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Automotive spark-ignited direct-injection gasoline engines

Automotive spark-ignited direct-injection gasoline engines

446 During the engine

446 During the engine cold crank and start, the high-pressure fuel pump generally cannot deliver fuel at the full design pressure due to the short time available and the low engine cranking speed. To solve this priming-time problem, the Mitsubishi GDI engine uses an in-tank fuel feed pump that is similar to that used in PFI engines. A bypass valve is used to allow the fuel to bypass the high-pressure regulator for engine crank and start. As a result, the electric feed pump supplies the fuel directly to the fuel rail. As the engine speed and fuel line pressure increase, the bypass valve is closed and the high-pressure regulator begins to regulate the fuel pressure to 5.0 MPa. As a result, it is reported that this engine can be started within 1.5 s for both cold and hot restart conditions [50,51]. Shelby et al. [52] visualized the spray structure of two Zexel GDI injectors with cone angles of 20 and 60 when operating at a cranking fuel pressure of 0.3 MPa. It was found that both injectors are still able to develop the hollow cone structure under such a low fuel injection pressure, however the atomization is noticeably degraded. It was reported that the injection duration required for a stoichiometric mixture is approximately four times as long as when operating from the main fuel pump, as the injection duration required is proportional to the square root of the fuel injection pressure. Based upon a review of the literature, the key points of information related to GDI fuel systems are: • 4.0–13.0 MPa have been utilized as fuel rail design pressures; • a fuel rail pressure of from 5.0 to 7.5 MPa is most common in current GDI systems; • 7.0–10.0 MPa is the most likely design rail pressure range for future systems; • pump life, noise and system priming rate are important concerns, particularly at pressures above 8.5 MPa; • variable injection pressure is a viable system strategy. A wide range of gasoline, having variable alcohol content, sulfur content and driveability indices are available in the field. This quality range makes it a necessity that a robust fuel system be developed and proven for GDI production engines [296]. To avoid dilution of the engine oil, most GDI fuel pumps are lubricated with gasoline [373]. However, gasoline has a lower lubricity, viscosity, and a higher volatility than diesel fuel, generally resulting in higher friction and a greater potential for wear and leakage as compared to diesel fuel pumps. It should be noted, however, that hydrodynamic lubrication may be used at high fuel pressures to compensate for the low viscosity. 2.2. Fuel injector considerations F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 The fuel injector is considered to be the most critical element in the GDI fuel system, and it should have the following attributes. Many of the required characteristics of the GDI injector are equivalent to those of the port fuel injector, which are [38]: • accurate fuel metering (generally a ^2% band over the linear flow range); • desirable fuel mass distribution pattern for the application; • minimal spray skew for both sac and main sprays; • good spray axisymmetry over the operating range; • minimal drippage and zero fuel leakage, particularly for cold operation; • small sac volume; • good low-end linearity between the dynamic flow and the fuel pulse width; • small pulse-to-pulse variation in fuel quantity and spray characteristics; • minimal variation in the above parameters from unit to unit. In certain critical areas the specification tolerances of a GDI injector design are more stringent than that of the port-fuel injector, or there are requirements that are not present for PFI applications. These areas are: • significantly enhanced atomization level; a smaller value of spray mean drop size; • expanded dynamic range; • combustion sealing capability; • avoidance of needle bounce that creates unwanted secondary injections; • reduced bandwidth tolerance for static flow and flow linearity specifications; • more emphasis on spray penetration control; • more emphasis on the control of the sac volume spray; • enhanced resistance to deposit formation; • smaller flow variability under larger thermal gradients; • ability to operate at higher injector body and tip temperatures; • leakage resistance at elevated fuel and cylinder pressures; • zero leakage at cold temperature; • more emphasis on packaging constraints; • flexibility in producing off-axis sprays in various inclined axes to meet different combustion system requirements. The GDI injector should be designed to deliver a precisely metered fuel quantity with a symmetric and highly repeatable spray geometry, and must provide a highly atomized fuel spray having a Sauter mean diameter (SMD) of generally less than 25 mm, and with a droplet diameter corresponding to the cumulative 90% volume point (DV90) not exceeding 45 mm [53–55]. The SMD is sometimes denoted more formally in the spray literature as D32. The DV90 statistic is a quantitative statistical measure of the largest droplets in the total distribution of all droplet sizes. Smaller values than these are even more beneficial, provided sufficient spray penetration is maintained for good air utilization. The fuel pressure required is at least 4.0 MPa for a single-fluid injector, with 5.0– 7.0 MPa being more desirable if the late-injection stratified mode is to be invoked. Even if successful levels of atomization could be achieved at fuel pressures lower than 4.0 MPa, significant metering errors could result from the variation of

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 447 metering pressure differential …P inj P cyl† with cylinder pressure, although this could be corrected by the engine control system (ECS) if cylinder pressures were monitored. The sac volume within the injector tip is basically the volume of fuel, resulting from the previous injection, which is not at the fuel line pressure; therefore it retards the acceleration of the injected fuel and generally degrades both the fuel atomization and the resulting combustion. There is a consensus that the sac spray influences the air entrainment and the resulting spray cone angle, particularly at elevated ambient densities. In general, the smaller the sac volume, the fewer large peripheral drops that will be generated when the injector opens. Needle bounce on closure is to be avoided, as a secondary injection generally results in uncontrolled atomization consisting of larger droplets of lower velocity or possible unatomized fuel ligaments. Needle bounce on closure also degrades the fuel metering accuracy and contributes to increases in the UBHC and particulate emissions. Needle bounce on opening is not nearly as important as that on closure, but should be controlled. The result of a needle bounce on opening is a small modulation in the injection rate. The ability to provide a high rate of injection, which is equivalent to delivering the required fuel with a short fuel pulse, is much more important for the GDI engine than for the PFI engine, particularly for light-load stratified-charge operation. Therefore, much more significance is attached to the low-pulse-width region of the GDI injector, effectively increasing the importance of the injector dynamic range requirement. Standard measures of injector capabilities are the dynamic (linear) range, the shortest operating pulse width on the linear flow curve and the lowest stable fuel pulse width. Fuel pulse widths that are shorter than the linear range limit can be used in the calibration by means of a lookup table of pulse width if the fuel mass delivery is stable at that operating point. The optimal design of the injector to resist coking is also one of the important requirements of the GDI injector, as is discussed in detail in the section on injector deposits. Sometimes overlooked are the voltage and power requirements of the injector solenoids and drivers. A number of current prototype GDI injectors have power requirements that would be considered unacceptable for a production application. It is also worth noting that it is advantageous to injector packaging to have the body as small as possible. This provides more flexibility in optimizing the injector location and in sizing and locating the engine ports and valves. In spite of decades of continuous development on diesel multi-hole injectors, it has been demonstrated that these nozzle-type injectors are generally poor choices for GDI applications. A multi-hole VCO nozzle used in a GDI engine application results in an unstable flame kernel when ignited by a single fixed spark plug. The rich mixture zones are close to the lean mixture zones; thus the flame front does not propagate uniformly through the combustion chamber. With the multi-hole nozzle, the orientation of the Fig. 4. Schematic of the inwardly opening, single-fluid, highpressure, swirl injector [272]. nozzle hole is an important factor in determining engine combustion performance. It has been found that the hole distribution that is effective in ensuring good spray dispersion and reliable flame propagation between the spray plumes generally provides good engine performance. The effect of the cone angle of individual spray plumes on GDI engine performance was studied by Fujieda et al. [56]. A multi-hole GDI injector similar to that used in diesel engines was designed and tested. It was found that with an increase in the spray cone angle the lean limit is extended, due to the improvement in air utilization. It was also determined that reducing the nozzle flow area and/or increasing the number of holes can achieve the same purpose. Currently, the most widely utilized GDI injector is the single-fluid, swirl-type unit that utilizes an inwardly opening needle, a single exit orifice and a fuel pressure in the range from 5.0 to 10 MPa. A schematic of this type of injector is illustrated in Fig. 4. This general configuration can be regarded conceptually as a multi-hole nozzle with an infinite number of holes, with a uniform distribution of the fuel around the cone circumference being obtained. As a consequence, wall impingement at full load can be minimized for an appropriate injector position and spray cone angle [57]. The needle-type, swirl-spray injector is designed to apply a strong rotational momentum to the fuel in the injector nozzle that adds vectorially to the axial momentum. In a number of nozzle designs, liquid flows through a series of tangential holes or slots into a swirl chamber. The liquid emerges from the single discharge orifice as an annular sheet that spreads radially outward to form an initially hollow-cone spray. The initial spray cone angle may range from a design minimum of 25 to almost 180, depending on the requirements of the application, with a delivered spray SMD ranging from 14 to 25 mm. In the swirl-type injector, the pressure energy is effectively transformed into rotational momentum, which enhances atomization. Moreover, the spray mass distribution of a swirl-type injector is generally more axisymmetric than that obtained without swirl [58,59]. Fig. 5 shows a comparison of the spray characteristics of hole-type and swirl-type nozzles. The reported spray atomization characteristics were obtained by means of laser

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