512 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 80. Mechanism of injector deposit formation [214]. • proximity of bulk fuel inside injector to injector tip area; • <strong>gasoline</strong> additives to inhibit cumulative deposit formation; • special coating or plating of tip surfaces; • internal and external surface finish of the nozzle tip; • operating cycle, including hot-soak intervals (to be verified by future research). The discussion above has dealt mainly with preventing or minimizing the formation of deposits, but there is another philosophy that can be invoked in parallel, although only by injector manufacturers. An injector can be designed with the important goal of minimizing the influence of deposits on the resulting flow rate and spray. This is known as an inherent tolerance or robustness to the presence of injector deposits. It is desirable, of course, to reduce the basic rate of deposition on the injector during the operation of the GDI engine. But, bearing in mind the inevitability of some deposition occurring with time, it is also desirable to have an injector flow-path design that exhibits a good tolerance to any deposits that do form. For example, 100 mg of deposits may form in 100 h of operation on two different injector designs, which is the same average rate of deposition. However, one injector design may exhibit a 3% flow reduction for this weight of deposits, while the flow in the second is reduced by only 1%. In this regard, some injector designs using swirl have proven to be quite sensitive to small amounts of deposits, particularly in terms of spray-symmetry degradation. Finally, it is fair to note that a reasonable portion of the GDI deposit problem results from the lack of proven and effective GDI anti-deposit additives in the fuel supply [337]. The fuel qualities are, of course, different in Europe, Japan, and North America, and the <strong>gasoline</strong> sulfur content in North America may contribute to differing levels of deposit problems in those markets. The additives that are currently in the <strong>gasoline</strong> supply in North America were developed and improved mainly over the time period from 1984 to 1993 to minimize the rates of port-deposit and intake-valve-deposit formation. In the early development of PFI systems, injector deposits were a very significant problem that had to be alleviated [38,212], and this will have to be done for GDI systems [216,217]. The dilemma is that with no GDI vehicle currently in production in North America, there is no strong incentive to develop and blend new fuel additives for this application. Even if such GDI additives were proven and available for reducing GDI injector deposits, it will have to be carefully verified that adding them to the North American fuel supply would not adversely affect the formation of deposits in the 100 million PFI vehicles now in operation. 5. Fuel economy and emissions 5.1. Fuel economy potential A current strategic objective for the automotive application of the four-stroke, <strong>gasoline</strong> engine is a substantial improvement in fuel consumption while meeting the required levels of pollutant emissions and engine durability. The improvement of passenger car fuel economy represents a very important goal that will determine the future use of SI <strong>engines</strong> instead of the small, high-speed, diesel <strong>engines</strong> [45,310,332,344–346,350,351,363]. The thermal efficiency of the GDI engine can be enhanced by increasing the compression ratio, or by using a lean mixture, thereby reducing the throttling losses and wall heat losses. As outlined conceptually by Karl et al. [193], the GDI engine using charge stratification offers the potential for reducing the part-loaded fuel consumption by 20–25% when the gas cycle, heat transfer and geometric configuration are optimized. Fig. 81(a) shows a computed balance of energy at 2000 rpm and 0.2 MPa BMEP. In this calculation the engine output and friction losses are maintained constant as the equivalence ratio is varied. In this study, the engine is operated in the homogeneous mixture mode at equivalence ratios of l ˆ 1:0–1:3; and in the stratified-charge mode at l ˆ 1:6–3:4 with decreasing amounts of throttling. It is reported that the exhaust energy and the wall heat transfer are reduced by 10.8 and 12.5%, respectively, even though the air mass is increased. As a result, the predicted improvement in fuel consumption is 23%. This considerable increase in
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 513 Fig. 81. Estimation of the energy balance for the GDI system under various degrees of charge stratification at 2000 rpm and 0.2 MPa BMEP [193]: (a) analyzed energy balance; (b) change of individual energies; and (c) change of individual energies.
Change language