Cycle-be-cycle Variation of Flame Initiation and Development in a GDI Engine: Numerical and Experimental Analysis

The early stage of combustion, namely ignition and flame inception, is well recognised as the source of cycle-to-cycle variability in spark ignition engines. Cyclic variability is an undesired phenomenon having a negative impact on vehicle drivability, level of pollutant emissions and fuel consumption. From a numerical perspective, it is hardly reproducible through 3D modelling based on RANS approaches, unless proper changes in the initial conditions of the flow transport equations are considered. This is often just possible on the ground of statistical considerations, namely without quantitative measurements relevant to, say, the spark discharge features, mixture inhomogeneity at the plug, turbulence intensity or even the main flow pattern in the vicinity of the ignition location.
Present work aims at exploiting information derived from images collected in a optically accessible single cylinder 4-stroke GDI engine to properly collocate the flame at the start of the combustion calculation. The collected data allows improving the numerical simulation and gaining hints about the guidelines one may follow in predicting the range of cyclic variability. A final comparison between the flame propagation images gained both experimentally and numerically is made in order to also highlight the correlation between the overall flame behaviour and the initiation duration.