Internal Flow and Spray Characteristics of Pintle-Type Outwards Opening Piezo Injectors for Gasoline Direct-Injection Engines

The near nozzle exit flow and spray structure generated by an enlarged model of a second generation pintle type outwards opening injector have been investigated under steady flow conditions as a function of flow-rate and needle lift. A high resolution CCD camera and high-speed video camera have been employed in this study to obtain high-magnification images of the internal nozzle exit flow in order to identify the origin of string ligaments/droplets formation at the nozzle exit. The images of the flow around the nozzle seat area showed clearly that air was entrained from outside into the nozzle seat area under certain flow operating conditions (low cavitation number, CN); the formed air pockets inside the annular nozzle proved to be the main cause of the breaking of the fuel liquid film into strings as it emerged from the nozzle with a structure consisting of alternating thin and thick liquid filaments. As the flow rate increased, the air pockets were suppressed, reduced in size and pushed towards the exit of the nozzle resulting in a smoother spray.

The results showed that the number of strings increased linearly, within the measured range, with liquid exit velocity and that the spray cone angle was smaller or larger than the nominal value depending on the attachment of air pockets to the cartridge or needle surfaces, respectively; these two distinct small and large cone angles were found to be dominant at low and high lifts. Increasing the flow rate further so that CN exceeds the critical value, gave rise to pockets of vapour that started to emerge in the nozzle seat region and disintegrated rapidly as they were convected towards the nozzle exit. The analysis of the near nozzle flow visualizations has shown the existence of air entrainment and cavitation as two different phenomena occurring under different operating conditions.