Automotive spark-ignited direct-injection gasoline engines

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510 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 77. Photograph of the appearance of a GDI injector with deposits after 30 h of engine operation [214]. at a location under the intake port. GDI combustion systems such as those of Mercedes-Benz, Isuzu and Ford that use a centrally mounted injector would be expected to yield 10– 15C higher injector tip temperatures than would be obtained with the Mitsubishi, Nissan, or Toyota system, in which the injector is located far from the exhaust valves and derive additional tip cooling from the intake air. Jackson et al. [195] reported that the upper limit for tip temperature of current GDI injectors is in the range of 150–200C. Twodimensional finite element analysis (FEA) and analytical calculations showed that the higher flame heat fluxes occur at the center of the combustion chamber. For a central injector the best location for minimum tip temperature is to be as close as possible to the intake valve seats. Fig. 76 shows the injector surface temperature as measured in the Mitsubishi GDI engine during the continuous high-speed full-load operation and a subsequent hot soak period [50]. It was reported that the injector tip temperature of the Mitsubishi GDI engine is not significantly different from that of the PFI injector. It was also claimed that no deposit problems were encountered with the tip temperature history shown in Fig. 76 during the durability test because the soot, lubricating oil or the deposit are washed away by the highpressure gasoline jet. Some deposit formation has been noted for combustion systems that use a centrally mounted injector, where full-load tip temperatures of more than 150C may be achieved. Engineers at Toyota [41] reported that the early injector deposit problem of the Toyota D-4 engine was alleviated by maintaining the injector tip temperature always below 150C, and using a special organic material coating on the injector tip. Kinoshita et al. [214] investigated the injector deposit formation mechanism using a Toyota D-4 engine. The injector deposit was classified into: soot deposited on the nozzle and needle surface; and fuel polymerized by thermal decomposition to form a gum-type deposit inside the nozzle. The outward appearance of an injector that contains significant deposits is shown in Fig. 77. An engine dynamometer test showed that the deposit effects were first evident after 2 h and that the flow rate continued to decrease during the first 8 h, after which no significant further decrease in the flow rate was observed. Fig. 78 shows a comparison of the nozzle Fig. 78. Photographs of a GDI injector nozzle for different dynamometer test hours [214]: (a) new injector; (b) 4 h of dynamometer testing; and (c) 8 h of dynamometer testing. hole appearance before and after the test. It was found that deposits were initially formed at the nozzle exit and progressed into the internal surfaces of the nozzle. The internal injector deposit occurred at the position where fuel resides after the end of each injection. Both the nozzle temperature and fuel distillation characteristics were found to significantly dominate the deposit buildup. Fig. 79 shows the effect of nozzle tip temperature on the injector flow rate reduction for a range of fuel types. As shown in Fig. 79(a), except for fuel E, all the other blended fuels have the same T90, which is the distillation temperature at which 90% of the fuel is vaporized. When comparing the blended fuels A and D in Fig. 79(b), it is evident that the flow rate is diminished significantly as the nozzle temperature exceeds T90. Even though the blended fuels A and E were tested at the same injector tip temperature, the flow rate reduction with fuel E is much less than that of fuel A, due to its high T90
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 511 Fig. 79. Effects of the nozzle-tip temperature and fuel T90 point on injector deposit formation [214]: (a) engine operating conditions; (b) engine dynamometer testing conditions; and (c) effect of the nozzle-tip temperature on injector flow rate reduction. point. A schematic explaining the mechanism of the injector deposit formation is illustrated in Fig. 80. When the injector tip is subjected to elevated temperatures, thermal decomposition of the fuel may occur inside the nozzle, where deposit precursors are formed. When the nozzle tip temperature is lower than T90, most of the fuel will stay as liquid even though some of the fuel may evaporate before the next injection event occurs. The continued presence of liquid fuel insures that the deposit precursors will be easily washed away during the next injection event. However, when the nozzle tip temperature exceeds the T90 of the tested fuel, most of the fuel will vaporize after the end of fuel injection. As a consequence, the deposit precursors are distributed on, and may attach to, the nozzle surface. This makes it more difficult to wash away all the deposit precursors during the next injection event. Eventually deposits may accumulate inside the nozzle. The theory suggests that it is important to retain some of the liquid fuel inside the nozzle between the injection events in order to prevent deposit buildup. Based upon this investigation, it was found that maintaining the injector tip temperature below the T90 point of the fuel being utilized is a critical factor in minimizing the formation and accumulation of deposits. Mao [215] studied the carbon formation process inside a tube in order to understand the deposit formation process inside a gas turbine fuel injector. Some of the findings are also useful for understanding the deposit formation process in a GDI injector. It was found that the wall temperature appears to be the most significant parameter in determining the carbon deposit rate, with the fuel flow rate ranked second, and the fuel inlet temperature ranked third in importance. It was noted that the variation of deposit rate depends on whether the test apparatus provides a constant heat flux or a constant wall temperature in the test tube. It was found that the deposition rate remained approximately constant for wall temperatures below certain threshold values. As the wall temperature exceeded this value, the deposit rate increased sharply with increasing wall temperature. This suggests that the controlling mechanism for deposit formation changed, depending on the wall temperature. This agrees well with the investigation of injector deposit formation using the Toyota GDI engine [214]. It was also found that the deposit rate exhibits a maximum as a function of fuel flow velocity. The fuel inlet temperature was found to be not as significant as the wall temperature, but it had a noticeable effect on deposit rates when wall temperatures were high. Experimental results indicated that, initially, the deposit rate decreased with increasing fuel temperature. After reaching a minimum value, the rate of deposition begins to increase with a further increase in fuel temperature. It is important to note that the interior wall material, the surface finish and surface coating all exhibit a significant influence on deposit formation. A fine micro-finish of the surface was found to significantly reduce the deposit rate, which, however, requires complex and time-consuming manufacturing operations. Surface coating was found to be able to delay the onset of deposition. But once a deposit layer is formed, the coated and uncoated surfaces exhibit very little difference in carbon deposition. It was also reported that the fuel sulfur content plays an important role during the initial process of deposit formation. The concentration of sulfur was relatively small in the case of heavy deposits when compared with those cases with moderate or small carbon deposits. It was speculated that the interfacial tension between the fuel deposit and the solid surface might be a key factor. Much remains to be learned with regard to the surface chemistry of GDI deposit precursors and deposit formation. The following items represent important considerations in minimizing the formation of GDI injector deposits: • injector tip temperature (maintained at less than 145C); • injector protrusion distance into chamber; • heat path from injector body to engine coolant passages; • air velocity variation at the injector tip during an engine cycle;
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Automotive spark-ignited direct-injection gasoline engines

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