Direct Injection technology for Spark Ignition engines is currently undergoing a significant development process in order to achieve its complete potential in terms of fuel conversion efficiency, while preserving the ability to achieve future, stringent emission limits. In this process, improving the fuel spray analysis capabilities is of primary importance. Among the available experimental techniques, the momentum flux measurement is one of the most interesting approaches as it allows a direct measurement of the spray-air mixing potential and hence it is currently considered an interesting complement to spray imaging and Phase Doppler Anemometry. The aim of the present paper is to investigate the fuel spray evolution when it undergoes flash boiling, a peculiar flow condition occurring when the ambient pressure in which the spray evolves is below the saturation pressure of the injected fluid. These thermodynamic conditions can occur in part load operation for GDI (Gasoline Direct Injection) engines, causing the spray flow structure and hence the mixture formation process to be completely altered with respect to standard flow conditions.
To investigate the effects of flash-boiling on the spray evolution, a single-hole GDI research injector designed by Magneti Marelli was analyzed in terms of both global spray shape evolution and of spray momentum flux. A preliminary injection rate analysis was also carried out to investigate the hydraulic behavior of the research injector. The spray tests were executed inside a quiescent vessel at ambient pressure ranging from 40 to 300 kPa. To obtain the flash-boiling conditions, both the injector fixture and the test fuel (n-heptane) temperatures were set between 30 °C and 120 °C. For the spray momentum flux tests, distances from 5 to 40 mm from the nozzle were used. Aiming to compare the internal spray structure under low and high temperature conditions, momentum spatial distribution was also investigated over planes at different distances from the nozzle. The results of this work, obtained in well-defined conditions in terms of fuel composition and spray configuration (single jet), can assist the development of CFD numerical tools as well as contribute to a better understanding of the flash-boiling phenomenon effect to the spray formation and evolution.