This work comprises of experimental and numerical investigations of the volumetric flow in a direct injection sparkignition engine to analyse the origin of cycle-to-cycle variations during stratified engine operation. High-speed twodimensional two-component particle image velocimetry measurements are carried out simultaneously in the central tumble and mid-intake valve plane of an optically accessible engine, to capture the three-dimensional characteristic of the in-cylinder flow. Early investigation showed spray formation of stratified operation to be sensitive to cyclic fluctuations of the flow in a specific region below the spark within the central plane prior the first injection. Conditional statistics are used to track the origin of these variations back to the tumble flow in the valve plane during early compression underlining the three-dimensional structure of the flow. Moreover, conditional statistics reveals that the combustion performance is sensitive to the same specific flow region. According to this, computational fluid dynamics simulations are used to describe the in-cylinder flow with a map of four dominant flow structures. A new intake port geometry is derived from computational fluid dynamics simulations, optimized for high tumble generation. High-speed twodimensional two-component particle image velocimetry in the central tumble plane is utilized to compare the optimized intake port with the original geometry. The new intake ports yield a considerable increased tumble movement with a favourable combustion performance and less cycle-to-cycle variation.

-A combination of Large-Eddy Simulation (LES) and Particle Image Velocimetry (PIV) was utilized to investigate the three-dimensional in-cylinder flow within an optically accessible Direct Injection Spark Ignition (DISI) engine at motored engine operation. The PIV measurements were used to guide the meshing procedure by identifying the regions were refinements and improvements were needed. From the iteratively optimized meshes LES results are shown from two selected meshes, an intermediate coarse mesh and the final optimized mesh, and compared to PIV measurements. The evolution of the intake flow and the tumble in the central tumble plane during compression are presented and discussed. Exploitation of the LES results allowed showing the influence of out-of-plane velocities along the cylinder liner impacting the formation of the tumble flow. The optimized mesh was then used to investigate the influence of the spark plug on the incylinder flow. For the studied engine the spark plug had a significant impact on the evolution of the tumble flow during compression. Finally 35 engine cycles were simulated using the optimized mesh with the spark plug in place. Velocity distributions in a region below the spark plug are shown and compared with PIV results. The two-sample Kolmogorov-Smirnov test revealed a strong similarity between the velocity distributions obtained by PIV and LES, thus validating the potential of LES for investigating cycle-to-cycle variability.