A new LES model coupling flame surface density and tabulated kinetics approaches to investigate knock and pre-ignition in piston engines

Abstract

Downsizing currently appears as a promising technique for reducing the CO 2 emissions of spark-ignition engines. Nevertheless boost levels are limited by the occurrence of uncontrolled auto-ignitions of fresh gases. These latter are sporadic and can have localized origins. The large-eddy simulation (LES) theoretically seems to be a promising numerical approach to study these abnormal combustion processes as a part of the turbulence spectrum is resolved and the effects of cycle-to-cycle variations are taken into account. In this context, a dedicated model integrating the auto-ignition model TKI (Tabulated Kinetics for Ignition) and ECFM-LES (Extended Coherent Flame for LES) is first detailed. It is then applied to the modeling of simple test cases representative of phenomena encountered during abnormal combustions. Finally, a qualitative study is performed to verify that well-known experimental trends related to knock can be retrieved numerically in a LES context. To this purpose, several operating points for a spark-ignition engine are computed. Signals of in-cylinder pressures that have the same appearance as those found in the literature are obtained. Retarding spark timing enables numerically to limit the occurrence of knock, as commonly used in engine control. A pre-ignition case is also regarded and shows the possibility to use the proposed model for further quantitative studies of this phenomenon.