Patent ID: 7448253

Claim:
A combustion state determination method of an internal combustion engine for detecting an ion current which is generated in a combustion-chamber at combustion of the internal combustion engine in a detection period to determine a combustion state, comprising steps of: detecting a primary combustion period on the basis of a first ion current which composes the detected ion current and occurs immediately after ignition; detecting a secondary combustion period on the basis of the ion current which occurs after disappearance of the first ion current when at least the detected primary combustion period is normal; determining the combustion state on the basis of the detected primary combustion period when the secondary combustion period is smaller than a predetermined value; and determining the combustion state on the basis of a whole combustion period from start of the detection period of the ion current to end of the last secondary combustion period in the detection period when the secondary combustion period is larger than the predetermined value.