The effects of ambient pressure and fuel temperature on spray characteristics of air-assisted injection system were investigated by using high-speed photography and phase Doppler particle analyzer (PDPA) in a constant volume chamber. The ambient pressure ranged from 0.01MPa to 0.1MPa

and the fuel temperature ranged from 25℃ to 100℃ during the experiment. The experimental results showed that spray condition transferred from the non-flash boiling to flash boiling gradually with the decrease of the ambient pressure that induced the intensified bubble bursting at the nozzle exit. When the ambient pressure was less than 0.02MPa

the spray was in flash boiling condition

bringing about the increase of the droplet velocity

the decrease of the droplet diameter and the spray expansion in the near field. Meanwhile

the rising fuel temperature led to the decrease of the fuel surface tension

which can accelerate the break of the droplet. As the fuel temperature reached 100℃

when the spray was in flash boiling condition

the spray expansion occurred in the far field.