Legislative restrictions due to global warming and human health issues as well as the limitation of fossil energy carriers drive engine developers to further optimize internal combustion engines with regard to efficiency and emissions. Three aspects that could lead to improvements in the performance of SI engines are discussed in the course of this thesis. First, Mie-scattering imaging (MSI) measurements are conducted to investigate the spray propagation of two biofuels, which spare fossil energy sources and are reported to yield improved engine performance. The MSI measurements are conducted in an optical DISI engine and in a constant-pressure flow vessel to isolate the impact of the in-cylinder flow on the spray behavior. Secondly, the influence of Miller intake valve timings on the engine air flow is investigated using high-speed stereoscopic PIV (HSSPIV). While the impact of Miller cycles on thermal efficiency, emissions, and knocking limits was extensively studied, investigations of the impact on the flow are quite rare in the literature. Therefore, the impact of two different Miller timings on the engine flow is extensively discussed and compared to a standard valve timing. In the third part of this thesis, fundamental 3D investigations of the in-cylinder air flow are conducted, since the behavior of the tumble vortex and the turbulence in the combustion chamber directly affect the mixture formation and enhance the turbulent flame propagation, which is a key factor for combustion performance. Furthermore, the in-cylinder flow is subject to cycle-to-cycle variations (CCV), which can severely hinder the performance of an instantaneous engine cycle. To provide the denoted investigations, a novel highspeed tomographic PIV (HS-TPIV) setup is implemented on the optical engine, for the first time. Since HS-TPIV measurements of the acquired temporal and spatial resolutions are lacking in the literature, an extensive error estimate and a validation are conducted to assess the quality and validity of the measurement setup. The HS-TPIV setup is utilized to investigate instantaneous 3D flow fields of selected engine cycles and to analyze the impact of the intake pressure on the ensemble-averaged engine flow, which also demonstrates the capabilities of the HS-TPIV setup on the optical engine. In subsequent measurements, the HS-TPIV setup is updated to increase the spatial and temporal resolution even further. Finally, a triple-velocity decomposition method for 2D and 3D measurement data is introduced that separates the instantaneous in-cylinder flow velocity into a mean part, a velocity fluctuation from CCV, and a turbulent velocity fluctuation. This is in contrast to the commonly used Reynolds decomposition that yields only a single fluctuation velocity term, which encompasses the combined fluctuations from CCV and turbulence. The provided methodologies enable a fundamental 3D investigation of the in-cylinder air flow and CCV at high temporal and spatial resolutions.