An ion current probe using a spark plug was applied to gasoline-fuelled homogeneous charge compression ignition (HCCI) combustion with hot residual gas in order to verify the possibility of using it as a combustion sensor. The ion current signal for single-cycle HCCI combustion had a simple profile and effectively one maximum value. There is a possibility that a similar ion current signal corresponding to the completed reaction can be obtained, depending on the location of the probe during HCCI combustion. The ionization reaction for HCCI combustion is affected by the chemical ionization reaction with heat release, and there is a possibility that the ion current can be used to detect heat release corresponding to the chemical ionization reaction. A strong correlation between the timing of the integrated ion current and the timing of the mass fraction burned is observed. The timing of the mass fraction burned is assumed from the timing of the integrated ion current, and the mass fraction burned (up to 70 per cent) can be determined, even if the engine driving condition changes within the scope of the test. There is a correlation between the timing of the maximum ion-current and the maximum rate of heat release, and there is a possibility that the maximum value of the rate of heat release can be inferred by detecting the timing of the maximum ion current. There is a correlation between the timing of the maximum ion current and the timing of the maximum pressure at each cycle. Therefore, it may be possible to monitor the variation of the HCCI combustion phasing.

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