ABNORMALITY DIAGNOSTIC DEVICE OF INTERNAL COMBUSTION ENGINE

PROBLEM TO BE SOLVED: To improve diagnostic accuracy in diagnosing each cylinder abnormality based on an output of an air-fuel ratio sensor arranged in an exhaust confluent part of an engine. SOLUTION: An inter-cylinder deviation X(#i) of the actual air-fuel ratio of each cylinder is accurately estimated by determining the inter-cylinder deviation X(#i) of the actual air-fuel ratio before starting air-fuel ratio dither control by using a variation ΔX(#i) of the actual air-fuel ratio when forcibly changing the air-fuel ratio of an (i)th cylinder #i by the air-fuel ratio dither control, a variation ΔY(#i)ä=Y2(#i)-Y1(#i)} of the detected air-fuel ratio of the air-fuel ratio sensor at this time and an inter-cylinder deviation Y1(#i) of the detected air-fuel ratio of the air-fuel ratio sensor before starting the air-fuel ratio dither control. A frequency of exceeding a predetermined determining value α in the inter-cylinder deviation X(#i) of the air-fuel ratio is counted for each cylinder, until the stopping from the starting of the engine 11, and when its count value becomes larger than a predetermined frequency K, it is determined that an abnormality is caused in the cylinder. COPYRIGHT: (C)2008,JPO&INPIT