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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.

In the measurement of engine oil consumption the conventional radiometric method is more rapid and precise than the drain-weigh method, but the former method-a batch type-is inapplicable to measurements in transient engine operations which are inevitable in real driving. In the present study, a reliable method was developed to measure continuously the consumption rate through all phases of engine operation. Oleic acid sulfide containing S35 was selected as a radioactive tracer to be added to the engine oil. Engine exhaust gas containing the discharged oil was burned in an electric furnace and a gas burner and reacted with aqueous H2O2 solution, converting the S35O2 into H2S35O4. A plastic bead scintillator used for the detection of β-rays from the aqueous solution was sensitive enough to measure the consumption rate as low as 0.4 g/h.