5 years ago

Automotive spark-ignited direct-injection gasoline engines

Automotive spark-ignited direct-injection gasoline engines

468 F. Zhao et al. /

468 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 29. Schematic and spray characteristics of the air-assisted injector [72]: (a) schematic of the injector design; (b) injected air-to-fuel ratio as a function of the engine load; and (c) effect of the air rail pressure on droplet size. overall mean drop size. However, it is generally agreed that it is more difficult to develop a portfolio of off-axis, angled sprays with this constraint. The off-axis spray is one of the developments that address the packaging of injection systems. Component packaging in GDI design is a primary constraint, and the movement from a 12 mm to a 10 mm to an 8 mm injector mounting diameter is being driven by the need to enhance packaging options. The availability of injectors that deliver fuel sprays that are directed 10, 20 or 30 from the injector mounting axis can offer significant flexibility in the original design configuration of a GDI combustion chamber. The high-pressure, single-fluid, swirl-channel injector using an inwardly opening pintle is by far the most common GDI injector type, and there are intense programs by injector manufacturers to continue to improve injector and spray performance. The non-spray performance parameters of the injector include the opening time, closing time, pintle bounce, durability, dynamic range, noise level, power consumption, leakage and operating pressure range. The spray performance parameters include the mean diameters of the delivered main spray and the sac spray, as well as the corresponding values for DV90. Other key spray parameters include the cone angles of the spray (both the initial angle and the final collapsed spray angle), the spray-deviation (skew) angles of the main and sac-volume sprays, the sac and main tip penetration rates and velocities, the drippage, after-injections or ligament formation upon injector closure and the fuel mass distribution within the spray. Additional measures of performance, both spray and nonspray, are related to injection-to-injection and unit-to-unit variability in all of the above parameters. With such rapid development occurring in GDI injector design and performance, it is obvious that a compilation of best performance values for each of the above parameters, which constitutes a table of best practice performance, is a living document. The injector performance envelope is continually being pushed as new and improved injectors are designed, built and tested. Table 1 contains a tabulation of current best practice for a number of key injector performance parameters. As injectors have a range of design fuel rail pressures, four categories are listed, with each category having current best-practice values for the performance metrics. All of the droplet size statistics presented in the table were obtained by a two-component, phase Doppler, real-time-system analyzer for a very wide range of injector hardwares. All injectors were measured for indolene fuel at 20C using a standard test protocol. 2.3.6. Future requirements of GDI fuel sprays Research by a number of workers in Japan, Europe and the United States indicates that a working design goal for current GDI fuel injection systems should be to achieve a symmetric fuel spray having a SMD of less than 20 mm, with 90% of the injected fuel volume (DV90) in drops smaller than 40 mm, while requiring fuel pressures that are on the order of 5.0–7.5 MPa. This fuel pressure range will generally provide a longer pump life than is obtained using 8.0–13 MPa, with less pump noise and a pressure rise to a greater percentage of

the design rail pressure during crank and start. It may also permit a fuel system without an accumulator to be used. The spray cone angle should ideally become smaller as the required engine air utilization is reduced. This can be achieved mechanically or electrically, or may occur due to the inherent collapse of a swirling cone of droplets that are injected into elevated air densities. For the more stringent emissions and performance requirements of future (2002–2008) GDI vehicles, the fuel spray requirements of future GDI engines set threshold performance limits for the fuel injectors and for the fuel system as a whole. Some of the key requirements for the next generation of injectors (2002–2008) are as follows. • SMD: 16 mm or less (main spray); F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 469 Fig. 30. Comparison of the spray characteristics of air-assisted and single-fluid swirl injectors [72]: (a) droplet size distribution; and (b) spray-tip penetration rate. • SMD: 21 mm or less (sac spray); • DV90: 30 mm or less (main spray); • DV90: 34 mm or less (sac spray); • sac volume of less than 6% of idle fuel delivery; • sufficient spray symmetry to permit injector rotation or rail mounting tolerance without combustion degradation; • narrower spray footprint (cone collapse) for injection into higher ambient density; • a design fuel pressure of 5.0–9.0 MPa; • stable injection at 0.75 ms pulse width; • 0.25 ms or less opening time; • 0.40 ms or less closing time; • no after injection; • reduced power consumption; • reduced noise level.

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