Pressure effects on transient plasma discharge in air

Nanosecond scale pulsed high voltage discharges in air/fuel mixtures can generate radicals which in turn have been shown to improve combustion efficiency in gasoline fueled internal combustion engines. We are exploring the possibility to extend such transient plasma generation and expected radical species generation to the range of pressures encountered in compression-ignition (diesel) engines having compression ratios of ~20:1, thereby improving lean burning efficiency and extending the range of lean combustion. Our preliminary experiments on streamer propagation velocities have shown deviations from simple (pd = const.) based similarity law in the pressure range of 1-8 bar in synthetic air [1]. Here we report the results of streamer propagation experiments in the extended range of air pressures, 6-18 bar. In order to extend our work to 18 bar, the gap size is reduced to 1.75 mm. Optical data obtained from PI-MAX 3 ICCD camera is complemented with electrical measurements to deduce average streamer velocities.