Patent ID: 8965665
Filing Date: 2015-02-24
Classification: F02D

Abstract:
1. An air-fuel ratio imbalance among cylinders determining apparatus, applied to a multi-cylinder internal combustion engine having a plurality of cylinders, for determining whether or not an air-fuel ratio imbalance state among cylinders is occurring, said state being a state in which an imbalance is occurring among individual cylinder air-fuel ratios, each of which is an air-fuel ratio of a mixture supplied to each of at least two cylinders of said plurality of cylinders, comprising: an air-fuel ratio sensor, which is disposed at an exhaust gas aggregated portion of an exhaust gas passage of said engine where exhaust gases discharged from said at least two cylinders aggregate or is disposed at a portion downstream of said exhaust gas aggregated portion of said exhaust gas passage, and which generates, as an air-fuel ratio sensor output, an output corresponding to an air-fuel ratio of exhaust gas which has reached said air-fuel ratio sensor; and imbalance determining means for obtaining a second-order differential value of a detected air-fuel ratio represented by said air-fuel ratio sensor output with respect to time based on said air-fuel ratio sensor output, obtaining an air-fuel ratio second-order differential corresponding value varying in accordance with said obtained second-order differential value based on said obtained second-order differential value, and determining whether or not said air-fuel ratio imbalance state among cylinders is occurring based on said obtained air-fuel ratio second-order differential corresponding value, wherein said imbalance determining means is configured so as to determine that said air-fuel ratio imbalance state among cylinders is occurring when an absolute value of said obtained air-fuel ratio second-order differential corresponding value is larger than a first threshold value, and wherein said imbalance determining means is configured so as to obtain said second-order differential value obtained every elapse of a predetermined time period in a unit combustion cycle period required for an arbitrary one of said at least two cylinders to complete one combustion cycle formed of an intake stroke, a compression stroke, an expansion stroke, and an exhaust gas stroke, and so as to obtain, as said air-fuel ratio second-order differential corresponding value, a second-order differential value whose absolute value is largest among a plurality of said second-order differential values obtained in said unit combustion cycle period.