Automotive spark-ignited direct-injection gasoline engines

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528 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 93. Predicted emissions of different aftertreatment technologies for the ECE EUDC cycle [136,137]: (a) control map for the Ricardo GDI engine; and (b) predicted results. technology into those markets. In order to comply with the stringent NO x emission regulations in Europe and North America, a more efficient NO x catalyst that is more robust to sulfur-poisoning is required [41,338]. Iwamoto et al. [50,51] reported that the Mitsubishi GDI engine employs a selective-reduction catalyst to further reduce the NOx emissions not only during the urban driving cycle, but also for the lean-burn condition associated with the highway driving cycle. This selective-reduction-type catalyst has three-way catalytic functions and a high inherent resistance against sulfur contamination; however, it currently has a relatively narrow working temperature range that limits its performance. It is claimed that the selective-reduction catalyst maintains a good efficiency in the mileage-accumulation durability test with high-sulfur gasoline. However, this type of catalyst may generate more N2O and NO2 emissions. Jackson et al. [145,173,195,196] subtracted emissions entering the catalyst from those measured at the catalyst outlet, with the results indicating that the catalyst increases N2O and NO2 emissions during the cycle. Another new technology that is being developed for potential application to GDI engines is a plasma system that provides for the simultaneous conversion of NOx, UBHC and CO. The system features a surface plasma having low temperature, low pressure and low energy that could be packaged in a volume similar to a
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 529 conventional catalyst. The potential advantages of this system include a lower light-off temperature and an expanded range of operating temperature. Improving NOx conversion efficiency while reducing the power consumption is a challenge of this emerging technology [311,369]. In order to assess the prospects of the GDI-powered vehicle for meeting the future emission regulations, Jackson et al. [136,137] predicted the vehicle emissions and fuel economy for various aftertreatment technologies using a driving-cycle simulation. The engine operating strategy was assumed to be as shown in Fig. 93(a). Multi-cylinder engine maps were constructed from steady-state emissions data that were obtained using a single-cylinder test engine. Adjustments were made in the cycle simulation for the effects of cold start and transient effects. The aftertreatment technologies considered in the simulation study and their conversion efficiencies (%) for the ECE/EUDC cycle are: Aftertreatment technologies Conversion efficiency (%) UBHC NOx CO 1. Underfloor three-way catalyst (UF TWC) 81/95 0/0 67/98 2. Close-coupled three-way catalyst (CC TWC) 90/95 0/0 85/98 3. Item 2 NOx catalyst 90/95 40/50 85/98 …CC TWC NOx CAT† 4. Item 2 NOx trap 90/95 70/80 85/98. …CC TWC NOx TRAP† The emission prediction results for the ECE EUDC cycle are shown in Fig. 93(b). It was reported that with EGR and a close-coupled three-way catalyst the GDI vehicle has a reasonable engineering margin for meeting the EURO II (current) standard. To meet the EURO III (2000) standard with an engineering margin, however, would require EGR, a close-coupled three-way catalyst, and a 40–50% efficient DeNO x aftertreatment system. Meeting the EURO IV (2005) emissions standard would require EGR, a closecoupled three-way catalyst, and a 70–80% efficient DeNOx aftertreatment system. In addition, light throttling would be required to control engine-out UBHC emissions. Andriesse et al. [190] reported that a GDI engine operating in the stoichiometric, homogeneous-charge mode is able to provide a fuel economy benefit of 7–10% while lowering NOx emissions by at least 50%. This allows the EURO II emissions standard to be met with a conventional three-way catalyst system. The stratified GDI engine provides a 15–20% enhancement in fuel economy, but requires a lean NOx catalyst with an efficiency of 80% to meet the EURO III standard, and an efficiency of 90% to meet the EURO IV standard. In summary, it is apparent from the current literature that the attainment of regulatory levels of NOx emissions is one of the major challenges facing GDI engine developers, particularly in the North American market [323]. 5.2.3. Particulate emissions In general, particulate matter (PM) is defined as all substances, other than unbound water, which are present in the exhaust gas in the solid (ash, carbon) or liquid phases. Engine particulates basically consist of combustion-generated, solid carbon particles, commonly referred to as soot, that result from agglomeration or cracking, and upon which some organic compounds have been absorbed. The carbon particles become coated with condensed and absorbed organic compounds, including unburned hydrocarbons and oxygenated hydrocarbons. The condensed matter can be comprised of inorganic compounds such as sulfur dioxide, nitrogen dioxide and sulfuric acid. The size distribution of an exhaust aerosol from an engine is reported to be trimodal, with particle sizes ranging from several nanometers to several microns [233,234]. Both the peak magnitude and the shape of the size distribution curve change dramatically with engine type and operating condition. The complete size range is most effectively divided into three regions, or modes, that have physical significance. The three modes that are used in the literature to describe the particulate size distribution are the nuclei, accumulation and coarse modes. The nuclei mode is generally considered to be comprised of particles with equivalent diameters of less than 50 nm. The particles in this mode are primarily formed during combustion and dilution, usually by means of both homogenous and heterogeneous nucleation mechanisms. This mode normally contains the largest number of particles, thus dominating the number-weighted size distribution, but having an insignificant impact on the mass-weighted size distribution. The second region is denoted as the accumulation mode, and consists of particles having an equivalent diameter in the range from 50 to 1000 nm. These particulates are generally formed through the agglomeration of nuclei mode particles, and may contain a layer of condensed or absorbed volatile material. The particles in the accumulation mode normally contribute only a modest amount to the total number-weighted size distribution, but are generally the most significant in the mass-weighted size distribution. The third region, denoted as the coarse mode, is comprised of particulates having an equivalent diameter larger than 1000 nm. These particles are generally not a direct product of the combustion process, but are normally formed from the deposits on the valves and chamber walls that occasionally leave the solid surfaces and enter the exhaust system. The number density of the coarse mode particles is generally quite low, but can, under some conditions, influence the mass-weighted size distribution. The significance of each mode is, to some degree, dependent on the specific particulate emissions regulations. The US Environmental Protection Agency (EPA) has regulated PM emissions using mass-weighting, and its PM10 designation includes all particulate matters having equivalent
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Automotive spark-ignited direct-injection gasoline engines

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