6

Powertrain Developments

POWERTRAIN TECHNOLOGIES BEING STUDIED

Research that provides substantial improvement in the efficiency of powertrains compared with that of current commercial systems is crucial to the success of the PNGV. The importance and difficulty of achieving this improvement is underscored by the fact that, after almost 100 years of internal combustion engine refinement and improvement, typically, only 12 to 20 percent of the energy in the fuel used reaches the wheels to propel today's passenger cars. To meet PNGV goals, roughly a twofold improvement in powertrain efficiency must be achieved while retaining the size, reliability, durability, safety, and affordability of today's cars; meeting even more stringent emissions and recyclability levels; and employing components capable of being mass produced and maintained in a manner similar to current powertrains.

Even when combined with reductions in vehicle mass, aerodynamic drag, tire rolling resistance, and other energy-saving vehicle design parameters, the PNGV technical team estimates that achieving the Goal 3 fuel economy target (up to three times fuel efficiency of today's comparable vehicle) will require a powerplant with at least 40 percent thermal efficiency (PNGV, 1995). Achieving this efficiency by incremental improvements to current gasoline engines is unlikely. Therefore, a variety of alternative energy conversion devices and drivetrain components are being considered by the PNGV. None of these alternatives is, at present, suitable for passenger car application without further development. Moreover, many combinations are possible; therefore, system tradeoff analyses must be performed to fully understand the fuel efficiency potential of each. For instance, adding hybrid and regenerative braking driveline1 components reduces the powerplant

1  

The term driveline (or drivetrain) typically refers to the transmission system from engine output shaft to driven road wheels.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT 6 Powertrain Developments POWERTRAIN TECHNOLOGIES BEING STUDIED Research that provides substantial improvement in the efficiency of powertrains compared with that of current commercial systems is crucial to the success of the PNGV. The importance and difficulty of achieving this improvement is underscored by the fact that, after almost 100 years of internal combustion engine refinement and improvement, typically, only 12 to 20 percent of the energy in the fuel used reaches the wheels to propel today's passenger cars. To meet PNGV goals, roughly a twofold improvement in powertrain efficiency must be achieved while retaining the size, reliability, durability, safety, and affordability of today's cars; meeting even more stringent emissions and recyclability levels; and employing components capable of being mass produced and maintained in a manner similar to current powertrains. Even when combined with reductions in vehicle mass, aerodynamic drag, tire rolling resistance, and other energy-saving vehicle design parameters, the PNGV technical team estimates that achieving the Goal 3 fuel economy target (up to three times fuel efficiency of today's comparable vehicle) will require a powerplant with at least 40 percent thermal efficiency (PNGV, 1995). Achieving this efficiency by incremental improvements to current gasoline engines is unlikely. Therefore, a variety of alternative energy conversion devices and drivetrain components are being considered by the PNGV. None of these alternatives is, at present, suitable for passenger car application without further development. Moreover, many combinations are possible; therefore, system tradeoff analyses must be performed to fully understand the fuel efficiency potential of each. For instance, adding hybrid and regenerative braking driveline1 components reduces the powerplant 1   The term driveline (or drivetrain) typically refers to the transmission system from engine output shaft to driven road wheels.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT efficiency gain needed for the PNGV Goal 3 vehicle but increases the size, weight, complexity, and cost of the complete powertrain. This kind of first-order qualitative analysis has resulted in the powertrain technologies listed below. These technologies are currently being pursued by PNGV for Goal 3 vehicles, all of which will operate as hybrid systems. The powertrain technologies being pursued by the PNGV for Goal 3 vehicles are as follows: four-stroke DICI engines gas turbines stirling engines fuel cells reversible energy-storage devices (namely, batteries, flywheels and ultracapacitors) electrical and electronic power-conversion devices Hybrid powertrain systems are attractive to increase powertrain efficiency for two reasons. When combined with a suitable energy-storage device, these systems allow the possibility of recovering a significant portion of the kinetic energy of the vehicle as it decelerates. They also allow the primary energy converter (engine or fuel cell) to be smaller and to operate under load and speed conditions that are independent of the vehicle's immediate needs. This reduces its size and permits its efficiency to be optimized. In addition, this arrangement allows an engine to operate at a speed and load that are independent of the vehicle, and increases the feasibility of using powerplants that would otherwise be unsuitable for passenger vehicles. Emissions can also be reduced significantly, especially at startup when the car can start without the engine. Both series and parallel hybrid configurations are being considered. In the series configuration, all of the engine power is transmitted to the wheels through electric machines. In a parallel configuration, the engine supplies some power directly to the drive wheels through a mechanical transmission, and this is supplemented by electrical machines and an electrical power source. Continuously variable transmissions (CVTs) allow the relationship between engine speed and vehicle speed to be changed at will and are candidates for the parallel hybrid application. It appears that little or no work with respect to CVTs is being conducted on behalf of the PNGV program in the United States. However, foreign firms are continuing to develop such transmissions (Dancey, 1995; Liebrand, 1995). The committee, therefore, believes that these developments should continue to be incorporated into the PNGV agenda.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT Under Goal 2, the PNGV is pursuing a number of powertrain-related developments with varying levels of effort. Some of these are also applicable to Goal 3 requirements, notably: various emission reduction mechanisms high dynamic range engines high expansion ratio engines direct injection diesels spark ignition engines fuel changes to improve efficiency and reduce emissions wide ratio and continuously variable transmissions reduced engine and driveline losses The PNGV has concluded that gasoline spark-ignited, internal-combustion engines and straight electric vehicles (as opposed to hybrid electric vehicles) do not have the potential to satisfy Goal 3 requirements within the PNGV time frame (see appendix E). The committee concurs with this conclusion, based on its understanding of the status of the associated technologies. However, improvements in the spark-ignited reciprocating engine continue to be pursued under Goal 2. Likewise, the USCAR partners are pursuing electric vehicles as independent developments. The committee concurs that the technologies listed above are relevant and appropriate choices. It is not obvious to the committee that any high potential candidates have been overlooked. At this time, however, there is no methodology or comprehensive study that justifies PNGV's technology slate on the basis of potential gains and probability of success in meeting Goal 3 objectives. Such a study would assure that other equally meritorious technologies have not been overlooked and would provide a framework for evaluating future developments. For this reason, the committee considers such a study to be worth undertaking and believes that the underlying methodology would be very useful in the selection process planned to focus research efforts past the year 1997. The PNGV Technical Roadmap represents an important accomplishment during the past year (PNGV, 1995). This document includes a master schedule for the selection and development of technology, together with parameter targets that each major component must meet by the end of 1997, 2000, and 2004. This compilation provides the technical teams with first order requirements for performance and schedules. More refined systems analyses and packaging studies are still needed to provide detailed specifications for each component and to enable interface and tradeoff studies between them. Although these analyses and studies are just starting on a cooperative basis in

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT PNGV, the individual car companies (Chrysler, Ford, and General Motors) have each conducted proprietary studies of their own. The absence at USCAR of any comprehensive systems analyses and quantified assessments of development risks for the technologies under consideration by PNGV means that resource allocation for the technology program must be based mainly on the past experience of the industry partners. However, the PNGV program involves technologies without automotive precedent and requires breakthroughs in both new and existing automotive technologies. Therefore, the committee believes the acquisition of effective systems analysis capability and tools at USCAR should be a very high priority for the PNGV. For example, systems analysis models must be sufficiently sophisticated to address safety issues for hybrid vehicles, which will need to meet all the safety requirements for conventional automobiles. KEY TECHNOLOGY SUMMARIES Four-Stroke Direct Injection Compression Ignition Engines Development objectives for the thermal efficiency, specific weight and specific power, cost, durability, NOx, and particulate emissions of the Direct Injection Compression Ignition (DICI) engine have been clearly specified and compared to today' s engines that use diesel fuel in the PNGV Technology Roadmap. This set of objectives provides a basis against which to evaluate the adequacy of the program effort to achieve the needed improvements. A thermal efficiency improvement of 8.4 percent (from 41.5 to 45 percent) is sought, but a gain of 40 percent in specific power is targeted to minimize the higher weight associated with structures to contain the high, peak cylinder pressures typical of these engines. The targeted emissions reductions of 50 percent in particulates and 67 percent in NOx appear to be the most challenging of the objectives. A thermal efficiency of 45 percent will still require a very large mass reduction in the vehicle to achieve a threefold increase in fuel economy. This may force the use of expensive composite structural materials, which puts pressure on the cost target. PNGV progress to date has been limited to benchmarking, target setting, and analytical studies with engine subsystem development scheduled to begin at the end of 1995. Some diesel engine research programs were underway prior to 1994 between the federal laboratories and USCAR, and program proposals were made to the DOD, Tank Automotive Command (TACOM) for funding in alternate fuels, engine structure, and materials tradeoffs. These activities are now coordinated by the PNGV. In addition to these efforts, some of the domestic original equipment manufacturers (OEMs)

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT have substantial in-house, largely overseas, DICI engine development programs. The objectives of these in-house programs are being realigned to address the specific needs of a smaller displacement, light-duty engine for the PNGV hybrid vehicle application. The most promising advancement noted by the committee in these programs has been the demonstration of a lean NOx catalyst with improved steady-state peak efficiency. This result has come from the cooperation of five national laboratories and three OEMs through the USCAR Low Emission Partnership. An effective, durable, low-cost, lean NOx catalyst would substantially improve the commercial practicality of both the DICI and the lean-burn, spark-ignited engine. Progress in the diesel research programs now being considered by the PNGV appears to the committee to be commensurate with the limited funding available to date. However, requests by the DOE to fund hybrid vehicle systems and light-duty engine development have been reduced for FY 1996. A detailed milestone list and schedule have been developed, and the program is about to begin an intensive and expensive engine subsystem hardware evaluation. The cost of building and testing advanced fuel systems, cylinder heads for combustion development and engine-out emission reduction, in addition to the development of unique exhaust treatment systems, will far exceed the funds available. Significant results from such testing must be obtained to support an appropriate technology-selection decision in 1997 by the PNGV prior to continued development for the concept and production prototype vehicles. Commercialization of an advanced DICI engine will be highly dependent upon the ultimate exhaust emission standards that are promulgated. If the standards are set to be more stringent than the current ultra low emission vehicle (ULEV), DICI engines could essentially be ruled out of the marketplace. Possible tightening of particulate emissions standards in the future, given (1) new knowledge on the health impacts resulting from particulate pollution and (2) public concerns about pollution from heavy-duty diesel trucks and buses, would also affect the commercialization of advanced DICI engines. The DICI engine development will also have to overcome the label of a “conventional engine” development program. In the committee's view, a variable geometry turbocharged, direct-injected, variable-swirl, compression-ignition engine with a unique lightweight structure, complex high-pressure fuel system, using sophisticated electronic controls and exhaust after treatment should not in any way be considered “conventional.” It currently is the most promising alternative to achieve the goals of the PNGV program within the prescribed time frame.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT Gas Turbine Engines Gas turbine engines have been identified as one of the most promising technologies for the hybrid vehicle. The primary reasons for this are the very high power-to-weight ratio of gas turbines, the relatively low uncontrolled exhaust emissions, and the fact that gas turbines can use virtually any type of fuel. In general, R&D within and outside the PNGV has demonstrated progress in developing gas turbine engines for automotive applications. Partly, this is because of the enormous advances in automotive electronics and precision control capability that can now be applied to the gas turbine system. Among the most notable achievements are the turbo-alternator designs and fuel and control systems developed by Allied Signal and Allison. There is also the expectation that high-performance bearings that do not require oil are far enough along in development to be considered practical for gas turbine applications. Other areas where significant progress has been made are in the area of combustion—especially reducing emissions through staged burners—and post-combustion controls through use of catalytic converters and the pre-heating of catalytic converters and other components by energy-storage systems; heat recovery systems, especially smaller, lighter weight heat exchangers and better seals; and in the design of high-efficiency, low-cost alternators. In each of the areas mentioned, PNGV has made some advances in the past year. However, with the possible exception of the fuel control system, the pace of achievements in these areas—and also in reducing costs—is as yet insufficient to predict at this time an unequivocal technology selection decision by 1997. PNGV's progress to date is behind schedule, in part for budgetary reasons. There have also been major technical roadblocks. Among these are the critical issues of manufacturing reproducibility of high-temperature, structural-ceramic components and the aerodynamic design of small compressors and turbines with sufficiently high efficiency in the relatively low-power hybrid-vehicle applications. Turbine inlet temperatures of around 2,500°F (1,370°C) (i.e., above normal operating temperatures for conventional turbine blades) are essential to achieve the thermal efficiency required to meet Goal 3 performance targets. Achieving these targets requires either ceramic blades, which must also provide the strength, anti-erosion, and other characteristics necessary for durability, or active cooling. Cooling is not promising in such small components and, as far as the committee is aware, is not being considered to any great extent within the PNGV. Although some aircraft gas turbines operate above 2,500°F (1,370°C), it is the committee's

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT understanding that efforts to adapt this technology for automotive powerplants have not been successful to date. The pursuit of ceramic materials for high-temperature turbomachinery has a long history and has yielded advances in the knowledge base. Indeed, many millions of dollars have been spent over many years in this area in support of automotive and military projects, especially in the United States. Unfortunately, in spite of this sizable effort, no satisfactory materials or fabrication techniques have been found that could extend to the type, size, and cost requirements of engines suitable for the PNGV application. Specifically, the material technology is expensive, even for the raw materials alone. Also, the process of forming them into the proper shapes is costly. There are problems in reducing the ceramic components to the size required, while still meeting the essential performance criteria (e.g., for strength, durability, and precision). The other important technical issue for the PNGV gas-turbine application is heat recovery. To achieve the efficiency necessary, it is essential to recover a significant portion of the turbine exhaust heat. Using heat exchangers to recover part of this energy for transfer back into the compressor discharge gas is difficult and requires large, heavy, and expensive heat exchangers. Although progress has been made in the past year, further work is needed to reduce the size, weight, and costs of the necessary heat exchangers. Consequently, the heat recovery area is critical and must be satisfactorily resolved if the gas turbine is to be seriously considered as a prime mover within the PNGV schedule. Gas turbine engines face a major cost hurdle to achieve the PNGV goal. Historically, high-speed, high-temperature turbomachinery has been more costly than less compact, heavier powerplants. The need for precision ceramic components may render the cost target difficult to achieve within the PNGV time schedule. Stirling Engines The Stirling engine is being considered as an alternative in the General Motors hybrid electric vehicle program. Stirling engines have some of the same advantages as the gas turbine engine: exhaust emissions are very low, and virtually any fuel can be satisfactorily used. Earlier attempts to use this engine in automotive applications showed it to be uncompetitive in size, weight, and cost. Being a closed cycle engine, all of the heat supplied to and rejected from the engine must go through heat exchangers, and this is a fundamental drawback. However, development of this engine for stationary applications has continued, and progress has been promising enough that its use

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT by the PNGV has not been ruled out. In the absence of further information from the PNGV on the current status of Stirling engine development, the committee chose not to comment on the attractiveness of this technology other than noting the advantages stated above. Fuel Cells The challenges faced for fuel cell system development are clearly set out in the PNGV Technical Roadmap, which lists them as: “...cost, efficiency...unattended reliability and durability.” In addition it states, “PNGV cost and efficiency targets are the major challenges for fuel processing, while cost and energy density targets are the major challenges for hydrogen storage.” System targets set forth in the Technical Roadmap include a peak power efficiency of 53 percent, compared with an estimate of 45 percent today; and a power density of 0.4 kW/l, and specific power of 0.4 kW/kg, both of which are about twice the estimated state of the art today. It is important to note that the efficiencies quoted above are based on the use of hydrogen as a fuel2. Since it is unlikely that hydrogen can be stored on board a vehicle under the constraints established by PNGV Goal 3, vehicle efficiency calculations based on fuel sources may be more appropriate. The cost reduction target is by far the most challenging—from over $200 per kW today to $30 per kW (in 1995 dollars) by 2004. Hydrogen storage targets of 2 kWh/l and 3 kWh/kg can only be approached with costly and energy-inefficient liquid storage systems. Today, such storage of gaseous hydrogen would require five times the PNGV target size and 2.7 times the target weight, as specified in the Technical Roadmap. The committee did not receive estimates for the current size and weight of fuel processors for converting hydrocarbon or methanol fuel to hydrogen on board the vehicle, but targets of 0.5 kW/l and 1.0 kW/kg have been established by the PNGV. A wide variety of fuel-cell-related activities have been sponsored under the DOE Fuel Cells for Transportation Program. Participants include the three USCAR partners, several national laboratories, private companies, and universities. There is general agreement that the proton exchange membrane (PEM) fuel cell is the best candidate for automotive applications. The General Motors program, in Phase 2, is concentrated on using a methanol reformer and has supported PEM fuel cell development at Ballard Power Systems, Inc., in Canada. (The PNGV Technical Roadmap lists system targets but not individual 2   Efficiencies of overall systems, including a reformer, are frequently not known and also depend on the reformer fuel. The use of an efficiency based on hydrogen is helpful as a standard and for comparison purposes.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT stack-power or energy-density targets.) The Chrysler and Ford programs are in Phase 1 and are focused on reducing system cost and addressing hydrogen infrastructure issues, respectively. Significant progress has been reported on membrane and electrode development, which has resulted in vastly improved power densities and reductions in platinum loading. Ballard reports that stack-power density has increased from 140 W/l in 1992 to 570 W/l in 1995 and projects a value of 720 W/l in 1996. With these improvements, the cost of the electrode materials for a fuel-cell stack are relatively low, but the fabrication costs of graphite bipolar plates are extremely high. Lower cost metallic or plastic bipolar plates and automated electrode fabrication techniques are being pursued, and it is reported that the current membrane costs might be reduced by a factor of 10 with high-volume production. Another major technical barrier is water, thermal, and air management. Current stack engineering efforts are aimed at self-humidifying the cells, and these efforts should be intensified. A great deal of progress will have to be made in order to meet the 1997 PNGV milestones. Fuel storage and supply represents another critical area. It appears unlikely that liquid hydrogen can be used on board the vehicle, mainly from an energy efficiency point of view but also due to safety and refueling concerns. In addition, supplying this fuel on a wide scale presents daunting problems, as indicated by work conducted by BMW and DaimlerBenz in Germany on technology and infrastructure issues for future hydrogen-powered vehicles (Klaiber, 1995; Tachtler, 1995). Several small (10 kW) methanol reformers have been designed; projections of the overall efficiency for the process of producing methanol to provide mechanical energy are on the order of 20 to 25 percent, which is approximately half the target value specificed in the PNGV Technical Roadmap3. Hydrocarbon partial-oxidation processors may be slightly more efficient overall and are beginning to be explored. This issue of fuel storage and supply faced by the fuel cell system may be the most difficult problem of all. Significant additional resources will be needed to develop an efficient, practical way of generating hydrogen on board personal passenger vehicles. In summary, fuel cells, in the long term, offer the potential of high-energy conversion efficiency. Cost remains a major problem. Mass production alone is not enough to drive the cost of today's state-of-the-art fuel cell systems down to acceptable levels. No satisfactory fuel supply solution is apparent, 3   Methanol is converted to hydrogen in a fuel processor composed of a reformer and a shift converter; the conversion efficiency is approximately 70 percent. In the reformer, methanol reacts with steam over a catalyst to produce hydrogen, carbon dioxide, and small amounts of carbon monoxide. Since the latter poisons the anodic reaction of hydrogen in the fuel cell, the mixture of gases is passed through a water-gas shift reactor to reduce the amount of carbon monoxide.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT although some candidates have certain attractive features. Hydrogen infrastructure and storage concerns force consideration of using on board fuel processors, which, in turn, substantially compromise the efficiency advantage of the fuel cell. As noted above, substantial progress is being made, but several major research breakthroughs are still needed for this technology to meet PNGV technical targets and schedules. Energy Storage Devices for Hybrid Electric Vehicles Energy storage remains an essential component of hybrid electric vehicles. Energy storage not only gives the capability for recovering a significant portion of the kinetic energy of a vehicle when being decelerated or stopped, it also adds another dimension to the flexibility of the overall powertrain in the case of the heat engine or fuel-cell hybrid vehicle. In the past a number of storage technologies have been considered and investigated to varying degrees. Among these were elastomers, hydraulic springs, compressed air, flywheels, ultracapacitors, and batteries. Only the latter three have been given major consideration by the PNGV and, while it seems likely that they are indeed the best candidates for energy storage, it would be useful if the criteria for their selection were better articulated. Batteries Batteries represent the most mature technology among the three energy-storage systems being considered by the PNGV. Further, from a systems integration standpoint, batteries probably offer the most advantages. However, there are significant problems with all of the batteries being considered for hybrid electric vehicles at this time, including the lead-acid, nickel-cadmium, nickel-metal hydride, lithium-ion, lithium-polymer, sodium-sulfur and zinc-air batteries. Most battery R&D (with the exception of the lead-acid and nickel-cadmium starting, lighting, and ignition batteries) has been devoted to increasing energy storage capacity, specifically aimed at straight electric-vehicle applications. The hybrid electric vehicle, however, has need for batteries with a higher power to energy ratio than does the straight electric vehicle. Relatively little electrical energy needs to be stored on board the hybrid vehicles compared with the electric vehicles. Consequently, there are several candidate batteries that could likely provide the required energy and power for application in hybrid vehicles. The lead-acid battery has the advantage of being much more near term but, in spite of the considerable

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT efforts that have been put into its development, it still has relatively lower energy density than some of the other candidates, and there are cycle-life limitations that are likely to persist. Thus, the lead-acid battery is a high-cost option. The lithium-ion and lithium-polymer batteries are leading candidates from the standpoint of cost and energy density. However, there are critical issues to be resolved, especially those concerning the manufacturing process, achieving high cycle life, and obtaining appropriate thin electrodes to achieve the high power outputs. The nickel-metal hydride battery is also a candidate, but has unresolved cost, cycle life and efficiency problems. Unless major breakthroughs are obtained within the next two years, nickel-cadmium batteries will probably be eliminated due mainly to higher cost and environmental concerns, and zinc-air batteries may be eliminated due to power-density limitations. The more promising battery technologies appear to have been selected by PNGV from a longer list, which includes such past candidates as nickel-iron, nickel-zinc, zinc-chlorine, zinc-bromine, and the like. In the committee 's view the best technologies are probably being considered by the PNGV for the hybrid vehicles, but no specific justifications were presented for their selection. Batteries represent an important component of a hybrid system, but it was not clear to the committee that sufficient effort is being put into battery research by the PNGV. The FY 1996 expenditure of less than $10 million seems modest in light of the overall technical problems to be resolved and the amount of funding that has been put into high energy batteries in the past. If the characteristics of the lithium and nickel-metal hydride batteries are as promising as they initially appear, it would be worthwhile for PNGV to focus the research efforts and funds on the critical problems to be resolved. Flywheels Flywheels represent an alternative to batteries as energy-storage devices and, in some aspects, have characteristics superior to batteries. In particular, they have the capability for very high power to weight, both in delivering power and, perhaps more importantly, in accepting power. The energy-storage capacity per unit weight and volume of flywheels makes it impractical to use them as the primary source of motive energy in a car. As a result, for the hybrid vehicle configurations, which might be loosely classified as either (1) range-extender hybrids, (2) dual mode, or (3) power-assist hybrids4, the flywheel is viewed as having potential only for the power-assist 4   Hybrid vehicle configurations are defined as follows: (1) Range extender—the engine (or fuel cell) provides only the average power needed to keep the electric storage system charged for normal operations; (2) dual mode—the engine (or flywheel or fuel cell) is sized so that either it or the electric-storage system may be used to provide the entire power needs of the vehicle; (3) power assist—the electric power system (battery or fuel cell) is assisted in providing peak power requirements by another energy source (battery or flywheel).

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT configuration. In this type of use, flywheels have very attractive power to weight and power-per-volume characteristics. Their performance is only marginal or fair with regard to other important characteristics, such as efficiency and self-discharge, and they currently present potential safety containment and cost concerns. Flywheel safety requires fracture-energy management in case of failure. From both a consumer acceptance and a product liability standpoint, it may be necessary for vehicle manufacturers to make flywheels “fracture proof,” that is, to eliminate the possibility they could come apart in a catastrophic fashion. The cost concern is associated with the high-cost of lightweight composite materials and the manufacturing process for flywheels using such materials. Other issues of concern are potential gyroscopic effects coming from shock loading, self-discharge on the order of one-half of one percent per hour of stored energy, and long-term durability. There are also appreciable challenges, as yet unsolved, regarding the appropriate matching of alternator and flywheel design, electrical safety, maintainability of the system, and providing adequate and appropriate cooling (Energy Storage for the Next Generation Vehicle, 1995). Ultracapacitors Ultracapacitors are in the same category as flywheels, being compatible only with the power-assist type of hybrid-vehicle configuration. They have also been proposed as a way to limit the surge current load on batteries used in hybrid vehicles. Problems identified thus far by PNGV concern energy density, cost, and self-discharge characteristics of ultracapacitors. Energy storage is limited, probably to less than 2 kWh, and there are, additionally, problems of energy input and output. Power electronic conditioning can be applied to manage the charge and discharge cycles efficiently, but at a cost. A potential hazard is unbalanced current flow between individual cells in the ultracapacitor stack, which has unknown safety implications. This is an area where there is very little, if any, available data. On the other hand, recent laboratory demonstrations of individual cells have indicated superior performance. Using carbon-based materials, devices have shown energy and power densities of 10 Wh/kg and 2 kW/kg, with 90 percent turnaround efficiency and 100,000 cycle lifetime —all of which

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT exceed the PNGV performance targets for this technology. Metal oxide ceramic device costs are now in the range of $100 per kJ, and the cost goal is about $1 per kJ. Presently, the leakage or self-discharge of such units is about 10 µA/cm2 compared with a PNGV target of two orders of magnitude lower. Clearly, substantial progress has been made; but key challenges remain, and ultracapacitor developments deserve additional coordination efforts from the PNGV management. Electrical and Electronic Power Conversion Devices This effort at PNGV is focused on the electric power conversion subsystems to be used with propulsion systems for hybrid vehicles. Key elements being considered are the electric motor/alternator, the electric power inverter, and the electronic/electric controllers. Also being addressed are the adjustable-speed drives required for vehicle subsystems, such as electric power steering, passenger compartment heating, ventilating and air conditioning, and other vehicle accessories. The committee noted that the technical team responsible for this area has shown good awareness of the state of the art for these devices and has established appropriate and challenging targets for performance, efficiency, weight, and cost, as given in the Technical Roadmap. Overall size targets have not been established, but these are highly dependent upon the vehicle configuration chosen. Realistic size and shape targets can only be derived from specific packaging studies. The milestones in the PNGV Technical Roadmap show the overall electric driveline efficiency improving from 70 percent to 85 percent in 10 years, and its weight decreasing by 47 percent. The electronic inverter/controller cost must be reduced by 90 percent, and the electric motor cost by 80 percent. In the committee's view these are very demanding challenges. The schedule in the PNGV Technical Roadmap indicates a need for design and evaluation of three generations of hardware. Baseline knowledge in this area is derived from several extensive electric-vehicle programs within Chrysler, Ford, and General Motors over past decades, as well as from work on numerous electrical and electronic control systems developed by the U.S. government for military and space applications. Progress under the PNGV program, to date, largely has been related to work performed under the DOE hybrid electric vehicle projects, but the Technical Roadmap targets several additional government agencies and other research programs for support of the developments needed for the PNGV Goal 3 vehicle. The challenge will be to coordinate these efforts such that they address the unique automotive requirements and focus on cost reduction as a primary target. In addition, this

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT program faces a major challenge to develop electrical and electronic components suitable for the wide variety of powertrain configurations currently being considered by the PNGV. The absence of complete systems studies for many of these configurations compounds this challenge. As these studies become available and the technology focus narrows, the development of appropriate electrical and electronic components capable of overcoming the significant technology and cost barriers outlined in the Technical Roadmap can be managed more effectively. LIKELIHOOD OF MEETING GOAL 3 POWERTRAIN NEEDS The challenge of meeting three times today's fuel economy in a prototype vehicle ready for production in 2004 (with all of the other requirements surrounding Goal 3) is, in the committee's view, extremely difficult. Clearly, recent reductions in funding for the government portion of the program increase the risk that this target will not be met within the schedule as currently specified. Nevertheless, significant progress is being made by the PNGV. Powertrain improvement is crucial for the program's success. The DICI engine is probably the only candidate today that could be combined with a conventional drivetrain and other vehicle changes and come close to meeting Goal 3 within the PNGV schedule. Hybrid configurations improve the likelihood of meeting powertrain efficiency targets but increase the challenges relative to size, weight, and cost. Gas turbine and Stirling engines are candidates only in conjunction with a hybrid powertrain, not as stand-alone prime movers. In addition, each engine has its own specific technical challenges that must be met for it to continue as a candidate past the 1997 technology selection date. Fuel cells offer the greatest long-term potential fuel economy gain, with by far the most aggressive improvements required, and have demonstrated substantial progress over the past few years. Each of the various energy-storage and electrical-conversion devices being considered for hybrid powertrains faces very difficult size, weight, efficiency, and cost hurdles. It was observed at the recent PNGV energy-storage meeting that there has been relatively little improvement in the past year on the critical issues of safety and cost and that there is a lack of coordination, which hampers progress. If this is true, this situation must be corrected for adequate information to be available for proper choices to be made in 1997. The PNGV program plan calls for technology selections to be made at the end of 1997 so that concept vehicles can be built using the most promising systems. Completion of more rigorous systems analyses before then will help

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT in this selection process, but it is likely that these decisions still will have to be made on a largely subjective basis. The committee continues to believe that this selection process should designate some of these technologies as being worthy of continued support but should recognize that they are outside of the time frame of the PNGV program. RECOMMENDATIONS Recommendation. The PNGV should devote substantial additional resources to the DICI hybrid powertrain in view of its relatively high potential to meet PNGV Goal 3 objectives. Recommendation. The PNGV should develop a powertrain systems analysis methodology to aid in the evaluation of the potential gains and probability of success for various technologies. Recommendation. The PNGV should perform vehicle packaging studies soon for each powertrain system that is likely to remain a candidate past 1997. Such studies would establish realistic size and shape goals for the component development programs. Recommendation. The PNGV should perform a study to establish the energy balance, in-use environmental effects, and resource requirements, as well as production and distribution costs, for any fuels other than gasoline or diesel fuel being considered for use in Goal 3 vehicles. Recommendation. PNGV should continue to develop flywheel and generator technologies. Recommendation. On a stand-alone basis, batteries still appear to be the best near-term candidates for energy storage, and PNGV should fund development of the most promising battery system consistent with this potential. Recommendation. PNGV should focus its ultracapacitor R&D on the most promising technologies, and serious efforts should be devoted to the investigation of a battery/ultracapacitor hybrid storage device.

OCR for page 46
REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT REFERENCES Dancey, J. 1995. Better Automatic Transmissions for Vehicles. Presented to the Standing Committee to Review the Research Program of the PNGV at National Academy of Sciences, Washington, D.C., October 31, 1995. Energy Storage for the Next Generation Vehicle. 1995. PNGV Symposium held September 12–13, 1995 in Washington, D.C. Klaiber, K. 1995. Fuel Cells for Transport—Can the Promise be Fulfilled: Technical Requirements and Demands from Customers. Presented at the 4th Grove Fuel Cell Symposium held 1995 in London, United Kingdom. Liebrand, N. 1995. Continuously Variable Transmission (CVT) Systems. Presented to the Standing Committee to Review the Research Program of the PNGV at the National Academy of Sciences, Washington, D.C., October 31, 1995. PNGV (Partnership for a New Generation of Vehicles). 1995. Technical Roadmap (draft). Dearborn, Michigan: PNGV. Tachtler, J. 1995. Hydrogen-Powered Vehicle Development at BMW. Presented to the Standing Committee to Review the Research Program of the PNGV at National Academy of Sciences, Washington, D.C., October 31, 1995.