Cylinder-to-cylinder air/fuel ratio imbalance determination system of internal combustion engine

A cylinder-to-cylinder air/fuel ratio imbalance determination system, comprising:

• an air/fuel ratio sensor provided in a multi-cylinder internal combustion engine having a plurality of cylinders, said air/fuel ratio sensor being disposed in an exhaust collecting portion of an exhaust passage of the engine into which exhaust gases emitted from combustion chambers of at least three cylinders, out of said plurality of cylinders, are collected, or disposed in a portion downstream of the exhaust collecting portion;
• a plurality of fuel injection valves provided in conjunction with said at least three cylinders, respectively, each of said fuel injection valves being adapted to inject a fuel contained in an air-fuel mixture supplied to each of combustion chambers of said at least three cylinders;
• and
• an imbalance determination device that obtains an imbalance determination parameter that increases or decreases as a difference between the air/fuel ratio of an air-fuel mixture supplied to one of said at least three cylinders and the air/fuel ratio of air-fuel mixtures supplied to the remaining ones of said at least three cylinders increases, based on at least an output value of the air/fuel ratio sensor, and makes an imbalance determination as to whether a cylinder-to-cylinder air/fuel ratio imbalance condition occurs, based on a result of comparison between the obtained imbalance determination parameter and a given threshold value for use in imbalance determination, wherein
• the imbalance determination device comprises:
• a forced imbalance condition creating device that changes an amount of fuel injected from at least one of said plurality of fuel injection valves, so as to create a forced imbalance condition in which the air/fuel ratio of an air-fuel mixture supplied to one of said at least three cylinders deviates from the air/fuel ratio of air-fuel mixtures supplied to the remaining ones of said at least three cylinders, before the imbalance determination device makes an imbalance determination;
• an evaluation parameter obtaining device that obtains an air/fuel ratio sensor evaluation parameter indicative of an output characteristic of the air/fuel ratio sensor, based on the output value of the air/fuel ratio sensor in a condition where the forced imbalance condition is established;
• an air/fuel ratio sensor evaluation device that makes a sensor adequacy determination as to whether the output characteristic of the air/fuel ratio sensor is adequate for making an imbalance determination, by comparing the obtained air/fuel ratio sensor evaluation parameter with a predetermined reference parameter;
• and
• an imbalance determination executing/inhibiting device that inhibits the imbalance determination device from making an imbalance determination or makes the result of imbalance determination invalid when the air/fuel ratio sensor evaluation device determines that the output characteristic of the air/fuel ratio sensor is not adequate for making an imbalance determination.