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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine Board on Energy and Environmental Studies Mailing Address: 500 Fifth Street, NW Washington, DC 20001 www.nationalacademies.org February 14, 2008 The Honorable Nicole Nason Administrator National Highway Traffic Safety Administration U.S. Department of Transportation 1200 New Jersey Avenue, S.E., West Building Washington, D.C. 20590 Dear Administrator Nason: The National Highway Traffic Safety Administration requested that the National Academies provide an objective and independent update of the 2001 National Research Council report Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards and add to its assessment other technologies that have emerged since that report was prepared. The National Research Council has therefore formed the Committee on Assessment of Technologies for Improving Light-Duty Vehicle Fuel Economy, and the committee has begun its review of vehicle technologies. In this letter, the committee provides, as called for in its task statement (Appendix C), its interim assessment of the technologies to be analyzed in the final report and of the computational models that will be used in its analysis. The committee presents this letter as its preliminary assessment of technologies and potential fuel-economy benefits. The estimated fuel-economy benefits, typically reported as the benefits that would be realized according to the procedure used to certify vehicles with respect to federal fuel-economy standards, reflect those that have been presented in the literature and have been presented to the committee. They represent the preliminary judgment of the committee based on those sources. The committee will continue to revise the list of technologies and fuel-economy benefits as it completes its study and writes its final report, to be provided in late spring of 2008. MOTIVATION AND PURPOSE OF INTERIM REPORT In the wake of the 1973 oil crisis, Congress passed the Energy Policy and Conservation Act in 1975 as a means of reducing the country’s dependence on imported oil. The act established the Corporate Average Fuel Economy (CAFE) program, which required automobile manufacturers to increase the average fuel economy of passenger and nonpassenger vehicles sold in the United States to standards of 27.5 miles per gallon (mpg) for passenger cars and 22.5 mpg for light trucks. The standards are administered by the U.S. Department of Transportation (DOT) on the basis of the U.S. Environmental Protection Agency (EPA) city-highway dynamometer test procedure. In 2000, Congress called on the National Academies to conduct a study of the impact and effectiveness of CAFE standards, looking both historically and to the future. The study resulted in the 2001 National Research Council report, which included a chapter focused on the potential of various technologies to improve the fuel economy of light-duty vehicles.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report The rapid rise in gasoline and diesel fuel prices experienced during 2005-2007 because of large increases in global demands and Middle East oil producers’ policies on oil production continues to make vehicle fuel economy an important policy issue, and growing recognition of the climate-change issue has drawn more attention to fuel economy. The recently passed Energy Independence and Security Act of 2007 requires DOT to raise vehicle fuel-economy standards, starting with model year 2011, until they achieve a combined average fuel economy of at least 35 mpg for model year 2020. A recent Supreme Court decision also requires EPA to regulate greenhouse-gas emissions from new light-duty vehicles under its Clean Air Act authority.1 DOT, through NHTSA, has continued to review estimates of the potential for various technologies to improve fuel economy. And a number of other investigations have been conducted to assess fuel economy or greenhouse-gas reduction potential, especially for California’s recent initiative to reduce greenhouse-gas emissions in the state. NHTSA would like to keep up to date on the potential for technologic improvements as it moves into planned regulatory activities. It was as part of its technologic assessment that NHTSA asked the National Academies to update the 2001 National Research Council report and add to its assessment other technologies that have emerged since that report was prepared. The task statement directs the committee to estimate the efficacy, cost, and applicability of technologies that might be used over the next 15 years. The list of technologies includes diesel and hybrid electric powertrains, which were not considered in the 2001 assessment. Weight and power reductions also are to be included. Updating the fuel economy-cost curves for different vehicle size classes that are in Chapter 3 of the 2001 report is central to the request. The current study focuses on technology and will not consider CAFE issues related to safety, economic effects on industry, or the structure of fuel-economy standards; these issues were addressed in the earlier report. It will look at lowering fuel consumption by reducing power requirements through such measures as reduced vehicle weight, lower tire rolling resistance, or improved vehicle aerodynamics and accessories; by reducing the amount of fuel needed to produce the required power through improved engine and transmission technologies; by recovering some of the exhaust thermal energy with turbochargers and other technologies; and by improving engine performance and recovering energy through regenerative braking in hybrid vehicles. This letter constitutes the interim report called for in the task statement. It discusses the technologies to be analyzed in the final report, the types of vehicles that may use them, the estimated improvements in fuel economy that may result, and the computational models that will be used in analysis. In producing this interim report, the committee met four times and received presentations from automobile manufacturers, suppliers of technologies to the automobile industry, federal agencies, and researchers in universities and government laboratories. The agendas for the committee’s public sessions are shown in Appendix D and demonstrate the committee’s commitment to hearing from diverse experts. There are two primary ways to show the effectiveness with which fuel is used in vehicles: fuel economy and fuel consumption. Both are used in this report. Fuel-economy standards are expressed as miles driven per gallon of fuel consumed. Fuel consumption is the inverse measure: the amount of fuel consumed in driving a given distance, such as gallons consumed per 100 miles traveled. Fuel consumption is a fundamental engineering measure and is useful because it is related directly to the goal of decreasing the amount of fuel required to travel a given distance.2 Figure 1 shows how the two measures—miles per gallon and gallons per 100 miles—are related to one another: an increase in fuel economy from 25 mpg to 40 mpg is a 60% improvement in fuel economy and a 38% improvement in fuel consumption. Note that the curve is relatively flat beyond 35 mpg. Although this report describes changes primarily in terms of fuel consumption, in many cases it also expresses them in terms of fuel economy. 1 See Massachusetts et al. v. Environmental Protection Agency et al. EPA can avoid promulgating regulations only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do. Such an explanation must be grounded in the provisions of the Clean Air Act, not based on policy or other grounds. 2 Furthermore, the “average” in CAFE standards is determined as the sales-weighted average of fuel consumption converted into fuel economy.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report FIGURE 1 Relationship of fuel consumption to fuel economy. FUEL-ECONOMY TECHNOLOGIES On the basis of a preliminary analysis of sales information on light-duty vehicles, the committee selected the vehicle classes shown in Table 1 for use in assessing the costs and fuel-economy benefits of engine, transmission, and vehicle technologies. Individual vehicles in those classes will be used for the modeling efforts described later. Fuel-Economy Technologies for Spark-Ignition Engines Although new vehicle powertrain systems, such as those relying on hybrid electric and diesel technologies, have begun penetrating into the U.S. light-duty vehicle fleet, the vast majority of vehicles that make up the fleet are powered solely by conventional gasoline-powered spark-ignition engines. Thus, any discussion of fuel-economy-improvement technologies for light-duty vehicles must focus extensively on such engines. Table 2 lists some of the techniques and technologies that will be considered in the committee’s final report. It is important to note that engine layouts and base powertrain configurations affect the types of improvements possible and their costs and benefits. Those issues will be discussed more extensively in the final report. Some well-developed techniques involve fast-burn combustion systems combined with strategic use of exhaust-gas recirculation (EGR) at part load. Those techniques were introduced in the 1980s and have traditionally afforded a fuel-economy improvement of 3-5% over vehicles not using these techniques. All comparisons that use other fuel-economy enhancements should refer to that base condition because it is used in the vast majority of light-duty vehicles. Typical implementation of fast-burn combustion involves the creation of large-scale in-cylinder flows that are degraded into small-scale turbulence just before combustion. When this fluid-mechanical approach is impractical, fast-burn combustion can be implemented with multiple spark plugs (not common but used occasionally in automotive applications). EGR, which reduces energy losses when the engine is operating under partial load conditions, then is introduced up to the combustion-stability limit. Typically, fast-burn combustion and EGR enable a compression-ratio increase of about 0.5, the maximum possible without increasing engine knock to an unacceptable level. A higher compression ratio generally allows increased thermodynamic efficiency.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report TABLE 1 Possible Classes of Light-Duty Vehicles for Analysis Vehicle Class Representative Models Type of Vehicle Compact car Honda Civic, Ford Focus, Toyota Corolla Passenger car Midsize car Toyota Camry, GM Malibu, Nissan Altima Passenger car Luxury car GM Cadillac CTS, Mercedes E350, Nissan G35 Passenger car Minivan Chrysler Town and Country, GM Uplander, Toyota Sienna Light truck Small sports utility vehicle (SUV), 2WD Honda CR-V, Ford Escape, Toyota RAV 4 Light truck Large SUV, 2WD Ford Expedition, Chevrolet Tahoe, Dodge Durango Light truck Crossover vehicle Toyota Highlander, Honda Pilot, Ford Edge Light truck Medium SUV Ford Explorer, Chevrolet Trailblazer, Toyota 4 Runner Light truck Large pickup, 2WD Ford F150, Dodge RAM 1500, Chevrolet Silverado Light truck Cylinder deactivation is an attractive and cost-effective option for increasing fuel economy when applied to overhead-valve engines. This technique essentially turns a six-cylinder (V6) or eight-cylinder (V8) engine into a three- or four-cylinder engine at light engine loads, improving fuel economy by increasing the load on the active cylinders and creating higher intake manifold pressures, thereby reducing pumping losses. It is most beneficial when applied to vehicles with higher power-to-weight ratios. Although cylinder deactivation may increase noise, vibration, and harshness (NVH), active engine mounts or active noise-cancellation techniques can moderate these effects and enable aggressive use of the strategy. Another technology, direct injection, offers the potential to increase both fuel economy and engine power output. As the fuel is forced to vaporize in the cylinder (as opposed to the intake port), an increase in the knock-limited compression ratio is possible. The technique can also increase the volumetric efficiency of the engine. Direct-injection gasoline engines have been studied for many years, but their application to mass-produced vehicles has been limited largely by injector deposit problems associated with hot shutdowns. Improvements in injector design, control strategies, and fuels, such as locating the fuel injector in the coolest part of the cylinder head, appear to be alleviating such problems. Turbocharging with downsizing is another technique for raising fuel economy. The improved size-to-output efficiency of a turbocharged engine allows a smaller displacement engine to be used for the same power output and thus reduces fuel consumption. Fuel consumption in a given vehicle is lower in smaller engines primarily because of higher manifold pressure (reduced pumping losses) and smaller contact area of moving surfaces (reduced friction losses). Turbocharging is most beneficial when it is applied to vehicles subjected to highly diverse driving conditions and if it permits the use of a smaller-displacement engine, although the effects of and remedies for potential launch-performance shortfall need to be investigated. Valve-event manipulation (VEM) technologies can also improve fuel economy. They come in many forms with many names (such as variable valve timing and variable valve lift). They can improve fuel economy by improving volumetric efficiency and reducing pumping losses. Their desirability tends to be related to the engine architecture and other characteristics of a vehicle. There is functional overlap among many VEM technologies. Spark-ignition engines used in any light-duty vehicle can benefit from some use of VEM, although some of the benefits may be achieved by other means, such as use of variable-geometry intake manifolds.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report TABLE 2 Technologies for Reducing Fuel Consumption in Spark-Ignition Enginesa Item Fuel-Consumption Reduction Comments Fast combustion with high dilution tolerance 3-5% vs. engines not specifically optimized with respect to this item This is the 2007 datum against which other items should be referenced, in that the vast majority of today’s vehicles already use these techniques Cylinder deactivation 3-8% depending on power-to-weight ratio Most cost-effective when applied to overhead-valve V6 and V8 engines NVH and drivability issues Direct injection 1-3% for constant-displacement engine Need for high-pressure fuel pump can increase parasitic loss, and increased volumetric efficiency increases pumping loss Turbocharging and downsizing 3-7% for equal performance at 0-60 mph Piston-oil squirters, oil coolers, and intercoolers will contribute to system merits About 1% increase in fuel consumption without engine downsizing Valve-event manipulation 1-7% based on pumping-loss reduction at part load Implementation methods include cam phasers and two-step-lift cams; timing is important, and lift is merely consequence of duration Small performance gains at wide-open throttle Benefit varies, depending on degree of engine downsizing; effects on different vehicle-performance measures need further analysis Intake-valve closing Valve-overlap control 0.5-1.5% above conventional EGR system Implementation includes exhaust-only and dual-cam phasers Reduces pumping losses Intake-valve “throttling”; implementations include analogue or stepwise control (an alternative to conventional pressure throttling) 3-8% owing to (net) pumping-loss reduction Goal is to shorten intake-valve-lift duration; short durations may reduce pumping losses; reduced valve lift is simply a consequence of shorter durations Manufacturing tolerance control is critical As intake-manifold vacuum decreases, alternative means must be found to implement power brakes and positive crankcase-ventilation valves Benefit will vary with engine size Other valve events: exhaust-valve closing, exhaust-valve opening, intake-valve opening Independently, relatively small effects outside the aforementioned Timing of exhaust-valve closing is important for maintaining peak power Friction reduction 0.3-1% owing to reduced-viscosity lubricants; other approaches require further investigation Roller-follower valve trains and piston-kit friction-reduction measures were nearly universally implemented in the middle 1980s Parasitic-loss reduction Electric coolant pump To be determined Camless valve actuation To be determined Variable charge motion To be determined Homogeneous-charge compression ignition To be determined aImprovements are over a 2007 naturally aspirated gasoline vehicle engine of similar performance characteristics.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report Other techniques for improving fuel economy include reducing internal friction and reducing parasitic losses. Major friction-reducing options, such as roller-follower valve trains and technologies that apply to the piston assembly, were implemented in the 1980s, but there still remain opportunities to increase fuel economy by reducing friction. The replacement of hydraulic water pumps with electric coolant pumps and replacing the thermostat with electronic coolant flow control could also improve fuel economy, although they may cause higher parasitic losses through the electric system than they relieve through mechanical decoupling; this issue will be explored in the final report. Three other technologies that could enhance fuel economy—variable compression ratios, camless valve trains, and homogeneous-charge compression ignition engines—have undergone substantial research efforts over decades. The committee will discuss them and their potential for improving fuel economy over the next 15 years in its final report. Fuel Economy Technologies for Compression-Ignition Engines The earlier NRC report did not consider diesel-powered, compression-ignition engines. At the time of that report, the technology available could not overcome the tradeoff between NOx and particulate emissions typical of light-duty diesel engines. The motivation for including light-duty diesel technology in the new report stems from the fact that the light-duty diesel vehicles in production and in widespread use in Europe have already demonstrated a 30-40% reduction in fuel consumption, depending on engine size, compared with 2007 model-year gasoline engines. In addition, the emissions performance of diesel vehicles has improved. In the United States, diesel technology holds the potential to improve fuel economy (compared with conventional gasoline vehicles on the market in the United States) while improving some aspects of vehicle performance. Performance advantages include higher low-end torque and possibly greater durability. U.S. manufacturers have committed to offering diesel technology as a higher-performance alternative to large V8 gasoline engines that also enables the downsizing of vehicle engines. Some of the technologies for reducing fuel consumption in diesel engines that the committee is assessing are shown in Table 3. Recent developments in NOx after-treatment systems further improve the prospects for diesel light-duty vehicles. Production vehicles using improved after-treatment systems have proved able to meet U.S. emissions regulations. Work on new technologies for controlling particulate and NOx emissions is continuing; most of it is in the proprietary development phase. Little detail is available in the open literature on the cost or effectiveness of these technologies. Assessments of the devices will need to consider any fuel-economy penalties associated with flow restrictions and the use of fuel or reagent for device regeneration, although developments to date appear to have minimized such penalties. It should be noted that part of the fuel-economy benefit of diesel engines stems from the differences in energy content between gasoline and diesel fuels. On the average, the energy content of gasoline and diesel fuel per unit mass (MJ/kg) are similar, but diesel fuel’s specific gravity is typically 0.82-0.85 and that of gasoline 0.72-0.78, so the energy content of a gallon of diesel fuel (MJ/gal) is about 11% higher than that of a gallon of gasoline. Moreover, the carbon content of a gallon of diesel fuel is about 14.7% higher than that of a gallon of gasoline. If carbon dioxide emissions are the concern rather than fuel economy, the higher energy density and carbon content of diesel fuel will have to be taken into account. Transmission Technologies for Improving Fuel Economy Transmission technologies that can improve fuel economy involve increasing electronic controls, continuing to reduce the mechanical losses in transmissions, and improving the mating of transmission operations with engines. Table 4 lists some of the technologies. In both automatic and manual transmissions, increasing the number of gear ratios can allow the engine to operate closer to its efficient optimum at a wider variety of speeds and thus allow improvements in fuel economy. Many manual
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report TABLE 3 Technologies for Reducing Fuel Consumption in Compression-Ignition Enginesa Technology Fuel-Consumption Reduction Comments Turbocharged diesel (current standard diesel technology for light-duty vehicles) 20-40% 2000 bar piezoelectric injectors To be determined Technology allows improved emission control, which has indirect effect on fuel economy Engine shut-off during idle 2% for city cycle Baseline diesel-similar performance characteristics Ceramic glow plugs To be determined Decreased fuel sensitivity, improved cold start Two-stage turbocharging To be determined Performance enhancement Improved particulate control To be determined Required for emissions certification Improved particulate and NOx after-treatment To be determined Required for emissions certification Improved starter-alternator To be determined Required for idle-stop operation Lean NOx trap To be determined Candidate after-treatment for NOx Urea selective catalytic reduction To be determined Candidate after-treatment for NOx Hydrogen-rich reactant to reduce NOx To be determined Candidate after-treatment for NOx Homogeneous-charge compression ignition combustion To be determined Exploration to reduce after treatment cost Diesel hybrid 5-15% Improvement is over a diesel vehicle of similar performance characteristics aImprovements are over a 2007 naturally aspirated gasoline vehicle engine of similar performance characteristics. TABLE 4 Transmission Technologies for Reducing Fuel Consumptiona Technology Fuel-Consumption Reduction Comments Five-speed automatic transmissions 2-3% Technology can also improve vehicle performance Six-speed automatic transmissions 3-5% Seven-speed automatic transmissions 5-7% Eight-speed automatic transmissions 6-8% Automated manual transmissions (six-speed) 6-8% Continuously variable transmissions 1-8% Some issues related to differences in feel and engine noise; improvements depend on engine size Early torque converter lockup 0.5% NVH issues Aggressive shift logic 1-5%b Potential effects on drivability aImprovements are over a 2007 naturally aspirated gasoline vehicle engine of similar performance characteristics. bPotential benefits of aggressive shift logic can vary even more widely depending on how aggressively it is implemented and the baseline against which fuel-economy benefits are estimated.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report transmissions today have four or five speeds, sometimes referred to as overdrive gearing. With the lower gear ratios, the engine is actually turning slower than the drive shaft and rear axle. The low gear ratios and lower engine speeds permit substantial improvements in fuel economy. The five-speed automatic transmission is already a standard for many vehicles; six-, seven-, and eight-speed automatic transmissions have been available on luxury cars and are penetrating into the mainstream market. Another technology is the automated manual transmission (AMT), which attempts to combine the efficiency of a manual transmission with the seamless shifting of an automatic transmission. The AMT does not require the driver to actuate the clutch or manually shift gears; instead, these functions are carried out with a hydraulic system or an electronically controlled electric motor. Most current transmissions feature a discrete number of gear ratios that determines the ratio of engine speed to vehicle speed. In contrast, a continuously variable transmission (CVT) offers a seemingly infinite choice of ratios between fixed limits, which allows engine operating conditions to be optimized for fuel economy. CVT technology has tended to be used in lower-horsepower vehicles because of materials limitations. Other fuel-economy improvements can be implemented through electronic transmission control (ETC), which is part of an automatic transmission. Electronic sensors monitor vehicle speed, gear-position selection, and throttle opening and send this information to the electronic control unit (ECU). The ECU controls the operation of the transmission. Two measures that can improve fuel economy, early lock-up and aggressive shift logic, can be implemented by the ETC through the ECU; both measures can also increase NVH and affect drivability. Vehicle Technologies for Improving Fuel Economy Vehicle technologies focus on nonpowertrain methods of reducing fuel consumption. The committee considers car-body design (aerodynamics and mass), vehicle interior materials (mass), tires, and vehicle accessories (power steering and heating, ventilation, and air-conditioning [HVAC] systems) to offer the greatest opportunity for achieving near-term, cost-effective reductions in fuel consumption. Those technologies are summarized in Table 5. The U.S. Council for Automotive Research and the U.S. Department of Energy continue to research ways to reduce body mass by substituting new materials—such as high-strength steel, advanced high-strength steel, aluminum, magnesium, and composites—for current materials. The materials industries also conduct research to advance new materials (for example, through the Auto-Steel Partnership, the Aluminum Association, and the American Chemistry Council). Increased costs for lighter and stronger materials result from higher material costs and higher costs of component fabrication and joining. Estimates of the body-mass reduction that can be achieved in the near term vary from 10% (with mostly conventional and high-strength steels) to 50% (with a mostly aluminum structure). Even greater reductions are feasible, but they require expensive composite structures that involve such materials as carbon fiber. A midsize-car body with closure panels (no trim or glass) can weigh roughly 800 lb (about 25% of the vehicle curb weight). Vehicle testing has confirmed the reductions in fuel consumption associated with reductions in vehicle mass. (See, for example, Pagerit et al., 2006 and U.S. EPA, 2006). For example, vehicles powered by internal-combustion engines (ICEs) can reduce fuel consumption by about 0.1 gal/100 miles driven for each decrease of 190 lb in mass. Potential improvements are smaller for hybrid vehicles because some of the increase in kinetic energy is captured by regenerative braking. If the total mass reduction is significant, a secondary benefit can accrue from reducing the size of the needed powertrain, braking systems, and crash-management structures; the secondary benefit is difficult to estimate but potentially can approach an additional 30% reduction in mass. Vehicle interiors also offer opportunities to reduce vehicle mass. Some changes can be implemented for little cost, and others high cost. For example, composite-intensive instrument panels, recycled seating materials, and lighter-weight trim panels can reduce mass by tens of pounds at virtually no cost. However, those options tend to affect vehicle character, and additional costs may be incurred in offsetting negative aesthetics.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report TABLE 5 Vehicle Technologies for Reducing Fuel Consumptiona Technology Fuel-Consumption Reduction Comments Mass reduction: body structure, closure panels, bumpers 3-7% Applicable to all sizes of vehicles; high-strength steel, aluminum, magnesium, and composites offer advantages, but generally at higher costs Mass reduction: interior body and trim (seating, trim, instrument panel, glass) 1-4% May incur adverse effects on aesthetics and vehicle character; material substitution and design tradeoffs; polycarbonate substitution for glass may be feasible Improved aerodynamics (coefficient of drag, Cd) 1-2% Affected significantly by vehicle design; affects vehicle character; varies with vehicle size Reduced tire rolling resistance 1-3% Need to investigate tradeoffs with tire wear and NVH Electric accessory technologies: power steering, power brakes, HVAC, thermoelectric materials 2-7% Opportunities for electrification and performance optimization of HVAC and power steering systems aImprovements are over a 2007 naturally aspirated gasoline vehicle engine of similar performance characteristics. The aerodynamic performance of a vehicle (represented by the coefficient of drag, Cd, which typically ranges from about 0.25 to 0.38 on production vehicles) depends on several factors. The primary influences are vehicle shape and height, but smaller influences come from, for example, external mirrors, rear spoilers, frontal inlet areas, wheel-well covers, and the vehicle underside. Vehicles with a high Cd may be able to reduce it by 5% or so (up to 10%) at low cost. The associated effect on fuel consumption and fuel economy could be 1-2%. A report on tires and fuel economy estimates that a 10% reduction in rolling resistance will improve fuel economy by 1-2% (NRC, 2006). The opportunity to improve fuel economy may differ between original-equipment tires and consumer-replaced tires because typical values of the coefficient of rolling resistance (Cr) associated with them differ. The total opportunity is defined by the fraction of the tires on the road that falls into each category. Tires with low rolling resistance do not appear to compromise traction but may wear faster than conventional tires. The incremental cost of low-resistance tires may not be great, but the cost-benefit tradeoff with increased wear and possibly NVH may be important to the consumer. Some automakers are beginning to introduce electric devices (such as motors) that can reduce the load on the engine, reduce weight, and optimize performance; the result is reduced fuel consumption. There may be an opportunity to decrease fuel consumption 3-4% with a variable-stroke HVAC compressor and better control of the amount of cooling and heating used to reduce humidity. Further reductions can be achieved by decreasing air-conditioner load through the use of low-transmissivity glazing, reflective “cool” paint, and cabin ventilation while parked. Electric power steering or electrohydraulic power steering might yield a decrease in fuel consumption of 3-5%; however, the benefits of electric or electrohydraulic power steering and greater efficiency in air-conditioning are not credited by current EPA fuel-economy tests (neither operates during the test), and they add cost, so manufacturers are reluctant to implement them. Recent improvements in thermoelectric materials for HVAC and exhaust-energy recovery appear promising with respect to the 15-year horizon and will be investigated further in the committee’s final report. Hybrid and Other Advanced Powertrain Fuel-Economy Technologies Hybrid electric vehicles achieve improved fuel economy by incorporating both an electric motor and an ICE in the drive train. In its most effective implementation, this permits the ICE to shut down
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report when the vehicle is stopped, permits brake energy to be recovered, permits the ICE to be downsized, and permits the ICE to function at more efficient operating points. As shown in Table 6, those operational characteristics in combination can result in a decrease in fuel consumption of 17-30%, depending on the vehicle class. Hybrid vehicles are the fastest-growing segment of the light-duty vehicle market, although they still make up less than 2% of the market in the United States. For purposes of this report, hybrid vehicles are defined as having one or more electric motors or generators and an ICE. There are four categories of hybrids, according to the size and function of the motor or generator: Type I—microhybrids. The starter and generator of a conventional vehicle are usually replaced with a single larger machine, still belt-driven and capable of both starting the engine and generating electric power. (Some microhybrid designs retain the starter for cold starts.) Fuel consumption is reduced by turning off and decoupling the engine at idle and during deceleration and by regenerating a small amount of braking energy (in some models). That reduces fuel consumption by about 6-9 %. larger machine connected between the engine and transmission. That enables some energy recovery Type II—integrated starter-generator (ISG). The starter and generator are replaced with a through regenerative braking, which results in a reduction in fuel consumption of 11-17% in the city driving cycle. Generally, these vehicles use a larger battery (such as a 42-V battery) and a higher voltage than gasoline or microhybrid vehicles. Type III—parallel hybrids. These generally involve two electric machines coupled to each other, the engine, and the vehicle wheels through a series of planetary gears and possibly clutches. Power flows to the wheels in parallel paths, directly through the mechanical gears or by conversion to electric power and back to mechanical power. Parallel hybrids allow a wide range of sizes of electric machinery and batteries. Typically, a much larger battery than in type I and type II hybrids is used (capacity, 1-2 kWh) at a voltage of 200-400 V. These hybrids often launch with electric power. The engine operates over a narrower speed and load range to improve efficiency and uses regeneration to recover braking energy. The reduction in fuel consumption is hard to estimate because all the systems discussed in the literature (such as the Prius, the Escape, and General Motors’ two-mode hybrid) involve changes in both the vehicle and the engine that contribute to their outstanding results. Reductions in fuel consumption can range from 17-30%. Type IV—plug-in hybrids. These optimally have a series configuration with the engine driving a generator that provides electric power to charge the battery. The wheels are driven by a large motor that gets its power from the battery through electronics. Because there is no mechanical connection between the engine and the wheels, the motor and the battery must be sized for the full torque and power needed by the vehicle. The potential advantages of this configuration are that a smaller engine may be used because it does not need to provide the full power needed for acceleration, the engine may be operated at its best efficiency point, and, with a larger battery, the vehicle can operate as a purely electric vehicle over a limited range (10-40 miles). The engine is used to recharge the battery. The battery can also be charged from the electric grid, in which case these vehicles are also known as plug-in hybrids. Such hybrids represent future technology and require a much larger battery than other hybrids (capacity, 4-20 kWh),3 depending on the desired electric-only range. General Motors has announced plans for such a vehicle for 2010 provided that a suitable battery is developed. Plug-in hybrids can also use a parallel configuration. For example, Toyota has announced plans for a plug-in hybrid for 2010 that will be built on a parallel hybrid architecture. 3 The Energy Independence and Security Act of 2007 defines a plug-in hybrid as a light-, medium-, or heavy-duty vehicle that draws motive power from a battery with a capacity of at least 4 kWh that can be recharged from an external source of electricity.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report TABLE 6 Reductions in Fuel Consumption of Hybrid Technologiesa Technology Fuel-Consumption Reduction (Combined City-Highway Cycle) Comments Type I—microhybrids, belt-driven starter-alternator (S/A) 6-9% In production Type II—integrated S/A (ISG) 10-17% In production Type III—parallel hybrid, NiMH battery 17-30% In production Type III—parallel hybrid, Li-ion battery 17%- (to be determined) Not in production; if successful, Li-ion batteries should be lighter and less expensive than NiMH batteries and have higher energy capacity Type IV—plug-in hybrid To be determined Highly dependent on battery developments, particularly Li-ion aImprovements are over a 2007 naturally aspirated gasoline vehicle engine of similar performance characteristics. Type I hybrids (microhybrids) use advanced lead-acid batteries designed for deeper and more frequent cycling than the conventional lead-acid batteries used for starting, lighting, and ignition in current vehicles. Type II hybrids (ISG) use either advanced lead-acid or nickel metal hydride (NiMH) batteries, depending on the required power and control philosophy. Type III hybrids of large commercial volume all use NiMH batteries; this technology is benefiting from improvements in cell packaging and cooling. There are no commercial type IV hybrids. Any type IV vehicles in use are conversions from type III. The current lithium-ion battery offers the promise of substantial improvements in both energy and power density relative to NiMH batteries. Several newer chemical formulations are showing promise because of their stability and performance at extreme temperatures; their cycle life expectancy and volume production costs are yet to be determined. For its final report, the committee will select hybrid vehicles for detailed analysis of the effect of hybridization on performance. Information from manufacturers, to the extent available, will be used to infer the performance improvements from hybridization that can be expected in the near term. Separately, the question of battery technology, a critical component of performance improvement, will be explored to determine the likely viability of batteries now being developed. Of particular importance is the prospect for the newer lithium-ion batteries, which, it is predicted, will substantially reduce safety problems and have longer cycle lives and considerably higher energy densities than the NiMH batteries now installed in all type III hybrids. Both all-electric and fuel-cell vehicles have the potential to reduce energy use and emissions (depending on how electricity and hydrogen are produced) and U.S. dependence on imported oil over the long term. The prospect of widespread introduction of all-electric vehicles is a function of the battery technologies discussed above. The commercial viability of the vehicles depends on a battery breakthrough. In the period of this study (15 years), all-electric vehicles probably will be limited to vehicles with less than full performance. In spite of the investment of hundreds of millions of dollars on the development of fuel cells by vehicle builders, equipment suppliers, and government organizations, serious problems requiring technical and economic resolution remain, including The higher cost of fuel cells than of other energy converters. The lack of a hydrogen-distribution infrastructure.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report The need for a low-carbon source of hydrogen (biomass or water electrolysis using electricity produced with low emissions). The need to demonstrate acceptable durability and reliability. The committee does not expect commercialization of fuel-cell vehicles or widespread marketing of all-electric vehicles before 2020 and therefore will not devote substantial resources to the assessment of these vehicles. MODELING OF FUEL-ECONOMY TECHNOLOGIES The 2001 National Research Council report was criticized by some in the automobile industry for its method of estimating fuel-economy improvements. In particular, it was claimed that stepwise application of technologies, the method used to estimate incremental improvements, could overestimate fuel-economy benefits. The committee that produced the 2001 report revisited the methodology in a public meeting, and a letter report released in 2002 reaffirmed the committee’s approach and general results. The issues have been revisited by the current committee, and it elected to review the attributes of several of the modeling approaches available. Modeling Approaches During the committee’s information-gathering, it has focused on two principal methods for assessing fuel-economy technologies for light-duty vehicles: Full-system simulation modeling (FSS). Partial discrete approximation (PDA). The committee has deliberated on the characteristics of the two approaches and how each might contribute to the study. The FSS approach attempts to capture the physics of the vehicle and powertrain system to some level of fidelity in the governing equations that are in the models, including the interactions of the various components in the vehicle (an “inside-out” approach). With the ability to simulate the interactions of vehicle subsystems directly, this approach has the potential to provide useful information about applying combinations of technologies to a given vehicle, to account for the nonlinear effects on overall vehicle performance that can result from the combinations, and to evaluate whether the estimated benefits of the individual technologies applied to vehicle subsystems are realized at the vehicle system level when technologies are combined. The approach may also provide useful information regarding potential fuel-economy improvements associated with new technologies or combinations of technologies when experimental data are sparse or unavailable. However, given the large number of simulations that would be needed, it will not be possible to use the FSS approach to develop the curves of cost vs. potential fuel-economy improvement called for in the study charge. The challenges related to implementing the FSS approach lie in obtaining sufficient specific data about the physical processes that are to be captured in the model equations, determining whether the data are of the appropriate type for the simulation (steady-state vs. transient data), balancing the fidelity of various component models within the vehicle system, and estimating how the modeling errors from each component accrue in the overall vehicle system simulation. Obtaining the data needed for FSS is often hampered by the fact that the owners of information consider it to be proprietary. Without valid models of the key components affecting fuel economy, FSS would not be able to predict either fuel-economy effects or synergies accurately. Not only must models of key components be accurate, but control strategies, such as transmission shift points and control of fuel injection and ignition timing, must be known. Such details also are generally considered proprietary by vehicle manufacturers. Lacking that information, numerous
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report engineering assumptions must be made at the subsystem model level, and these assumptions can significantly influence predictions. The PDA approach gathers data from a variety of sources on a wide array of proven technologies and uses lumped-parameter modeling to account for first-order interactions among technologies. The information used in the PDA approach includes vehicle-certification data collected by EPA (EPA, 2007), data from the literature, and data from manufacturers. As a general rule, the PDA method is limited to “proven” technologies, that is, technologies that have already been implemented by motor-vehicle manufacturers on some mass-produced vehicle that is available somewhere in the world. The method uses the available estimates of the effects of individual technologies and their synergies to estimate the expected fuel-economy improvement afforded by the technologies when combined in other vehicles (an “outside-in” approach). When possible, the approach can be informed by observation of the synergies that occur when subsystem technologies are combined in mass-produced vehicles. Expert judgment must also be used in combining technologies to avoid engineering incompatibilities. An advantage of the PDA method is that it has the potential to estimate the effects of fuel-economy technologies in a broad array of vehicle types. It allows assessment of fuel-economy technologies for a class of vehicles composed of many vehicle models without the need to simulate each model within the class separately. There are different ways to implement the PDA approach. In one, multiplication or addition is used to aggregate technologies, and engineering judgment and vehicle-testing are relied on to capture interactions of multiple technologies. Another is the energy consumption-balance PDA model developed by Energy and Environmental Associates, Inc. (EEA), which uses the computation methods outlined by Sovran and Bohn (1981). The EEA method is used to estimate the interactions among technologies and to ensure that pumping and friction losses are not reduced to below zero. It does not calculate absolute fuel economy, but only the changes from a given measured baseline vehicle. Although the EEA method still relies on data from proven technologies, it is able to provide an energy-balance accounting for system losses. It has been evaluated through comparisons with independent vehicle data and with results from FSS models, which have shown that results from energy consumption-balance PDAs are consistent with measured FSS values (Duleep, 2007). The committee will evaluate the EEA approach further as part of its work. The challenges related to the PDA approach lie in the difficulty of inferring specific details about the synergies among subsystem technologies from the input data used to generate the summary. As opposed to the FSS approach, in which these interactions are represented in the formulation of the model, in the PDA approach synergies are approximated by using simple lumped-parameter models, are embedded in the available vehicle data, or are introduced by means of engineering judgment. The PDA approach is generally not used for estimating the fuel-economy effects of novel vehicle systems on which there are no observed data, partly because of the difficulty of estimating synergistic effects among components. The effects of technologies applied to vehicle subsystems can be nonlinear, so the principle of superposition (whereby the effects of multiple technologies can be obtained by summing the effects of the individual technologies) may not represent the potential synergies of new systems. Applications of the FSS and PDA Approaches Each approach has its own strengths and weaknesses, so it is important to use each in a manner that capitalizes on its strengths to maximize the usefulness and accuracy of the committee’s conclusions. The committee’s analysis must also draw on and augment recently completed and current assessments of the effects of vehicle technologies on fuel economy or greenhouse-gas emissions.4 Both methods must be anchored by comparing with vehicle data. 4 For example, NSCCAF (2004) details FSS modeling of greenhouse-gas emissions reductions from a variety of vehicle technologies. And in response to a May 14, 2007, executive order to develop regulations that would cut gasoline consumption and emission of greenhouse gases by motor vehicles, EPA and NHTSA are using both FSS
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report It is proposed to use the PDA approach to assess a large dataset on current vehicles and to estimate the contribution of each technology to potential fuel-economy improvements in broad classes of vehicles. Examples of the approach and various assessments of technology contributions to overall vehicle fuel economy have already been presented to the committee. One important modification of the final results from the PDA examples that have been presented will be the inclusion of estimates of the range and accuracy associated with the approach. For example, it might be estimated that a given technology would improve fuel economy by 1.3%, but an assessment of the variability of this result (perhaps in the form of a range) should also be included; this will require an evaluation of model accuracy through a comparison of model estimates with vehicle data and FSS results. The addition of ranges and the comparison of estimates with other data and results will be important for interpreting the results of the PDA approach and assessing it for possible use by NHTSA. Given that the PDA approach depends on experimental data showing the effects of technologies on vehicles, its use to extrapolate potential fuel-economy effects to new technologies on which such data are not available should be limited. It is proposed to use the FSS approach in two ways: to assess incremental changes in fuel economy that result from adding individual technologies to or removing them from a base vehicle system, and to provide a comparison with the results obtained with the PDA approach. A few base vehicles that span a significant range of the classes of vehicles of interest will be chosen. They will be limited to two to four vehicles, depending on the resources available to the committee for this modeling activity. For each vehicle, a set of appropriate technologies, to be chosen by the committee, will be added to the base vehicle incrementally, and the incremental fuel-economy improvement associated with each technology will be assessed. The technologies included in the modeling should include combinations not currently available or an extrapolation beyond the dataset used in the PDA approach. The rationale for assessing vehicle technologies incrementally is that when the fuel economy of a particular vehicle is estimated with the FSS approach, the probability that the final estimate will be consistent with the actual value is reduced because of the accumulation of modeling errors for each component in the system. Often, the modeling errors are multiplicative; even if the component models represent the physics of the process accurately, the multiplicative effect at the system level can produce large estimation errors. However, if the models are used to assess incremental fuel-economy improvements rather than absolute fuel economy, the effects of small component modeling errors on the final result will usually be much smaller. That is, the sensitivity of the final result to component modeling errors is usually substantially reduced. Assessment of such incremental fuel-economy improvements is also what NHTSA has asked the committee to provide. The FSS approach can be compared with a similar analysis that uses a PDA method to assess the importance of synergies among the technologies; the synergies are represented directly in the FSS approach and inferred in the PDA approach. Results of both approaches must be compared with vehicle data. That will allow the committee to assess the adequacy of the PDA approach for producing the set of cost and potential-efficiency-improvement curves called for in the committee’s charge and to determine whether the inability of the PDA approach to capture such synergies implies that the FSS approach is required to assess the benefits of fuel-economy technologies. If the committee is satisfied that the PDA approach is sufficient for representing potential fuel-economy improvements in different classes of vehicles, it will undertake such analysis for the classes of vehicles listed in Table 1. Thermodynamics-Based Modeling Analysis In addition to modeling fuel economy with the FSS and PDA approaches, the committee proposes to do cycle simulations based on the second law of thermodynamics. Such simulations could demonstrate how changes in thermodynamic losses (changes in dissipative or irreversible processes) change vehicle and PDA methods to assess fuel-economy improvements and reductions in greenhouse-gas emissions that result from new vehicle technologies.
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report fuel economy. The simulations would be conducted under conditions that are typical during the EPA city-highway dynamometer test for fuel economy (for example, 2.4-bar brake mean effective pressure and 1800-2000 revolutions per minute). From a fundamental point of view, the fuel in a vehicle’s fuel tank supplies “available energy,” the potential for doing useful work, to the engine. The fuel’s chemical energy is changed by the combustion process into a form that can do work that powers the vehicle. A thermodynamic analysis involves tracking what is occurring in the engine that makes some of the energy unavailable to do useful work. For example, some energy is lost to the cooling system, and some flows out of the engine as thermal energy and as chemical energy in the unburned fuel components of the hot exhaust gases. Some energy is dissipated as mechanical friction in the engine. Those types of thermodynamic losses occur in all vehicle components and subsystems. Once the flows of available energy in a vehicle engine have been described mathematically, data and software packages can be used to simulate the changes in thermodynamic losses caused by fuel-economy technologies. This approach should lead to a much clearer understanding of the benefits of the more complicated changes in individual subsystems and of changes that affect multiple subsystems at the same time. For example, the addition of exhaust-gas recirculation (EGR) reduces thermodynamic losses that are affected by gas temperatures and pressures and thus reduces engine fuel consumption. Such effects would be highlighted directly by analysis based on the second law of thermodynamics. Not all technologic system changes would be subjected to that kind of analysis. Rather, it would be applied only to combinations of changes that are most complex and difficult to simulate. If the analysis does not yield a clearer understanding of the effects of such changes, it will not be included in the committee’s final report. PLAN AND INFORMATION NEEDS FOR FINAL REPORT The committee’s task statement calls for it to estimate, in its final report, the efficacy, cost, and applicability to different vehicle classes of fuel-economy technologies that might be used in the next 15 years. That will require the committee to move from its preliminary list of fuel-economy technologies and its preliminary assessment of the fuel-economy effects of the technologies, as contained in this interim report, to final decisions concerning the technologies and their effects. It will also require the committee to move from its assessment of the attributes of models that are used to project vehicle fuel economy to the application of the models. The committee’s immediate next steps will be to select one PDA model and one FSS model to use in its analysis. The committee will select a set of appropriate technologies to use in assessing the ability of both models to simulate actual vehicle fuel economy and in assessing the importance of synergies among technologies; this will be done during and after the committee’s January 2008 meeting. The committee will continue to collect information for use in its analysis during the coming months. As noted earlier, a May 14, 2007, executive order called for EPA and NHTSA to jointly develop a proposal for improving fuel economy and reducing greenhouse-gas emissions. The two agencies are developing data on costs and fuel-economy potential and are conducting analyses with both FSS and PDA methods. The draft rule, which will be accompanied by the release of the NHTSA and EPA information, will be a critical source of information for the committee. It is the committee’s understanding that the rule is scheduled to be released at the end of 2007. However, the recently passed Energy Independence and Security Act of 2007 has caused NHTSA and EPA to delay release of the rule. The need for additional information is especially acute with respect to costs. Although the costs of fuel-economy technologies are central to the committee’s final report, the task statement did not call for costs to be part of the interim report. The committee notes that the information provided to it by automobile manufacturers and technology suppliers was insufficient to allow a discussion of costs in this report. Preliminary cost information has been supplied in a consultant’s report commissioned by the committee (EEA, 2007), and NHTSA has informed the committee that it has held extensive discussions
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Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy: Letter Report with automobile manufacturers and technology suppliers concerning costs. The committee concludes that obtaining cost information from NHTSA, from which proprietary information has been removed, will be important in accomplishing its task. Sincerely, Trevor O. Jones, Chair Committee on Assessment of Technologies for Improving Light-Duty Vehicle Fuel Economy