Automotive spark-ignited direct-injection gasoline engines

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442 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 2. Comparison of the fuel quantity required to start GDI and PFI engines at different ambient temperatures [41]. whether intentional or unintentional, does introduce the important variable of transient wall film formation and evaporation. The GDI concept does indeed offer many opportunities for circumventing the basic limitations of the PFI engine, particularly those associated with port wall wetting. The fuel film in the intake port of a PFI engine acts as an integrating capacitor, and the engine actually operates on fuel inaccurately metered from the pool in the film, not from the current fuel being accurately metered by the injector [38]. During a cold start, the fuel from more than 10 cycles must be injected to achieve a steady, oscillatory film of liquid fuel in the intake port. This means that the cold PFI engine does not fire or start on the first few cycles, even though fuel is being repetitively injected into the film pool. Control algorithms must be used to provide significant over-fueling if acceptable PFI start times are to be achieved, even though the catalyst temperature is below the light-off threshold at this condition and UBHC emissions will be increased. Thus it is not unusual for PFI systems to generate 90% of the total UBHC emissions in the US FTP emission test within the first 90 s [39]. The direct injection of gasoline into the cylinder of a fourstroke, gasoline, spark-ignition engine eliminates the integrating fuel film in the intake port. It is well established that the direct injection of gasoline with little or no cold enrichment can provide starts on the second cranking cycle [40], and can exhibit significant reductions in UBHC spikes during load transients. An excellent example of the comparison of the fuel quantity required to start GDI and PFI engines is provided in Fig. 2 [41]. It is quite evident that the GDI engine requires much less fuel to start the engine, and that this difference in the minimum fuel requirement becomes larger as the ambient temperature decreases. Another limitation of the PFI engine is the requirement of throttling for basic load control. Even though throttling is a well-established and reliable mechanism of load control in the PFI engine, the thermodynamic loss associated with throttling is substantial. Any system that utilizes throttling to adjust load levels will experience the thermodynamic loss that is associated with this pumping loop, and will exhibit thermal efficiency degradation at low levels of engine load. Current advanced PFI engines still utilize, and will continue to require, throttling for basic load control. They also have, and will continue to have, an operating film of liquid fuel in the intake port. These two basic PFI operating requirements represent major impediments to achieving significant breakthroughs in PFI fuel economy or emissions. Continuous incremental improvements in the older PFI technology will be made, but it is unlikely that the long-range fuel economy and emission objectives can be simultaneously achieved. The GDI engine, in theory, has neither of these two significant limitations, nor the performance boundaries that are associated with them. The theoretical advantages of the GDI engine over the contemporary PFI engine are summarized as follows, along with the enabling mechanism: • Improved fuel economy (up to 25% potential improvement, depending on test cycle resulting from: ◦ less pumping loss (unthrottled, stratified mode); ◦ less heat losses (unthrottled, stratified mode); ◦ higher compression ratio (charge cooling with injection during induction); ◦ lower octane requirement (charge cooling with injection during induction); ◦ increased volumetric efficiency (charge cooling with injection during induction); ◦ fuel cutoff during vehicle deceleration (no manifold film). • Improved transient response. ◦ less acceleration-enrichment required (no manifold film). • More precise air–fuel ratio control. ◦ more rapid starting; ◦ less cold-start over-fueling required. • Extended EGR tolerance limit (to minimize the use of throttling). • Selective emissions advantages. ◦ reduced cold-start UBHC emissions; ◦ reduced CO 2 emissions. • Enhanced potential for system optimization. The significantly higher injection pressures used in common-rail GDI injection systems as compared to PFI fuel systems increase both the degree of fuel atomization and the fuel vaporization rate, and, as a result, it is possible to achieve stable combustion from the first or second injection cycle without supplying excess fuel. Therefore, GDI engines have the potential of achieving cold-start UBHC emissions that can approach the level observed for steady
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 443 running conditions. Takagi [23] reported that the cold-start UBHC emissions obtained with the Nissan prototype GDI engine are approximately 30% lower than that of an optimized PFI engine under comparable conditions. Another potential advantage of the GDI engine is the option of using fuel cutoff on deceleration. If implemented successfully, fuel cutoff can provide additional incremental improvements in both fuel economy and engine-out UBHC emission levels. For the PFI engine, which operates from an established film of fuel in the intake port, the cutoff of fuel during vehicle deceleration is not a viable option, as it causes a reduction or elimination of the liquid fuel film in the port. This generates very lean mixtures in the combustion chamber for a few cycles following the restoration of the load, generally resulting in an engine misfire. It should be noted that design engineers, managers and researchers who must evaluate and prioritize the published information on the advantages of GDI engines over PFI engines should be aware of one area of data comparison and reporting that is disconcerting. In many papers the GDI performance is compared to PFI baselines that are not well defined, thus making it very difficult for the reader to make a direct engineering comparison between GDI and PFI performance. One extreme example is the comparison of GDI and PFI fuel economy data that was obtained using two different vehicles with two different inertial weights. An example of a more subtle difference is the evaluation of the BSFC reduction resulting from the complete elimination of throttling in a GDI engine, but not noting or subtracting the parasitic loss of a vacuum pump that would have to be added for braking and other functions. A number of published comparisons lie between these two extremes. The readers are cautioned to review all claims of comparative GDI/PFI data carefully as to the precise test conditions for each, and the degree to which the systems were tested under different conditions or constraints. PFI engines do have some limited advantages over GDI engines due to the fact that the intake system acts as a prevaporizing chamber. When fuel is injected directly into the engine cylinder, the time available for mixture preparation is reduced significantly. As a result, the atomization of the fuel spray must be fine enough to permit fuel evaporation in the limited time available between injection and ignition. Fuel droplets that are not evaporated are very likely to participate in diffusion burning, or to exit the engine as UBHC emissions. Also, directly injecting fuel into the engine cylinder can result in unintended fuel impingement on the piston or the cylinder wall. These factors, if present in the design, can contribute to levels of UBHC and/or particulate emissions, and to cylinder bore wear that can easily exceed that of an optimized PFI engine. Some other advantages of PFI engines such as lowpressure fuel system hardware, higher power density at part load and the feasibility of using three-way catalysis and higher exhaust temperatures for improved catalyst efficiency present an evolving challenge to the GDI engine. Although the GDI engine provides important potential advantages, it does have a number of inherent problems that are similar to those of the early DISC engines. The replacement of the PFI engine by the GDI engine as the primary production automotive powerplant is constrained by the following areas of concern: • difficulty in controlling the stratified charge combustion over the required operating range; • complexity of the control and injection technologies required for seamless load changes; • relatively high rate of formation of injector deposits and/ or ignition fouling; • relatively high light-load UBHC emissions; • relatively high high-load NOx emissions; • high local NOx production under part-load, stratifiedcharge operation; • soot formation for high-load operation; • increased particulate emissions; • three-way catalysis cannot be utilized to full advantage; • increased fuel system component wear due to the combination of high-pressure and low fuel lubricity; • increased rates of cylinder bore wear; • increased electrical power and voltage requirements of the injectors and drivers; • elevated fuel system pressure and fuel pump parasitic loss. The above concerns must be addressed and alleviated in any specific design if the GDI engine is to supplant the current PFI engine. If future emission regulations such as the ultra-low-emission-vehicle (ULEV), the super ultralow-emission-vehicle (SULEV), and corporate average fuel economy (CAFE) requirements can be achieved using PFI engines without the requirement of complex new hardware, the market penetration rate for GDI engines will be reduced, as there will most assuredly be a GDI requirement for sophisticated fuel injection hardware, a high-pressure fuel pump and a more complex engine control system. An important constraint on GDI engine designs has been relatively high UBHC and NOx emissions, and the fact that three-way catalysts could not be effectively utilized. Operating the engine under overall lean conditions does reduce the engine-out NOx emissions, but this generally cannot achieve the minimum 90% reduction level that can be attained using a three-way catalyst. Much work is underway worldwide to develop lean-NOx catalysts, but at this time the attainable conversion efficiency is still much less than that of three-way catalysts. The excessive UBHC emissions at light-load also represent a significant research problem to be solved. In spite of these concerns and difficulties, the GDI engine offers an expanded new horizon for future applications as compared to the well-developed PFI engine. In summary, the potential advantages of the GDI concept are too significant to receive other than priority status. The concept offers many opportunities for achieving significant improvements in engine fuel consumption, while simultaneously realizing large reductions in engine-out UBHC
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