Effect of Electrode Geometry on Flame Kernel Development in a DI SI Engine

The interest in lean burn combustion is related to the necessity of developing effective solutions for improving fuel economy and reducing exhaust emissions, in particular for new generation direct injection (DI) spark ignition (SI) engines. However, it is also consolidated that lean operation is associated with increased cycle-by-cycle variability. In this context, turbulent mixture motion can play a critical role, since burning rates are much lower than in stoichiometric engines. Turbulence levels are correlated to combustion chamber geometry and intake flow organization; however, it is still not clear how swirl and tumble affect the ignition phase and flame kernel inception in lean DISI engines. In particular, the effect of spark plug geometry on ignition has been minimally treated. It is exactly this aspect that constituted the starting point of this work, with an interest in flame kernel development and cyclic combustion variability. Experiments were performed on a gasoline fueled DISI engine by comparing three electrode geometries at fixed crankshaft rotational velocity and maximum brake torque spark timing, in stoichiometric and lean burn conditions. Specifically, standard J-plug, double electrode and surface discharge spark configurations were tested in an optically accessible single cylinder engine, by using UV-visible visualization and in-cylinder pressure analysis. Digital imaging at fixed delay from spark timing allowed the evaluation of cycle-by-cycle variations of the main morphological parameters of the flame front during the early combustion stages. Particular interest is devoted to the flame kernel development and its displacement with respect to the geometrical center of the combustion chamber.