In present GDI engines, multiple injection strategies are often employed for engine cold start mixture formation. In the future, these strategies may also be used to control the combustion process, and to prevent misfiring or high emission levels. While the processes occurring during individual injections of GDI injectors have been investigated by a number of researchers, this paper concentrates on the interactions of multiple injection events. Even though multiple injection strategies are already applied in most GDI engines, the impact of the first injection event on the second injection event has not been analyzed in detail yet.
Different optical measurement techniques are used in order to investigate the interaction of the two closely timed injection events, as well as the effect of dwell time and the in-cylinder conditions. The injector investigated is a GDI piezo injector with an outwardly opening needle. The laser diagnostics applied include measurement of the spray injection velocity profile close to the nozzle exit by Laser Correlation Velocimetry (LCV) and the droplet velocities at nozzle distances between 10 to 15 mm by Phase Doppler Anemometry (PDA). The penetration length and the overall spray shape, including the spray fingers and toroidal vortices, are compared using light scattering and laser light sheet measurements. A comparison of the spray structures shows that as well in-cylinder temperature and pressure as the dwell time have a significant impact on the intensity of the interaction.
