Automotive spark-ignited direct-injection gasoline engines

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526 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 distributed uniformly to individual cylinders at a much lower pressure differential than is typical with PFI engines. Such an increased EGR rate may result in a substantial temperature increase of the intake air, which may degrade the full load performance. It is also quite difficult to provide an appropriate amount of EGR during GDI mode transitions. Fraidl et al. [142] designed a variable EGR distributing system to circumvent the disadvantages of current EGR systems. A distribution plenum in close proximity to the cylinder is designed to be closed by means of a rotary disk valve. The opening of the cylinder feed lines is synchronized with the firing order, with the actual metering occurring within an electrically operated EGR valve. At part load, a large cross-section is opened to each individual cylinder, allowing a high EGR mass flow rate while maintaining an even distribution to all cylinders. At full load, or during engine transients, the EGR system close to the cylinder is shut off, improving the EGR dynamics. For the transition into part load operation, EGR is available close to the cylinder head by opening the distributing plenum. As only a small section of intake manifold is exposed to hot exhaust gas, the temperature increase of the induction air is less than with the conventional central feed design. Even though EGR is widely employed for reducing the NOx emissions in SI engines, there is a general view in the literature that the progress of GDI engines is strongly coupled to the development of lean-NO x catalysts. This is because EGR cannot reduce the NOx emissions over the entire engine speed-load map to meet the scheduled emission limits in North America, Europe and Japan. For example, within the US FTP emission test cycle the GDI engine operates for a significant fraction of the time with a lean homogeneous mixture at a stable lean limit. In such an operating condition, large amounts of EGR cannot be used, as the engine combustion stability would be significantly degraded. Also, the stratified-charge region in the operating map is narrowed when restricted to low NO x levels, thus reducing the achievable fuel economy improvement. Therefore, a proven lean-NOx-catalyst technology would definitely be a welcome tool in order to make optimum use of stratified combustion technology, as the conversion efficiencies and the durability of lean-NOx catalysts are still lacking when compared with the current three-way catalyst. A significant research effort is currently being directed toward this desired technology [232]. A stoichiometric homogeneous mixture could be used in the GDI engine in combination with the three-way catalyst, but the achievable level of fuel economy would be theoretically lower for this class of GDI engine. A number of technologies are currently being explored for NO x reduction in lean-burn and stratified-charge engines. These, with the associated technical issues, are summarized as follows. • Selective-reduction, lean NO x catalysis: ◦ reduces tailpipe NOx emissions when engine is operated lean; Fig. 91. Comparison of the selective-reduction-type and storagetype lean-NOx catalysts [17]. ◦ requires some UBHC to reduce NO x; ◦ lower conversion efficiency; ◦ narrow working temperature range, depending on catalyst materials; ◦ not as durable as conventional three-way catalysis; ◦ high resistance against sulfur contamination. • NO x absorber: ◦ stores NOx while engine is operated lean; ◦ converts stored NOx during a rich excursion in air– fuel ratio; ◦ no UBHC required to store NOx; ◦ can be installed downstream from other catalysts; ◦ higher conversion efficiency over an operating window of 350–500C;
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 527 Fig. 92. Comparison of the gasoline sulfur levels in European, US and Japanese markets [17]. ◦ rich mixture required for regeneration; ◦ complex control system required for regeneration; ◦ 2% fuel economy degradation due to regeneration; ◦ not as durable as conventional three-way catalysts; ◦ limited to low-sulfur gasoline. • Plasma three-way catalyst: ◦ expanded operating temperature range; ◦ improving NOx conversion efficiency while reducing the power consumption is a challenge. The family of lean NOx catalysts include NOx reducing catalysts of zeolite and precious metal for oxygen-rich conditions, as well as an NO x storage catalyst that is capable of trapping NOx when the exhaust is oxygen-rich, then converting the stored NOx during intermittent short periods of controlled over-fuelling. The NO x storage catalyst has a high conversion efficiency; however, it is subject to sulfur poisoning and must be used with low-sulfur fuel. Fig. 91 shows a comparison of the selective reduction catalyst and NOx trap catalyst [17]. This may be contrasted to the selective-reduction catalyst that incorporates the direct reduction reaction of NOx by hydrocarbons, and exhibits a stable conversion capability even after lengthy operation with a high-sulfur fuel. However, the selective reduction catalyst has a much lower conversion efficiency and a narrower window of operating temperature. One of the problems that complicates the application of NOx storage-catalyst technology to stratified lean combustion is the generation of a precisely controlled rich excursion in the exhaust flow during extended periods of part-load operation. During lean operation, any brief increase in the fuel rate will result in a momentary increase in engine power, unless it is anticipated and controlled. During homogeneous lean mixture operation in the Toyota D-4 GDI system, fuel is injected during the intake stroke and the desired rich-mixture spike is obtained at 50 s intervals by the synchronized control of both fuel pulse width and simultaneous spark retard to maintain a constant output torque. For stratified-charge operation this spike is obtained by a complex control system command that not only provides a brief fuel pulse width and spark retard, but also by a simultaneous brief adjustment to the injection timing, throttle opening and the positions of the swirl control valve (SCV) and the EGR valve. It was reported that the NOx emission level for this engine without either EGR or a NOx storage catalyst is 1.85 g/km for the Japanese 10–15 mode test. With EGR control only, NOx is reduced to 0.6 g/km, which is a 67% reduction. This can then be further reduced to 0.1 g/km with a stabilized NO x storage catalyst. With the use of an NO x storage-reduction catalyst, the periodic introduction of the required rich mixture results in a 2% loss of fuel economy potential [21]. Another possible strategy is to have a controlled secondary injection of fuel during the exhaust stroke. The use of a NOx sensor is expected to contribute to an optimization of the regeneration process for the storage-type catalyst. NO x absorption catalysts show a high potential for NO x removal under periodic lean–rich conditions. Engine and model bench tests have proven that significant NOx reduction is feasible over a wide temperature range. A closely coupled interaction between the engine control system and the catalyst characteristics is required for best performance and lowest fuel consumption. However, as shown in Fig. 92, the high sulfur level in standard gasoline grades, particularly in North America, pose a major impediment to the wider application of this promising catalyst technology. Relatively small amounts of absorbed sulfur can cause a severe suppression of NOx storage activity. Some evidence exists that SO3 reacts preferentially with the NOx absorbents, forming surface sulfate species that block the active sites from further NO x absorption [232]. As the average sulfur levels of gasoline in the European and North American markets are substantially higher than that in the Japanese market, a significant challenge is presented to automotive engineers who are striving to introduce the GDI
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470 3. Combustion chamber geometry
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Automotive spark-ignited direct-injection gasoline engines

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