A Large-Eddy-Simulation (LES) approach is applied to the calculation of multiple SI-engine cycles in order to study the causes of cycle-to-cycle combustion variations. The single-cylinder research engine adopted in the present study is equipped with direct fuel-injection and variable valve timing for both the intake and exhaust side. Operating conditions representing cases with considerably different scatter of the in-cylinder pressure traces are selected to investigate the causes of the cycle-to-cycle combustion variations.
In the simulation the engine is represented by a coupled 1D/3D-CFD model, with the combustion chamber and the intake/exhaust ports modeled in 3D-CFD, and the intake/exhaust pipework set-up adopting a 1D-CFD approach. The adopted LES flow model is based upon the well-established Smagorinsky approach. Simulation of the fuel spray propagation process is based upon the discrete droplet model. Modeling of flame propagation is realized on the basis of the coherent flame concept. Spark ignition and early flame kernel formation is approximated by prescribing the initial flame surface density in a sphere around the spark location.
For the LES study of the causes of the cycle-to-cycle combustion variations the simulations are run for more than 20 cycles for each of the selected operating points in order to obtain a statistically relevant set of results for the in-cylinder flow, mixture formation and combustion quantities. Assessment of the impact of the instantaneous, cycle-resolved flow quantities on the early flame kernel formation and the subsequent main combustion process is achieved by analysis of both spatially resolved and cylinder-averaged result data.