Numerical modeling of knocking combustion in spark ignition engines

At present, around 85% of the 1.1 billion passenger cars in the world are powered by gasoline spark ignition (SI) engines. Engine downsizing coupled with boosted operation and transition to sustainable fuels is considered an attractive strategy to enhance power density and reduce fuel consumption of SI engines. However, these operating conditions result in severe thermodynamic conditions, thereby promoting the likelihood of abnormal combustion phenomena such as knock. The severity of knock can vary significantly, and the efficiency of engines at high loads is limited in practice by heavy knocking phenomena. Since, a thorough analysis of such recurrent but non-cyclic phenomena via experiments alone becomes highly cumbersome, in the present work, a multi-cycle large-eddy simulation study was performed to quantitatively predict cyclic variability in the combustion process and cyclic knock intensity variability in a direct injection spark-ignition engine. For both the mild knock and heavy knock conditions, the numerical results were validated against experimental measurements. Based on the simulation results, a correlation analysis was performed considering combustion phasing, peak cylinder pressure and maximum amplitude of pressure oscillation. Furthermore, a detailed three-dimensional spatial analysis illustrated the evolution of auto-ignition kernel development and propagation of pressure waves during knocking combustion.