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The goal of this work is to develop a new approach for characterizing transient spray behavior from a multi-hole, inward opening gasoline direct injection (GDI) injector. For a modern engine, it is well understood that transient spray performance is inferior from the perspective of emissions, yet few studies currently exists which experimentally isolate spray in this region. All the while, new injection strategies and designs continue to exacerbate the problem by spending less time at full lift. These injection strategies result from a void in understanding in how a needle’s partial-lift can impact spray behavior for both virtual and real experiments. To partially fill this void, this dissertation seeks to demonstrate the following: 1. Design a modified injector capable of reliably operating at specific partial-lift set-points with accuracy on the order of one micrometer.2. Develop a new experiment to isolate the impact of the injector needle position on spray characteristic quantities such as flow rate, droplet sizing, and spray shape.3. Demonstrate how the experiments can be leveraged to generate inlet conditions that more realistically represent spray during needle transients in CFD.4. Leverage computational fluid dynamics (CFD) to explore the viability of approximating what is normally a moving needle with fixed, partial-lift tests. This will involve comparison of in-nozzle flow between a moving and fixed needle.For this injector, performing the new experiments by discretizing the needle's stroke into a series of steady partial-lift tests seems a viable approach for quantifying transient behavior. Comparing sac flow between a moving and stationary needle in CFD demonstrates similar nozzle-hole flow rates and effective exit velocities. These observations suggest similar downstream spray break-up. The similarities persist in this injector above lifts around 10 μm.To facilitate the proposed experiments, an inward-opening multi-hole GDI injector is minimally modified with an apparatus to finely control needle lift by limiting the needle's stroke with a fine micrometer head. The modified injector is capable of discretizing the stroke in steps as small as 2 μm. To accompany the new injector, an experiment process is developed to associate the micrometer set-point with the imposed needle lift, compensating for pressure induced component flex. This flex is significant and of a similar magnitude as the needle’s stroke. The combination of the modified injector and experiment calibration process results in unpreceded control on operational needle stroke.The modified injector is then used in two subsequent spray characterization tests: 1. down-stream droplet sizing statistics as measured using a laser diffraction system; and 2. Spray angle, bend angle, and penetration as measured using spray imaging. The results demonstrate a 2x inflation of droplet diameters as the needle reduces to 10 μm, and a significant reduction in spray penetration, but minimal impacts on spray angle or bend angle.This information is then leveraged in a virtual CFD spray box to demonstrate how a more realistic spray can be obtained through the improvement of the spray boundary condition. This additional spray transient data could be immediately used to better inform control/calibration efforts on in-production vehicles by exposing injector specific impacts from multi-shot spray or other short injection windows where transients become significant. |
