Transient needle motion of an outwardly opening GDI injector and its effects on initial spray formation

• Transient needle motion of an outwardly opening GDI injector was analyzed.
• The effect of needle motion on initial spray formation was investigated.
• X-ray imaging technique was used for needle motion and initial flow analysis.
• Needle vibration was observed and its amplitude damped down with time.
• Initial flow dynamics and breakup varied following the needle vibration.

Outwardly opening injectors for gasoline direct-injection engines form thin-sheet hollow-cone sprays. The spray formation of the outwardly opening injectors is governed by the transient needle motion that alters the initial flow conditions such as liquid sheet thickness, emerging flow velocity and turbulence strength. However, the transient needle motion and its effects on the initial spray formation have been difficult to be studied by conventional lasers due to severe absorption and scattering of the laser lights from dense liquid features in the near-field. The current study investigates the transient needle motion of an outwardly opening injector and discusses its effects on initial spray formation using an X-ray phase-contrast imaging technique. An X-ray pulse with 80 ps duration in FWHM was used to visualize the transient needle motion and near-nozzle flow morphology. On the other hand, three X-ray pulses with 165.2 ns period were used to analyze the dynamics of near-nozzle flow by tracking the movement of the liquid features. An obvious needle vibration was observed through the injection process and the amplitude of the vibration damped down with time. This needle vibration accelerated and decelerated the near-nozzle flow in turn that altered the undisturbed liquid length and flow breakup with time. The temporal variation of the initial flow characteristics caused an irregular droplet size distribution with the distance from the nozzle tip other than monotonous decrease.