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Cycle-to-cycle variations of combustion processes strongly affect the emissions, specific fuel consumption as well as work output. Especially Direct Injection Spark-Ignition (DISI) engines are very sensitive to cyclic fluctuations within the combustion chamber. Multi-cycle Large Eddy Simulation (LES) based analysis has been used for investigating unsteady effects of spray combustion processes and misfires. A realistic four-stroke DISI internal combustion engine configuration was taken under consideration. The effects of variable spray boundary conditions on spray combustion are discussed first. A qualitative analysis of the intensity of cycle-to-cycle variations of in-cylinder pressure is presented for various combinations of injection parameters and ignition points. Finally, the effect of ignition probability and analysis of misfires are pointed out. The described above processes were discussed in terms of mean and standard deviation of temperature, velocity and pressure.

Large eddy simulation (LES) of two-phase non-reacting flow in an internal combustion (IC) engine has been carried out. The impact of cyclic velocity variations, unsteady effects of fuel spray injection as well as their joint effect on in-cylinder flow field pattern were investigated. Influence of these factors on globally averaged charge motion was discussed in details. LES of 40 consecutive engine cycles have been performed using KIVA-3V for each considered case. The configuration under investigation represents a four stroke direct fuel injection engine with variable tumble system.

Investigation of unsteady mixing and combustion effects presents a significant interest in the design and optimization of direct injection spark-ignition (DISI) engines. The main attention focused on the analysis of impact of in-cylinder velocity cyclic variations on mixing and combustion processes. For this purpose multi-cycle large eddy simulation (LES) of the flow, mixing and combustion have been done in a realistic four-stroke internal combustion (IC) engine. The LES method has been used to simulate up to 50 consecutive engine cycles. The paper summarizes the main findings regarding cycle-to-cycle variations of in-cylinder velocity flow filed as well as mixing processes. Then the unsteady effects of spray combustion processes are discussed in details.

In the present study a LES numerical modeling is carried out for a GDI using KIVA-4 CFD code. Thereby a comprehensive model for the fuel injection process as encountered in IC engine injectors is integrated in a Eulerian-Lagrangian framework. The injector represents a continental piezoinjector with outwardly opening nozzle. The model includes atomization, collision, evaporation and SGS turbulence models. The atomization is described with a combined primary and secondary atomization model. For the primary atomization, a LISA model is used, which is based on the assumption of formation of unstable hollow sheet close to nozzle exit, while a TAB based model is used for the secondary atomization. A new appropriate collision-coalescence model that is independent of mesh size and type is suggested and integrated into the spray model. It accounts for different regimes of droplet-droplet interactions (e.g. bouncing separation, stretching separation, reflective separation and coalescence).