Standing at the gas pump, watching the dollar total spin up into the stratosphere, haven't many of us dreamed of owning an electric-powered car that can thumb its hood ornament at the oil companies as it whizzes us ever so quietly by their filling stations? Aside from the promise of oil independence, there's the very real need to reduce environmentally polluting vehicle emissions. For now, electric vehicles are the only cars available to consumers that qualify as zero-emissions vehicles (ZEV) under emerging regulatory standards.
Yet despite some progress in the development and production of electric vehicles, their advancement seems stuck in the stop-and-go traffic jam of battery development. Building pure electric vehicles with a just-adequate driving range still requires a heavy array of expensive batteries, which pushes up the vehicle's sticker price to levels that discourage all but the most gung ho electric-car enthusiasts. In time, that situation may change. But for now, it appears that hybrid-electric vehicles stand a much better chance of electrifying the automotive world.
The current crop of hybrid-electric vehicles (HEVs), both those in production and those in the preproduction prototype stage, supplement the driving power supplied by the conventional internal-combustion engine. They do this by incorporating an electric motor, a more powerful alternator, and a higher-capacity battery into the vehicle's power train. HEVs can be configured as either series, parallel, or series-parallel combinations of gas and electric power (see "Construction And Classification Of Electric Hybrid Vehicles," p. 92).
While these cars don't eliminate vehicle emissions, they do significantly reduce them. Even organizations such as the California Air Resources Board (CARB), which is pushing car makers to ramp up production and sales of electric vehicles, have recognized that HEVs will play a key role in satisfying stricter environmental regulations.
CARB says that carmakers may use HEVs to partially satisfy the CARB mandate for sales of ZEVs. CARB is requiring that zero-emissions vehicles make up 10% of the cars sold in California by 2003. Using a complex formula that takes a vehicle's driving range and fuel efficiency into account, CARB rates HEV models as partial ZEVs. For example, a Toyota Prius rates as 0.3 ZEVs. Note that at the moment, the production of electric vehicles has greatly declined as vendors reassess the viability of their products.
Ford Motor Co. is one automaker that believes the future for electric vehicles may lie in niche markets such as neighborhood vehicles. By developing the Think city vehicles, this maker is developing cars that aren't powerful enough for highway driving but are sufficient for use in urban environments or gated communities.
There are a few reasons why HEVs are now considered better candidates for high-volume production than pure electric vehicles. Above all else, hybrids offer a better ratio of price to performance. This reflects a number of differences between the two vehicle types. Because the hybrid doesn't rely strictly on electric drive, HEVs get by with few batteries and a smaller electric motor, cutting down on both the cost and weight of these elements.
Moreover, a hybrid power train doesn't require lengthy and perhaps inconvenient recharging of batteries. And, despite its limitations in the amount of electric drive power and energy storage, hybrid-concept vehicles are achieving fuel economy ratings of 80 miles per gallon. How do they do it?
HEVs exploit a number of gas-saving techniques. One is stop-start operation, a method that shuts down the gas engine when the vehicle stops, saving fuel consumed during idle periods. When the driver accelerates after stopping, the electric motor kicks in, propelling the car forward and restarting the combustion engine. The electric drive may also provide a boost to the engine as needed. That electric-motor assistance allows the use of a smaller, lighter engine, as it can be sized to accommodate average rather than peak loads. In turn, this allows an improvement in the operating efficiency of the engine.
Another tool available to HEVs is regenerative braking. It recovers the energy used to slow down or stop a vehicle, converting mechanical braking energy from the combustion engine back to electrical energy, which is then stored in the battery. Optimizing all of the electrical and electronic elements of the power train achieves additional gains in fuel efficiency. These components include the electric motor, alternator, batteries, and power-conversion and management circuitry.
Beyond the electrical system, there also is the need to reduce the total weight of the entire vehicle. To this end, HEV designs are turning to special light-weight materials that replace steel autobody components with aluminum and molded plastic parts. With all elements of HEV design, the availability of low-cost manufacturing processes is critical to make the technology feasible. One major incentive for HEV development is that it makes maximum use of the existing automotive manufacturing infrastructure and supply chain.
Another factor favoring hybrids is the increasing demand for electrical power in the car. According to data presented by Delphi Automotive Systems, the steadily rising demand for electrical power will push the level of electrical power consumed in the car past that consumed in the home (Fig. 1). HEVs, which boost the car's power-generating capacity for propulsion, also make more electrical power available to power-train components and the growing list of vehicle accessories. Without a migration to hybrid designs, vehicles will either suffer a degradation in fuel efficiency or else face more-restrictive electrical power budgets.
Hybrid designs will not only make more electrical power available for running the car and its accessories, they also will foster the development of better energy management systems. These will implement higher-voltage systems, improvements in batteries and other energy storage devices, advanced electrical generator design/motor design, more-efficient power-conversion components, and sophisticated control schemes to optimize power management.
PowerNet Will Help
The implementation of the 42-V PowerNet during the next few years will likely bring a number of light or mild hybrid vehicles to the marketplace. The higher-voltage system will be used to implement the integrated starter-alternator (ISA) and 36-V battery storage, which immediately boosts the car's abilities to generate and store electrical energy.
Cars that implement the 42-V electrical system are deemed mild hybrids. The capacity of the components used in these systems will limit the degree to which the electric motor can propel the vehicle, as well as the amount of energy that can be generated and stored. Yet it will be possible to deploy hybrid functions like start-stop operation, torque and engine pulse smoothing, electric turbo boost, and to a limited extent, regenerative braking.
The 42-V systems might be only the first step on a road leading to higher-voltage, higher-power electrical systems (Fig. 2). Furthermore, mild hybrid designs could evolve into full parallel hybrids where the electric motor/generator provides more of the propulsion power and recovers a greater percentage of braking energy.
Hybrid vehicles have been on the market for some time, although they have been limited in availability. One of the first, Toyota's Prius, was introduced in Japan in 1997. A revamped version appeared this past year in the U.S. and Europe. The Prius employs a combination series-parallel design to achieve better than 50-mpg fuel efficiency. It seems to support the contention that HEVs are truly on the rise. Supposedly, a little more than a year and a half after the Prius was introduced in Japan, it sold more units than any of the electric vehicles introduced over the past 30 years.
Another recent import, Honda's Insight, is a two-seater able to achieve greater than 60 mpg. But even more impressive HEV performance is in the works, thanks to an ongoing effort known as the Partnership for a New Generation of Vehicles (PNGV).
The PNGV is an initiative sponsored by both the domestic auto industry and the U.S. government. Its goal has been to produce vehicles with up to 80-mpg fuel efficiency or roughly three times the fuel economy of a 1993 model, family-size sedan. Makers have to produce these cars at costs equivalent to conventional vehicles.
The program proposes that car makers achieve these goals by reducing vehicle weight up to 40%, by raising engine efficiency as much as 40% to 55%, by implementing regenerative braking, and by increasing energy storage up to 90%. The PNGV timeline calls for automakers to build their concept vehicles by 2000 and have preproduction prototypes ready by 2004.
All three automakers—Ford, General Motors Corp., and DaimlerChrysler Corp.—did in fact introduce their PNGV concept cars early last year. A peek under the hood reveals some commonalities among the designs (Fig. 3). For instance, each vehicle employs parallel-hybrid designs with diesel direct injection. Battery types, voltage levels, the amount of regeneration used, motor types, and other design factors vary from company to company, however.
Meeting the goals set forth by the PNGV presents complex system-level design challenges. In terms of the PNGV car's electrical system, all elements and subsystems associated with power generation, conversion, storage, distribution, and management must be optimized. Given the extensive nature of development on hybrid component and system design, it's only possible to offer a sampling of notable developments here. But these advances might indicate some of the ways in which HEV designs are evolving.
The U.S. Department of Energy (DOE) is sponsoring much of the R&D required to build the electrical and electronic components and systems necessary for PNGV cars. Some research is being conducted by DOE scientists, such as those at Oak Ridge National Laboratory (ORNL), while some of the effort is being contracted out to vendors. For example, external vendors have signed up to develop electric motor drives and integrated power modules.
Within DOE labs, scientists also are working to develop power inverters, controls, motors, and generators. To further develop these devices, the scientists are exploring more elemental components and materials. This work includes development of improved permanent magnets, capacitors, sensors, connectors, and heatsink materials. In addition to work carried out by the DOE, some federal PNGV research is being sponsored by the U.S. military via the Army Tactical Automotive Command (TACOM).
Development of electric motors/alternators lies at the heart of PNGV research. To promote work in this area, DOE has established a series of performance targets for the electric motor/generator (see the table). In striving to reach these goals, designers must reduce cost, size, and weight while providing the necessary electrical performance and reliability.
Various motor types are being considered for the starter/alternator, including induction motors, brushless dc permanent-magnet motors, interior permanent-magnet motors, and switched reluctance motors. These motors impose tradeoffs of factors including spin loss, inverter loss due to pole count, inverter sizing, and cost.
At present, brushless dc permanent-magnet motors and induction motors are said to be the top candidates for the motor/generator application. The brushless dc motor has the advantage of maximum efficiency, but the induction motor may be a better choice when considering tradeoffs in electronics, packaging, and performance.
A key system-level parameter is motor/generator efficiency at the operating point. In many hybrid designs, the motor might be operating mostly under a partial load. According to researchers at Delphi Automotive Systems, overall system-level efficiency depends more on charging algorithms than the optimization of motor design for peak efficiency.
ORNL has shown that it's possible to reduce the size of a permanent-magnet motor by field weakening. The key point is that it's possible to do this without damaging the magnets. The design adds a smaller auxiliary field coil to the stator and employs a simplified inverter. With the laboratory prototype, researchers demonstrated a field-weakening ratio of 10:1. A more complete prototype of this motor is under development to improve motor performance at a lower weight and cost.
Another project addresses two limitations of brushed dc motors: their weight and electrical noise. Brushed dc motors weigh more than induction or permanent-magnet motors. At the same time, the brushes generate EMI. To counter these problems, ORNL has developed a soft-commutated dc motor that quells the EMI while increasing brush life to the point where maintenance isn't an issue. The resulting motor design costs less than alternative motor types and simplifies system design by eliminating the inverter.
Switched reluctance motors are being considered as well for future hybrid applications because they are relatively inexpensive and rugged traction drives. Unfortunately, they don't satisfy PNGV design criteria. Additionally, these machines suffer from harsh waveforms, EMI, and high torque ripple, which causes torsional vibration and audible noise. ORNL is working with motor suppliers to advance switched-reluctance motor technology to overcome its current limitations. Nevertheless, motor suppliers are said to be reluctant to pursue the hybrid vehicle application without proof of a sizable market for their product.
Meanwhile, the lab also is taking steps to reduce the cost of rare-earth magnets used in permanent-magnet motors. In particular, Argonne National Laboratory and ORNL are working to develop a lower-cost alternative to the traditional powder metallurgy used to build NdFeB magnets. By applying high-strength superconducting magnets, researchers can improve the magnetic alignment of grains in the magnetic material before it's pressed and sintered, creating magnets that are up to 25% stronger than existing ones.
Creating control circuits and power inverters with greater efficiency, reliability, and lower cost is another objective of PNGV research. As with electric machinery, target specifications have been established for the power circuits (see the table, again). ORNL has developed a number of new inverter designs as well as dc-dc converters.
In one such project, the lab demonstrated the viability of multilevel inverters. These power devices use isolated dc sources to generate smooth, EMI-free ac power with high efficiency. Additionally, the dc sources can be isolated for battery charging. Such inverters also enhance safety. Once system power is turned off, no high voltage is present in the vehicle despite the presence of a high-voltage motor. ORNL tests show that multilevel inverters can be built with cost, size, and reliability levels comparable to those of traditional pulse-width-modulated (PWM) inverters.
Concurrently, the Army's TACOM and its PNGV partners are investigating soft-switching topologies to find the one most suitable for use in automotive electric drives. Topologies such as zero-current transition, auxiliary resonant clamped-pole zero-voltage transition, a low-cost zero-voltage transition, and resonant dc links are being compared against a standard hard-switching inverter topology.
Inverter technology also promises to better the performance of permanent-magnet motors. One of the constraints in permanent-magnet motor design is that the voltage required by the motor can't be greater than the available battery voltage. These motors are typically oversized so they can deliver the necessary torque at low speeds and the required power at high speeds.
If the field of the permanent magnets could be weakened, however, both of these objectives could be met without oversizing the motor. But other issues cause concern, too. Certain motor failures appear uncontrollably when applying full power. Plus, there's a tradeoff in low inductance and efficiency due to eddy currents (spin losses).
To answer these concerns, ORNL developed a dual-mode inverter control for permanent-magnet motors. This design provides a constant power region of five times base speed, the ability to drive motors with low inductance, and the ability to interrupt the circuit during motor failures to protect the inverter and the vehicle. ORNL is currently working to achieve cost, size, and efficiency goals to make this design viable.
Power inverters normally need external sensors to measure the current and voltages of motor waveforms, so development work is being carried out in this area too. Sensors for PNGVs must resist EMI and withstand the temperature, vibration, and other rigors of the automotive environment. Unfortunately, the presently available sensors are large and costly, and they have high power dissipation. ORNL has demonstrated a low-cost alternative, less than $5 per device, based on fiber optics and MEMS technology that overcomes existing size and power problems.
The current sensor consists of a microcantilever coated with a magnetic material. When placed near a current source, the microcantilever flexes in proportion to the induced magnetic field. An optical fiber placed close to the cantilever senses the motion of the cantilever. In the voltage sensor, the microcantilever is combined with another plate to form a capacitor whose value varies with voltage.
The DOE also is leading research into the capacitors needed to build the power inverters. Existing capacitors are said to be too expensive and too large (accounting for about 40% of the volume in existing automotive inverters) with inadequate life expectancies, reliability, and temperature limits for the application. Current research is intended to improve the viability of electrolytic, polymer-film, and ceramic capacitors for automotive inverters.
For electrolytics, the goal is to improve operating life, reliability, size, cost, and the ability to handle ripple current. Regarding polymer-film capacitors, researchers seek to develop inexpensive higher-dielectric materials. Ceramic capacitors, on the other hand, require fail-safe mechanisms and low-cost techniques for their production.
One of the capacitor-related projects is being conducted by Sandia National Laboratories. The company is working to replace the aluminum electrolytics that serve as dc bus capacitors with high-temperature polymer dielectric film capacitors. The polymer dielectric film capacitors under development exhibit better dielectric properties than aluminum electrolytics in the same or smaller form factors.
Thermal management is another aspect of power conversion that impacts PNGV design goals. With that in mind, ORNL developed a carbon-foam material that has much higher thermal conductivity than currently available materials, making it possible to shrink the size and weight of the inverter's heatsink. The open-cell foam can be formed into almost any shape and is easily machined. ORNL is working to improve the material's performance and to lower costs through new manufacturing processes.
According to Poco Graphite, a company that has licensed the manufacturing process for this technology, carbon foam's thermal conductivity is 150 W/(m × k), which is in the same ballpark as aluminum. But the new material's weight is just 20% that of aluminum. More significant is carbon foam's very porous nature. Its high porosity gives it a much greater surface area than aluminum and enables it to dissipate more power in less time.
A great deal of the current research into hybrid-electric vehicles targets the development of high-efficiency batteries. In hybrid vehicles, battery pack performance affects the ability of the electric motor to assist the engine as well as the amount of braking energy that can be recaptured. Because the cost, durability, and life of battery packs further affect the cost and reliability of the vehicle, battery packs must be optimized to further the use of HEVs.
The DOE holds that no current battery technology has demonstrated an economically acceptable combination of power, energy efficiency, and life cycle for high-volume production vehicles. Current lead-acid technology gives batteries an energy-to-weight ratio of 30 to 40 W-h/kg at up to $150/kW-h.
The PNGV has determined that future HEV batteries must have a power-to-energy ratio of greater than 20 W/W-h. The designated target life is 10 years, and the battery needs to ensure at least 120,000 cycles. These batteries must offer high peak and pulse-specific power, high specific energy at pulse power, and high charge acceptance to maximize regenerative braking efficiency. Five major issues in the successful development of future HEV batteries were identified by the PNGV.
First, the battery must ensure both abuse tolerance and ad-vanced safety measures. Currently, battery configurations with multiple strings of cells are troubling because they don't possess overcharge or overdischarge protection. The group maintains that electrical and mechanical safety device development has to advance in order to enable the development of efficient HEV batteries.
Another issue is cost reduction. According to the PNGV, the current high cost of nickel-metal hydride and lithium-based batteries is detrimental to HEV advancement.
Effective thermal management is the third major issue. The present candidates' operating temperatures don't cover the entire range required for automotive vehicles.
These thermal issues are of particular concern in an HEV pack due to its high power and aggressive charge/discharge profile, which is the fourth issue. An active thermal-control system is needed to maintain the integrity of the battery and reduce cell imbalance. An ideal thermal-management system must maintain the desired uniform temperature in a pack in varying climates while also ventilating the battery if it generates potentially hazardous gases.
Generally, for parallel-HEV configurations, a thermal-management system using air as a medium is adequate. For series HEVs, though, liquid-based thermal-management systems might be required for optimum thermal performance. In addition, the PNGV requires a 10-year calendar life for future batteries to reduce overall system cost.
Recyclability Is An Issue
The final issue is the targeted goal of 80% recyclability of future HEV technology. Care must also be taken to develop accurate techniques for determining a battery's state of charge.
At present, researchers are focusing on the replacement of lead-acid batteries with an alternative to raise the efficiency of HEVs. Generally, lead-acid batteries possess a specific energy of 40 W-h/kg and a specific power of 130 W/kg. These batteries are typically 65% energy efficient. Also, a recycling infrastructure is in place for lead-acid battery technology. Still, these batteries possess low specific energy as well as a short calendar and cycle life.
In accordance with the recommendations of the National Research Council, the PNGV has designated two candidate battery chemistries to serve as the near-term replacement of the lead-acid battery and meet the HEV high-power requirements. The PNGV recommends these batteries for use with highly fuel-efficient, direct-injection (DI) engines where fuel is injected directly into each engine cylinder.
The nickel-metal hydride (NiMH) and lithium-ion (Li-ion) batteries are the most likely chemistries for near-term HEV use. It has been determined that although nickel-cadmium batteries offer a high specific energy and a better life cycle than lead-acid batteries, they lack sufficient power for HEV applications.
Formed in 1991, the United States Advanced Battery Consortium (USABC) is a partnership between Chrysler (now DaimlerChrysler), Ford, and General Motors Corp. Overseen by the DOE, it set a near-term goal for the prototype production of batteries with energy-to-weight ratios of 50 to 100 W-h/kg at a cost of less than $150/kW.
The NiMH battery is being investigated as the likely candidate to reach these goals. The midterm goal is for batteries with power-to-weight ratios of 150 to 200 W/kg and a five-year useful life. Furthermore, the USABC believes the NiMH and Li-ion chemistries are the most probable candidates to achieve these goals. For the long term, the goal is a battery with a power-to-weight ratio of 300 W/kg and a 10-year useful life. The USABC has designated lithium-polymer batteries as the most likely candidate (see "HEV Battery Research Rolls On," p. 96)
Down the road, many in the industry see great promise in fuel cells. They offer much greater energy efficiency when converting fuel to electricity than the combination of an internal-combustion engine, an alternator, and a battery. By converting chemical energy directly to electrical energy, fuel cells achieve a conversion efficiency of better than 50%. Initially, fuel cells may be used to provide on-board auxiliary power and to add emissions-lowering reformate to fuel in conventional combustion engines.
Solid-oxide fuel cells are under consideration for these auxiliary power applications. One reason is because the technology is scalable. Another is its use of a smaller and less complex fuel reformer than the proton-exchange membrane (PEM) fuel cells. These cells are candidates for propulsion systems and in the long run might provide a superior alternative to battery-powered electric vehicles. In fact, many in the industry view fuel cells as the only reasonable path to building affordable zero-emissions vehicles.
Fuel-cell-powered cars promise greater driving range than battery-powered electrics, eliminating the need for battery charging while producing zero or near-zero emissions. With hydrogen-fueled PEM cells, the only waste byproduct is water. With those running off methanol, emissions qualify as near zero.
Although not as far advanced as HEVs, fuel-cell-powered vehicles are already in the prototype stage. Ford claims its P2000, a four-door sedan, is the first full-size passenger vehicle powered by fuel cells. Its performance, Ford says, is on par with that of some of the current combustion-engine vehicles.
The P2000 powertrain consists of an electric (ac induction) motor and transaxle plus a hydrogen-powered fuel cell developed jointly by Ford and XCELLSiS. Last November, a production prototype of this vehicle was delivered to the California Fuel Cell Partnership. The company expects to offer fuel-cell vehicles to customers by 2004.
Fuel Cells In The Future
Ford isn't the only car company banking on fuel-cell technology. DaimlerChrysler has committed to building fuel-cell buses in 2002 and fuel-cell cars in 2004. Of course, the success of these vehicles depends upon the creation of a refueling infrastructure, which presents challenges not faced by either HEVs or pure electric vehicles.
But it's not unlikely that HEVs and electric vehicles will lay the groundwork for fuel-cell vehicles as many of the technologies employed in developing hybrids and electrics may be applied in the design of fuel-cell cars. Aside from the technical feasibility of building these vehicles, there's also the issue of market acceptance. As car suppliers and consumers gain confidence in alternatives to conventional vehicles, the industry's willingness to push the technology envelope will grow. That type of environment will foster the development of HEVs, electrics, fuel-cell cars, and other approaches that for now are merely pipe dreams.
- "Consumers, Electronics, and the Link to Hybrid Vehicles and the Environment," by Gary A. Cameron and Richard Lind, Delphi Automotive Systems. Convergence2000 proceedings, p. 279-286. (Paper 2000-01-C045 available from SAE.)
- "Solid Oxide Fuel Cell Auxiliary Power Unit-A Paradigm Shift in Electrical Supply for Transportation," by James Zizelman, Jean Botti, Delphi Automotive Systems and Joachim Tachtler and Wolfgang Strobl BMW Group. Convergence2000 proceedings, p. 471-479. (Paper 2000-01-C070 available from SAE.)
- "Power electronics and Electric Machinery Innovations—U.S. Government's Role in PNGV," by Donald J. Adams, Oak Ridge National Laboratory, Convergence2000 proceedings, p. 427-435. (Paper 2000-01-C063 available from SAE.)
For a list of relevant manufacturers and organizations see page 6 on the web.
|Companies And Organizations Mentioned In This Report|
Acme Electric Corp.
Argonne National Laboratory
Contact Max Gates
[email protected]sler. com
Daewoo Motor Corp.
Delphi Automotive Systems
Ford Motor Co.
Media Information Center
General Motors Corp.
Contact Kristi Thoel
(do keyword search for
Oak Ridge National
Power Electronics and
Contact Donald J. Adams
Ovonic Battery Co.
Partnership for a New
Contact Patrick Lloyd
Saft America Inc.
+33 1 49 15 36 00
Society of Automotive
For Convergence2000 papers
or full proceedings call
T/J Technologies Inc.
Toyota Motor Corp.