Ion Current Waveform Characteristic
Basically, the shape of the measured ion current can be separated in four parts.
The first part starts immediate after ignition and as long the ignition spark is present. At this time (Spark-Time) a high negative current is flowing, which is constrained to zero by the measurement circuit. The second part starts at the point the coil energy remaining is not sufficient to keep the spark alive. The coil and measurement part build a R-L-C circuit, which causes an oscillating current out of the remaining coil energy. The third part is the ionization cause by the flame front (chemical ionization). At this point the air fuel mixture starts to burn. The fourth and last part corresponds to the internal cylinder pressure and reflects a high ionization caused by the high temperature and the combustion process.