Experimental Analysis of Three-Dimensional Flow Structures in Two Four-Valve Combustion Engines

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. Engine B is a one-cylinder motorcycle engine optimized for high filling. For both engines, the flow is investigated in a set of eight vertical measurement planes and at thirteen crank angles using 2D/2C PIV. The main area of interest is in the center of the combustion chamber beneath the spark plug and injection nozzle. The in-plane velocity-components derived from the PIV measurements are used to visualize the main vortical structures, i.e., the ring vortices beneath the two inlet valves and the main tumble in the symmetry plane between the inlet and outlet valves revealing the typical spin-up towards the end of the compression stroke. The temporal development of the mean turbulent kinetic energy is calculated for crank angles from 80° to 300° after top dead center. The results show high turbulent kinetic energy during the intake stroke which dissipates at increasing crank angles and remains at an almost stable level during the compression stroke. The temporal analysis based on the ensemble and plane averaged mean and turbulent kinetic energies also show that the very well conserved tumble vortex dominates the flow structure in engine A during intake and compression. The influence of the tumble vortex in engine B is significantly smaller. The temporal analysis of the vorticity shows a comparatively small decay of the vorticity during the intake stroke in engine A, which is likely to be caused by the intake port transporting rotating flow structures into the combustion chamber and thereby equalizing the negative effect of volume growth. The vorticity of engine B, where the tumble is generated within the combustion chamber, decreases significantly due to the latter effect.