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In gasoline engines with direct injection, fuel deposition on the walls that immediately follow the injector bore can lead to increased particulate and hydrocarbon emissions if the liquid does not fully evaporate before the beginning of combustion. Apart from surface wetting, this “injector pocket” region can also indirectly affect engine performance: although the details are poorly understood, it is well known that walls block air entrainment into sprays and thus can change spray characteristics. Hence, this study investigates the influence of the injector pocket on the spray shape quantitatively by using a methodical series of tests. The present work analyzes the changes of spray geometry caused by nearby walls using high speed shadowgraphy in an optically accessible pressure chamber. Furthermore, Fluorescent Particle Image Velocimetry and Structural Image Velocimetry are used to investigate the air entrainment, which lies at the origin of the interaction between the spray and nearby walls. A flexible experimental setup with generic, quasi-two-dimensional injector pockts was designed, providing excellent optical access to the complete spray. Due to the Coanda˘ effect, the gasoline spray tilts towards the walls by about 5°. The injector pocket restricts the entrainment of air at the spray periphery. Instead of the perpendicular flow into the spray that characterizes air motion around an undisturbed spray, a flow of opposite direction to the spray and parallel to the injector pocket outline is observed. Since air entrainment carries momentum into the spray, the change in air flow explains the observed changes of the spray direction. Similar effects are observed for different two-dimensional injector pocket outlines, and also for a geometry which was extracted from a series engine and combined with the matching series injector. However, once the distance between injector axis and wall exceeds 6 mm, the magnitude of the deflection of the spray towards the wall becomes very small and tends to disappear at a distance of roughly 10 mm. At low back pressure, the deflection of the spray towards the walls is somewhat stronger since the drastically with fuel temperature and pressure, the shift of spray direction due to the injector pocket remains relatively independent of fuel temperature and injection pressure, at least within the range that usually occurs in gasoline engines. At low distances from the injector axis, significant wetting of the injector pocket surface and stripping of large, slow droplets from the wall film is observed for the generic models that allow optical access to the edge of the wall. |
