output of said auxiliary air-fuel ratio sensor for said at least one of said cylinder banks to be in antiphase with the output of said auxiliary air-fuel ratio sensor for the other of said cylinder banks when the outputs of said auxiliary air-fuel ratio sensors are in phase with each other and on the lean side.