Spark Mechanism in High Speed Flow

An experimental study was performed to investigate spark ignition and subsequent spark stretch evolution in an inert environment at high- flow velocities up to 32 m/s across the spark plug gap in a constant-volume optical combustion-vessel at pressures representative of those in an engine. The vessel is capable of generating various in-cylinder thermodynamic conditions representative of light-duty spark ignition engines. The characteristic behavior of the spark was investigated using both a high-speed optical diagnostics and electrical measurement. Charge gas pressures were varied from 15 to 45 bar. Results show that the spark, flowing downstream the spark plug, is subject to short circuits of the spark channel and/or restrikes. The frequency of the restrike increased with increased flow velocity and charge gas pressure and decreased discharge current level. The position of the ground electrode with respect to the flow and the gap size, as well as the flow velocity and charge gas pressure, were determined to have a significant influence on spark plasma development and electrical discharge prediction. It was observed that a wider spark plug gap led to reducing the electric field resulting in a higher breakdown voltage and subsequently, a larger gap impedance and a shorter discharge. It was shown that the discharge duration and energy at 15 bar and the flow velocity of 10 m/s were 1.73 ms and 66.4 mJ. However, the discharge duration reduced by 20.2% with the increased flow velocity of 32 m/s at the same pressure. The corresponding discharge energy increased by 17.3%. Further combustion tests were conducted to investigate the flame propagation rate under high-cross flow using stoichiometric ethane mixture. It was concluded that the most favorable position of the ground electrode to produce a faster development of flame propagation was the perpendicular orientation at which the flame propagation rate was 11.5% greater than the upstream case.