Enhancement of Cold Plasma Reactions and Energy Deposition by Multi-Voltage Pulse Discharges

The use of cold plasma for ignition and combustion improvement in internal combustion engines has been widely investigated in the past two decades. Usually, high voltage nanosecond pulses are repeatedly applied to facilitate such a process. Compared to traditional ignition, nanosecond repetitive pulse ignition systems demonstrate improvement in heat release rate during combustion, ignition delay time reduction and enhanced ignition process in flowing and lean reactive gas mixtures. In this research, we conducted a preliminary, theoretical/numerical investigation into the possibility of use of on-board control of cold plasma processes, known to enhance ignition and combustion of reactive gas mixtures. The focus was on diagnostics and enhancement of energy deposition in specific modes, by application of bi-polar short duration voltage pulses in low-pressure air. The physical model couples the electric field, potential and current, with the relevant conservation equations for 24 species via 168 kinetic reactions, including molecules’ rotation, vibration, electronic excitation, dissociation, and ionization inside the electrodes gap. Evaluation using various pulse repetition frequencies and different pulse shapes was conducted. Special attention was given to the overall coupled energy deposited during the discharge, and to energy channeled to known ignition supportive modes such as nitrogen electronic excitation and oxygen radicals’ generation. The results of the analysis show that for the considered conditions, energy deposition can be divided into two main stages, characterized by high and low voltage magnitudes, respectively. It was found for the first time, that the (low voltage) second stage’s energy deposition can be higher than that of the first (high voltage) stage. At the second stage, the deposition of energy into specific modes can be tuned by setting appropriate voltage magnitudes. In addition, the energy deposited in modes important for ignition exhibits a simple linear relation to the overall energy deposition.