Effects of Spark Discharge Energy Scheduling on Flame Kernel Formation under Quiescent and Flow Conditions

The breakdown phase is considered to have the highest electric-thermal energy transfer efficiency among all the discharge modes in a conventional spark ignition process. In this study, an external capacitor is connected in parallel with the spark plug in order to enhance the discharge energy and power during the breakdown phase. A constant volume combustion chamber is used to investigate the high power spark discharge under different background pressures and with varied flow velocities. Results show that the added parallel capacitance is effective in redistributing the spark energy. With the increase in parallel capacitance, the breakdown power and energy increase, though at the cost of reduced glow phase energy. The breakdown energy also increases with the increased background pressure. Then combustion tests are carried out to study the effects of the breakdown power enhanced spark on flame propagation under both quiescent and flow conditions via optical diagnosis. Firstly, the spark discharge characteristics are investigated from the shadowgraph images recorded during the spark discharge in air where no combustion is involved. The high temperature field generated by spark plasma can be viewed from the shadowgraph images. A larger high-temperature region with enhanced turbulence is obtained with the enhanced breakdown power. Combustion test results prove that this enlarged turbulent high-temperature region is effective in promoting the flame propagation. Subsequently, the spark discharge and combustion under flow conditions are studied. Results reveal that the high power discharge does not bring benefit under the tested flow conditions with a flow velocity of about 15 m/s and background pressure of 4 bar absolute.