Apparatus for detecting misfire in internal combustion engine

Abstract

A rotation signal outputting device outputs a rotation signal at every predetermined rotational angle of an engine output shaft. An actual measurement value calculating device measures an amount of time spent by multiple engine cylinders in rotation between predetermined rotational angles during an expansion stroke, the measurements being made based on rotation signals output by the rotation signal outputting device. The actual measurement value calculating device also calculates an actual measurement value in response to the measured time interval. A first variation amount calculating device calculates a first difference between actual measurement values in two cylinders of the engine, and calculates a first variation amount in response to the calculated first different. A second variation amount calculating device calculates a second difference between a current first variation amount and a previous first variation amount calculated during a time which precedes a time of calculation of the current first variation amount by a period equal to an integer times a crank angle of 360 degrees in the engine. The second variation amount calculating device also calculates a second variation amount in response to the calculated Second difference. A smoothing device smooths the second variation amount into a third variation amount. A consecutive misfire detecting device detects a misfiring condition in response to the third variation amount.