Device for detecting misfire of an internal combustion engine by comparing voltage waveforms associated with ignition system

Apparatus for recognizing misfire in an internal combustion engine equipped with an ignition circuit (100a) having a primary coil (1a) and a secondary coil (1b), means for sending a primary current flowing through the primary coil (1a) to induce a secondary voltage across the secondary coil (1v), and a distributor (2) for applying the secondary voltage across electrodes (3a, 3b) of a spark plug (3) so as to establish a spark between the electrodes (3a, 3B) of the spark plug (3), comprising:

• (a) a secondary voltage detector (40) electrically connected between the secondary coil of the ignition circuit and the spark plug (3) so as to detect a secondary voltage value based on a spark in which the spark plug (3) is discharged by the ignition circuit;
• (b) an integration means (53) which is electrically connected to the secondary voltage detector (40) to integrate the secondary voltage value so as to produce an integration voltage;
• (c) a comparator means (54) electrically connected to the secondary voltage detector (40) and the integration means (53) so as to compare the secondary voltage value with the integration voltage, and producing an output pulse when the secondary voltage value exceeds the integration voltage;
• (d) a microcomputer means (55) electrically connected to the comparator means (54) to detect a width of the output pulse from the comparator means (54) so as to determine a misfire due to an abnormal ignition of an air-fuel mixture in the internal combustion engine when the width of the output pulse of the comparator means (54) exceeds a predetermined level.