Large Eddy Simulation of Ignition and Combustion Stability in a Lean SI Optical Access Engine

Large-Eddy simulations (LES) are becoming an engineering tool for studying internal combustion engines (ICE) thanks to their ability to capture cycle-to-cycle variability (CCV) resolving most of the turbulent flow structures. ICEs can operate under lean combustion conditions to maximize efficiency. However, instabilities associated with lean combustion may cause problems, such as excessive levels of CCV or even misfires. In this context, the energy released by the spark during the ignition and its interaction with the flow field are fundamental parameters that affect ignition stability and how combustion takes place and develops. The aim of this paper is the characterization of the combustion stability in a SI optical access engine, by means of multicycle LES simulations, using CONVERGE software. Sub-grid-scale turbulence is modeled with a viscous one-equation model. Two different combustion approaches are used combined with local adaptive mesh refinement (AMR): G-equation combustion model and the perfectly stirred reactor (PSR) combustion model with a skeletal kinetic mechanism for primary reference fuel (PRF). First, a mesh sensitivity study was carried out comparing three different resolutions and discussing pros and cons. Then, 15 engine cycles for each condition were simulated, investigating three different mixture cases: stoichiometric condition (relative air-fuel ratio, λ = 1.0) and lean conditions with λ = 1.4 and λ = 1.5. Computational fluid dynamics (CFD) results are compared with experimental data collected in a low-speed low-load engine point. Numerical results are able to match experimental data, capturing the transition from stable combustion at λ = 1.4 to unstable combustion at λ = 1.5. Crank-angle resolved flame front structures are also compared and discussed.