METHOD FOR DETERMINING COMBUSTION STATE OF SPARK-IGNITION INTERNAL COMBUSTION ENGINE

Provided is a method for determining a combustion state of a spark-ignited internal combustion engine including an ignition plug to ignite a fuel-air mixture by a plasma generated by a reaction between a spark discharge generated by a high voltage applied through an ignition coil connected to the ignition plug and an electric field generated in a combustion chamber. In the method, a change in a secondary voltage secondarily generated in the ignition plug after the spark discharge in association with the spark discharge is detected. A case where the change in the secondary voltage deviates from a predetermined range is determined as an occurrence of misfire. This enables detection of a misfire state without being hindered from an ion current due to the plasma remaining in the combustion chamber.