Misfire detecting apparatus for internal combustion engine

Abstract

A misfire detecting apparatus for an internal combustion engine detects only an ion current due to combustion of an air/fuel mixture regardless of a leakage current when combustion: takes place normally. The apparatus includes circuitry for detecting an ion current generated within the engine cylinder immediately after combustion, circuitry for detecting a leakage current occurring between electrodes of a spark plug , a first waveform shaper for shaping the ion current into a pulse signal indicating a combustion or misfire event by comparing the pulse signal with a first threshold value for deciding the ion current, the first threshold value being set on the basis of a level of the ion current upon rising thereof, the ion current being superposed on the leakage current, a second waveform shaper for shaping the ion current as detected into a pulse signal through comparison with a second fixed threshold value which is fixed at a value smaller than the first threshold value, and a selector for selectively outputting as the combustion pulse signal either a shaped pulse signal outputted from the first waveform shaper upon detection of occurrence of the leakage current or a shaped pulse signal outputted from the second waveform shaper when no leakage current is detected.