Detailed Simulations of Stratified Ignition and Combustion Processes in a Spray-Guided Gasoline Engine using the Spark CIMM/G-Equation Modeling Framework

摘要

Recently, high-speed optical imaging data for a single operating point of a spray-guided gasoline engine has, along with the flamelet model and the G-equation theory, enabled the development of the new spark-ignition model SparkCIMM. Within its framework, detailed chemistry flamelet models capture the experimental feature of multiple localized ignition events along the excessively stretched and restriking spark channel, as well as the observations of non-spherical highly corrugated early turbulent flame fronts. The developed flamelet models account for the substantial turbulent fluctuations in equivalence ratio and enthalpy present under spray-guided conditions. A non-unity Lewis number formulation captures the deficient species diffusion into the highly curved flame reaction zone. Previously, these diffusion processes were shown to have a significant effect on the early flame development in spray-guided engines due to, initially, locally rich and highly diluted low turbulent Damkohler number mixtures. Localized flame extinction phenomena are captured by corresponding flamelet criteria. In this paper, the theoretical-numerical SparkCIMM/G-equation framework is applied to three different spray-guided engine operating conditions. These conditions vary substantially in engine speed, load, EGR rate, and in injection and spark-ignition timings. Accurate wall temperatures, flow fields and mixture property conditions at ignition timing were obtained by multiple 3D detailed intake simulations. Swirl numbers, trapped masses, pressure traces before spark advance, and the integral heat release were aimed to be in agreement with experimental data. Spray model parameters were optimized by comparison to spray-rig experiments. Providing such a proper setup for each operating point, the simulation results show excellent agreement with local and cylinder-averaged experimental data. The simulation captures the effects of the operating-point-dependent varying mixture pathways of the spark channel and the propagating flame on ignition and combustion. Turbulent mixture property fluctuations in equivalence ratio, enthalpy, flame stretch, and scalar dissipation are successfully reproduced.