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đây em nhờ các bác!mong các bác đừng làm em thất vọng!
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 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.
còn đoạn này có từ slug mong các bác giúp đỡ thêm!
The initial fuel slug or sac spray associated with the sac volume of a high-pressure swirl injector was also observed by Salters et al. [98] inside a firing engine having a four valve Pentroof head and a centrally mounted injector. It was found that this slug, consisting of relatively larger droplets with high velocity, penetrates tens of millimeters prior to the formation of the main spray cone, and impacts directly on the piston crown for early injection. An important point is that observations for an injection timing of 80 ° ATDC on the intake stroke confirmed that there is no major impingement of the fuel spray upon the piston except for this initial slug of fuel from the sac volume. For earlier injection timings, however, a large portion of the main spray directly impacts the piston crown.
