This study aims to examine the combustion process in a spark-ignition (SI) engine, and the correlation between the in-cylinder pressure, local gas temperature, and ion current under different operating conditions (i.e.,spark timing, engine speed, equivalence ratio, throttle position). A commercial fast thermocouple was modified in order to measure both the local gas temperature and the ion current, and placed inside the combustion chamber. The fast thermocouple was charged on its stainless steel sheath isolated from the combustion chamber by an external 64-V direct current (DC) source. All three stages of the ion current as well as the local gas temperature were successfully measured in the experiment using an ion-current sensor integrated to the fast thermocouple. A significant positive relation between periods of combustion, ion-current signals, and local gas temperature was observed. A new correlation between local gas temperature and ion-current signals was obtained with an ion-current sensor mounted to the fast response thermocouple for combustion control with the ion-current method, which appears to be quite significant. The new ion-current sensor, which was developed for the purposes of this study, can be conveniently used as a combustion control sensor in the management of internal combustion engines. (C) 2016 American Society of Civil Engineers.