3 Vehicle Engineering Developments
Systems models integrate models of individual vehicle and power-train components to predict component and overall vehicle system performance. They are used to examine the effect of varying individual component design and performance characteristics on overall system performance. Thus, they are used to develop optimum system configurations and the component performance specifications for such configurations for various types of vehicles over various driving patterns. They can also provide information on system cost and reliability, using hypothetical or actual future components, to perform trade-off studies.
The PNGV Systems Analysis Team has developed the PSAT (PNGV Systems Analysis Tool) model over the past several years for these purposes. This tool is now well developed and is being used to further the objectives of the PNGV program. PSAT is a driver-driven or forward model in which the performance of the vehicle and of its components is determined based on vehicle driver inputs. In 2000, DOE assumed responsibility for the ongoing development of the PSAT model in parallel with another earlier vehicle systems model, Advisor, developed by the National Renewable Energy Laboratory (NREL). Advisor is a backward, or drive-cycle-driven, model in which the power-train performance is calculated from the torque and speed requirements needed to drive the vehicle through the specified drive cycle.
In its last report, the committee recommended greater emphasis on validating PSAT component and overall vehicle predictions, developing emissions modeling capabilities, developing a generic system and subsystem cost model, and increasing fuel cell component and system modeling (NRC, 2000). During 2000–
2001, substantial emphasis was placed on validating the PS AT model, using data from the Toyota Prius and Honda Insight HEVs and the Ford P2000 fuel cell vehicle. Vehicle testing and simulation evaluations are still in progress, and the results to date are encouraging.
Major efforts have been initiated on modeling emissions from internal combustion engines and their exhaus-gas after-treatment systems (catalysts and particulate traps). While this is a challenging technical task, the initial progress is encouraging, and approaches have been developed that should provide useful estimates of vehicle emissions to assess compliance with future emissions standards. It is apparent that the challenge of modeling the effects of engine acceleration and deceleration transients on vehicle emissions with HEV systems is less severe than with stand-alone engine transient modeling because the HEV system reduces the impact of these vehicle drive transients on the engine. However, the HEV internal combustion engine undergoes many start-ups and shut-downs during normal driving. Modeling the emissions produced by these events is especially challenging. The committee is encouraged by this progress in emissions modeling and recommends continuing emphasis on this topic. One important use for a vehicle emissions model is to quantify the trade-off between efficiency and emissions for the various propulsion system options, as discussed in Chapter 5.
Fuel cell HEV system simulation studies of a large sport utility vehicle (SUV) are under way to examine the trade-off in relative sizing of the battery pack and fuel cell. This indicates that a useable fuel cell HEV system model is now available. An especially important fuel cell system modeling area that was not part of the fuel cell modeling review is the liquid fuel reformer system. An effective gasoline-to-hydrogen reformer is a critical component in the most practical shorter-term fuel cell system because it avoids the challenges of developing a hydrogen production and distribution system and the need to store hydrogen on the vehicle. The committee encourages increased effort in modeling fuel cell system-component and overall system performance and especially in the fuel-reformer technology area. Because fuel cell technology is developing rapidly, well-validated system models are important tools for extrapolating from the performance of current prototype system data to likely future system performance.
Progress in cost modeling was not as encouraging as in the areas described above. However, a Cost Analysis Task Group has recently been formed. The committee urges that a framework be developed for using the systems model to assist in cost estimation studies for the internal combustion engine (ICE) and fuel cell HEV systems, and the effort on detailed modeling required to implement effective cost models should be intensified.
HYBRID PRODUCTION VEHICLES
Since the last committee report (NRC, 2000), both Toyota and Honda have introduced HEVs into the U.S. market. An overview of these two vehicles illus-
trates the wide range of design philosophies and component configurations that are classified as HEVs. It also points out the differences between the Toyota and Honda hybrids and how they compare to developments being pursued by the USCAR partners.
In the simplest terms, an HEV power train can be defined as a power propulsion system in which both an engine and an electric motor work together to propel the vehicle. Hybridization allows three distinct design principles to be incorporated into the vehicle: engine downsizing (by using electric power assist), battery-only electric driving, and regenerative braking. Additional approaches to improve efficiency, such as engine stop/start at idle and operating along the best efficiency versus power curve, may be incorporated into hybrid power trains, but they are not exclusive to them.
The Honda HEV (the Insight) and the Toyota HEV (the Prius) are smaller vehicles than the PNGV development vehicle (the family sedan). The Prius is advertised as a five-passenger sedan, albeit small, while the Honda Insight is a two-passenger sedan. The Prius has four doors and weighs approximately 2,700 lb, while the Insight is a two-door vehicle that has very limited interior space and weighs approximately 2,125 lb. Both use relatively conventional body and structural design and materials. Although exact numbers are held in confidence, it is generally accepted that both vehicles are being sold at a loss to the respective parent companies.
An examination of the power-train configurations of the two vehicles helps to illustrate the wide range of power-train configurations that are possible under the HEV classification, as well as the different optimization schemes that can be employed for improving fuel economy (see Figures 3–1 and 3–2). Both vehicles are parallel hybrid configurations; that is, in principle the vehicles could be driven either by the engine or the electric motor or both.
Gas mileage tests of the two vehicles have recently been performed and reported by Argonne National Laboratory (ANL) (Duoba and Ng, 2001; Duoba et
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al., 2001). Fuel-economy (gas-mileage) data were presented for three driving cycle tests: the Japanese 10–15 mode test, the Federal Test Procedure (FTP), and the highway test. Typically, in the United States a combined schedule of 55 percent urban and 45 percent highway fuel economies is used as the basis for vehicle gas-mileage comparison. Even though the reported results do not represent EPA-certified numbers, they are instructive for comparison purposes. The FTP fuel economy results were 2.07 gallons per 100 miles (48.2 mpg) and 1.65 gallons per 100 miles (60.7 mpg) for the Prius and Insight, respectively, somewhat lower than the results cited in the committee's sixth report (NRC, 2000). The difference presumably is caused by the variation in production vehicles. The FTP fuel economy results are impressive; however, it is also insightful to observe the differences in fuel economy of the two vehicles for the different types of driving, for example, highway driving versus city driving.
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The Honda Insight is described by its manufacturer as having an integrated motor assist (IMA). The motor, integrated with a small engine, provides acceptable vehicle acceleration and recaptures energy during braking operations. The regenerative braking is simply done in parallel with the friction brake. In fact, if the driver disengages the clutch during braking, there is no regenerative braking. The Insight does not use the electric motor to produce power at low vehicle speeds or at low engine loads. The primary benefit of this hybridization scheme is reducing the engine size, thereby allowing it to operate more efficiently. As a result the driver benefits from excellent gas mileage during steady-speed driving, with good transient response supplied by the motor assist. Steady-speed driving tests of the Insight resulted in very low fuel consumption (high gas mileage).
In contrast to the Insight, the Toyota Prius uses a much more complicated power train and system control logic. Toyota uses a planetary gear system to
couple the engine, generator, and motor in a way that provides a continuously variable transmission (AuYeung et al., 2001). This torque split is shown in Figure 3–2. In this configuration electrical power is required to keep the effective gear ratio fixed during operation at steady vehicle speed. This arrangement provides a great deal of flexibility. The engine can be turned off and on rapidly without using a clutch, electric power can be generated when the vehicle is at rest, and the vehicle can be driven in the electric-only mode. The regenerative braking scheme is significantly more complicated than that of the Insight. It is an active system that diminishes the friction braking depending on the power absorption capability of the electrical system and the driver-demanded deceleration rate to maximize the amount of energy recovery. This vehicle power train is optimized for both stop-and-go as well as low-speed driving. As part of the vehicle reconfiguration for introduction into the U.S. market, Toyota installed a larger engine to accommodate a higher percentage of highway driving, more typical in this country compared with Japan. When one considers the different approaches taken in hybridizing the two vehicles, it is perhaps not surprising that the Prius gets better gas mileage than the Insight on city-driving dynamometer tests, even though it is a larger and more massive vehicle (Duoba et al., 2001).
One approach to putting the comparison between the Honda Insight and the Toyota Prius in perspective is to describe the degree of hybridization of each vehicle. Decisions on the extent to which the fuel economy benefits of hybridization are to be incorporated into the vehicle's power train must be countered by assessments of the increased complexity, expense, and diminishing return of implementing the larger degree of hybrid technologies. From this perspective it becomes convenient to describe the approach to vehicle hybridization as a continuum, from no hybridization, to mild hybridization, to extensive hybridization. In this continuum the Honda Insight would be classified as a very mild hybrid and the Toyota Prius would be classified as having moderate hybridization.
How do these two vehicles compare with the hybridization effort being pursued by the PNGV partnership? There are similarities and some major differences. The PNGV concept cars are more similar to the Prius. In pursuit of maximum fuel economy, the concept cars incorporated all the design principles allowed by hybridization, namely, engine downsizing, battery-only electric driving at low loads, and regenerative braking. However, the extent to which the individual vehicles are hybridized and the control logic being pursued for each PNGV concept car are quite different. To meet the Goal 3 fuel economy target the PNGV hybrid vehicles will require a much larger degree of hybridization than has been demonstrated in either the Insight or the Pruis. The PNGV hybrid power trains and control systems are targeted for a different class of vehicle and a more lofty mileage target than those pursued by the Prius and Insight. The Toyota Prius and the Honda Insight are both smaller than the PNGV target vehicle; yet neither of them comes close to the PNGV target gas mileage of 80 mpg (1.25 gallons per 100 miles) for the FTP.
The USCAR partners face the same problem as both Honda and Toyota, in that they must overcome significant cost barriers in order to produce HEVs that have little or no cost penalty over today's single-power-plant vehicle. In this context the HEVs introduced by Toyota and Honda represent very interesting case studies. Both companies are gaining experience in manufacturing, marketing, and maintaining HEVs by subsidizing these vehicles and certainly must be looking for ways to migrate aspects of this technology into the rest of their vehicle fleet for competitive advantage. Consequently, the introduction of these hybrid vehicles is viewed as a significant occurrence and should be followed closely.
CONCEPT CARS AND PRODUCTION PROTOTYPES
The concept cars introduced in 2000 marked the completion of a major milestone for the PNGV program. Each company designed a car meeting its own vision of how to best approach the challenge of Goal 3 using both proprietary technology and technology developed in the cooperative program. Subsequent development of these vehicles has proceeded during 2000–2001. A concept car is the embodiment of a complete design using component part designs that represent final production intent but are not yet validated for production readiness. These concept cars have been used to validate simulation models, develop control systems, run performance tests, and evaluate driving characteristics. This activity generates knowledge from which components can be modified and a production-ready design can be developed. Because of its proprietary nature, most of this activity is carried out as a part of the PNGV but is not a partnership activity.
The next step called for in the PNGV Program Plan is the design, development, manufacture, and assembly of a production prototype by 2004. A production prototype is a car with components that have been validated as production ready, meaning that, at a minimum, a production process has been identified that is capable of manufacturing all the car's components in volume and with the required quality. This prototype car should also demonstrate all the characteristics required in order to make it an attractive, salable product. There should be a well-defined path to resolve any deficiencies from this standard.
At the time of the committee review the car companies were not ready to discuss their plans for moving from the concept-car stage into a production prototype. In proprietary discussions with the committee each company reviewed its plans to move major portions of the technology developed in the PNGV program into a variety of production vehicle programs. None of these vehicle programs was a simple extension of the power-train and vehicle design aspects of the concept cars.
As discussed in Chapter 4, “Program Overview,” the committee believes that it is unrealistic to expect production prototypes to be built in the configurations of
the concept cars. The cost and complexity of such vehicles, and the changed market environment since the program began, call into question the wisdom of following this original plan. Each of the companies is considering how to deal with this issue. The logical business decision is to apply derivative forms of the technology developed in the PNGV program to types of vehicles in which the increased costs may be better supported by market forces. This activity is exactly what was envisioned under Goal 2, but it leaves open the question of what course should be pursued under Goal 3. The committee recommends that the parties involved redefine this goal along the lines suggested in Chapter 4.