Automotive spark-ignited direct-injection gasoline engines

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484 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 45. Effect of the spray characteristics on UBHC emissions for early injection [165]: (a) PDA velocity vector plots of two injectors; and (b) effect of spray characteristics on early-injection UBHC emissions. effective in increasing the mixing rate at high ambient temperature and pressure. This effect can only be observed when the injection pressure is higher than 10 MPa. When the in-cylinder pressure is higher than the fuel saturation pressure and the in-cylinder temperature is high, which is the stratified charge case, no significant difference in the evaporation rate can be expected for fuels with different saturation vapor pressures. Unintended fuel impingement on the combustion chamber surfaces, either the cylinder wall or the piston crown or both, is often observed in current GDI engines. Nonoptimal fuel injection strategies may lead to fuel wetting of the cylinder wall, leading to the occurrence of gasoline being scraped off by the moving piston, and transported into the oil pan [170–172]. This can degrade the oil very quickly. Oil dilution resulting from the gasoline impingement on the cylinder wall is one of the factors to be investigated during GDI development. The key factors that affect the amount of fuel impacting the cylinder wall are: • spray-tip penetration curves for sac and main sprays; • injector spray characteristics (cone angle, mean drop size, sac volume); • mounting location and orientation of spray axis; • combustion chamber geometry (piston bowl, dam or baffle); • in-cylinder charge motion, especially during injection; • engine operating conditions (in-cylinder air density and temperature); • fuel injection strategies (injection timing, injection pressure). Engineers at Toyota [170] investigated the extent of oil
dilution in the GDI D-4 engine for a range of engine operating conditions. The experimental results from this study are summarized in Fig. 46. As shown in Fig. 46(a), a significant oil dilution was measured in the GDI D-4 engine at high load, but was not observed in a conventional PFI engine. For late injection corresponding to stratified charge operation, a similar degree of oil dilution was observed in both the PFI and GDI engines. For this operating point, fuel is trapped inside the piston bowl and fuel wetting of the cylinder wall is effectively avoided. For a similar reason, earlier injection timing seems to be effective in limiting the oil dilution, as shown in Fig. 46(c). It was also found that the oil temperature, coolant temperature and engine operating time are important parameters in minimizing the oil dilution. Closing the SCV valve was found to be another effective way of reducing the oil dilution, due to the fact that the particular air–fuel pattern introduced by the SCV affects the amount of liquid fuel that impinges on the cylinder wall. The use of a solid-cone spray was also reported to be useful in reducing the oil dilution. This was attributed to the low fuel mass fraction at the periphery of the spray as compared to that of a hollow-cone spray. In general, the Toyota GDI combustion system with a side-mounted injector and the spray penetration associated with a very high injection pressure (up to 13 MPa) was expected to experience more cylinder wall fuel wetting, especially for early injection. However, vehicle test results indicate that the overall level of oil dilution in the field is similar to that observed for PFI vehicles. This is quite different from what has been observed in the dynamometer tests. The rapid rise in the engine oil and the coolant temperatures in the vehicle were considered to be the reason. In summary, the preparation of the fuel–air mixture is one of the most important processes in ensuring a successful GDI combustion system. The spray–air–wall interaction and the spray-induced air motion all must be well examined in order to optimize the mixture formation process. Some of the guidelines for an optimal mixture preparation strategy are summarized as follows. • Spray characteristics: F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 485 ◦ appropriate cone angle to insure good air utilization for early injection; ◦ appropriate fuel mass distribution within the cone to avoid fuel wetting of the cylinder wall and the piston crown outside of the bowl; ◦ appropriate penetration characteristics to insure good air utilization for early injection while avoiding wall impingement; ◦ small sac volume and no after-injection. • Optimized spray-axis angle to achieve a combustible mixture at the spark plug gap while avoiding wall impingement. • Injection timing for the early injection, homogeneous operating mode should be advanced to take full advantage of in-cylinder charge cooling. • Spray should “chase” the piston to minimize spray/ piston-crown impingement during early injection. • Injection timing for the stratified-charge operating mode: ◦ timing as retarded as possible to minimize excessive fuel diffusion; ◦ timing advanced enough to enable reliable ignition; ◦ sufficient injection rate and low-pulse-width stability to reliably inject small fuel quantities. • Optimized combustion chamber geometry. • Optimized combination of the above parameters. 4. Combustion process and control strategies 4.1. Combustion chamber geometry The location and orientation of the fuel injector relative to the ignition source are critical geometric parameters in the design and optimization of a GDI combustion system. During high-load operation, the selected injector spray axis orientation and cone angle must promote good fuel– air mixing with the induction air in order to maximize the air utilization. For late injection, the spark plug and injector locations should ideally provide an ignitable mixture cloud at the spark gap at an ignition timing that will yield the maximum work from the cycle for a range of engine speeds. In general, no single set of positions is optimum for all speed-load combinations; thus the positioning of the injector and spark plug is nearly always a compromise. In extending the time-honored considerations for designing PFI engine combustion chambers to the design of GDI combustion chambers, a number of additional requirements must be satisfied. To minimize the flame travel distance, and to increase the knock-limited power for a specified octane requirement, a single spark plug is generally positioned in a near-central location. As with PFI systems, this location usually provides the lowest heat losses during combustion. As pointed out by Jackson et al. [173], spark plug eccentricity should be less than 12% of the bore diameter for achieving a low octane requirement. For a majority of the concepts, the only constant factor in the variety seems to be the central position of the spark plug in the cylinder head [174]. The use of two spark plugs may also be a possible choice for increasing the probability of effective ignition, however, two ignition sources contribute to already difficult packaging problems. The main reasons for a near-centrallymounted spark plug are to obtain a symmetric flame propagation, to maximize the burn rate and specific power, and to decrease the heat losses and autoignition tendency. Once a near-central location has been specified for the spark plug, numerous factors must be taken into account in positioning and orienting the fuel injector. The location selection process can be aided by laser diagnostics
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