Evaluation of Geometry-Dependent Spray Hole Individual Mass Flow Rates of Multi-Hole High-Pressure GDI-Injectors Utilizing a Novel Measurement Setup

In order to optimize spray layouts of commonly used high-pressure injectors for gasoline direct injection (GDI) engines featuring multi-hole valve seats, a detailed understanding of the cause-effect relation between inner spray hole geometries and inner flow conditions, initializing the process of internal mixture formation, is needed. Therefore, a novel measurement setup, capable of determining spray hole individual mass flow rates, is introduced and discussed. To prove its feasibility, a 2-hole configuration is chosen. The injected fuel quantities are separated mechanically and guided to separate pressure tight measurement chambers. Each measurement chamber allows for time resolved mass flow rate measurements based on the HDA measurement principle (German: “Hydraulisches Druck-Anstiegsverfahren”). The setup is utilized for a systematic parameter study, comprising four almost identical generic real-size GDI-injector variants, where one spray hole per variant is subject to a diameter variation. To evaluate the influence of different spray hole diameter ratios, injection pressures and back pressures on the resulting internal flow conditions and the relative mass flow rates between both spray holes, two data reduction schemes are presented. The chosen pressure boundary conditions allow the investigation of the transition from unchoked to choked flow conditions. Consistent onsets of choked flow conditions can be observed for both spray holes of all injector variants, leading the conclusion that the occurrence of choked flow is not affected by the spray hole diameter. A close correlation between the relative mass flow rates and the ratio of spray hole cross sectional areas can be observed throughout all injector variants and internal flow conditions. The influence of spray hole diameter outweighs the influence of varying discharge coefficients. The results prove the capability of the introduced measurement setup in terms of individually characterizing the internal flow conditions in each spray hole and evaluating relative mass flow rates with high reproducibility.