Modeling of combustion in spray-guided spark-ignition engines

摘要

Subject of this work is the advancement of detailed physical models to capture the three-dimensional combustion process in modern spray-guided spark-ignition engines. Particular emphasis is placed on the ignition process, which requires the reproduction of the complex interactions of fuel injection, ignition, and early flame front propagation. In spray-guided spark-ignition engines, the major part of the distinctively stratified fuel/air-mixture is consumed by a propagating turbulent flame front. Its modeling approach is based on the concept of the G-equation, simulating the propagation of the flame front by solving a kinematic equation for its surface. The propagation results from the flow velocity on the one hand, and from the turbulent burning velocity, which models the burning rate, on the other hand, respectively. This thesis is structured as follows: After the introduction, the conservation equations of fluid mechanics, along with the applied turbulence model, are presented in chapter two. Afterwards, the fundamentals of the physical and numerical modeling concepts of turbulent flame front propagation and auto-ignition are discussed in the third and fourth chapter, respectively. In chapter five, the developed ignition model for spray-guided spark-ignition engines is presented. There, the already introduced modeling fundamentals are extended to capture both the auto-ignition process of the mixture along the spark channel and the consequent quasi-laminar and eventually turbulent flame front propagation. In chapter six, the numerical implementation of the combustion model into the used 3D CFD code is exemplified and validated. Afterwards, the physical combustion model is validated in chapter seven, using experimental data of premixed turbulent combustion in a constant-volume vessel and in a natural gas spark-ignition engine. In chapter eight, the developed model is applied to simulate combustion in a spray-guided spark-ignition engine. The results are qualitatively and quantitatively compared to measurements. The thesis closes with a discussion of the results and an outlook.