PIV measurements and numerical analysis of in-cylinder tumble flow in a motored engine

The purpose of this study is to present an experimental methodology and a numerical analysis to characterize the in-cylinder tumbling flow generated by a motored four-valve spark ignition single-cylinder optical engine. High-speed two-dimensional particle image velocimetry measurements were made in the symmetry vertical plane between the inlet and outlet valves. The velocity flow fields were recorded during the intake and compression strokes for three different engine speeds (1000, 1500, and 2000 rpm) at several crank angles. Computational fluid dynamics (CFD) simulations were carried out using the Star-CD software with es-ICE module, and the two different turbulence models, RNG-k-epsilon and k-w SST, were analyzed with experimental data of the engine running at 1000 rpm in motored conditions. Regarding experimental results, vorticity, tumble ratio, and kinetic energy were calculated and compared. The initially large mean kinetic energy and tumble ratio, which is caused by the high-velocity jets of the inlet valves and in-cylinder bulk flow, decrease during intake in response to the piston motion. The air jet entering the cylinder space strikes the piston top surface generating a counter-clockwise rotating tumble vortex. Flow field cyclic variation could be noticed and quantified. In addition, an evolution analysis of the main vortex center and engine speed influence were made. Moreover, a qualitative analysis of the CFD scalar flow field in the region of interest is presented. The numerical results showed a better agreement with the RNG-k-epsilon and experimental mean velocity magnitude curve than k-omega-SST. However, the qualitative scalar velocity field of the k-omega model captured more details of the flow than the k-epsilon model. The methodology contributes to a better understanding of flow motion behavior, piston speed and valve timing influences, and, consequently, the mixture formation process in spark ignition engines.