Effect of sac-volume on the relationship among ball behavior, injection and initial spray characteristics of ultra-high pressure GDI injector

•Ball behavior inside injector was visualized and analyzed using X-ray technique.•Reducing the sac volume will decrease ball lift and shorten the closing delay.•Injectors with small sac volume have a greater advantage in reducing post-injection, but also have problems with larger ball deviation and longer STP.•Some proposals were made to optimize the design of injectors with smaller sac volume.