Time-resolved ballistic imaging of a transient diesel spray

Pilot injections play a crucial role in enhancing thermal efficiency and reducing NO_x emissions in large-scale dual-fuel (DF) combustion engines. These injection strategies are characterized by extremely short dwell times and compact spray events, which are highly sensitive to the transient dynamics of the injector. The near-nozzle region is particularly important, as it governs spray initiation, primary breakup, and fuel-air mixing. Spray behavior in this region is strongly influenced by injection pressure, primarily due to the needle's opening and closing dynamics. However, conventional imaging techniques are inadequate for capturing spray development and the dense liquid core within a few millimeters of the nozzle, owing to limitations in temporal resolution. To overcome this limitation, ballistic imaging was employed under ambient conditions to visualize spray evolution within 10 mm of the nozzle at injection pressures of 1700 and 2100 bar. Ballistic imaging is an optical technique that uses an ultrafast shutter (1.5-2 ps) to capture high-resolution images of dense sprays by filtering out multiple scattered light. The results show the temporal evolution of the spray liquid core morphology and highlight the influence of injection pressure on spray characteristics; particularly, the spray cone angles increase at lower injection pressures due to reduced needle lift and lower spray momentum.