3D CFD Analyses of Intake Duct Geometry Impact on Tumble Motion and Turbulence Production in SI Engines

In recent years, engine manufacturers have been continuously involved in the research of proper technical solutions to meet more and more stringent CO2 emission targets, defined by international regulations. Many strategies have been already developed, or are currently under study, to attain the above objective. A tendency is however emerging towards more innovative combustion concepts, able to efficiently burn lean or highly diluted mixtures. To this aim, the enhancement of turbulence intensity inside the combustion chamber has a significant importance, contributing to improve the burning rate, to increase the thermal efficiency, and to reduce the cyclic variability. It is well-known that turbulence production is mainly achieved during the intake stroke. Moreover, it is strictly affected by the intake port geometry and orientation. In this paper, different geometries of the intake port are analyzed by means of a 3D-CFD approach, to foresee the flow evolution and tumble motion development during intake and compression strokes. Tumble vortex collapse and turbulence production at the end of the compression stroke are analyzed in detail, since turbulence levels just before TDC have a direct impact on the combustion process. Analyses are carried out under motored operation, with time-varying boundary conditions provided by a 1D model of the whole engine. The mass-averaged intensities of tumble motion and turbulence are evaluated for different intake port orientations and throat areas. An analytical correlation between intake duct orientation and turbulence intensity is identified. The latter represents a useful tool to easily recognize the optimal geometrical configuration of the intake port promoting the required in-cylinder turbulence level. The numerical results will constitute the basis for the development of a phenomenological turbulence model, able to sense the main geometric parameters of the intake system.