In recent years, technology using a spark plug as an ion probe was developed. Utilizing this technology, we could detect some combustion information, e.g., misfire and knocking, without sensors in the engine cylinders. We examined the mechanism for detecting the knocking using the ion current. In our experiments, heavy knocking occurred in a single-cylinder engine on supplying the mixture of n-pentane, oxygen and argon. The knocking was analyzed by measuring the ion current and the pressure at some positions and the results of the combustion observation with shadowgraphs. When the shock wave which occurred due to auto-ignition in the end gas arrived at the ion probes, the ion currents rapidly increased with the pressure. Moreover, when the polarity of the ion probe was altered, the power spectrum of the ion current changed, too. It appeared that the power spectrum of the ion current did not always agree with that of the pressure during the knocking. However, we could describe these phenomena as the difference between the mobility of an ion and that of an electron. The region where the ion probe catching the ions then changes by altering the polarity. From these results, it was understood that the ion current vibrates because the ion density changes with the coarseness and minuteness of the pressure wave.
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