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Stereoscopic particle-image velocimetry (PIV) is used to investigate the non-reacting flow field in the combustion chamber of a motored direct-injection spark ignition (DISI) engine with tumble intake port. The in-cylinder flow is controlled by variable valve timing (VVT), i.e., shifting of the intake cam shaft to earlier or later crank angles (cam phasing). VVT systems are already implemented in production combustion engines, e.g., BMW's Vanos system, to improve the volumetric efficiency and to reduce pumping losses. In the present study, the underlying flow phenomena, i.e., the effect of VVT on the tumble development and turbulent kinetic energy, are analyzed. The flow field is investigated at a set of early, intermediate, and late intake valve opening (IVO) positions during the intake and compression strokes, thus enabling the analysis of the temporal development of the main flow structures.

The development of the flow field in the cylinder of a piston engine possesses a distinct influence on the fuel-air mixing and thus, on the combustion process. In particular, the flow structures that evolve during the intake and compression stroke are of major importance and at constant flow parameters, the intake port geometry influences these structures. To show this impact, the flow field of two engines with different intake port geometries is measured using particle-image velocimetry in the present study. The data are compared regarding the temporal and spatial development of the main flow phenomena and the turbulent kinetic energy. The study focuses on the impact of the two different formation mechanisms of tumble vortices due to the different intake port geometries on the flow structure. Engine A is an optical research engine optimized for high tumble ratios for combustion stability in combustion processes of tailor-made fuels.