Patent ID: 8452517
Filing Date: 2013-05-28
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, comprising: an air-fuel ratio sensor, disposed either at an exhaust-gas-aggregated-portion onto which exhaust gases discharged from at least two or more of cylinders among a plurality of said cylinders merge in an exhaust gas passage of said engine or at a position downstream of said exhaust-gas-aggregated-portion in said exhaust gas passage, said air-fuel ratio sensor including an air-fuel ratio detection element and a protective cover which accommodates said air-fuel ratio detection element in its inside so as to cover said air-fuel ratio detection element, said protective cover having an inflow hole which allows said exhaust gas flowing through said exhaust gas passage to flow into said inside, and an outflow hole which allows said exhaust gas which has flowed into said inside to flow out to said exhaust gas passage, wherein said air-fuel ratio detection element generates, as an output of said air-fuel ratio sensor, an output in accordance with said exhaust gas reaching said air-fuel ratio detection element; and imbalance determining portion configured so as to obtain, when a target value of an air-fuel ratio of a mixture supplied to each of at least two or more of said cylinders is set at a constant target air-fuel ratio, a value for a unit combustion cycle period, based on said output of said air-fuel ratio sensor in said unit combustion cycle period, said value corresponding to a detected air-fuel ratio change rate which corresponds to a change amount per unit time of an air-fuel ratio represented by said output of said air-fuel ratio sensor, and said unit combustion cycle period being a period necessary for anyone of said cylinders among at least said two or more of said cylinders discharging exhaust gases which reach said exhaust-gas-aggregated-portion to complete one combustion cycle including an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, and so as to perform a determination, based on an indicating amount of air-fuel ratio change rate which is determined in accordance with said value corresponding to said detected air-fuel ratio change rate, as to whether or not an air-fuel ratio imbalance among cylinders state is occurring in which an imbalance among individual-cylinder-air-fuel-ratios, each being an air-fuel ratio of a mixture supplied to each of at least said two or more of said cylinders, is occurring.