Engine In-Cylinder Flow Control via Variable Intake Valve Timing 2013-24-0055
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. Two parallel, vertical measurement planes, the symmetry plane and the valve plane, are investigated. It is shown that VVT has a strong influence on the mean vorticity and the local and temporal distribution of the kinetic energy and turbulent kinetic energy. The tumble stability is improved by a late opening of the intake valves, i.e., the kinetic energy and vorticity are increased. This is due to the formation of an additional positive vortex which adds to the vorticity and kinetic energy of the main tumble. An early intake valve closure, however, leads to a stronger rear part of the ring vortex, which contains negative vorticity and decelerates the main tumble. Furthermore, the amount of turbulent kinetic energy in the intake phase strongly depends on IVO, which is important for the fuel injection and mixing.