GDI spray evolution and sizing characteristics in Flash-Boiling conditions

In the optimization of Direct Injection Spark Ignitions engines, the injector operation is playing an increasingly important role in the effort to gain a proper control of the combustion process, so to improve the engine efficiency and to reduce the pollutant emissions. The essential task is to obtain the required fuel spray distribution and atomization level inside the combustion chamber, in a wide range of engine operating conditions.
In this frame, the effect of the flash-boiling phenomenon is not negligible. In fact, when hot fuel is introduced in an ambient in which the static pressure is below the fuel saturation pressure, drastic physical changes occur. Under superheated state, vapor bubbles inside the liquid phase are generated and the abrupt fuel vaporization takes place. The injected fluid becomes unstable and the spray evolution and consequently the later combustion are significantly affected.
In order to deepen the important role played by the flash-boiling effect on the spray formation and evolution, a single-hole research injector was designed by Magneti Marelli and tested at Perugia University SprayLab. Spray atomization and morphology changes as caused by injection pressure, ambient pressure and fuel temperature variation were analyzed using optical techniques. In order to simulate both part-load and wide open throttle conditions, the tests were carried out in a constant volume chamber at ambient pressure of 40, 100, 300 kPa. To reach the flash boiling conditions, both the test fuel (n-heptane) and the injector body were heated in the range 30-120°C.
High speed imaging was used to investigate the global effect exerted by flash boiling on the macroscopic spray structure. In flash boiling conditions a penetration reduction and a wider diffusion angle were observed. An imaging close-view setup made also possible to investigate the vortex formation and the spray angle at the nozzle outlet. Sizing and velocity spray characteristics in flash boiling conditions were investigated by Phase Doppler Anemometry, proving a significant effect on the atomization level with respect to standard spray evolution conditions. Further, the spray sizing characteristics resulted to be more uniform over the spray cross section and the droplets diameter distribution was significantly affected, with a considerable Sauter Mean Diameter reduction.