Automotive spark-ignited direct-injection gasoline engines

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454 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 14. Comparison of the spray structure between the conventional and CASTING NET injectors [66]: (a) spray structure from a conventional injector; (b) spray structure from a CASTING NET injector; and (c) key parameters controlling the spray structure of the CASTING NET injector.
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 455 Fig. 15. Schematic of other GDI injector concepts: (a) two-step nozzle [65]; and (b) PZT nozzle [22]. size distributions between swirl-type and hole-type highpressure fuel injectors [60], it is clear that even though the difference in the mean droplet size (SMD) between the sprays from these two injectors is only 4 mm, the holetype nozzle produces a wider droplet-size distribution having many larger droplets that are theorized to be responsible for the observed increase in the UBHC emissions. It should be noted that the use of finer atomization may or may not reduce the UBHC emissions, depending upon the incylinder turbulence level, due to small pockets of very lean fuel–air mixtures [75–77,348]. A strong turbulence level in the combustion chamber is required to enhance the fuel–air mixing process by eliminating small pockets of very lean mixture. An important point to consider is that the increased droplet drag that is associated with finer atomization reduces the spray penetration, which can degrade air utilization. Even though a design fuel rail pressure of 5.0 MPa seems Fig. 16. Schematic of the AlliedSignal high-pressure swirl injector [83]. to be high enough for producing an acceptable GDI spray, Xu and Markle [58] recommended a higher fuel pressure for some of the reasons listed below. From their measurements, the SMD of the Delphi outwardly opening GDI injector is reduced from 15.4 to 13.6 mm as the fuel pressure is increased from 5.0 to 10 MPa. Such a small SMD reduction does not seem to be significant, but the total surface area for an injected quantity of 14 mg of fuel is increased 13%, which should lead to a direct improvement in the fuel vaporization rate. More importantly, an elevated high injection pressure may be required to reduce the key statistic for the maximum droplet size of the spray, namely the DV90. It was reported that the DV90 of the Delphi GDI injector spray at 30 mm downstream from the injector tip is reduced from 40 to 28 mm when the fuel injection pressure is increased from 5.0 to 10 MPa. In addition to its effect on spray characteristics, an elevated fuel rail pressure may also reduce the injector flow rate sensitivity to injector stroke variations. 2.3.2. Single-fluid high-pressure swirl injector The development of the spray from a GDI injector may be divided into discrete stages. The first is the initial atomization process that occurs at or near the injector exit. This is mainly dependent on the injector design factors such as nozzle geometry, opening characteristics, and the fuel pressure [275,289]. The second stage of spray development is the atomization that occurs during the spray penetration process, which is dominated by the interaction of the fuel droplets with the surrounding airflow field. The spray cone angle is an important parameter that is nominally determined by the injector design; however, in the actual application the spray cone angle of a swirl injector also varies with the in-cylinder air density and, to a lesser degree, with the fuel injection pressure [78–81]. With the pressure-swirl
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Automotive spark-ignited direct-injection gasoline engines

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