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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report 2 Development of Vehicle Subsystems CANDIDATE SYSTEMS The success of the PNGV program will depend on the integration of R&D programs that can collectively improve the fuel efficiency of automobiles and meet the requirements of comparable size, reliability, durability, safety, and affordability of today’s vehicles. At the same time, the PNGV vehicles must meet even more stringent emission standards and recycling levels and must use components that can be mass produced and maintained in a manner similar to current automotive products. In order to achieve the Goal 3 fuel economy objective of 80 mpg, the energy conversion efficiency of the chemical conversion system (e.g., a power plant, such as a CIDI engine or a fuel cell) averaged over a driving cycle will have to be at least 40 percent, approximately double today’s efficiency. In addition to improving the energy-conversion efficiency of the power train (including energy converters and transmissions) and reducing other energy losses in the vehicle, achieving the Goal 3 fuel economy objective will require a very large weight reduction (> 40 percent) through new vehicle designs and lightweight materials. In short, meeting the Goal 3 fuel economy target will require extensive innovation. For example, the primary power plant, when used in a hybrid electric vehicle (HEV) configuration, will have to be integrated with energy-recovery and energy-storage devices. Radically new vehicle structures will be necessary to reduce vehicle weight. The design and performance of essentially every aspect and function of the vehicle, from the passenger heating and cooling systems to the conversion efficiency of the exhaust-gas after-treatment systems, will have to be evaluated for possible energy savings.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Hybrid drive systems, the PNGV near-term power trains of choice, use energy-storage devices to reduce fluctuations in demand on the primary power plant, thereby permitting the use of a smaller power plant operating closer to optimum conditions for increased energy-conversion efficiency and reduced emissions. These storage devices also allow recovery of a portion of the vehicle’s kinetic energy during braking operations. In this chapter, the committee continues its evaluation of the candidate energy-conversion and energy-storage technologies that survived the 1997 technology selection process, as well as of candidate electrical and electronic systems and advanced structural materials for the vehicle body. The committee reviewed R&D programs for: four-stroke internal-combustion reciprocating engines, fuel cells, electrochemical storage systems (rechargeable batteries), power electronics and electrical systems, and structural materials and safety to assess their progress toward commercial applicability. In the committee’s opinion, PNGV has continued to make significant progress on the development of candidate systems and the identification of critical technologies that must be addressed to make each system commercially viable. Indeed, global competitiveness is one of the objectives of the PNGV program. In fact, previous committee reports have specifically addressed the directions and trends of international development programs. The committee is aware that the PNGV partnership continues to maintain awareness of international programs and, in many cases, the USCAR partners are participating in those developments through international operations and coalitions. Therefore, a separate section on international developments has not been included in this report. INTERNAL COMBUSTION RECIPROCATING ENGINES The internal combustion engine continues to be the primary candidate power plant for meeting near-term PNGV program goals. To meet the fuel economy target of Goal 3, the internal combustion engine will have to be integrated into an HEV configuration. The CIDI or diesel engine is the most fuel efficient internal combustion engine being developed. Consequently, in the near term, maximizing the efficiency of a diesel engine and integrating it into an HEV will provide maximum vehicle efficiency. However, as will be discussed below, the challenges of meeting the new California Air Resources Board (CARB) and EPA Tier 2 emission standards have called into question the viability of the CIDI engine as the primary energy converter in PNGV’s time frame. As a result of the new standards, the technical team working on the 4SDI (four-stroke direct-injection) engine began focusing more on emissions control research for the CIDI engine. In addition, they are continuing research programs on other combustion systems for internal combustion engines, namely the homogeneous-charge compression-ignition engine, the gasoline direct-injection engine, and the homogeneous-charge spark-ignition engine.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Figure 2-1, which was included in previous committee reports, is a convenient summary of the advantages and challenges of the internal combustion engines being considered as primary energy converters for the PNGV program. Program Status and Plans In the past year, the 4SDI technical team has made excellent progress in continuing the development of a power system to meet the PNGV goals. In addition, each USCAR partner developed and built its own concept car. Based on proprietary visits to each partner’s laboratories to be briefed on concept-car programs, the committee recognizes and commends the tremendous and impressive efforts by each USCAR partner to develop and build these concept cars and integrate promising technologies into the vehicles. The extent to which each concept vehicle represents a different approach toward meeting the PNGV goals is testimony to the dedication, creativity, and industry of the USCAR partners. All three partners are developing diesel-powered electric hybrid systems for their concept vehicles. However, overall vehicle objectives and the logical basis for hybridization vary among the partners, whose approaches to trade-offs among fuel economy, degree of hybridization, emission reduction strategies, attaining manufacturability, and cost reduction are quite different. Because of the importance of these vehicles as a program milestone, a separate section of this report is devoted to them (see Chapter 4). In addition to proprietary internal work on the concept vehicles, PNGV continued its collaborative programs on precompetitive fundamental technologies, including interactions between fuel composition and engine performance (as recommended in the committee’s fifth review [NRC, 1999]); the fundamentals of combustion, including some very novel concepts; and in response to Tier 2 emission standards, a stepped-up investigation of emission control technologies and sensors. Progress to date and the future plans of the 4SDI technical team are discussed below in three sections: engine-fuel interactions; engine-combustion system developments; and after-treatment and controls and sensors. Engine-Fuel Interactions The effects of fuel chemistry and physical properties on engine performance and emissions was the focus of intense activity during the past year motivated by data reported in the literature showing that fuel composition affects nitrogen oxides (NOx) and particulate matter (PM) emissions from a variety of diesel engines (Ryan et al., 1998; Takatori et al., 1998; Tanaka and Takizawa, 1998; Wall and Hoekman, 1984). The data also indicated that fuel composition could affect the performance of exhaust-gas after-treatment systems. However, the extent to which these changes would carry over to a state-of-the-art, highly
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report FIGURE 2-1 Alternatives for energy conversion. The baseline engine is the port fuel injection homogeneous charge spark injection for model year 1993. Source: Low et al., 1999.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report controllable engine system in which the engine and fuel have been optimized together was not clear. Similarly, as new exhaust-gas after-treatment systems are developed, it will be important to determine whether they retain their sensitivity to fuel composition. These are the general areas of focus for PNGV’s studies on fuel-engine systems and exhaust-gas after-treatment systems. PNGV is working cooperatively with the low-emission partnership of USCAR, the fuels industry, the U.S. Department of Energy (DOE), and the national laboratories to evaluate the effects of fuel composition and physical properties on engine performance and exhaust-gas after-treatment conversion efficiencies. The working relationships range from formal (e.g., the Engine Manufacturers Association-EPA-American Petroleum Institute Diesel Fuel for 2004 Program and the DOE Advanced Fuels Technology Initiative) to ad hoc (e.g., arrangements between individual USCAR partners with fuel companies [BP Amoco, Shell, and ExxonMobil] to include fuel composition in their matrix for in-house engine development programs). Collaborative in-house testing by PNGV teams and their energy company partners is being done on a CARB commercial #2 diesel; a petroleum-based, low-sulfur (<10 ppm), low-aromatic (<9 percent by weight) fuel; a neat Fischer-Tropsch diesel fuel; and an oxygenated fuel made by blending 15 percent dimethoxymethane with the low-sulfur, low-aromatic petroleum-based fuel. Standard fuel properties, such as distillation, cloud point, pour point, density, and viscosity were measured for all of the fuels tested. The choice of fuels was based on practical considerations of what might be feasible to introduce into the current fuel infrastructure. The fuels were run in four different state-of-the-art engines by DaimlerChrysler, Ford, and GM at a range of speeds and loads. At the time of this review, the initial testing had been completed and the data were being analyzed. Although the detailed results and conclusions have not been released yet, one conclusion seems apparent. No clearly identifiable practical fuel composition or property will enable the CIDI engine to meet the new emission standards simply through improved combustion. Although control of the fuel composition and/or properties yields some benefits, the gains in a highly controlled and optimized engine were not as large as expected. The effects of fuel properties on exhaust-gas after-treatment systems are also being investigated. As the committee has noted in previous reports, the PNGV technical team has determined that none of the 4SDI technologies will be capable of meeting the emission standards without extensive exhaust-gas after-treatment (NRC, 1998, 1999). Therefore, determining the effects of fuel properties, especially sulfur content, on the conversion efficiency of the after-treatment devices will be critical. PNGV’s activities in the last year in this area have been intense. PNGV teams are working cooperatively with the Diesel Emission Control Sulfur Effects
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report (DECSE) test program, which is attempting to determine the impact of diesel fuel sulfur level on emission control systems. The DECSE program is evaluating the effects of fuel sulfur concentrations ranging from 3 to 350 ppm on NOx absorbers, PM traps, lean-NOx systems, and diesel oxidation catalysts. The data are still being analyzed, but a consensus has already emerged that sulfur degrades the performance of exhaust-gas after-treatment systems. This consensus is supported by the results of fundamental research on advanced after-treatment technologies at the national laboratories (discussed in more detail below). When researchers incorporate sulfur concentration into their test matrices, the data show a strong correlation between the system conversion efficiencies and sulfur concentration. In general, the less sulfur the better. But, because the cost-benefit analysis of incremental sulfur removal has not been conclusively evaluated, work in this area should be continued. PNGV plans to continue its very active program in the area of fuel-engine system interactions. Ad hoc working relationships with fuel companies, as well as participation in formal programs, are critical aspects of the program on fuel-engine system interactions. These ad hoc relationships seem to be working well and provide flexibility to the individual companies to pursue appropriate research. Engine-Combustion System Developments Understanding the challenges facing PNGV requires understanding the fuel economy and emission characteristics of different engine types. To date, the approach of the 4SDI technical team has been to pursue technologies that promise maximum fuel efficiency and then address the more stringent emission targets. For an internal combustion engine, the maximum work per unit fuel is obtained when the engine is operated in a fuel-lean (i.e., excess air) condition. If the engine load can be controlled by varying the quantity of fuel introduced into the cylinder without altering the amount of inducted air, energy losses associated with drawing air into the engine across a throttle (throttling losses) can be eliminated. Throttling losses in a typical homogeneous spark-ignition engine range from a few percent at high loads to more than 90 percent at very light loads. The desirable engine attributes described above are inherent characteristics of diesel engines, the most efficient internal combustion engines available. For this reason, as well as because extensive manufacturing and operating experience are available, the diesel engine is the engine of choice as the principal energy converter for the PNGV program. However, because the diesel engine attains lean combustion by maintaining a heterogeneous combustion system, there are large gradients in air-fuel mixtures in the combustion chamber during energy release. As a result, PM forms in fuel-rich regions, and NOx forms in high-temperature stoichiometric regions. The challenge is to control NOx and PM emissions from diesel engines simultaneously.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report The implementation of the EPA Tier 2 standards and the necessity of meeting CARB Ultra Low Emission Vehicle (ULEV) II requirements in California, as well as in other states that may choose to adopt them, has significantly increased the challenge of meeting emission regulations with any internal combustion engine, especially a diesel engine. The initial design targets for NOx and PM emissions at the start of the PNGV program were 0.2 g/mile NOx and 0.04 g/mile PM. These targets seemed to be attainable with an advanced technology diesel engine and some exhaust-gas after-treatment. In October 1997, immediately prior to the PNGV technology selection process (downselect), new research targets were introduced, namely 0.20 g/mile NOx and 0.01 g/mile PM. These levels required substantial improvements in the efficiency of combustion and after-treatment systems. At that time PNGV introduced R&D on the effects of fuel composition into the program. The new EPA Tier 2 standards tighten the emission requirements further. The new standards mandate fleet averages of 0.07 g/mile NOx and 0.01 g/mile PM, including light trucks and sport utility vehicles (SUVs) of up to 8,500 pounds. The Tier 2 regulations set a phase-in time schedule for different classes of vehicles, as well as emission level “bins” into which different vehicles can be grouped, as long as the fleet average emission level is met. This regulatory flexibility is intended to allow for emerging technologies being introduced into the market by giving manufactures time to gain production experience and continue their development. However, for each vehicle that exceeds the fleet average emission level, a number of vehicles with emission levels below the fleet average will have to be sold to maintain the fleet average. Producing ultralow emission vehicles will be a substantial challenge, and, from a business perspective, if the manufacturer’s lightest and most fuel-efficient vehicle, the PNGV vehicle, cannot meet the mandated fleet average, in all likelihood it will not be pursued. Clearly, PNGV needs to reassess the technological avenues and roadblocks to attaining fleet average standards with the diesel-powered HEV and decide whether or not it should be the primary choice for the PNGV vehicle. In an attempt to put current work on the diesel-powered HEV in perspective, the 4SDI team assessed to what extent the current projections of after-treatment system performance would have to be modified to meet the new emission standards. The results of this sobering exercise are summarized below: With a gasoline port-fuel-injected, stoichiometric, homogeneous-charge engine, the emission standards could probably be met with advanced three-way catalysts of different combinations. With a direct-injection, spark-ignited gasoline engine as the principal energy converter, advanced NOx traps and PM traps would be required. With a diesel engine, exhaust-gas after-treatment systems would require 75 percent and 50 percent reduction efficiencies for NOx and PM, respectively.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Because the fuel economy projections are based on extrapolations of emissions reduction technology, the fuel economy penalty associated with meeting the emission standards could not be quantified.1 The immediate effect of the announcement of the Tier 2 standards has been a shift in resources toward after-treatment technologies to reduce emissions and away from improvements in fuel economy. Prior to the announcement, PNGV believed that the best chance of reaching 80 mpg in its time frame was with a diesel engine as the primary energy converter. Based on the current assessment, the challenges of meeting the Tier 2 emission standards with the diesel engine will be enormous, and the technology most likely to meet the new standards is an engine that uses homogeneous stoichiometric combustion, for which the exhaust-gas after-treatment is most advanced. Unfortunately, this type of engine is the least efficient of all of the candidate engines being investigated. Ironically, in the near term, the likely effect of the Tier 2 emission standards will be to promote the development of the less efficient engine systems. Despite the increased emphasis on emission reduction technologies, which are being pursued primarily through improved exhaust-gas after-treatment systems, the 4SDI technical team has also continued to perfect in-cylinder combustion processes. This R&D is being performed by the PNGV partners in collaboration with DOE national laboratories, the Department of Defense Tank Automotive Command, EPA, AVL List GmbH, FEV, Wayne State University, and the University of Wisconsin at Madison. Projects have been divided into two categories: (1) enabling technologies that will be critical in the near term (less than three years); and (2) technologies that could be important in the longer term (more than three years). Near-term projects include: the development of advanced fuel-injection systems, studies of the cylinder-to-cylinder distribution and transient response of exhaust-gas recirculation (EGR); the development of pressure-reactive pistons for dynamic changes in the compression ratio; and detailed comparisons of combustion data obtained in an optical research engine, a similar metal engine, and the predictions of sophisticated, three-dimensional computer simulations. The results of these projects during the past year have varied. To date, using injection-rate shaping and multiple injection events for emissions control have shown no significant reduction in emissions. However, this 1 For example, the diesel engine operates in an air-fuel regime that is overall fuel lean (i.e., with excess air). The combustion process is not premixed or homogeneous. Injecting the fuel directly into the combustion chamber results in large gradients of air-fuel mixture. The cylinder contents vary spatially from extremely lean through stoichiometric to very rich air-fuel mixtures. In addition, the fuel economy penalty of the NOx trap or lean-NOx catalytic reduction technology has not been quantified.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report technology has not been completely explored. In addition, this technology may have a critical effect on the composition of the exhaust gas that enters the exhaust-gas after-treatment system. A special fuel system using dimethyl ether was successfully constructed and tested, and more extensive testing of this interesting fuel can now be done in a Ford DIATA (direct-injection, aluminum-block, through-bolt assembly) diesel engine. An instrumentation system for monitoring the transient EGR distribution was developed by FEV this year, which will enable the detailed investigation of the cylinder-to-cylinder distribution of EGR and the transient response of the engine-EGR system. Because EGR is an important method of in-cylinder NOx control, the results of this investigation will be significant. Through the collaborative efforts of Sandia National Laboratories (SNL), Wayne State University, and the University of Wisconsin at Madison, the program on the optical engine, the metal engine, and advanced simulation is now fully operational. Data showing soot luminosity and liquid fuel distribution were mapped in the optical engine, and tests of EGR tolerance, from zero to the maximum possible amount, were performed in the metal engine. Both of these data sets were used as a basis of comparison for the predictions of the simulation. The objective of the project is to characterize fully the in-cylinder flow field with the effects of swirl and to validate the full combustion simulation. Projects in the longer time frame include: continued investigation of homogeneous-charge, compression-ignition combustion; studies on advanced variable compression-ratio engines; studies of electromagnetically actuated engine valves (the camless engine); and the investigation of several novel engine concepts. Progress was also made in these projects. Researchers at the Combustion Research Facility at SNL in Livermore, California, are close to completing a dedicated laboratory for studying the fundamentals of homogeneous-charge, compression-ignition combustion. Some preliminary data have already been obtained. Experiments and comparisons of the results with predictions from detailed chemical-kinetic computer codes are planned for this facility. PNGV also continues to investigate novel engine concepts, such as a variable compression-ratio small engine that would operate either as a port fuel-injected or gasoline direct-injected engine. The idea behind this concept is to minimize throttling by varying the compression ratio and boost pressure. If perfected, this engine could operate at stoichiometric air-fuel ratios and take advantage of the advanced state of technology in exhaust-gas after-treatment for stoichiometric systems. The results of initial testing of this engine concept at AVL List GmbH have encouraged researchers to design a functional variable compression-ratio mechanism. Other novel concepts being investigated include an engine that could switch between four-stroke operation and two-stroke operation as the power demands on the engine change and a small engine that would
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report operate at close to full load for normal operation, which is predominantly light load, and operate in “burst power” mode when high power is needed.2 The 4SDI technical team was also involved in the solicitation of competitive bids for the development of emission control for PNGV-sized CIDI engines. Cooperative agreements were awarded in September 1999 to Detroit Diesel Corporation and Cummins Engine Company. The 4SDI technical team supplied the committee with an estimate of the cost and affordability challenge of the proposed engine and emissions control system. The projected cost is $900 higher than the target cost of $2,100 for the combined engine and exhaust control system. The estimate of funding for collaborative research on emission control for PNGV-sized CIDI engines for 1999 was $52 million. After-Treatment and Controls and Sensors Meeting the new EPA Tier 2 emission standards will be a major challenge for any internal combustion engine, especially the diesel engine. Any internal combustion engine used as the primary energy converter will require substantial emissions reduction in the exhaust system. Thus, exhaust-gas after-treatment systems will be critical enabling technologies for meeting the PNGV goals. Highly effective exhaust-gas after-treatment is even more difficult for lean combustion processes, which are the most efficient processes. PNGV has been aggressively pursuing exhaust-gas after-treatment systems in the past year. Participants in collaborative research with the national laboratories and catalyst manufacturers have included: DOE’s Office of Advanced Automotive Technologies; Los Alamos National Laboratory (LANL); Oak Ridge National Laboratory (ORNL); Pacific Northwest National Laboratory (PNNL); SNL; the Low Emission Technologies Research and Development Partnership; and DaimlerChrysler, Ford, and GM. The work performed directly by PNGV and through cooperative research and development agreements (CRADAs) ranged from the fundamental development of catalysts to the testing of multicylinder engines. SNL continued its testing of platinum-based hydrous metal oxide (Pt-based/ HMO) catalysts with simulated exhaust gas and sulfur dioxide (SO2). A patentable technology, lower light-off temperatures, and a wider temperature window for effective NOx reductions has been transferred to the suppliers. At best, the maximum NOx conversion efficiency was 60 percent; but a significant degradation in performance was observed when SO2 was present in the exhaust. 2 “Burst power” is a term used to describe engine operation when engine power has been temporarily increased. For example, the “burst power” could come from a significant increase in cylinder pressure charging or a jump to very high engine speed. “Burst power” would only be used temporarily and would not be a sustainable operating condition.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Suppliers sent a test catalyst with the SNL formulations to ORNL for testing on a Navistar 7.3-liter direct-injection diesel and a Volkswagen (VW) 1.9-liter turbocharged direct injection engines. There were significant differences in the conversion efficiencies of the catalyst for the two engines. For the VW engine, the conversion efficiency was less than one-half of the conversion efficiency for the Navistar engine. An evaluation showed that this difference was not related to the different levels of NOx emitted by the two engines. Further evaluation showed that the hydrocarbon conversion in the catalyst was lower for the VW TDI engine, and there was a buildup of soot at the entry point of the catalyst. Explanations of these phenomena may be relevant to the performance differences in after-treatment systems for larger and smaller (PNGV size) engines. In any case, the tests showed that Tier 2 standards would not be attainable using Pt-based catalysts. LANL is investigating synthesis and characterization of high-temperature supports and catalysts, including NOx absorber technologies. One goal of these studies is to optimize the support selection, pretreatment, and catalyst selection. More than 170 materials were prepared and 150 screening tests performed. Fifty tests were made to evaluate SO2/temperature/water stability of the materials. LANL has developed several new catalysts, filed patent applications, and is in the process of transferring the technologies to the catalyst suppliers. Although the results of these investigations have been encouraging, the catalysts are still very much in the experimental stage of development. Many questions, such as catalyst stability and overall conversion efficiencies, have not yet been addressed. PNNL has continued its research on plasma-assisted catalysis, a process in which an electrical potential difference generates nitrogen ions that combine with NO to form molecular nitrogen and atomic oxygen. This system is used to precondition the exhaust gas from the engine before it enters the catalyst after-treatment system. The device was tested with simulated exhaust gas in the exhaust stream of a diesel-powered generator, and at ORNL on an engine dynamometer system. Bench test results of one catalyst showed up to 50-percent conversion of NOx; some NO and unidentified nitrogen-containing species were detected. Results of long-term stability tests (up to eight days) and sulfur tolerance tests (50 ppm SO2) have been very encouraging. Chemical modeling and mechanistic studies of this catalyst have been started. The results, although still in the early development stage, are promising, and the work will be continued for the next year to explore the full potential of this technology. An evaluation of combined NOx and PM reduction using catalyst-in-plasma designs is also planned.. Another approach to eliminating PM from engine exhaust is to use a PM filter. Filter systems physically trap PM and, therefore, require a method of cleaning out the filter. Continuously regenerative traps, which require low-sulfur fuel, are being developed outside of PNGV by catalyst manufacturers. In addition, in the past year, the 4SDI team has investigated the potential of using a microwave generator to initiate the burn-out of PM captured in the filter. The microwave regeneration efficiencies were evaluated under actual diesel engine
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report system. Two teams have been selected for this work, one led by Delco Remy International and the other by Delphi Automotive Systems. The Delphi team has proposed using the AIPM in combination with an alternating-current (AC) induction motor; the Delco Remy team will use a direct-current (DC) brushless motor. Projects on system development are focusing on integrated packaging of the power electronics and machine, as well as the modeling of system cost. Assessment of the Program The EE Tech Team has done an excellent job of identifying the key barriers to meeting the PNGV cost and performance targets for the electrical and electronic systems and of leveraging ongoing work by industry, universities, and the national laboratories. As the committee has stated previously, the functional specifications of the power electronics and electrical systems have been met (NRC, 1999), and the remaining challenges include meeting the physical (packaging), efficiency, and, particularly, cost targets. In its fifth report, the committee expressed its concern about the PNGV’s prospects for meeting its cost targets for power electronics and motors. The extensive discussion of this issue will not be repeated here (NRC, 1999). The contractors for the development of AIPM and AEMD all claim they can meet or exceed the cost targets, but the EE Tech Team and the committee have been shown little supporting data for these claims. The committee was shown drawings of proposed AIPM designs but no detailed cost analyses. The committee reviewed production cost goals for SatCon’s proposed design, which showed that the current cost of $20 to $30/kW is expected to be reduced to $5 to $9/kW in the final production prototype. This implies a substantial reduction in component cost, as well as manufacturing process cost. However, this projection is predicated on developments by component suppliers, whose level of confidence regarding the success of these developments was not clear. For example, the EE Tech Team expects that the cost of the high-voltage bus capacitors will be reduced from $0.20/microfarad (μF) to $0.02/μF but did not explain how this would be achieved (Malcolm et al., 1999). In fairness to the EE Tech Team, many cost projections must be taken on faith because they depend on developments that are under way but not yet demonstrated. However, given the importance of the electrical and electronic systems to the viability of the entire PNGV vehicle, the EE Tech Team must always be aware of progress toward cost goals and must communicate this information to the systems analysis team. In its fifth report, the committee recommended that the EE Tech Team perform an analysis of electrical accessory loads to verify system needs. The vehicle engineering team has taken the responsibility for reducing energy consumption at the vehicle system level, and the EE Tech Team intends to initiate a program to
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report develop efficient motors for accessories. However, to the committee’s knowledge a system level study of electrical requirements has not yet been done. Recommendations Recommendation. The Electrical and Electronics Systems Technical Team should closely monitor the progress toward meeting the cost goals of the automotive integrated power module and automotive electric motor drive and update and communicate realistic expectations for costs in 2004 to the systems-analysis team. Recommendation. The Electrical and Electronics Systems Technical Team, in collaboration with the vehicle engineering team, should undertake a comprehensive study to identify the electrical load requirements of the accessory system. Although the details of the accessory system will differ among the three USCAR partners, the impact of the accessory load is important enough that it should be considered explicitly by the systems analysis team. STRUCTURAL MATERIALS AND SAFETY The reduction of vehicle mass through improved design, lightweight materials, and new manufacturing techniques is one of the key strategic approaches to meeting the PNGV Goal 3 fuel economy target. To achieve the 80 mpg target, systems analyses showed that a 40-percent reduction in vehicle weight would be necessary, together with: 40 to 45 percent power train energy conversion efficiency, 70 percent efficiency5 for regenerative braking, improved driveline efficiency, and reduced aerodynamic drag. At the same time, the baseline vehicle performance, size, utility, and affordable cost of ownership must be maintained. The substantial vehicle weight reduction targets for various subsystems (Table 2-3) would result in an overall reduction in curb weight of 1,200 lbs (40 percent), and a 2,000 lb vehicle. Materials Selection, Design, and Manufacturing In its search for lightweight materials that would enable the targeted large weight reductions (50 percent for the body-in-white [BIW],6 for example), PNGV has focused on materials with densities substantially lower than the density of 5 The regenerative braking efficiency is the proportion of total braking energy that is returned to the vehicle as useful propulsion energy. 6 Body-in-white (BIW) includes all the structural components of the body, the roof panel, and the subframes, but not the closure panels.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report TABLE 2-3 Weight-Reduction Targets for the Goal 3 Vehicle Subsystem Current Vehicle (lbs) PNGV Vehicle Target (lbs) Mass Reduction (%) Body 1,134 566 50 Body-in-White 590 Chassis 1,101 550 50 Power train 868 781 10 Fuel/other 137 63 55 Curb weight 3,240 1,960 40 Source: Adapted from Stuef, 1997. steels used in the baseline vehicle (NRC, 1998, 1999). Components and BIW structures fabricated primarily from aluminum, glass fiber-reinforced polymer composites (GFRP), and carbon fiber-reinforced polymer composites (CFRP), including hybrid structures (NRC, 1999), are being investigated. Lower density materials entail an incremental increase in cost compared to the baseline steel BIW and closure panels, probably on the order of $1,400 (Schultz, 1999), depending on several variables. The cost penalty for CFRP BIW would be substantially higher. Another approach to weight reduction has been taken by the steel industry. A consortium of 35 producers of sheet steel was formed in September 1995, under the auspices of the American Iron and Steel Institute, to design and build a prototype of an ultralight steel auto body (ULSAB) (Jeannes and van Schaik, 2000; USLAB, 1999). This consortium is not part of the PNGV program. Porsche Engineering Services carried out the engineering analyses and manufacturing management of the project. The development of the ULSAB BIW, completed in 1999, cost $22 million over four years. In carrying out this project, the steel industry investigated a class of steel grades with varying strength but with essentially the same density. They decided to explore the weight reductions that could be achieved through: (1) greater use of higher strength steels than in the baseline vehicle and steel/plastic sandwich structures; (2) finite element method (FEM) modeling for performance, particularly for torsional and bending stiffness and crash performance; and (3) innovative manufacturing processes, such as laser-welded tailored blanks and hydroformed tube structures and roof panels. All of these processes were integrated into a unified approach to reduce the average thickness of steel sheet and thereby save weight. At the end of the study, the ULSAB BIW weighed only 447 lbs, a 24-percent reduction over the PNGV baseline vehicle described in Table 2-3, and as much as 36 percent lower in weight than the heaviest PNGV vehicle benchmarked in the ULSAB study. Because the ULSAB study is very well documented, the PNGV partners should
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report be able to verify the results (USLAB, 1999). The study also concluded that the ULSAB BIW would cost $154 less than the baseline PNGV BIW. The American Iron and Steel Institute has embarked on a follow-on study to ULSAB, named ULSAB-AVC (advanced vehicle concepts), which will result in complete design concepts for an ultralight steel-intensive car that meets 2004 vehicle and crash requirements, including closures, suspensions, engine cradle, and all structural and safety-relevant components, for which steel offers mass and/or cost-efficient solutions. The ULSAB-AVC study, which will be completed in early 2001, will be based entirely on computer modeling. If the modeling results are promising, the American Iron and Steel Institute may build a prototype in the next phase of development (USLAB, 1999), which would be a PNGV-type vehicle (i.e. a five-passenger, four-door sedan, with an overall length of 187 in [4,750 mm] and a total weight of 2,275 lbs [1,032 kg] when powered by a gasoline-fueled internal combustion engine). The ULSAB-AVC study has adopted an interesting approach to achieving the low total weight of 2,275 lbs. The BIW weight is assumed to be 447 lbs (203 kg), the level achieved in the ULSAB prototype. Actually, crash standards in 2004 will require that 55 lbs (25 kg) in new or strengthened structures be added to the BIW. Therefore, to maintain a 447 lb BIW, design efficiencies would have to be found in BIW components that are less involved in crash performance. As pointed out earlier, a 447 lb BIW represents a 24-percent improvement over the PNGV baseline vehicle, whereas a 50-percent reduction is required (Table 2-3). The shortfall of 152 lbs (69 kg) would have to be offset by additional reductions in other subsystems, such as closures, chassis, and the power train. The study will identify potential reductions and investigate their performance through modeling. Thus far, the ULSAB-AVC concept does not include a hybrid diesel/electric power train, which will make it difficult to make direct comparisons between the efficient steel concept and the aluminum-intensive vehicle approach. Vehicle Crashworthiness The crashworthiness of lightweight vehicles, such as the PNGV concept cars, depends significantly on the design innovations and lightweight materials used in the crash-energy-absorbing structures of the vehicle body. Increasingly stringent crash standards usually result in heavier body weights. For example, the ULSAB BIW was designed to meet future crash requirements, such as 35 mph frontal and rear impacts (compared to the 30 mph impacts for current federal motor vehicle safety standards). These standards were not in place in 1994 when the PNGV benchmark vehicles (midsized family sedans) were selected. The more stringent standards will require that 55 lbs be added to the ULSAB 447-lb BIW. Current crash design practices based on barrier crash requirements (NRC, 1999) have used low-stiffness front ends to ensure that decelerations are in the range of l5g to 25g. If a lightweight, full-sized vehicle, such as the PNGV Goal 3
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report vehicle, followed current design practices, it would be at a disadvantage in crashes with heavier cars because the soft front end would cause most of the crash energy to be absorbed by the lighter car while significant deformation of the heavier car had barely begun. To ensure compatibility in car-to-car collisions, it has been argued that light and heavy cars should crush at the same load level, which implies that the deceleration of the lighter car would have to be faster than for the heavier car by the ratio of the mass of the heavier car to the lighter car (Frei et al., 1997). Of course, the resisting force of the lighter car structure could be designed so that the deceleration does not exceed a safe level of 40g to 50g. With this approach, the crush distance of each vehicle would be proportional to the mass of the vehicle. The crush zone of the lighter car would have to be long enough to absorb at least its own kinetic energy. These concepts have been confirmed in car-to-car crash tests (Frei et al., 1997; Niederer, 1993). Proprietary crash modeling is being used extensively by the USCAR partners to develop crashworthy 2000 concept vehicles. The PNGV program, however, is exploring advanced crash modeling methods at ORNL by running modern crash models on high-speed, parallel computers to develop crashworthy structures of steel, aluminum, and composites, as well as hybrid-material structures (Simunovic and Carpenter, 1999). The ORNL models are state of the art and include materials-related effects, such as strain rate sensitivity, and materials processing effects, such as springback. ONRL has built complex FEM models of several vehicles: ULSAB BIW, a Ford Taurus, an AUDI A8, and a Ford Explorer. The models are sufficiently complete so that the exterior can be peeled away to reveal interior components, and sectional views can also be taken. The primary use of these models will be to simulate car-to-car collisions and barrier crashes. These studies should be continued and used in developing design practices to mitigate the disadvantage of lightweight cars in collisions with heavier vehicles. Recent studies are addressing such issues, for example, Evans (2000) has empirically analyzed the on-highway effects of changing car mass without varying car size from National Highway Traffic and Safety data. In addition, the safety implications of the sensitivity of lighter vehicles to wind gusts should be considered in the design of PNGV vehicles. Materials Road Map The PNGV materials team has developed a materials road map identifying lightweight material alternatives for the major subsystems of the vehicle (PNGV, 1999a). Alternative materials are prioritized based on total weight saving potential and feasibility to meet the PNGV time frame requirements. Technical challenges are also identified and prioritized by the likelihood that promising solutions will be found to overcome the challenges. The road map, which was summarized in the committee’s fifth report, is essentially the same this year (NRC, 1999).
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report More than 40 materials projects have been initiated to tackle the technical challenges identified in the materials road map. The committee believes that these projects have the potential to remove the barriers to the use of alternative materials in current vehicles. The requirement that the overall cost of vehicle ownership not be increased presents a major hurdle to meeting the fuel economy goal. The road map reflects this requirement by defining reduction in the cost of feedstock as a common challenge for all alternative materials. The committee is pleased to note that PNGV personnel are working closely with materials suppliers to develop less costly manufacturing processes and new design practices that use materials more efficiently. Program Status Body System The existing baseline vehicle body system weighs approximately 1,100 lbs (500 kg) and is targeted for a 50-percent weight reduction. Because the BIW (590 lbs [268 kg]) and the closure panels (220 lbs [100 kg]) account for a large fraction of the weight, the PNGV team has concentrated on these components and has identified several alternatives: the ULSAB steel-efficient concept, aluminum sheet and extrusions, fiber-reinforced polymer composites with GFRP or CFRP reinforcements, and hybrid material concepts (e.g., an aluminum space frame that supports polymer composite panels). Steel. The ULSAB steel-efficient designs for a high-strength steel BIW (discussed above) have demonstrated a 24-percent weight saving over the baseline vehicle in Table 2-3. ULSAB is carrying out a modeling study to reach a steel-intensive vehicle weighing 2,275 lbs (1,033 kg), which may be further reduced by hybridizing with aluminum or other low-density materials. Aluminum. The low-cost aluminum sheet project to develop 5000-Series continuous cast sheet for body structures has been completed successfully. Belt-cast thin sheet, using several process/alloy variations, has been used to form several large and challenging prototype parts for PNGV. Progress is also being made in aluminum manufacturing and assembly. An apparatus has been built to control the binder force during the forming stroke, which will increase the percentage of good parts made. To improve the formability of aluminum, warm forming trials are being conducted. Another project to evaluate high-pressure water pulse joining has been initiated. A project to study laser welding of aluminum stamping blanks with different thicknesses and/or strengths (tailor-welded blanks) has been completed and a follow-on project is planned. An aluminum alloy scrap-sorting project has also been initiated.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Polymer Composites. A lightweight hybrid body employing bonded CFRP and aluminum structures successfully achieved a 68.5-percent weight reduction over the baseline vehicle (NRC, 1999). In a successful 30-mph frontal crash test, this lightweight body exhibited less deceleration than the baseline vehicle. Projects are in place with suppliers and ORNL to develop low-cost carbon fibers through lower cost precursors and by microwave processing. Costs could be reduced by as much as 20 percent. DaimlerChrysler has been exploring injection molding of large integrated moldings for body panels from thermoplastic resins, both nonreinforced resins (e.g., polyethylene-terephthalate) and glass-reinforced thermoplastic resins. DaimlerChrysler has reported building a Composites Concept Vehicle and a concept Jeep Commander from such materials and believes it can achieve 50-percent weight savings. Achieving a class “A” surface on outer panels and required crash impact response could be barriers to this approach (Brown, 2000; Pryweller, 1999a, 1999b). This technology was also used in the DaimlerChrysler PNGV concept car, the ESX3 (see Chapter 4). Power Train and Chassis System The total weight in the power train (868 lbs [394 kg]) and chassis (1,101 lbs [500 kg]) subsystems for the baseline PNGV vehicle is substantial. The total targeted weight savings for the two subsystems is 648 lbs (294 kg), whereas the total identified weight savings, based on the PNGV materials road map, is 325 lbs (148 kg), a shortfall of 323 lbs (147 kg) (NRC, 1999). This shortfall, which was pointed out in the committee’s fifth report, has not changed significantly. Aluminum. Cast aluminum has been used extensively in the power train subsystem, replacing cast iron over the past two decades in most cylinder heads and intake manifolds and increasingly replacing cast iron in cylinder blocks. These trends are expected to continue, although reinforced nylon composites, which provide a cost and weight savings, are replacing aluminum in many intake manifold applications. The PNGV materials road map describes major efforts to reduce the costs of feedstock and improve the casting processes for cast aluminum (PNGV, 1999a). Progress in the cast light-metals programs includes: creation of a cast light-metal material property database (CD-ROM) that correlates with cast microstructures; the development of a fiber-optic infrared temperature sensor for on-line process control; and the development and validation of a simulation model that predicts monotonic tensile properties of cast metals. This phase of the project is now nearing completion. Aluminum metal matrix composites applications in chassis and power-train subsystems account for only 30 to 50 lbs. The major hurdles to developing applications of this material are feedstock costs and the development of a reliable
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report TABLE 2-4 Material Cost Targets Material Current Cost ($/lb) Target Cost ($/lb) Product Form Steel 0.30 to 0.40 N/A Sheet Aluminum 1.40 to 1.60 1.00 Sheet Magnesium 1.65 1.20 Ingot Carbon fiber > 8.00 3.00 Fiber Aluminum metal matrix composites 2.00 1.40 Ingot Titanium 8.00 2.00 Bar, sheet Source: Adapted from Sherman, 1998. process for making the composites (Table 2-4). A low-cost powder metal process is under development. Progress includes: the development of materials for a gerotor application; the production of gerotors from production tooling; and initial wear testing. Magnesium. Projects are under way to improve the machinability of magnesium castings, to develop a lower cost magnesium through the use of plasma torch technology, to develop more creep-resistant alloys, and to utilize semisolid forming (Thixomat, Inc., process) to produce higher strength, low-cost magnesium components. These projects are in the early stages of development. Titanium. The applications for titanium are in the chassis (40 lb [18 kg] potential savings) and power train (10 lb [4.5 kg] potential savings). The components of interest are springs, piston pins, connecting rods, and engine valves. A program designed to lower the cost of titanium feedstock is in an early stage of development. Program Assessment and Plans Body System Steel/Aluminum Hybrid Body. In the PNGV concept vehicle phase, the emphasis has been on aluminum and composites to meet the 50-percent weight reduction target. As the PNGV program moves toward 2004 and the production of affordable production prototypes, the PNGV team should attempt to balance the opposing requirements of weight reduction and affordability. In the preceding discussion, the committee presented a rationale for considering steel in addition to aluminum and composites (Table 2-4). The PNGV team should carefully follow the progress on the new ULSAB-AVC project. If this modeling study shows the feasibility of a 2,275 lb steel-intensive vehicle, PNGV might be able to
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report add some aluminum and/or magnesium components (hybridize) to the body to produce a 2,000 lb (908 kg) vehicle at a much lower cost than with an aluminum-intensive body. Aluminum. As a matter of public record, several aluminum-intensive prototype vehicles have been built outside the PNGV program by the USCAR partners and evaluated for ride, handling, noise, vibration, and handling, crashworthiness, and production processes, such as stamping, extruding, joining, and painting (Jewett, 1997). The three PNGV concept cars are all different in this respect (Askari, 2000). The Ford Prodigy has an aluminum body; GM’s Precept has an aluminum skin and frame and composites in the body; the DaimlerChrysler’s ESX3 has large integrated plastic body panels supported by an extruded aluminum frame (see Chapter 4). Thus, PNGV partners already have extensive design and manufacturing expertise with aluminum, and a change to an aluminum-intensive vehicle would not be a major technological challenge. The development of new processing methods, however, would be a challenge especially for feedstock materials, to bring the cost of an aluminum-intensive vehicle to the level of a steel vehicle. As mentioned earlier, the cost penalty of an aluminum BIW over steel is on the order of $1,400 (Schultz, 1999). Current PNGV programs that involve direct casting of thin aluminum sheet, which eliminates expensive hot rolling processes, have the potential to reduce the cost of aluminum sheet to $1.00/lb (see Table 2-4). Injection Molded Composites. The injection molding of large integrated components from GFRPs being investigated by DaimlerChrysler is conventionally thought to offer weight savings of 25 to 35 percent, which is considerably less than the weight savings with aluminum or CFRP. However, DaimlerChrysler claims a 46-percent weight savings and a 15-percent cost savings over a conventional steel body, including the cost of an aluminum space frame to support the composite moldings (DaimlerChrysler, 2000). Through low-cost tooling and part integration, this approach has the potential to be a low-cost solution to body construction. Carbon Fiber-Reinforced Polymers/Aluminum Hybrid. The committee’s fifth report discussed the development of a hybrid-material BIW fabricated largely from thin (1mm) CFRP sheet, which resulted in a weight of 90 kg for the PNGV BIW weighing 285 kg in steel, a 68.5-percent weight savings (NRC, 1999). Early work by the USCAR Automotive Composites Consortium showed that it was difficult to achieve good frontal crash response from polymer composite front-end structures. Some of these early problems have now been overcome. In the PNGV hybrid-material BIW, lower cost, extruded or hydroformed aluminum beams were used in the front-end structure to ensure adequate crash response, which was verified by 30 mph frontal crash simulations.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report The committee considers these results very encouraging, even though there is still much to be learned about: (1) composite design techniques; (2) the consistency of mechanical properties in high production volumes; and (3) the reduction of manufacturing cycle times. Finally, methods of recycling the material into high-value applications to take advantage of its intrinsic properties, as opposed to using it only as filler, must still be developed. But the major barrier to the intensive use of CFRPs for a Goal 3 vehicle is the high cost of the carbon fibers. As shown in Table 2-4, the cost of carbon fiber will have to be reduced from $8.00/lb to $3.00/lb. Currently, the cost penalty of a CFRP BIW is much higher than for a steel or aluminum BIW. Although the weight savings are impressive, CFRP technology must be considered a long-term alternative to steel and aluminum. Power Train and Chassis System The committee was pleased to see that several important R&D studies are now in place to reduce the costs and improve the properties of aluminum and magnesium castings. A program is also in place to reduce the cost of titanium feedstock. Cost is the main barrier to increased use of these materials. Also, the committee notes that programs to help reduce the processing costs of aluminum-metal matrix composite materials have made progress. Gerotors fabricated by the low-cost powder metal-metal matrix composite technology will be wear tested in the coming year. Also, a new project has been initiated on low-cost cast metal matrix composites. RECOMMENDATIONS Recommendation. As the PNGV project moves toward 2004 production prototype vehicles, affordability will be a key requirement. Therefore, the development of an efficiently designed and fabricated steel-intensive vehicle being worked on by the American Iron and Steel Institute in the Ultralight Steel Autobody-Advanced Vehicle Concepts (ULSAB-AVC) project should be closely followed, and the possibility of applying the ULSAB concepts to a hybrid steel-aluminum vehicle should be explored. Recommendation. The committee recognizes the cost reduction potential of DaimlerChrysler’s thermoplastic composite injection-molding technology and urges that this work be continued to bring the technology to successful commercialization. The committee encourages the earliest possible generation of vehicle and component test data to define better the structural properties and performance of various composite materials and structures.
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Review of the Research Program of the Partnership for a New Generation of Vehicles: Sixth Report Recommendation. The committee regards structural crashworthiness, and safety in general, extremely important in the design of lightweight PNGV vehicles. Using the Oak Ridge National Laboratory car-to-car collision simulation capability, the National Highway Traffic and Safety Administration should support a major study to determine how well lightweight PNGV vehicles would fare in collisions with heavier vehicles and to assess potential improvements.
Representative terms from entire chapter: