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Automotive spark-ignited direct-injection gasoline engines

Automotive spark-ignited direct-injection gasoline engines

464 F. Zhao et al. /

464 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 25. Schematic of the air entrainment into the spray at an ambient pressure of 0.1 MPa [50,51]. by Salters et al. [98] inside a firing engine having a fourvalve 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. It is well established that different GDI operating modes require different spray characteristics for optimum performance. For homogeneous operation, a spray with a wide cone angle is generally optimum; whereas a narrowercone spray may be effective in creating a highly stratified charge. However, these general guidelines may be less applicable for complex GDI combustion systems. The compromise between spray penetration and cone angle needs to be considered in optimizing air utilization. Engineers at Nissan [27,99,100] and at Hitachi [101] utilized the initial sac volume spray to meet these two conflicting cone angle requirements. The spray tends to collapse to a narrower cone when injected into elevated ambient air densities corresponding to late injection. It was noted that this tendency becomes more pronounced as the fuel quantity in the sac spray is increased, resulting in a further reduction in cone angle. Fig. 28(a) shows the difference of the collected mass for two sprays with different sac volumes. The initial center spray was defined on the basis of the amount of fuel collected within an angle of 20 from the spray center by a 37-ring patternator. The 5% COV limit map between ignition timing and injection end timing is illustrated in Fig. 28(b). It is claimed that increasing the quantity of fuel in the initial sac spray improves engine combustion stability and that the region of stable combustion becomes wider. For those reasons, an injector with a relatively large cone angle (70) and an appropriate quantity of initial center spray was developed in order to accommodate the requirements of both homogeneous and stratified operations. However, the possible increase in overall mean droplet size and engine UBHC emissions with increasing sac volume must be carefully evaluated for a particular application before invoking this strategy. 2.3.4. Air-assisted injection A significant number of references exist for the application of air-assisted injectors to gasoline direct-injection in two-stroke engines [102–120,317] and PFI engines [38,82,121–124,374]. Unfortunately, the number dealing with four-stroke DI gasoline engines is much more limited [71,72,125–129], although it is important to note that a large portion of the basic information is applicable to both types of engines. The majority of air-assisted GDI injectors utilize outwardly opening poppet valves, whereas the majority of single-fluid swirl injectors are inwardly opening. The airassisted injectors also use two solenoids per injector, although it should be noted that there is an increasing use of two solenoids on single-fluid swirl injectors to enhance opening and closing characteristics. A further point of information is that injector designs that rely on a poppet cracking pressure tend to exhibit some injection rate variations due to lift oscillations, and tend to exhibit poppet bounce on closure. Each injector design should be evaluated for these tendencies. Air-assisted fuel injection systems provide an interesting alternative for future GDI applications and this option is receiving increased attention among four-stroke GDI developers [72,73]. Air-assisted GDI injectors are being evaluated for several four-stroke GDI applications, and a schematic of the performance of an Orbital GDI injector is illustrated in Fig. 29. As shown, for injection

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 465 Fig. 26. Spray characteristics of a high-pressure swirl injector at an injection duration of 1.06 ms and an injection pressure of 4.83 MPa [96]: (a) SMD at different time scales; (b) obscuration characteristics of the spray at different time scales; and (c) transient characteristics of the spray volume flux.

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