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Automotive Fuel Economy: How Far Should We Go? 2 FUEL USE IN AUTOMOBILES AND LIGHT TRUCKS The fuel economy of an automobile or light truck is determined by its technology and design and by vehicle use, driver behavior and the conditions under which it is used—for example, speed, road design, weather, and traffic.1 This chapter examines how the design and technical characteristics of an automobile or light truck affect its fuel economy, with a focus on technologies that might be employed to achieve fuel economy gains in the future.2 High fuel economy is only one of many desirable vehicle attributes. Consumers also value acceleration and handling, safety, comfort, reliability, passenger- and load-carrying capacity, size, styling, and low noise and vibration, not to mention low purchase and ownership costs. Society at large requires vehicles to have certain additional attributes, such as low exhaust emissions. All of these attributes influence vehicle design and technology, and most of them affect fuel economy. As a consequence, the fuel economy of a vehicle results from the trade-offs, guided in part by costs and benefits, that must be made among a variety of vehicle characteristics.3 1 To control for the effects of the variables that are external to the vehicle, it is customary to evaluate the fuel economy of a vehicle using the Environmental Protection Agency's (EPA's)Federal Test Procedure (FTP), which involves laboratory measurement of fuel economy over specific simulated urban and highway driving cycles. 2 It was beyond the scope of this study to examine the determinants and effects on fuel consumption of vehicle use (typically measured by vehicle miles traveled, or VMT), driver behavior, and driving conditions. All contribute importantly to the total national consumption of gasoline, diesel, and other fuels for automobiles and light trucks. 3 It is possible to produce vehicles with extraordinarily high fuel economy, but only at the sacrifice of nearly all other important vehicle characteristics. For example, an experimental passenger-carrying vehicle, optimized for fuel economy alone, has achieved a world record 6,409 miles per gallon (mpg) (Associated Press, 1988). However, it can carry only a child or small adult in the prone position, and can achieve a speed of only 15 miles per hour (mph) on level ground. It has very limited hill-climbing ability, would fail federal safety tests, and might fail emissions tests as well. An automobile with maximized fuel economy would simply be unusable and unacceptable to consumers.
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Automotive Fuel Economy: How Far Should We Go? Changes over the years in vehicle design and in automotive technologies have made it possible to achieve substantial increases in fuel economy while maintaining acceptable levels of other attributes, including price. This chapter illustrates some of the fundamental principles underlying the fuel economy characteristics of automotive technologies. It also identifies some of the improved technologies that are available now or that are in the R&D stage. The details of how some of the technologies discussed in this chapter might contribute to fuel economy improvement are presented in Appendix B, and their implications for size-class and fleet fuel economy are discussed in Chapters 7 and 8. Certain emerging technologies—especially alternative engine designs—are described later in this chapter and discussed further in Appendix C. THE VEHICLE AS A SYSTEM In any vehicle that is propelled by burning fuel in an engine, the chemical energy in the fuel is released by combustion to fulfill a number of functions. Part of the fuel's energy is lost to the vehicle's surroundings as low-grade (low-temperature) heat in the exhaust gas and the air that cools the engine and radiator. Part of the chemical energy is transformed into mechanical energy, or work. A portion of the work propels the vehicle by overcoming (a) inertia (weight) when accelerating or gravity when hill climbing, (b) the resistance of the air to the motion of the vehicle, and (c) the rolling resistance of the tires on the road. Another portion operates the vehicle's essential and optional accessories. Yet another portion operates the engine itself and overcomes the frictional energy losses that occur in every part of the vehicle system. The proportions of the chemical energy that are used to fulfill each of the several functions for a typical vehicle of modern design are not fixed. Rather, they depend on the details of the vehicle's design and the nature and efficiencies of the vehicle's technologies. Improvements in fuel economy are usually accompanied by changes in the proportion of the fuel's energy that goes to satisfy each function. For example, the energy required to propel the vehicle can be decreased by reducing inertia, the drag imposed by the air on the moving vehicle, or the rolling resistance of the tires. The energy required to operate accessories can be reduced by changing their design, increasing their efficiency, or, in the case of optional accessories, eliminating them. The energy required to operate the engine itself can be reduced by adopting different engine technology or making improvements in the detailed engineering design of the engine. The energy lost to friction in the rest of the vehicle system can be reduced by making any of a large number of design changes and technical improvements in, for example, the transmission and other parts of the drivetrain or by improving lubrication of their elements. It is also not possible to attach precise quantitative values to each of the energy flows because the distribution varies greatly depending on operating conditions. A vehicle that is optimized for fuel economy at highway speeds might have unacceptable performance in the stop-start usage of urban travel. In recognition of this fact, the FTP covers a wide range of operating conditions, including cold start, idle, acceleration,
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Automotive Fuel Economy: How Far Should We Go? deceleration, and cruise at low and high speeds. Consequently, the measured fuel economy (and emissions performance) is a composite of energy use under a variety of operating conditions. Automotive designers and engineers must optimize the vehicle and its power train to meet the often-conflicting demands of customer satisfaction, fuel economy requirements, emissions standards, and cost in the variety of operating conditions under which the vehicle will be used. Complete consideration of the effects on fuel economy of changes in design or technology requires examination of the vehicle as a total system. For example, reducing the size of the engine may reduce the vehicle's weight and thus reduce the fuel needed to accelerate the vehicle and propel it up hills. But, unless engine performance is improved or the drivetrain is altered, the smaller engine may accelerate the vehicle less rapidly. If those changes are introduced, however, the "feel" of the vehicle will change, with associated impacts on consumer acceptance. Modifications of technology change the system and hence have complex effects that are difficult to capture and analyze. It is usually possible, however, to estimate the impacts of specific technologies in terms of a percentage savings in fuel use for a typical vehicle without a full examination of all the system-level effects. This approach is used in most of this report. The analysis in Chapter 7 of the fuel economy potential of future vehicles takes into account some of the most important systems implications for fuel economy of certain alternative technologies. Nonetheless, changes in technology have both obvious and subtle effects that could prove significant, and the committee's analysis cannot capture all of these impacts. ENGINE TECHNOLOGY AND FUEL USE Engine design and technology are key determinants of the fuel economy of an automobile or light truck. The engine's efficiency in converting fuel energy to useful work affects fuel economy directly, and its design influences most of the other characteristics of the vehicle, especially its weight and size, which further influence fuel economy.
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Automotive Fuel Economy: How Far Should We Go? Standard Engine Technology The standard engine in today's vehicles is a spark-ignition, four-stroke, internal combustion engine of four, five, six, or eight cylinders. 4 The basic concepts underlying this engine have been understood and applied for over a century. Yet, improving the efficiency, operability, and emissions characteristics of this engine is the subject of continued fruitful study and evolutionary innovation even today. Many variations on the standard engine design have been used in vehicle applications. Some important variations, which may be used in combination, include the multivalve engine (which employs two valves or more per cylinder for intake of the air/fuel mixture and for exhaust of the combustion products), the variable-valve-timing engine (in which the timing of valve operation can be varied with respect to the crankshaft position), the two-stroke engine (which combines the functions of the four strokes into two), and the lean-burn engine (which burns the fuel in the presence of excess air and/or exhaust gases and which may also include alternative configurations for, and precise control of, the pattern of combustion of the fuel). These variations, and the details of design within each, offer the possibility of different and improved combinations of fuel economy, performance, and cost. The compression-ignition, or diesel, engine differs in several ways from the sparkignition engine, although it is based on the same twoor four-stroke cycle. In the diesel, air alone is compressed until late in the compression stroke, when fuel is injected into the cylinder at very high pressure. Because the fuel is injected late in the cycle, a diesel engine may use a much higher compression ratio. This, combined with its ability to burn a ''lean" air/fuel mixture (see below), makes the diesel more efficient than a spark-ignition engine. On the other hand, the diesel engine is typically heavier than an equivalent spark engine, and it produces a different set of exhaust emissions. (See Appendix C for further discussion of diesel technologies.) In a spark-ignition, four-stroke engine of standard design, a portion of the energy released in combustion does work as the hot, high-pressure combustion gases expand 4 In such an engine, a mixture of fuel and air is delivered by a carburetor or fuel-injection system to the cylinders, where it is compressed by pistons moving up in the cylinders (the compression stroke) and is ignited by an electrical spark. As the fuel burns, it produces a mixture of hot, high-pressure gases whose controlled expansion in the cylinders causes the pistons to move down (the expansion stroke). Through a mechanical linkage, this downward motion of the piston in the cylinder is translated into a rotating motion of the crankshaft. The piston then moves upward again (the exhaust stroke) and, with the simultaneous opening of the exhaust valve on the engine, the burned fuel products (exhaust gases) are pushed out of the cylinder and into the exhaust manifold. Finally, the piston again moves down (the intake stroke), with the inlet valves open this time to admit fresh fuel/air mixture. When the piston reverses motion, it begins again to retrace the sequence starting with the first stroke. In this way, a portion of the fuel energy released during engine operation is transformed into mechanical energy of the rotating crankshaft, which is then available to propel the car, operate accessories, and overcome frictional losses in the drivetrain. All the pistons are attached to a common crankshaft in such a way that the expansion stroke of each cylinder occurs at different times. Thus, the torque (twisting force) transmitted to the crankshaft from the pistons through steel shafts, or connecting rods, is more nearly uniform in time than it would be if all the pistons were in the expansion stroke at the same time. The greater the number of cylinders, the more uniform is the torque on the crankshaft and the smoother is engine operation.
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Automotive Fuel Economy: How Far Should We Go? and force the piston down during the expansion stroke.5 However, not all of this work is available for moving the vehicle, in part, because a portion of it must be used to compress the air/fuel mixture during the compression stroke. The difference between the work made available during the expansion stroke and that used during the compression stroke is conventionally defined as the indicated work. A portion of the indicated work—the pumping loss—is required to pump the air and fuel into the cylinders during the intake stroke and to pump the exhaust gas out during the exhaust stroke. Yet another portion of the indicated work is transformed into heat by friction among the moving parts of the engine. The remaining work, called the brake work, is available at the engine crankshaft. This brake work, less friction losses in the drivetrain and accessory losses, is transferred by the drivetrain to the wheels of the vehicle. Figure 2-1 illustrates the pathways for the use of the indicated work during vehicle operation on a level road. Each of the energy pathways offers the potential for fuel economy improvement. For example, fuel economy might be increased by redesigning the vehicle exterior to reduce the aerodynamic drag forces that consume a substantial portion of the fuel energy at high speeds. Or, fuel economy might be increased by reducing friction in the engine, drivetrain, accessories, or other parts of the vehicle through use of different technologies or better lubricants. The indicated work, pumping loss, and friction loss vary with the load (see Figure 2-2) and, to a lesser extent, the engine speed. At decreased load, a greater fraction of the indicated work goes to overcoming pumping loss and friction. Consequently, reducing pumping losses and engine friction will significantly improve fuel economy for engines that operate at light load much of the time. Figure 2-2 also illustrates why driving in a higher gear at a given vehicle speed generally improves fuel economy—in a higher gear, the engine runs at lower revolutions per minute (rpm), where friction is lower, and at a higher load, where pumping and friction losses are proportionately smaller. 5 Indicated thermal efficiency is usually defined as the proportion of the energy of the fuel that is converted to work and delivered to the top of the piston during the compression and expansion strokes. It is determined primarily by the rate of burning, heat losses, compression ratio, and air/fuel ratio. The rate of burning is affected by in-cylinder turbulence, combustion chamber design, spark plug location, and air/fuel ratio. A high rate of burning is desirable, but, if excessive, it can cause undesirable engine noise, vibration, and harshness. Cooling of the air/fuel mixture late in the compression stroke minimizes the tendency of the engine to "knock" (premature burning and detonation in the cylinder), but cooling of the exhaust gases after combustion decreases the work obtained during expansion. The compression ratio is limited in the spark-ignition engine by knock, which is affected by fuel octane rating, combustion chamber design, spark timing, combustion chamber cooling, and deposits on the combustion chamber wall. Within limits, knocking can be controlled by using sensors to detect it and by delaying, or retarding, the spark—an action that decreases fuel economy. The air/fuel ratio, with a few exceptions, is fixed by emissions considerations because the catalytic system used in modern vehicles requires a stoichiometric air/fuel mixture (i.e., a mixture that contains precisely the correct amount of oxygen needed for complete combustion of the fuel). There is a fuel economy advantage, but an emissions problem, when excess air is used, as discussed in Appendix C. For homogeneous mixtures of air and fuel, the rate of burning and combustion stability decrease on either side of an approximately stoichiometric mixture, so there is a practical lean-combustion limit for homogeneous mixtures. This limit does not apply to lean, stratified-charge engines, such as the diesel. (See Chapter 4 and Appendix C.)
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Automotive Fuel Economy: How Far Should We Go? FIGURE 2-1 Where the energy in the fuel goes (proportions vary with vehicle design and operating conditions).
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Automotive Fuel Economy: How Far Should We Go? FIGURE 2-2 Illustrative distribution of energy released in the engine as a function of load. SOURCE: Adapted from Amann (1991).
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Automotive Fuel Economy: How Far Should We Go? Table 2-1 shows how the brake work delivered to the drive wheels is distributed among the three major categories of force (aerodynamic drag, rolling resistance, and inertia) against which the engine must work during three different "cycles" of driving, all on a level road. One cycle is driving at a steady speed of 55 to 65 mph. The other two cycles are the urban and highway parts of the EPA's FTP. The urban cycle includes numerous periods of acceleration and deceleration, accompanied by periods of moderate, steady speeds. The highway cycle includes fewer acceleration and deceleration periods and more steady driving at speeds higher than in the urban cycle, although at an average speed less than the 55 to 65 mph of the steady-state cycle. As shown in Table 2-1, the proportion of the energy used to overcome aerodynamic drag increases substantially at higher speeds. (Drag increases as the square of vehicle speed.) The proportion of the energy used to overcome inertia is greatest during the urban cycle, with its frequent stops and accelerations. Performance/Fuel Economy Trade-Offs One of the more significant trade-offs in automotive design arises from the tendency for high performance to require more fuel. For example, engine design can affect both fuel economy and the capability of a vehicle to accelerate or climb hills quickly. A vehicle's acceleration performance is directly related to the torque exerted on the driving wheels, which is proportional to the engine torque conveyed through the transmission and drivetrain. Thus, deliverable torque, which depends on both engine speed and load, is a significant parameter of engine performance. TABLE 2-1 Illustrative Distribution of the Energy Delivered to the Drive Wheels Operating Conditions Force to be Overcome Steady-State Cycle (55-65 mph) EPA Highway Driving Cycle EPA Urban Driving Cycle Aerodynamic drag 75-80% 50-55% 28-25% Rolling resistance 25-20 35-40 28-25 Inertia 0 15-5 44-50 Total 100% 100% 100% SOURCE: MacCready (1989).
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Automotive Fuel Economy: How Far Should We Go? For a given engine design and air/fuel ratio, the torque produced by an engine increases as greater amounts of air are introduced into its cylinders during the intake stroke. The flow rate of air can be increased by increasing the size of the engine or by introducing air at higher pressures so as to increase its density. (Engine size is measured by its displacement, that is, the volume swept by the pistons during one stroke.) Increased engine size increases engine friction and reduces fuel economy. Efforts to increase air density may have similar effects.6 Engines with different designs but the same displacement may have very different curves of engine torque versus engine rpm, as illustrated in Figure 2-3, which was prepared by Honda Motor Company to explain the torque characteristics of its new VTEC engine (see Appendix C). The VTEC engine has a mechanism that permits it to change from 2-valve to 4-valve operation at the shift point, as shown in Figure 2-3. The valve timing also differs between 2- and 4-valve operation. As shown in Figure 2-3, for a 2-valve engine, the combination of valve timing and increased flow restriction creates a torque peak at about 2,300 to 2,400 rpm. However, a 4-valve engine with different valve timing could have a torque curve that peaks near 4,800 rpm. Honda has designed its valve train for 2-valve operation at low engine speed and for 4-valve operation at high engine speed. This produces the upper torque curve for the VTEC, which is relatively flat over a wide range of engine speed. Thus, used in the same vehicle, the VTEC engine, which delivers higher horsepower as well as higher torque at the same engine speed, delivers higher acceleration performance (and probably equal or higher fuel economy) than the standard Honda engine with the same displacement. Alternatively, the VTEC engine 6 Increasing the ambient air density increases the mass of air in the cylinder and consequently increases torque and power output. One way to increase ambient air density is to connect an air compressor to the intake manifold. The power required to drive the air compressor can come from the engine crankshaft (using a supercharger) or from a gas turbine driven by the engine exhaust (a turbocharger). The supercharger is not activated until wide-open throttle is reached, and when it is activated, it momentarily reduces the power available to move the vehicle. (Because of the benefits of increased air density, however, the net effect is a power increase.) The response of a supercharger to sudden power demands is instantaneous, but there is a small delay in the response of a turbocharger because of the delay in the exhaust-gas flow rate and the need to overcome the inertia of the turbocharger. A variable-speed drive between the crankshaft and the supercharger is sometimes used because of the mismatch between the flow characteristics of the reciprocating engine and the rotating compressor usually used. A variable-displacement compressor has also been proposed for the same purpose. There is considerable disagreement about the effects of these arrangements on fuel economy. When ambient air density is increased, a smaller displacement engine can be used to produce the required power. The smaller engine should have less friction and may be lighter, both of which would improve fuel economy. However, the higher effective compression ratio that accompanies use of turbo/supercharging results in an increased tendency for the engine to knock and may require the use of knock sensors to retard the spark. Retarding the spark is detrimental to fuel economy. Also, some engineers believe that the higher cylinder pressures encountered when ambient air pressure is increased will require, for equal durability, a stronger engine structure, which in turn, will increase engine weight and engine friction and decrease fuel economy. As a result of this controversy, the committee did not take turbo- or supercharging into account in its analyses, even though both are in limited use today to enhance vehicle performance.
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Automotive Fuel Economy: How Far Should We Go? FIGURE 2-3 Torque curves for various engines. SOURCE: Honda Motor Company, Ltd. (1991). could be accompanied by changes in the drivetrain to give increased fuel economy rather than higher acceleration performance. In short, a range of fuel economy performance can be obtained not only from different engines with the same displacement, but also from the same basic engine. The 4-valve engine also provides an excellent example of how a technology can be used for different purposes. Most, if not all, implementations of the 4-value engine to date have been to improve performance rather than fuel economy. Numerous design details of engines and drivetrains could be changed to maximize vehicle fuel economy rather than performance. Technologies to improve fuel economy can also affect performance attributes other than acceleration. Downsizing an engine from six to four cylinders may improve fuel economy, but at the cost of increased engine-induced vehicle vibration. Although a 5-speed transmission weighs more than a 4- or 3-speed transmission, by allowing more efficient engine operation it may yield both better performance and higher fuel economy. Substituting a continuously variable transmission for an automatic one also can save fuel, but with modifications to the "feel" of a car in ways that consumers may not like. However, not all technologies to improve fuel economy necessarily require compromises of other performance attributes. For example, multipoint fuel injection, which enables more precise fuel metering and control, not only improves fuel
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Automotive Fuel Economy: How Far Should We Go? economy, but also can yield lower emissions and improved responsiveness. As a result, a substantial proportion of all new passenger cars use multipoint fuel injection, despite the fact that this technology may not be cost-effective as a fuel economy technology alone (SRI, 1991). FUEL ECONOMY TECHNOLOGIES FOR THE NEXT DECADE Table 2-2 lists the two categories of technologies—proven and emerging —that the committee determined to be important to the fuel economy of future automobiles and light trucks over the period to 2006. The selection was based on published studies and presentations to the committee (Berger et al., 1990; DeCicco, 1991; Energy and Environmental Analysis, 1991a,b; Ledbetter and Ross, 1990; Office of Technology Assessment, 1991; SRI, 1991). Proven Technologies Many concepts, technologies, and designs have the potential to improve the fuel economy of motor vehicles. Proven technologies are ones that can reasonably be expected to be available for incorporation in the future vehicle fleet by automotive manufacturers. To be considered "proven" by the committee, a technology had to be currently available in at least one mass-produced light-duty vehicle. An emphasis on proven technology is warranted because of the need for assurance that a fuel economy gain can in fact be achieved by manufacturers, who must be concerned with safety, emissions, reliability, cost, manufacturability, and consumer acceptance, as well as fuel use.7 The proven technologies vary widely in their potential contribution to increased automotive fuel economy, their costs, and the degree to which they are used in mass-produced vehicles today. There is a considerable divergence of views in the literature about these aspects of a number of the proven technologies. The committee examined each of the technologies at length, and the details of that inquiry are presented in Appendixes B and E. Appendix B includes (1) a discussion of the technology and the aspects of vehicle energy use it affects, (2) estimates of the improvement in fuel economy that may be achievable, compared with a baseline technology, and (3) estimates of the cost of using the technology. Among other data, Appendix E includes estimates of the market share of each of the proven technologies in the current fleet and the committee's judgments about the potential future market share of each. 7 Many other technologies have been demonstrated to the public in prototype and concept vehicles. The constraints applied during the design of concept vehicles are different from the constraints applied during the design of production vehicles. Consequently, fuel economy values obtained from concept vehicles are not directly useful in estimating the potential fuel economy of production vehicles (see Appendix C).
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Automotive Fuel Economy: How Far Should We Go? TABLE 2-2 Fuel Economy Technologies for Automobiles and Light Trucks Proven technologiesa Proven technologies (continued) Engine technologies Accessories Roller cam followers Accessories General friction reduction Electric power steering Deceleration fuel restriction Compression ratio increase Rolling resistance Throttle-body fuel injection Advanced tires Multipoint fuel injection Overhead camshaft Inertia Four valves/cylinder Weight reduction Variable valve timing Front-wheel drive Four-cylinder engine Six-cylinder engine Aerodynamic drag Advanced lubricants Aerodynamics Turbo/superchargingb Dieselc Transmission technologies Torque converter lockup Emerging technologies Electronic control Four-speed automatic Lean-burn engine Five-speed automatic Two-stroke engine Continuously variable transmission Active noise control Five-speed manual Lean NOx catalyst NOTE: Descriptions of the technologies and their fuel-use characteristics, along with estimates of their costs, are provided in Appendix B. a The categories of proven technologies correspond to the energy pathways in Figure 2-1. b Currently used primarily to enhance performance. Because their ability to improve fuel economy is controversial, these technologies were not included in the analyses (see footnote 7). c Light trucks only. Discussed in Appendix C as an emerging technology.
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Automotive Fuel Economy: How Far Should We Go? The proven technologies list includes technologies that will be familiar to most American consumers who are attentive to the new car and truck markets in the United States, including mass-market imported vehicles. Better engines, more efficient transmissions, body designs with improved aerodynamics, and lighter weight vehicles are all staples of the sales and marketing activities of major automobile and light-truck producers. On the other hand, most of these technologies have reached only a fraction of their potential application in vehicles sold in the United States, and as detailed in Chapter 7 and Appendix E, rather substantial increases in new-car and light-truck fuel economy by vehicle type and class could be achieved if they were to be employed to their maximum potential. A substantial portion of the proven technologies relates to changes in the technology of the standard four-stroke, spark-ignition engine or to use of transmissions with larger numbers of gear ratios (including the continuously variable transmission, which conceptually has an unlimited number of ratios). The discussion of the standard engine earlier in this chapter provides the fundamentals of the technical background for understanding how each of these technologies works and why it is potentially important to fuel consumption. More details on the fuel economy aspects of these technologies are given in Appendix B. It is difficult to compare the different published costs for each proven technology because the costs are usually reported as the marginal costs above some baseline technology, and the selection of the baseline for comparison can make a large difference in the reported cost. The same issue arises in examining the potential fuel economy contributions of a technology. This matter is examined in detail for the proven technologies in Appendix B, and all of the numbers reported there have been put on as consistent a basis as possible by the committee. The choice of a baseline for comparison is a somewhat arbitrary matter—for example, in choosing the type of transmission to use as the base for comparison with the other types. The choice can also reflect the more fundamental question of whether a technology is treated as an addition to, or replacement of, part of an existing vehicle already in production, or whether it is incorporated in an entirely new design. Some proven technologies offer the promise of major increases in fuel economy for cars and light trucks, yet their costs are also quite high. Typical of these are such technologies as multipoint fuel injection, engines with four valves per cylinder, and vehicle weight reduction. Others appear to offer relatively limited fuel economy benefits, yet their costs are relatively low as well. In making decisions about which of the proven technologies to incorporate in a vehicle, manufacturers and their customers, if acting on a financially reasonable basis, will adopt proven technologies in the order of their cost-effectiveness—that is, in the order in which they provide the most fuel economy improvement for each dollar invested in them. Beyond higher fuel economy, some proven technologies offer other desirable attributes to producers and consumers, such as enhanced acceleration performance or better control over emissions. Multipoint fuel injection is such a technology—it has been adopted for many makes and models of vehicles, even though on fuel economy grounds alone it may not be cost-effective.
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Automotive Fuel Economy: How Far Should We Go? The precise ordering of new technologies according to their cost-effectiveness is sensitive, then, to the cost and potential for fuel economy improvement of each one. These factors differ not only among authors, but also among vehicle types and sizes, as explained in Appendixes B and E. Thus, it is not possible to make generalized statements about which technologies are the most cost-effective in the general case. Therefore, for each technology, the committee carried out a detailed examination of its cost, fuel economy improvement potential, and cost-effectiveness for cars and light trucks of different sizes and types, using data from different sources as the basis for the examinations. The implications of this analysis for fuel economy improvements within vehicle size classes are discussed in Chapter 7. Emerging Technologies The committee defined emerging technologies as those that hold promise of improving fuel economy, but whose future success is uncertain. Because a number of such technologies are being studied, and intense development is under way at a number of places, the driving forces toward successful application are strong. Thus, it seems reasonably certain that one or more emerging technologies (probably including some not foreseen in this report) will begin to make significant contributions to fuel economy by the year 2006. Two widely discussed emerging technologies, lean-burn and two-stroke engines, exemplify the types of new engine technologies that may offer promise of enhanced fuel economy in the long term. In the committee's judgment, however, neither alternative is likely to achieve widespread practical vehicle application during the next decade, despite current commercial uses. Technical details of their operation, their fuel economy potential, and barriers to their adoption, including environmental limitations, are discussed in Appendix C. Lean-burn engines are designed and operated so that excess air over and above that needed for complete combustion is introduced into the combustion chambers.8 Because of its potential for increased fuel economy, the homogeneous lean-burn approach was investigated extensively in the 1960s and early 1970s as an alternative emissions control approach to the three-way catalyst, which requires use of a stoichiometric air/fuel mixture that leads to lower fuel economy. 9 However, the lean-burn engine was unable at that time to meet emissions and drivability requirements and its development was discontinued. Its current revival is due to its acknowledged fuel economy advantage and the availability of electronic fuel injection, which makes possible operation under lean-burn conditions in selected portions of the driving cycle. The difficulty in limiting NOx emissions is a major problem with lean-burn engines, however. A lean NOx catalyst, discussed in detail in Chapter 4, would markedly 8 Lean burn is also sometimes used to describe an engine in which exhaust gases, rather than excess air, are used to dilute the air/fuel mixture. The diesel engine, discussed earlier and detailed in Appendix C, is a stratified-charge, lean-burn engine. 9 A three-way catalyst allows the simultaneous control of unburned hydrocarbons (HC), oxides of nitrogen (NOx), and carbon monoxide (CO).
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Automotive Fuel Economy: How Far Should We Go? improve the potential for use of lean-burn engines, but much fundamental research remains to be done in this area. Two-stroke engines use an air pump other than the engine's pistons and cylinders to help accomplish the four tasks of compression, expansion, intake, and exhaust in only two strokes (one revolution of the crankshaft). In the two-stroke engine, exhaust and air (or air/fuel) intake are accomplished late in the expansion stroke and early in the compression stroke, respectively, by blowing the compressed air or mixture into the cylinder through the intake valve or port. Ideally, all of the products of combustion (and none of the incoming gases) would be blown out of the open exhaust valve or port, leaving primarily fresh gases in the cylinder. In practice, however, a significant portion of the compressed input mixture escapes through the exhaust valve, and a significant portion of the exhaust gases remains in the cylinder. If the incoming gases contain fresh fuel, the exhaust will contain unburned fuel, leading to high levels of unburned HC in the exhaust. The potential benefits from automotive applications of two-stroke technology include reduced engine weight, size, and cost. Further, since each cylinder undergoes a power stroke on every revolution of the crankshaft rather than on every second, for the same number of cylinders a two-stroke engine generates somewhat less than twice the output power and operates more smoothly than a four-stroke engine. If the vehicle is optimized around a two-stroke engine, there is the potential for improved fuel economy. Significant problems related to mechanical components and exhaust emissions must be overcome, however, before the two-stroke engine can be a serious competitor to the four-stroke engine. Although NO x emissions are reduced by the high internal exhaust gas recirculation (EGR),10 HC and particulate emissions can be a problem. Emissions control to meet strict new and emerging standards will prove difficult. Addressing these problems may increase the weight, cost, and complexity of two-stroke engines, and the outcome of the current intense development efforts is not clear. Active noise control is another emerging technology of interest. Standard mufflers used to control exhaust noise slightly restrict the flow of the exhaust gases. This increases the pressure in the engine's cylinders during the exhaust stroke and, consequently, increases engine pumping losses, particularly at high engine speed and load. Active noise control can reduce ambient noise by using an electronic device to detect noise (which consists largely of rapid, small variations of ambient air pressure) and to create variations of pressure of equal magnitude and opposite sign that cancel out the original noise. In automotive applications, this approach offers the promise of noise control with little or no restriction on exhaust flow other than that due to the catalytic muffler required for emissions control. However, important uncertainties remain, including the magnitude of the fuel economy advantage as well as the durability, size, and cost of the necessary electronic equipment and generators of 10 EGR involves recycling a portion of the exhaust gases with the fresh air charge.
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Automotive Fuel Economy: How Far Should We Go? offsetting noise. Active noise control is used commercially to reduce noise from large air blowers, but it is still in the R & D stage for automotive use. To explore the bounds of technological feasibility, as well as to achieve favorable publicity, automobile companies develop and present to the public ''concept" and "prototype" cars. Such cars may embody extreme styling, new interior concepts, ultrahigh speed capability, or other characteristics, such as high fuel economy, low emissions, or extraordinarily safety-conscious design. Data on concept vehicles are useful in establishing achievable boundaries and to confirm and help quantify directional gains. However, the performance of concept and prototype vehicles cannot be directly translated into production vehicles because they do not achieve the variety of objectives that consumers or regulatory agencies require. Typical concept cars and their limitations are discussed in Appendix C. SUMMARY The energy content of automotive fuel is used or dissipated in a variety of ways that are affected by vehicle design and technology and by operating conditions. Each of the pathways of energy use or loss offers the potential for fuel economy improvements. Automobile engineers must seek to optimize the vehicle and its power train to meet the often-conflicting demands of improved fuel economy, customer satisfaction, emission standards, safety, and cost. A variety of engine technologies may be applied to achieve improved performance or improved fuel economy. Others offer the promise of improved fuel economy, but at the expense of performance or other attributes that may be valued by consumers. Modifying the details of a vehicle's design or technology affects the balance of all of its energy flows. Full consideration of the effects on fuel economy of design or technological changes requires examination of the vehicle as a total system. Modifications may have very complex effects that are difficult to analyze. Many proven technologies are available to improve fuel economy. Each technology makes possible small, but important, improvements in fuel economy. A number of emerging technologies hold the promise of better fuel economy. At this stage of their development, however, it is impossible to estimate with any accuracy the probability of their success. Nonetheless, it seems reasonably certain that one or more emerging technologies (including some not foreseen in this study) will begin to make significant contributions to fuel economy by 2006.
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Automotive Fuel Economy: How Far Should We Go? REFERENCES Amann, C.A. 1991. Fuel economy factors. Paper presented at the workshop of the Committee on Fuel Economy of Automobiles and Light Trucks, Irvine, Calif., July 8-12. Associated Press. 1988. Japanese car sets record at 6,509 mpg. Reported in Knoxville News Sentinel July 3. Berger, J.O., M.H. Smith, and R.W. Andrews. 1990. A system for estimating fuel economy potential due to technology improvements. Paper presented at the workshop of the Committee on Fuel Economy of Automobiles and Light Trucks, Irvine, Calif., July 8-12. University of Michigan, Ann Arbor. DeCicco, J.M. 1991. Cost-effectiveness of fuel economy improvements. Paper presented at the workshop of the Committee on Fuel Economy of Automobiles and Light Trucks, Irvine, Calif., July 8-12. American Council for an Energy-Efficient Economy, Washington, D.C. Energy and Environmental Analysis, Inc. 1991a. Documentation of Attributes of Technologies to Improve Automotive Fuel Economy. Prepared for Martin Marietta, Energy Systems, Oak Ridge, Tenn. Arlington, Va. Energy and Environmental Analysis, Inc. 1991b. Fuel economy technology benefits. Presented to the Technology Subgroup, Committee on Fuel Economy of Automobiles and Light Trucks, Detroit, Mich., July 31. Honda Motor Company, Ltd. 1991. VTEC-E engine. English version of Honda Press Release in Japanese, July 30. Honda North America, Inc., Washington, D.C. Ledbetter, M., and M. Ross. 1990. Supply curves of conserved energy for automobiles. Proceedings of the 25th Intersociety Energy Conservation Engineering Conference. New York: American Institute of Chemical Engineers. MacCready, P.B., Jr. 1989. Perspectives in transportation. Journal of the Air Pollution Control Association November:1428-1429. Office of Technology Assessment, U.S. Congress. 1991. Improving Automobile Fuel Economy: New Standards, New Approaches. Washington, D.C.: U.S. Government Printing Office. SRI International. 1991. Potential for Improved Fuel Economy in Passenger Cars and Light Trucks. Prepared for Motor Vehicle Manufacturers Association. Menlo Park, Calif.
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