An investigation of near-spark-plug flow field and its effect on spark behavior

In the recent decades, the emission and fuel efficiency regulations put forth by the emission regulation agencies have become increasingly stringent and this trend is expected to continue in future. The advanced spark ignition (SI) engines can operate under lean conditions to improve efficiency and reduce emissions. Under such lean conditions, the ignition and complete combustion of the charge mixture is a challenge because of the reduced charge reactivity. Enhancement of the in-cylinder charge motion and turbulence to increase the flame velocity, and consequently reduce the combustion duration is one possible way to improve lean combustion. The role of air motion in better air-fuel mixing and increasing the flame velocity, by enhancing turbulence has been researched extensively. However, during the ignition process, the charge motion can influence the initial spark discharge, resulting flame kernel formation, and flame propagation. Therefore, a combined empirical and simulation study is undertaken to investigate the flow field around the spark gap. The flow field generated by a steady flow of air across the spark gap of a conventional J-type spark plug under ambient conditions is studied using optical particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations. The flow characteristics are compared to the high-speed direct imaging, and voltage and current measurement results of the spark channel in an effort to correlate the spark behavior to the local flow velocity. The flow field near the spark gap in an SI engine under motoring conditions is simulated, and the results are compared to the empirical current and voltage measurements taken during engine operation. The results show that the turbulence is generated in the wake of the spark plug as expected and flow velocity in the spark gap is higher than the free stream velocity. Optical and electrical measurements show the spark stretching and restrikes increase, and discharge duration decreases with increase in flow velocity. Similar behavior is observed during engine operation as well