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In present GDI engines, multiple injection strategies are often employed for engine cold start mixture formation. In the future, these strategies may also be used to control the combustion process, and to prevent misfiring or high emission levels. While the processes occurring during individual injections of GDI injectors have been investigated by a number of researchers, this paper concentrates on the interactions of multiple injection events. Even though multiple injection strategies are already applied in most GDI engines, the impact of the first injection event on the second injection event has not been analyzed in detail yet. Different optical measurement techniques are used in order to investigate the interaction of the two closely timed injection events, as well as the effect of dwell time and the in-cylinder conditions. The injector investigated is a GDI piezo injector with an outwardly opening needle.

Spray penetration and mixture formation in GDI engines are crucial to a reliable ignition and the subsequent combustion. For the prediction of the mixture formation process, computational fluid dynamic simulations are applied. Therefore, details about the nozzle exit conditions are essential, either as boundary conditions to be set, or to validate the numerical results. This paper presents experimental results on the influence of boundary conditions on the spray structure at the nozzle exit of a GDI injector. The injector investigated is an outwardly opening piezo injector, generating a hollow cone spray with a string structure. The distribution of the strings (the so-called "string structure") is needed for the starting conditions of the computational fluid dynamic simulations, as the origin of the strings is unresolved so far.

In the GDI engines, the accuracy of the numerical results and their contribution to the design analysis and optimization tasks strongly depend on the predictive capabilities of the physical processes. While most of the studies apply RANS concept, in this contribution LES methodology is suggested as suitable unsteady approach for simulating spray dynamics including GDI processes using KIVA-4 CFD-code. A comprehensive model is integrated in a Eulerian-Lagrangian framework allowing to describe the spray evolving from the injector nozzle and propagating within the combustion chamber. It includes sub-models to account for various relevant sub-processes. The atomization is described using combined primary and secondary atomization sub-models. Instead of performing costly level set method or VOF technique, a LISA-based sub-model is applied for the primary atomization. The secondary atomization is modeled by a TAB model.