Improvement of Flame Kernel Growth by Microwave-Assisted Plasma Ignition

Abstract

Due to the depletion of petroleum resources and environmental concerns, automobile industry has been developing new engine technologies with acceptable cost range to consumers. Among many new technologies, application of non-thermal plasma ignition system is considered as a promising path to achieve high-efficiency clean gasoline vehicles. In this study, we developed a microwave-assisted plasma ignition using 3 kW, 2.45 GHz magnetron with customized electric components and ignitor. This system was tested in a constant volume combustion vessel to investigate the effects of microwave ejection on ignition kernel growth. High-speed shadowgraph imaging and hydroxyl (OH) radical imaging were carried out under various air-fuel ratio, ambient pressure, and ignition strategy conditions. The in-cylinder pressure measurement was also performed to compare combustion phase between conventional spark and microwave-assisted plasma ignition system. The experimental result showed that the microwave ejection on the thermal plasma created by conventional discharge had a significant improvement on initial flame development. The microwave-assisted plasma ignition system indicated advanced combustion phase with extended lean limit where conventional spark ignition failed to achieve flame propagation. The OH imaging on propagating flame presented much higher intensity with microwave-assisted plasma ignition case. The analysis on light emission spectrum showed 7,000 K higher electron temperature in the plasma created with microwave ejection. This implies that chemical reactions which could not be progressed with conventional spark ignition was enabled with additional non-thermal plasma induced by electro-magnetic wave. On the other hand, however, the enhancement in flame development was decreased under high pressure condition due to lower reduced electric field.