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The corelation between flame ion density and engine operating condition, such as air fuel ratio mixture and cylinder pressure peak value, etc., is commonly known. However, due to the difficulty of ion probe installation in the engine and the interruption of the high voltage ignition spark, the ionization current method is not optimally utilized. This paper presents the Spark Plug Voltage-ion method which requires a minor modification to the ignition system and will indirectly detect flame ion density. The voltage sensor is a voltage divider by capacitance which measures the spark plug voltage. The resulting behavior of the voltage wave form is noteworthy. After spark discharge, the stored residual charge of the spark plug starts to discharge by ion in a flame. Therefore, ion density in a flame is detected by the decay time of spark plug voltage.