In-Cylinder Tumble Flow Characteristics and Implications for Fuel/Air Mixing in Direct Injection Gasoline Engines

The present investigation is centered around two motored research gasoline direct-injection engines, equipped with a pressure-swirl atomizer closely spaced with the centrally located spark plug. At first a Laser Doppler Velocimetry system was employed to characterize the in-cylinder airflow in one of the engines. A comparison was made to velocity profiles in a port-fuel injected engine of similar design characteristics, which revealed a different decay mechanism of the large-scale flow structure and associated higher turbulence levels in the pentroof of the cylinder. Second, images of the hollow cone fuel spray generated by the direct injector were recorded for three different injection timings in order to discuss the temporal and spatial development of the liquid phase in the engine cylinder in terms of its interaction with the gas motion. The inclined annular gas jet entering the cylinder through the intake valves was found to affect the air entrainment in the fuel spray and its symmetry for injection during the intake stroke. For fuel injection at bottom dead center, the effect of the gas momentum was small and only noticeable late in the injection event far downstream of the injector nozzle. Images obtained for injection during compression show the effect of the elevated gas densities on the fuel spray but no visible effect of the intake generated in-cylinder gas motion.