No abstract

High-pressure multi-hole injectors for gasoline direct injection (GDI) engines allow a flexible optimisation of the individual spray hole properties to specific engines and combustion strategies. One degree of freedom within this spray targeting process is the mass flow rate of each spray plume, which is mainly controlled by the corresponding spray hole diameter. As a first approximation, it can safely be assumed that the flow rate scales with the spray hole crosssectional area, but due to the complex flow pattern within the valve seat, some deviations are likely to occur. This was investigated using four specially designed real-size research injectors with two almost identical spray holes -except for their diameters, one of them constant and the other variable. Combining the results of spray-hole-individual mass flow rate and spray force measurements, the spray-hole-individual discharge coefficients can be separated into area and velocity coefficients. Under the choked flow conditions typical of GDI, the discharge and area coefficients increase while the velocity coefficients decrease as spray holes become smaller. At the same time, the flow inside the unaltered spray hole is affected as well, although the discharge coefficient remains constant because of reciprocal trends between the area and velocity coefficients.