A model for flame kernel development in a spark-ignition engine

A First-Law-based model for the growth of the spark-generated flame kernel in a spark-ignition engine has been developed and applied. The purpose of the study was to show how the basic physics relevant to the early stages of flame growth—electrical energy deposition, chemical energy release, and heat losses to the spark plug—can explain the observed kernel behavior under real engine operating conditions. The primary focus was on how the heat losses to the spark plug electrodes, insulator and ground case effect the rate of flame kernel development and its variation, cycle-by-cycle. It is shown that the heat loss rate from the kernel to the spark plug, and the electrical power fed into the kernel from the discharge positive column, are comparable in magnitude and vary significantly cycle-by-cycle. For flame kernels which remain centered on the plug electrodes and have a high contact area with the plug, the heat losses are higher and spark energy deposited is lower than for the cycles where the kernel is convected away from the electrodes by the flow. With higher heat losses, which can be comparble in magnitude to the combustion energy release for kernels up to a about 1.5 mm radius, the kernel grows significantly more slowly. The model also shows that excessive heat loss to the spark plug can lead to misfire.