Modelling of internal and near-nozzle flow of a pintle-type outwards-opening gasoline piezo-injector

Computational fluid dynamics (CFD) methods have been used to investigate the internal nozzle flow of an outwards-opening piezo-driven pintle injector designed for spray-guided direct injection gasoline engines. The internal nozzle flow has been investigated for various nozzle designs, with emphasis placed on the effect of manufacturing tolerances on the internal and near-nozzle flow characteristics. The methods employed include features such as moving wall boundaries and time-dependent pressure or flowrate inlet conditions, cavitation as well as Eulerian and Lagrangian near-nozzle and spray models. The results reveal that not only the nozzle internal geometric details and manufacturing tolerances influence significantly the flow conditions at the exit of the pintle injector, but the actual spray characteristics are significantly influenced by the external geometry of the nozzle housing and the pintle shape. The atomization process of the liquid emerging from the pintle nozzle seems to be different from that realized in other nozzle designs used in direct injection gasoline engines; a so-called string-type of spray is formed at the nozzle exit, as confirmed by near-nozzle CCD spray images. The mechanism of string formation is attributed to the limited liquid volume passing through the needle seat area and partly occupying the available volume, while the details of the geometry in this location may enhance local air entrainment. The velocity differences along the circumference of the nozzle exit magnify those flow instabilities and have a predictable effect on the dispersion of the liquid droplets near the nozzle exit.