Combustion state-detecting system for internal combustion engines

Abstract

A combustion state-detecting system for an internal combustion engine is provided, which detects misfires occurring in the engine. A crank angle sensor generates a crank angle signal with a predetermined period shorter than a firing period of the engine whenever the crankshaft rotates through a predetermined angle. An ECU detects the rotational speed of the engine, based on the crank angle signal whenever the crank angle signal is generated to generate a rotational speed signal, and filters the rotational speed signal to extract a specific frequency component therefrom. A rate of variation in the extracted specific frequency component is calculated every first predetermined time period, and an average value of the rate of variation is calculated every second predetermined time period longer than the first predetermined time period. Further, the difference between the average value and the rate of variation is calculated, and absolute values of the difference is cumulated over the second predetermined time period to obtain a difference cumulative value. Then, the difference cumulative value is compared with a predetermined value, and it is determined that the engine is in a continuous degraded combustion state when the difference cumulative value has continued to exceed the predetermined value, and that the engine is in a sporadic degraded combustion state when the difference cumulative value once exceeds the predetermined value once and then becomes smaller than the predetermined value, before it again exceeds the predetermined value.