Combustion Diagnostics Using a Spark Plug as Probe.

In order to investigate the relation between ion current and combustion characteristics, ion current signal from a spark plug used as an ion probe, pressure history and flame development were measured in a homogeneous propane-air mixture in closed combustion chambers. When negative bias is applied to the central electrode of the spark plug, the ion current flows only due to an early flame kernel existing near the spark plug. When positive bias is applied to the central electrode, the ion current flows from the central electrode to the combustion chamber wall and to the ground electrode. Consequently, the ion current is dominated by the contact area between the flame and combustion chamber wall. The appearance time of ion-current peaks is related with combustion duration.