Effect of Spark Plug Electrodes on Flame Kernel Development in an Optically Accessible DISI Engine

In a historical moment in which the automotive sector is asked to improve its efficiency in terms of fuel economy and polluting emissions in order to mitigate the emerging climate crisis, the investigation and in-depth understanding of the flow phenomena and flame-fluid interactions occurring in the combustion process becomes fundamental. The basis of this analysis, aimed at the development of new combustion concepts of better efficiency, lies in the experimental activity that is providing over the years detailed studies of in-cylinder processes supported by the development of predictive CFD numerical models. The purpose of this thesis is to contribute to research by building up a wide thermodynamic and optical database concerning CCV phenomena, flame kernel formation and flame front propagation. The analysis was conducted on a Direct Injection Spark Ignition (DISI) engine featuring a Bowditch design, characterized by an extended piston combined with a quartz window mounted within the piston crown, that allowed optical access. The focus was on the effect of spark plug geometry and orientation under different engine operating conditions in terms of AFR, Spark Advance, engine load and speed. Two different types of spark plugs, respectively double-electrode J-type spark plug (in uniflow and crossflow orientations) and concentric-electrode spark plug, were examined by means of in-cylinder pressure measurements which allowed the analysis of thermodynamic parameters (IMEP, COVIMEP, Pmax, COVPmax, NHRR, MFB, VFB etc..) combined with results from optical measurements. The optical analysis of images, acquired through cycle-resolved digital imaging using a high-speed camera, allowed a spatial characterisation of the combustion process through the study of kernel displacement and flame expansion speed. One of the main findings is the significant role in flame kernel development played by spark plug orientation and its influence on the direction of propagation of the flame front.