By definition, flashboiling is referred to as superheated injections. The sudden occurrence of boiling inside the fuel can change the spray structure dramatically. Up to 99% of all injection processes during the New European Driving Cycle and 95% during 'Real Driving Emissions' tests are, with respect to mid-range cars, in a state of thermodynamic nonequilibrium below the specific vapor pressure of gasoline. Considering this fact, the scientific question is not the appearance of flashboiling during the operation of stoichiometric homogeneous charge direct injection gasoline engines but the intensity of occurring spray processes and their influence on nominal spray designs. As a consequence of induced targeting changes, the positive influence of flashboiling on the droplet size distribution and the penetration depth can be counteracted. As main driving factors for targeting changes, jet-to-jet interactions can be identified. By applying appropriate nozzle design features, the potential of flashboiling can be exploited and the targeting changes of the nominal spray designs, considered negatively, are avoided mostly. This work focuses on flashboiling-induced targeting changes, the socalled phenomenon of ''spray collapse'': its root cause, development and avoidance.

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Investigation of the primary breakup region of gasoline sprays is important for future nozzle development. It improves the principal understanding of inner nozzle flow and spray breakup. It also allows validating and developing CFD models. Due to the high optical density common measurement techniques like Phase Doppler Anemometry reach their limit in optical dense sprays as in the primary breakup region. High Speed X-Ray Imaging is capable to measure 2D velocity distributions directly at the spray hole exit. For generating the intense X-Ray beam the synchrotron Advanced Photon Source at Argonne National Laboratory is used. Passing through the spray the X-Ray beam is changed by two different physical principles: absorption and phase contrast. Absorption can be applied to measure the density of the spray. Phase contrast is used to visualize the borders of droplets and ligaments with high contrast. The accelerated electron bunches inside the synchrotron have a constant period length to each other. This leads to an accurate pulsed X-Ray beam (periodicity: 68 ns). The use of multi exposure with very short X-Ray pulses (17 ns) shows the traveled distance of the spray droplets and ligaments. The spray speed (150-250 m/s) is calculated by dividing these distances with the time gap between two X-Ray pulses. The X-Ray measured density distributions and velocity distributions are combined to calculate the spray force rate. The so gained force rate is validated with a spray force measurement performed at the Spray Momentum Test Bench (SMTB) at Continental Automotive GmbH. The study is focusing on the measurement setup of High Speed X-Ray Imaging at Argonne National Laboratory and the evaluation algorithms. KeywordsHigh Speed X-Ray, Gasoline Direct Injection, Gasoline Spray, Primary Breakup, Spray Density, Spray Velocity IntroductionThe spray performance of a GDI injector is highly influencing the efficiency and pollutant emissions of modern gasoline combustion engines. To optimize the whole process from injection over mixture formation and the final combustion, the behavior of a single spray plume has to be understood. To find a direct link between nozzle geometry and generated spray, the primary breakup region has to be analyzed. The primary spray velocity and density distribution are the key factors for further spray breakup. Measurement techniques like Phase Doppler Anemometry and Laser Induced Fluorescence reach their limit in optical dense sprays as in the primary breakup region. Therefore no information about the velocity and density distribution at the nozzle exit is available for comparison of nozzles and as input for CFD simulations. High Speed X-Ray Imaging is able to capture both. Out of the density and velocity distributions, the spray momentum can be calculated and for verification compared with a mechanically measured spray momentum distribution. [1,2,3,4]