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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.

The aim of this work has been to investigate the influence of bio-diesel (FAME - fatty acid methyl ester) and reasonable blends of FAME and diesel, on the behavior of the mixture formation, the combustion process and the emission levels of a modern diesel engine. Therefore experimental investigations have been carried out on a single cylinder engine which is a modification of a four cylinder production HSDI (high speed direct injection) diesel engine. Cylinder pressure and tail-pipe emissions have been measured. Additionally the injector has been mounted on a flow-metering device to gather information about the pure flow through the injection system depending on the fuel type. In parallel with the experimental investigations CFD (computational fluid dynamics) calculations have been carried out in order to closely check local effects inside the injector and the combustion chamber.