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Assessment of Fuel Economy Technologies for Light-Duty Vehicles
output as the 2.0-L engine, requiring that exhaust gas flow rates remain virtually unchanged. For the SUV, a smaller 10 percent emissions system cost drop was observed due to the lower PGM usage with SCR-urea aftertreatment for out-of-engine NOx control for the SUV. With SCR-urea systems, only the SCR device contains no PGM. As can be observed from examination of the entries in Table 5.6, DOC1, DOC2, and the coated DPF (called CDPF) all utilize PGM washcoats. As noted earlier, the spreadsheet used to generate the aftertreatment cost estimates shown in Table 5.6 is available for recomputing the aftertreatment system cost estimates should PGM commodity prices change significantly.
Finally, there is a technology choice involved in DPF systems. The four substrate options currently available for particulate filters are silicon carbide (Si-C), conventional cordierite, advanced cordierite, and acicular mullite. Conventional cordierite is used for most nonparticulate filter substrates (e.g., DOC and NSC catalysts), whereas Si-C has been the predominant choice for light-duty DPF usage in Europe. Conventional cordierite is less expensive and lower in mass than Si-C. On the other hand, Si-C has much higher thermal conductivity and strength, which are very favorable properties for withstanding regeneration without local hot spots causing thermal stress cracking and ultimate failure of the filter. As a result of these property differences, Si-C filters are typically filled (i.e., loaded) with about twice the amount of particulate (e.g., 8-9 g/L) during vehicle operation before regeneration is carried out, whereas conventional cordierite filters must be regenerated after about half that loading (e.g., 4-5 g/L) of particulate.
There are two results from this difference. First, conventional cordierite-based filter systems tend to require more frequent regenerations with associated FC increases. Second, since during regeneration fuel is injected into the engine cylinder during the expansion stroke with the piston part way down the cylinder to raise the temperature of the gases by partial oxidation of this regeneration fuel in the cylinder and completion of oxidation of that fuel in the oxidation catalyst, some fuel from the high-pressure spray reaches the cylinder wall and some of that fuel escapes past the piston rings down into the crankcase, where it dilutes the lubricating oil with fuel. This dilution requires more frequent oil changes to protect engine durability. Since frequency of oil changes is a marketing attribute, the choice of substrate has multiple implications, namely cost, durability, mass, and oil-change interval.
Advanced cordierite is emerging as a compromise between the properties of Si-C and conventional cordierite ( Tilgner et al., 2008). Therefore, for the purpose of this report, it has been assumed that new DPF applications will utilize advanced cordierite (as was assumed for the estimates in the Martec  report) and that existing Si-C applications will be converted to advanced cordierite for the next design and development cycle. Thus the cost estimates shown in Table 5.7 are based on the use of advanced cordierite for DPF monoliths.
Finally, acicular mullite has recently been introduced to the market. This new material has a number of properties that are potentially advantageous for exhaust filtration. First, this material appears to have lower pressure drop than the other materials due to higher porosity. According to material property specifications (Dow, 2009), this higher porosity and lower pressure drop remain when catalytic coatings are applied. As a result, it may be possible to integrate additional exhaust aftertreatment system components (e.g., combining SCR and DPF units into one component), thus reducing system cost, packaging volume, and complexity. The first production application of this material is expected in 2011, after which its technical potential and cost tradeoff relative to other materials will become clearer.
TABLE 5.7 Comparison of CI Engine Cost Estimates from Different Sources and the Committee’s Estimates
I4 CI Engine ($)
V6 CI Engine
Engine Sizing Methodology Specified
Aftertreatment System Configurations and PGM Loadings
aEPA 2009 estimates provided were for dollar-year-basis 2002 for engine and 2006 for aftertreatment. The numbers shown have been corrected by applying the ratios of the yearly producer’s price index (1.0169 for 2002 to 2007 and 1.0084 for 2006 to 2007). However, significant technology development has taken place since 2002, and so it is likely that technology-based component specifications and associated costs have changed.
bCosts from Tables IV-21, IV-22, and IV-23 of DOT/NHTSA (2009) were divided by 1.5 to convert from RPE (retail price equivalent) to cost.
cNRC (2010) refers to the present report. The CI engine costs are for base-level specifications. Detailed breakdowns of the committee’s cost estimates are given in Tables 5.4 and 5.5.
dThe spreadsheet used to compute aftertreatment system costs for the present work utilizes the configuration, sizing, and washcoat loadings included in the December 2008 version of the Martec Group, Inc. (2008) study.