Near-field spray atomization characteristics at the end of injection under flash-boiling and non-flash-boiling conditions

Spray at the end of injection is a significant source of soot and particle number (PN) emissions from gasoline direct injection (GDI) engines owing to poor atomization and incomplete combustion. This study aimed to understand the spray characteristics at the end stage under various engine operating conditions. The near-field spray was recorded using a back-lit optical method coupled with a signal control device, a heating system and a fuel supply device. An innovative image processing method for quantifying spray atomization characteristics at the end of spraying was developed, and more comprehensive spray atomization data, such as the droplet area, droplet number, and Sauter mean diameter (SMD), were extracted from the image. It was found that after injection, the spray area decreased and then increased again, followed by large droplet formation, which is believed to be caused by the ball bouncing. The averaged SMD decreased 49.0 % by increasing SD from −73 °C to 42 °C. During ball bouncing (approximately 0.1 ms AEOI), all SD conditions showed more droplets, and the number of droplets under slight flash boiling conditions is the most. Ball bouncing and flash boiling jointly promote the large spray structure to break into large droplets due to limited breakup. In addition, under flash boiling conditions, as the injection pressure increases from 10 MPa to 35 MPa, the averaged SMD decreases by 35.0 % due to the promoted atomization and droplet accelerated evaporation.