Abstract: A method for detecting a misfire by fluctuation in crankshaft rotation in which the occurrence of a misfire can be detected exactly by eliminating the effect of detection error in detecting the fluctuation in crankshaft rotation, caused by the error in the construction of a crank angle sensor. By succeedingly receiving pulses from the crank angle sensor, a processor detects a time interval taken from the entering into a crankshaft rotation angle region, relating to a cylinder corresponding to both pulses, to the leaving therefrom (S2), calculates the crankshaft angular acceleration D.omega..sub.n by using the correction factor calculated on the basis of the above detected time interval, and the time interval determined by dividing the time taken for one rotation of the crankshaft by the number of cylinders (S3, S4), the latter interval being free from sensor error, and detects the occurrence of a misfire when the calculated value D.omega..sub.n is less than the decision value (S5 through S7).

Abstract: A misfire detection method is provided, which method makes it possible to accurately detect the occurrence/absence of misfire by eliminating a detection error in misfire detection. This error is caused by a vibrational increase or decrease of the rotational changing rate of a crankshaft after a misfire state occurs. A processor, which receives pulse outputs from a crank angle sensor, calculates in sequence angular accelerations D.omega..sub.n-2, D.omega..sub.n-1, D.omega..sub.n, and D.omega..sub.n+1 in crank angle areas which correspond to a preceding cylinder, two cylinders checked for misfire, and a succeeding cylinder, respectively. If the values D.omega..sub.n-1 and D.omega..sub.n n significantly decrease in comparison with the values D.omega..sub.n-2 and D.omega..sub.n-1, respectively, then it is tentatively determined that misfire has occurred in both cylinders checked. Then If the value D.omega..sub.n+1 significantly increases compared to the value D.omega..sub.