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

摘要

An inter-cylinder air/fuel ratio imbalance judgmental device has an upstream-side air/fuel ratio sensor (67), a catalyzer (53) and a downstream-side air/fuel ratio sensor (68) that are provided at the downstream from an exhaust-collector part for an exhaust pathway, and calculates a sub-feedback quantity for matching an output value of the downstream-side air/fuel ratio sensor to a value corresponding to the theoretical air/fuel ratio, while performing an air/fuel ratio feedback control for matching an air/fuel ratio of the air-fuel mixture to be supplied to an internal-combustion engine to the theoretical air/fuel ratio in accordance with the sub-feedback quantity and an output value of the upstream-side air/fuel ratio sensor. At this time, the judgmental device acquires an imbalance judgmental parameter on the basis of a learning value for the sub-feedback quantity, judging that an inter-cylinder air/fuel ratio imbalance has occurred when the imbalance judgmental parameter exceeds a threshold. On the other hand, when the update frequency of a learning value for the evaporative fuel gas concentration after startup of the internal-combustion engine is less than a given frequency, the judgmental device prohibits a judgment for the inter-cylinder air/fuel ratio imbalance because the imbalance judgmental parameter is affected by purging of the evaporative fuel gas.