INTER-CYLINDER AIR/FUEL-RATIO IMBALANCE DECIDING DEVICE FOR MULTICYLINDER INTERNAL COMBUSTION ENGINE

When, for example, a predetermined operating condition such as a fuel cut operating condition is satisfied (time-point tfc), an air-fuel ratio imbalance among cylinder determining apparatus sequentially stops a fuel injection for each of cylinders in accordance with a firing (ignition) order, and changes intake valves and exhaust valves of the cylinder whose fuel injection is stopped from a valve operating state to a valve resting state. Consequently, after a time-point at which an exhaust gas (final gas) is discharged from a cylinder into an exhaust passage (the exhaust gas being generated from a fuel injected lastly for that cylinder), no new gas is discharged from that cylinder. Accordingly, when a predetermined time elapses (time-point tp), an upstream-side air-fuel ratio abyfs obtained based on an output value of an upstream air-fuel ratio sensor disposed in the exhaust passage becomes a value corresponding to an air-fuel ratio of the final gas. The determining apparatus obtains that value as data relating to an air-fuel ratio of the cylinder which discharged the final gas, and determines whether or not an air-fuel ratio imbalance is occurring based on the data relating to the air-fuel ratio of each of the cylinders.