Automotive spark-ignited direct-injection gasoline engines

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462 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 22. Spray-tip penetration as a function of the injection duration and fuel-rail pressure [95]: (a) spray-tip penetrations of the initial and main sprays as a function of injection duration; and (b) effect of the fuel rail pressures on spray-tip penetration for different loads. chamber filled with tracer particles of polymer microballoons. The airflow structure was mapped by the trajectories of the tracer particles, which are shown in Fig. 25. It was found that an intense, turbulent airflow field is generated at the center of the hollow cone due to the movement of the fuel spray. 2.3.3. Effect of injector sac volume The sac volume within the injector tip between the pintle sealing surface and the tip discharge orifice plays an important role in the transient spray formation process because it contributes to the formation of large droplets at the initiation of fuel injection and can influence the spray cone dynamics. The sac volume in any particular injector design is the geometric space within the injector tip that contains fuel which is not at the fuel line pressure. Fuel remaining from the previous injection resides in this space, and as it is downstream from the pintle sealing line, this stagnant fuel is not at the rail pressure. When the pintle is first lifted, this portion of liquid fuel does not have enough tangential velocity to form a hollow cone spray, and thus is generally injected directly along the injector axis as poorly atomized droplets having relatively high velocities. This is denoted as the sac spray, but descriptive terms such as sling spray, core spray and center spike may also be encountered in the literature. Thus, the sac and main sprays in most GDI injectors are actually two distinct sprays, each having its own drop size distribution. Therefore, a large sac volume can significantly
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 463 Fig. 23. Schematic and spray characteristics of the Toyota GDI injector [21]: (a) nozzle geometry; and (b) spray atomization characteristics. degrade the mixture formation process and can contribute to an increase in UBHC emissions, especially for light-load, stratified-charge operation in which the sac volume may constitute a major fraction of the fuel required. Parrish and Farrell [96] characterized the spray structure of a high-pressure GDI injector for an ambient pressure of 1 atm using laser diffraction and backlighting techniques. In addition to particle size measurements, obscuration measurements were performed. These measurements determined the ratio of unscattered laser light intensity with and without a spray sample, with large values of obscuration indicating optically dense sprays. From the spray visualization, a clear leading mass, or slug, was observed along the center line of the spray. It is generally accepted that this initial slug is comprised of fuel that was in the sac volume. For the fuel pressure range of 3.45–6.21 MPa, the initial axial velocity of the leading mass near the injector tip was in the range of 68–86 m/s, which is higher than the velocity of 45–58 m/s for the main body of the spray. Both the leading-mass and main-spray velocities decrease rapidly with time. Fig. 26(a) shows the SMD measurements with two measurement time scales. The shorter time scale is used to show the initial spray characteristics while the longer time scale is useful to indicate the entire development process of the transient spray. The droplet size measurements in Fig. 26(a) show that the SMD initially rises rapidly as the leading Fig. 24. Effect of the fuel rail pressure on the fuel economy improvement of the Toyota GDI D-4 engine [21]. mass of fuel first enters the measurement location of 38.75 mm from the injector tip along the center line of the spray. A peak SMD value is recorded for this initial sacvolume slug before the instantaneous mean droplet size decreases rapidly for the main spray body. This can also be observed from the obscuration characteristics as a function of time in Fig. 26(b). The result from the transient patternator measurements of the spray volume flux is shown in Fig. 26(c). It is clear that as the closing time is retarded, more fuel is collected. Part of the leading mass is captured in the 1.5 ms closing time curve. At a closing time of 2 ms, the outer regions of the spray begin to be collected, and for a 4 ms closing time the mass collected off-axis is larger than that collected on-axis. It was also found that short injection duration concentrates more fuel mass in the center than is achieved for longer durations. Wirth et al. [97] measured the droplet size distributions during the entire spray development process. Fig. 27 shows the typical droplet size distributions for a swirl-type GDI injector at ambient conditions, corresponding to a mediumload injection quantity. It was reported that, in the first phase of the injection process, a narrow cone jet associated with the sac volume penetrates with high velocity but poor atomization. The initial quantity of fuel is injected with very low swirl, resulting in larger droplets [35]. It was found that the acceleration of the swirl inside the injector swirl cavity leads to the transient evolution of the initially hollow spray cone. The sac volume quantity affects the spray plume geometry even in the later phases of the injection process when a stable cone is established at the injector nozzle. The intermediate phase of spray development clearly shows the turbulent spray cone breakup in a distance of approximately 25 mm from the nozzle exit. After the end of injection, a well-distributed plume with a low penetration is present with slightly increasing droplet sizes due to slightly degraded atomization at pintle closure and possible droplet coalescence within the spray plume. The initial fuel slug or sac spray associated with the sac volume of a high-pressure swirl injector was also observed
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Automotive spark-ignited direct-injection gasoline engines

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