Air-fuel ratio control method for internal combustion engines

Claim 1

A method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine having a plurality of cylinders divided into at least two cylinder groups, an exhaust passage having at least two divided portions connected to respective ones of said at least two cylinder groups, and at least two exhaust gas ingredient concentration sensors arranged in respsective ones of said at least two divided portions of said exhaust passage, wherein when each of said at least two cylinder groups is in a first predetermined operating condition, the air-fuel ratio of a mixture being supplied to said each cylinder group is controlled in a feedback control manner responsive to an output of corresponding one of said at least two exhaust gas ingredient concentration sensors, while when said each cylinder group is in a second predetermined operating condition, the air-fuel ratio of the mixture is controlled in an open loop control manner corresponding to said second predetermined operating condition, the method comprising the steps of: (a) determining whether each of said at least two cylinder groups is in said first predetermined operating condition or in said second predetermined operating condition, said determination being made with respect to each one of said at least two cylinder groups independently of the other cylinder group; (b) when one of said at least two cylinder, groups shifts from said first predetermined operating condition to said second predetermined operating condition, or vice versa, continually effecting control of the air-fuel ratio of a mixture being supplied to said one cylinder group in one of said control manners corresponding to one of said first and second predetermined operating conditions in which said engine was operating before said shift, until all the cylinder groups other than said one cyliner group shift to the other of said first and second predetermined operating conditions in which said one cylinder group is operating after said shift.

Claim 2

A method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine having a plurality of cylinders divided into at least two cylinder groups, an exhaust passage having at least two divided portions connected to respective ones of said at least two cylinder groups, and at least two exhaust gas ingredient concentration sensors arranged in respsective ones of said at least two divided portions of said exhaust passage, wherein when each of said at least two cylinder groups is in a first predetermined operating condition, the air-fuel ratio of a mixture being supplied to said each cylinder group is controlled in a feedback control manner responsive to an output of corresponding one of said at least two exhaust gas ingredient concentration sensors, while when said each cylinder group is in a second predetermined operating condition, the air-fuel ratio of the mixture is controlled in an open loop control manner corresponding to said second predetermined operating condition, the method comprising the steps of: (a) determining whether each of said at least two cylinder groups is in said first predetermined operating condition or in said second predetermined operating condition, said determination being made with respect to each one of said at least two cylinder groups independently of the other cylinder group; (b) when one of said at least two cylinder, groups shifts from said first predetermined operating condition to said second predetermined operating condition, or vice versa, continually effecting control of the air-fuel ratio of a mixture being supplied to said one cylinder group in one of said control manners corresponding to one of said first and second predetermined operating conditions in which said engine was operating before said shift, until all the cylinder groups other than said one cyliner group shift to the other of said first and second predetermined operating conditions in which said one cylinder group is operating after said shift.