Effect of Ignition Energy on Combustion Duration under Flow Conditions

Utilizing low carbon fuel in lean burn combustion presents a compelling strategy for improving thermal efficiency and reducing NOx emissions. Methane, the main content of natural gas, still receives challenge of a rapid and complete combustion process because of its low chemical reactivity. Further increase of excess air ratio would decrease the flame propagation speed significantly, leading to prolonged combustion durations. The long combustion duration deteriorates the performance of a spark ignition engine, in terms of poor combustion instability and misfire. Although ignition timing can be utilized to adjust the combustion phasing, the ignition process faces challenges due to reduced background pressure and temperature at advanced spark timings. In this paper, a rapid compression machine equipped with a specially designed combustion flow chamber is utilized to enhance the mean flow speed across the spark gap under the background pressure 6 bar abs. and temperature 600 K. A computational fluid dynamics simulation is conducted to quantify the in-chamber flow field, then verified by the spark anemometry measurements. The impact of flow characteristics on the combustion process is analyzed in detail, including the mean flow speed in the vicinity of spark gap, average turbulence velocity and vorticity. Previous studies have demonstrated that an increase in flow speed is effective in decreasing the combustion duration up to a certain optimum threshold, beyond which the combustion duration will be increased or even leading to misfire events. A custom-built ignition energy management module is utilized to further increase the discharge current at fixed discharge duration to support the flame kernel formation process, which can effectively reduce the combustion duration under lean burn conditions. A minimum spark energy required to reduce the combustion duration was discovered under each excess air fuel ratio. Further increase of discharge current amplitude beyond this boundary yields minimal impact on the flame propagation process. This study will offer important insights for developing an on-demand ignition energy profiling strategy to reduce sparkplug electrode erosion.