CFD-Based Assessment of the Effect of End-Gas Temperature Stratification on Acoustic Knock Generation in an Ultra-Lean Burn Spark Ignition Engine 2023-01-0250

End-gas temperature stratification has long been studied with respect to its effect on stoichiometric spark-ignition (SI) engine knock. The role of temperature stratification for homogeneous-charge compression ignition (HCCI) engine operation is also reasonably well understood. However, the role of temperature stratification in ultra-lean SI engines has had less coverage. Literature is lacking well-controlled studies of how knock is affected by changes in the full cylinder temperature fields, especially since cycle-to-cycle variability can impede a determination of cause and effect. In this work, the knocking propensity of specific cylinder conditions is investigated via 3D computational fluid dynamics (CFD) simulations utilizing a large eddy simulation (LES) framework. The end-gas temperature stratification is systematically varied such that all other parameters remain relatively constant (including the flow field, thereby avoiding cycle-to-cycle variability) allowing for direct conclusions to be drawn. It is found that a substantial paradigm shift in acoustic knock generation is brought on by having large volumes in the end-gas being under a stratification limit set by the time it takes for an acoustic wave to traverse the cylinder domain. Hence, it is shown that temperature stratification of the end gas plays a key role, enabling the combustion system to accept some degree of end-gas autoignition without inducing unwanted acoustic knock.