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Spark ignited engines are today operated more and more often under high load conditions, where one reason can be identified in the necessity of increasing the efficiency and hence reducing fuel consumption and specific CO2 emissions. Since the gasoline engine operation is inherently limited by knocking at high loads, strategies must be identified, which allow reliable identification and simulation of the appearance of this undesirable type of combustion. A new numerical model for the description of those kinds of pre-flame reactions in a CFD framework is discussed in this paper. Despite emphasis is put here on the auto-ignition effects, it will also be explained that the model is capable of supporting the engine development process in all combustion and emission related aspects.

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.

Research into internal combustion engines requires the development of engine simulation models which should ensure acceptable results of engine performances over a wide range of engine speeds and loads. Due to high costs of experiments and a rapid increase in the computer power, researches all over the world devote great effort to the development and improvement of simulation models. Well-known multi-dimensional simulation models (CFD models) of the engine cycle are the most demanding models in terms of computational resources. On the other hand, there are multi-zone models that are very robust and that are able to capture a certain in-cylinder property during the engine operating cycle. It is known that turbulence effects inside engine cylinder play an important role in the combustion process. In order to properly predict combustion process, characteristics of the turbulent flow field should also be accurately defined.