Automotive spark-ignited direct-injection gasoline engines

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458 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 of 10–25 mm was found to be larger than those in the other ranges. It was predicted that the mean size of droplets forming the toroidal vortex is about 20 mm, and the droplets that move directly down the cone have a diameter of the order of 50 mm. The spray atomization process of a Zexel high-pressure swirl injector was analyzed by Naitoh and Takagi [88] using the synthesized-spheroid-particle (SSP) method developed at Nissan Motor Co. The configuration of this nozzle is shown in Fig. 18 where sections A–A and B–B are the cross sections of the needle guide and nozzle hole. It was found that both the initial spray cone angle and the spray penetration decrease with an increase in the ambient pressure. The mean droplet size in the spray cross section 50 mm from the injector tip also increases with the ambient pressure. The SMD of drops at a cross section 50 mm from the injector tip is larger than the total spray-averaged SMD during the injection event, indicating that the smallest droplets do not penetrate 50 mm. Shelby et al. [52] investigated the spray structure of three GDI injectors during early injection using the PLIF technique. Four distinct phases were identified during the spray development process, which were categorized as the delay phase, liquid jet phase, wide-spray-cone phase and fully developed phase. It was found that the delay phase, which is the period of time when the signal to inject is sent to the injector driver and fuel is first detected with the PLIF system varies significantly with injector design. It was also observed that once a conical spray structure has developed, entrainment of the ambient air causes a low-pressure zone to develop in the spray center, resulting in a cone contraction for the fully developed spray [360,366]. The structure of an evaporating hollow-cone spray from a high-pressure swirl-type injector was predicted by Han et al. [89,307,308] using the KIVA code for the conditions of 500 K initial air temperature, 4.76 MPa fuel injection pressure and 1 atm ambient pressure. The air temperature used is somewhat higher than occurs in the actual engine. It was predicted that the liquid phase maintains the hollow-cone spray structure although it is deformed due to the air entrainment. However, the recirculating airflow carries the vapor fuel to the center region of the spray and the vapor phase is distributed in a solid cone. Lippert et al. [90] modeled the effects of fuel components on the structure of a spray from a high-pressure swirl injector using the KIVA-II code. Nearly all of the CFD analyses of GDI spray mixing in the literature have used a single component fuel to predict the fuel–air mixing process. For a fuel with a multi-component blend, there will be regions of lighter components and other regions where the composition is dominated by heavier components. This implies that the local equivalence ratio is not only a function of the fuel vapor concentration, but also of the local composition. The prediction conditions are 0.5 MPa and 500 K, corresponding approximately to the condition in an engine when injection is initiated at 60 BTDC during the compression stroke. The volume of fuel injected is 19 mm 3 , the injection duration is 1.32 ms, and the initial injection delay is 0.2 ms. It was found that the initial vaporization of the multi-component spray is only slightly higher than for the single component case, and that the rate decreases more rapidly toward the end. This was due to the more volatile components vaporizing slightly more rapidly at first, while at later stages the heavier components take longer to vaporize. However, the two cases show almost identical predictions of the spray penetration in the axial and radial directions. As the tip penetration of the spray is dominated by the trajectories of the largest droplets, and as the diameters of the largest droplets are similar, the close agreement between the two cases is not unexpected. The predictions for a single component also show a predominantly hollow-cone liquid spray, with very few liquid droplets present near the spray centerline. This does not hold true for the multi-component case, in which many smaller droplets are predicted near the spray center. For a multi-component fuel, more rich-mixture areas are observed during the initial stages of spray development. A decrease in the area of maximum equivalence ratio is predicted for the multi-component case as compared with the single-component case. It was also found that the lighter components are present toward the tip and center of the spray, and that the overall composition exhibits an increase in molecular weight as the heavier (less volatile) species in the liquid phase are vaporized. In order to quantify certain spray characteristics that are not obtainable by other means, Hoffman et al. [91] measured the spray mass flux distributions using a timed-sampling spray patternator [312]. Stoddard solvent at 6.9 MPa was used instead of gasoline, with an injected quantity of 90 mm 3 in an injection duration of 5 ms. The experimental visualization of the spray structure of a swirl GDI injector shows that the core region of the spray is initially skewed from the injector axis. Near the end of the injection event, there is no longer an observable central core vortex. Furthermore, the penetration of the core spray region results in the formation of a toroidal vortex. Fig. 19(a) shows the average liquid volume per injection that passed through each of the 23 measurement points equally spaced on a line 30 mm from the nozzle exit. The measurement results are plotted for four different times after the initiation of the injection. It is evident that the distribution of the liquid mass within the spray is that of a hollow-cone, with over 99% of the liquid mass located in an angular ring with an inner radius of 5 mm and an outer radius exceeding 20 mm. The highest liquid flux was observed at a radius of about 15 mm. The substantial changes in the spray geometry with time can be easily observed from the figure. The thickness of the spray cone walls was found to increase with time, while the outer boundary of the spray, which would typically be used to identify the spray cone angle, does not change at all. Fig. 19(b) shows the estimates of the average liquid mass flux passing through various points below the injector. The designation of “infinite” after BOI indicates the measurement
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 459 Fig. 19. Liquid fuel distributions inside a hollow-cone spray [91]: (a) averaged liquid fuel volume per injection passing through each of the 23 sample points at a plane of 30 mm below the nozzle tip; and (b) averaged liquid mass flux contours for the moments of 3.3 and 4.21 ms after the start of injection. is for steady injection. The spray structure shown is similar to that visualized by the pulsed-laser-light-sheet method. Pontoppidan et al. [64] conducted a basic study of the effect of the nozzle-tip design on the resulting spray characteristics. The three different nozzle configurations that were tested are shown in Fig. 20. Type I is a simple needle-type nozzle having a single cylindrical metering hole, which produces a single jet solid-cone spray. The
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