Effect of flash boiling on microscopic and macroscopic spray characteristics in optical GDI engine

•PDPA was firstly used to measure the droplet size in optical GDI engines.•Under idle operation, spray collapsed and penetrated faster at high temperatures.•Under idle operation, SMD slumped by 33.2% from 20 °C to 60 °C.•The droplet size distribution during flash boiling fits the Rosin-Rammler function.

The flash boiling sprays in a single-cylinder optical gasoline direct injection (GDI) engine were studied using phase Doppler particle analyzer (PDPA) and high-speed photography. Initially, the factors which may influence the in-cylinder measurement reliability of PDPA were carefully examined under atmospheric conditions. Afterwards, engine experiments were conducted under idle and wide open throttle (WOT) conditions with the fuel temperature ranging from 20 °C to 90 °C. The results show that under idle operation, the spray collapsed towards the injector axis as the fuel temperature increased. At high fuel temperatures, the spray penetration increased so that the probability of fuel-wall impingement rises. As the fuel temperature increased from 20 °C to 60 °C, the AMD and SMD slumped by 43.6% and 33.2%, indicating the dramatic improvement in atomization quality. Concurrently, the droplet uniformity decreased sharply. As the temperature further increased to 90 °C, the AMD and SMD only dropped slightly, but the droplet uniformity became better. Under WOT operation, no distinct evidence of spray deformation was found except for the case at 90 °C. The AMD and SMD dropped steadily with the increase in fuel temperature, which is mainly attributed to the reduction in viscosity and surface tension. Besides, the droplet uniformity increased gradually along with the fuel temperature.