Multicylinder internal combustion engine, inter-cylinder air/fuel ratio imbalance determination apparatus, and method therefor

Abstract

A multicylinder engine includes: combustion chambers; fuel injection valves corresponding to the individual combustion chambers; an exhaust control catalyst; an upstream-side air/fuel ratio sensor disposed upstream of the exhaust control catalyst; and a downstream-side A/F sensor disposed downstream of the catalyst. A first abnormality determination-purpose rich A/F control of controlling the A/F of the mixture formed in each combustion chamber to an A/F richer than stoichiometric is executed when it needs to be determined whether the downstream-side A/F sensor is abnormal. An inter-cylinder A/F imbalance determination of estimating the A/F of the mixture in each combustion chamber based on the output of the upstream-side A/F sensor is executed when the first abnormality determination-purpose rich A/F control is being executed, and it is determined whether there is a difference between the estimated A/Fs of the mixtures.

1 . A multicylinder internal combustion engine comprising:
 a plurality of combustion chambers;   fuel injection valves disposed corresponding to the individual combustion chambers;   an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers;   an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers;   a downstream-side air/fuel ratio sensor disposed in the exhaust passageway downstream of the exhaust control catalyst so as to detect the air/fuel ratio of the exhaust gas that flows out from the exhaust control catalyst;   a control device that executes a first abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the downstream-side air/fuel ratio sensor is abnormal; and   a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the first abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.   
     
     
         2 . The multicylinder internal combustion engine according to  claim 1 , wherein:
 the control device executes a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal; and   the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the second abnormality determination-purpose rich air/fuel ratio control is being executed.   
     
     
         3 . The multicylinder internal combustion engine according to  claim 2 , wherein
 the determination device executes the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.   
     
     
         4 . The multicylinder internal combustion engine according to  claim 1 , wherein
 the control device executes an engine start-time rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when operation of the multicylinder internal combustion engine is started, a post-fuel injection stop rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when injection of fuel from the fuel injection valves is re-started after the injection of the fuel from the fuel injection valves is stopped, or an exhaust control catalyst-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when temperature of the exhaust control catalyst is higher than a predetermined permissible upper-limit temperature; and   the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the engine start-time rich air/fuel ratio control is being executed, or when the post-fuel injection stop rich air/fuel ratio control is being executed, or when the exhaust control catalyst-purpose rich air/fuel ratio control is being executed.   
     
     
         5 . A multicylinder internal combustion engine comprising:
 a plurality of combustion chambers;   fuel injection valves disposed corresponding to the individual combustion chambers;   an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers;   an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers;   a control device that executes a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal; and   a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the second abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.   
     
     
         6 . The multicylinder internal combustion engine according to  claim 5 , wherein
 the determination device executes the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.   
     
     
         7 . The multicylinder internal combustion engine according to  claim 6 , wherein:
 the control device executes an engine start-time rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when operation of the multicylinder internal combustion engine is started, a post-fuel injection stop rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when injection of fuel from the fuel injection valves is re-started after the injection of the fuel from the fuel injection valves is stopped, or an exhaust control catalyst-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when temperature of the exhaust control catalyst is higher than a predetermined permissible upper-limit temperature; and   the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the engine start-time rich air/fuel ratio control is being executed, or when the post-fuel injection stop rich air/fuel ratio control is being executed, or when the exhaust control catalyst-purpose rich air/fuel ratio control is being executed.   
     
     
         8 . A multicylinder internal combustion engine comprising:
 a plurality of combustion chambers;   fuel injection valves disposed corresponding to the individual combustion chambers;   an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers;   an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers;   a control device that controls the air/fuel ratio of a mixture formed in each of the combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and   a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference between in the air/fuel ratio of the mixture between the combustion chambers.   
     
     
         9 . An inter-cylinder air/fuel ratio imbalance determination apparatus comprising:
 an upstream-side air/fuel ratio sensor disposed in an exhaust passageway upstream of an exhaust control catalyst so as to detect air/fuel ratio of exhaust gas discharged from a plurality of combustion chambers of a multicylinder internal combustion engine, the exhaust control catalyst being disposed in the exhaust passageway so as to remove a specific component of the exhaust gas discharged from the combustion chambers;   a control device that controls the air/fuel ratio of a mixture in each of the plurality of combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and   a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers.   
     
     
         10 . An inter-cylinder air/fuel ratio imbalance determination method comprising:
 determining whether a first condition for determining whether there is a difference in air/fuel ratio between a plurality of combustion chambers of a multicylinder internal combustion engine is satisfied;   determining whether a second condition for controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio between the combustion chambers;   detecting the air/fuel ratios of exhaust gas discharged from the plurality of combustion chambers of the multicylinder internal combustion engine; and   controlling the air/fuel ratio of the mixture in each of the combustion chambers to the air/fuel ratio richer than the stoichiometric air/fuel ratio when the first condition and the second condition are satisfied, and then determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on the detected air/fuel ratios.