Assessment of Fuel Economy Technologies for Light-Duty Vehicles
Chapter: 2 Fundamentals of Fuel Consumption
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Fundamentals of Fuel Consumption
This chapter provides an overview of the various elements that determine fuel consumption in a light-duty vehicle (LDV). The primary concern here is with power trains that convert hydrocarbon fuel into mechanical energy using an internal combustion engine and which propel a vehicle though a drive train that may be a combination of a mechanical transmission and electrical machines (hybrid propulsion). A brief overview is given here of spark-ignition (SI) and compression-ignition (CI) engines as well as hybrids that combine electric drive with an internal combustion engine; these topics are discussed in detail in Chapters 4 through 6 . The amount of fuel consumed depends on the engine, the type of fuel used, and the efficiency with which the output of the engine is transmitted to the wheels. This fuel energy is used to overcome (1) rolling resistance primarily due to flexing of the tires, (2) aerodynamic drag as the vehicle motion is resisted by air, and (3) inertia and hill-climbing forces that resist vehicle acceleration, as well as engine and drive line losses. Although modeling is discussed in detail in later chapters (Chapters 8 and 9 ), a simple model to describe tractive energy requirements and vehicle energy losses is given here as well to understand fuel consumption fundamentals. Also included is a brief discussion of customer expectations, since performance, utility, and comfort as well as fuel consumption are primary objectives in designing a vehicle.
Fuel efficiency is a historical goal of automotive engineering. As early as 1918, General Motors Company automotive pioneer Charles Kettering was predicting the demise of the internal combustion engine within 5 years because of its wasteful use of fuel energy: “[T]he good Lord has tolerated this foolishness of throwing away 90 percent of the energy in the fuel long enough” (Kettering, 1918). And indeed, in the 1920s through the 1950s peak efficiencies went from 10 percent to as much as 40 percent, with improvements in fuels, combustion system design, friction reduction, and more precise manufacturing processes. Engines became more powerful, and vehicles became heavier, bigger, and faster. However, by the late 1950s, fuel economy had become important, leading to the first large wave of foreign imports. In the wake of the 1973 oil crisis, the issue of energy security arose, and Congress passed the Energy Policy and Conservation Act of 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 cars sold in the United States in 1990 to a standard of 27.5 miles per gallon (mpg) and allowed the U.S. Department of Transportation (DOT) to set appropriate standards for light trucks. The standards are administered in DOT by the National Highway Traffic Safety Administration (NHTSA) on the basis of U.S. Environmental Protection Agency (EPA) city-highway dynamometer test procedures.
FUEL CONSUMPTION AND FUEL ECONOMY
Before proceeding, it is necessary to define the terms fuel economy and fuel consumption; these two terms are widely used, but very often interchangeably and incorrectly, which can generate confusion and incorrect interpretations:
Fuel economy is a measure of how far a vehicle will travel with a gallon of fuel; it is expressed in miles per gallon. This is a popular measure used for a long time by consumers in the United States; it is used also by vehicle manufacturers and regulators, mostly to communicate with the public. As a metric, fuel economy actually measures distance traveled per unit of fuel.
Fuel consumption is the inverse of fuel economy. It is the amount of fuel consumed in driving a given distance. It is measured in the United States in gallons per 100 miles, and in liters per 100 kilometers in Europe and elsewhere throughout the world. Fuel consumption is a fundamental engineering measure that is directly related to fuel consumed per 100 miles and is useful because it can be employed as a direct measure of volumetric fuel savings. It is actually fuel consumption
that is used in the CAFE standard to calculate the fleet average fuel economy (the sales weighted average) for the city and highway cycles. The details of this calculation are shown in Appendix E . Fuel consumption is also the appropriate metric for determining the yearly fuel savings if one goes from a vehicle with a given fuel consumption to one with a lower fuel consumption.
Because fuel economy and fuel consumption are reciprocal, each of the two metrics can be computed in a straight-forward manner if the other is known. In mathematical terms, if fuel economy is X and fuel consumption is Y, their relationship is expressed by XY = 1. This relationship is not linear, as illustrated by Figure 2.1 , in which fuel consumption is shown in units of gallons per 100 miles, and fuel economy is shown in units of miles per gallon. Also shown in the figure is the decreasing influence on fuel savings that accompanies increasing the fuel economy of high-mpg vehicles. Each bar represents an increase of fuel economy by 100 percent or the corresponding decrease in fuel consumption by 50 percent. The data on the graph show the resulting decrease in fuel consumption per 100 miles and the total fuel saved in driving 10,000 miles. The dramatic decrease in the impact of increasing miles per gallon by 100 percent for a high-mpg vehicle is most visible in the case of increasing the miles per gallon rating from 40 mpg to 80 mpg, where the total fuel saved in driving 10,000 miles is only 125 gallons, compared to 500 gallons for a change from 10 mpg to 20 mpg. Likewise, it is instructive to compare the same absolute value of fuel economy changes—for example, 10-20 mpg and 40-50 mpg. The 40-50 mpg fuel saved in driving 10,000 miles would be 50 gallons, as compared to the 500 gallons in going from 10-20 mpg. Appendix E discusses further implications of the relationship between fuel consumption and fuel economy for various fuel economy values, and particularly for those greater than 40 mpg.
Figure 2.2 illustrates the relationship between the percentage of fuel consumption decrease and that of fuel economy increase. Figures 2.1 and 2.2 illustrate that the amount of fuel saved by converting to a more economical vehicle depends on where one is on the curve.
Because of the nonlinear relationship in Figure 2.1 , consumers can have difficulty using fuel economy as a measure of fuel efficiency in judging the benefits of replacing the most inefficient vehicles (Larrick and Soll, 2008). Larrick and Soll further conducted three experiments to test whether people reason in a linear but incorrect manner about fuel economy. These experimental studies demonstrated a systemic misunderstanding of fuel economy as a measure of fuel efficiency. Using linear reasoning about fuel economy leads people to undervalue small improvements (1-4 mpg) in lower-fuel-economy (15-30 mpg range) vehicles where there are large decreases in fuel consumption (Larrick and Soll, 2008) in this range, as shown in Figure 2.1 . Fischer (2009) further discusses the potential benefits of utilizing a metric based on fuel consumption as a means to aid consumers in calculating fuel and cost savings resulting from improved vehicle fuel efficiency.
Throughout this report, fuel consumption is used as the metric owing to its fundamental characteristic and its suitability for judging fuel savings by consumers. In cases where the committee has used fuel economy data from the
FIGURE 2.1 Relationship between fuel consumption (FC) and fuel economy (FE) illustrating the decreasing reward of improving fuel economy (miles per gallon [mpg]) for high-mile-per-gallon vehicles. The width of each rectangle represents a 50 percent decrease in FC or a 100 percent increase in FE. The number within the rectangle is the decrease in FC per 100 miles, and the number to the right of the rectangle is the total fuel saved over 10,000 miles by the corresponding 50 percent decrease in FC.
FIGURE 2.2 Percent decrease in fuel consumption (FC) as a function of percent increase in fuel economy (FE), illustrating the decreasing benefit of improving the fuel economy of vehicles with an already high fuel economy.
literature, the data were converted to fuel consumption, using the curve of either Figure 2.1 or 2.2 for changes in fuel economy. Because of this, the committee recommends that the fuel economy information sticker on new cars and trucks should include fuel consumption data in addition to the fuel economy data so that consumers can be familiar with this fundamental metric since fuel consumption difference between two vehicles relates directly to fuel savings. The fuel consumption metric is also more directly related to overall emissions of carbon dioxide than is the fuel economy metric.
Motor vehicles have been powered by gasoline, diesel, steam, gas turbine, and Stirling engines as well as by electric and hydraulic motors. This discussion of engines is limited to power plants involving the combustion of a fuel inside a chamber that results in the expansion of the air/fuel mixture to produce mechanical work. These internal combustion engines are of two types: gasoline spark-ignition and diesel compression-ignition. The discussion also addresses alternative power trains, including hybrid electrics.
Basic Engine Types
Gasoline engines, which operate on a relatively volatile fuel, also go by the name Otto cycle engines (after the person who is credited with building the first working four-stroke internal combustion engine). In these engines, a spark plug is used to ignite the air/fuel mixture. Over the years, variations of the conventional operating cycle of gasoline engines have been proposed. A recently popular variation is the Atkinson cycle, which relies on changes in valve timing to improve efficiency at the expense of lower peak power capability. Since in all cases the air/fuel mixture is ignited by a spark, this report refers to gasoline engines as spark-ignition engines.
Diesel engines—which operate on “diesel” fuels, named after inventor Rudolf Diesel—rely on compression heating of the air/fuel mixture to achieve ignition. This report uses the generic term compression-ignition engines to refer to diesel engines.
The distinction between these two types of engines is changing with the development of engines having some of the characteristics of both the Otto and the diesel cycles. Although technologies to implement homogeneous charge compression ignition (HCCI) will most likely not be available until beyond the time horizon of this report, the use of a homogeneous mixture in a diesel cycle confers the characteristic of the Otto cycle. Likewise the present widespread use of direct injection in gasoline engines confers some of the characteristics of the diesel cycle. Both types of engines are moving in a direction to utilize the best features of both cycles’ high efficiency and low particulate emissions.
In a conventional vehicle propelled by an internal combustion engine, either SI or CI, most of the energy in the fuel goes to the exhaust and to the coolant (radiator), with about a quarter of the energy doing mechanical work to propel the vehicle. This is partially due to the fact that both engine types have thermodynamic limitations, but it is also because in a given drive schedule the engine has to provide power over a range of speeds and loads; it rarely operates at its most efficient point.
This is illustrated by Figure 2.3 , which shows what is known as an engine efficiency map for an SI engine. It plots the engine efficiency as functions of torque and speed. The plot in Figure 2.3 represents the engine efficiency contours in units of brake-specific fuel consumption (grams per kilowatt-hour) and relates torque in units of brake mean effective pressure (kilopascals). For best efficiency, the engine should operate over the narrow range indicated by the roughly round contour in the middle; this is also referred to later in the chapter as the maximum engine brake thermal efficiency (ηb,max). In conventional vehicles, however, the engine needs to cover
FIGURE 2.3 An example of an engine efficiency map for a spark-ignition engine. SOURCE: Reprinted with permission from Heywood (1988). Copyright 1988 by the McGraw-Hill Companies, Inc.
the entire range of torque and speeds, and so, on average, the efficiency is lower. One way to improve efficiency is to use a smaller engine and to use a turbocharger to increase its power output back to its original level. This reduces friction in both SI and CI engines as well as pumping losses. 1 Increasing the number of gear ratios in the transmission also enables the engine to operate closer to the maximum engine brake thermal efficiency. Other methods to expand the high-efficiency operating region of the engine, particularly in the lower torque region, are discussed in Chapters 4 and 5 . As discussed in Chapter 6 , part of the reason that hybrid electric vehicles show lower fuel consumption is that they permit the internal combustion engine to operate at more efficient speed-load points.
Computer control, first introduced to meet the air/fuel mixture ratio requirements for reduced emissions in both CI and SI engines, now allows the dynamic optimization of engine operations, including precise air/fuel mixture control, spark timing, fuel injection, and valve timing. The monitoring of engine and emission control parameters by the onboard diagnostic system identifies emission control system malfunctions.
A more recent development in propulsion systems is to add one or two electrical machines and a battery to create a hybrid vehicle. Such vehicles can permit the internal combustion engine to shut down when the vehicle is stopped and allow brake energy to be recovered and stored for later use. Hybrid systems also enable the engine to be downsized and to operate at more efficient operating points. Although there were hybrid vehicles in production in the 1920s, they could not compete with conventional internal combustion engines. What has changed is the greater need to reduce fuel consumption and the developments in controls, batteries, and electric drives. Hybrids are discussed in Chapter 6 , but it is safe to say that the long-term future of motor vehicle propulsion may likely include advanced combustion engines, combustion engine-electric hybrids, electric plug-in hybrids, hydrogen fuel cell electric hybrids, battery electrics, and more. The challenge of the next generation of propulsion systems depends not only on the development of the propulsion technology but also on the associated fuel or energy infrastructure. The large capital investment in manufacturing capacity, the motor vehicle fleet, and the associated fuel infrastructure all constrain the rate of transition to new technologies.
Combustion-Related Traits of SI Versus CI Engines
The combustion process within internal combustion engines is critical for understanding the performance of SI versus CI engines. SI-engine combustion occurs mainly by turbulent flame propagation, and as turbulence intensity
“Pumping loss” refers to the energy dissipated through fluid friction and pressure gradients developed from the air flow through the engine. A more detailed explanation is provided in Chapter 4 of this report.
tends to scale with engine speed, the combustion interval in the crank-angle domain remains relatively constant throughout the speed range (at constant intake-manifold pressure and engines having a conventional throttle). Thus, combustion characteristics have little effect on the ability of this type of engine to operate successfully at high speeds. Therefore, this type of engine tends to have high power density (e.g., horsepower per cubic inch or kilowatts per liter) compared to its CI counterpart. CI engine combustion is governed largely by means of the processes of spray atomization, vaporization, turbulent diffusion, and molecular diffusion. Therefore, CI combustion, in comparison with SI combustion, is less impacted by engine speed. As engine speed increases, the combustion interval in the crank-angle domain also increases and thus delays the end of combustion. This late end of combustion delays burnout of the particulates that are the last to form, subjecting these particulates to thermal quenching. The consequence of this quenching process is that particulate emissions become problematic at engine speeds well below those associated with peak power in SI engines. This ultimately limits the power density (i.e., power per unit of displacement) of CI diesel engines.
While power density gets much attention, torque density in many ways is more relevant. Thermal auto ignition in SI engines is the process that limits torque density and fuel efficiency potential. Typically at low to moderate engine speeds and high loads, this process yields combustion of any fuel/air mixture not yet consumed by the desired flame-propagation process. This type of combustion is typically referred to as engine knock, or simply knock. If this process occurs prior to spark ignition, it is referred to as pre-ignition. (This is typically observed at high power settings.) Knock and pre-ignition are to be avoided, as they both lead to very high rates of combustion pressure and ultimately to component failure. While approaches such as turbocharging and direct injection of SI engines alter this picture somewhat, the fundamentals remain. CI diesel engines, however, are not knock limited and have excellent torque characteristics at low engine speed. In the European market, the popularity of turbocharged CI diesel engines in light-duty vehicle segments is not only driven by the economics of fuel economy but also by the “fun-to-drive” element. That is, at equal engine displacement, the turbocharged diesel tends to deliver superior vehicle launch performance as compared with that of its naturally aspirated SI engine counterpart.
The fuels and the SI and CI engines that use them have co-evolved in the past 100 years in response to improved technology and customer demands. Engine efficiencies have improved due to better fuels, and refineries are able to provide the fuels demanded by modern engines at a lower cost. Thus, the potential for fuel economy improvement may depend on fuel attributes as well as on engine technology. Implementing certain engine technologies may require changes in fuel properties, and vice versa. Although the committee charge is not to assess alternative liquid fuels (such as ethanol or coal-derived liquids) that might replace gasoline or diesel fuels, it is within the committee charge to consider fuels and the properties of fuels as they pertain to implementing the fuel economy technologies discussed within this report.
Early engines burned coal and vegetable oils, but their use was very limited until the discovery and exploitation of inexpensive petroleum. The lighter, more volatile fraction of petroleum, called gasoline, was relatively easy to burn and met the early needs of the SI engine. A heavier, less volatile fraction, called distillate, which was slower to burn, met the early needs of the CI engine. The power and efficiency of early SI engines were limited by the low compression ratios required for resistance to pre-ignition or knocking. This limitation had been addressed by adding a lead additive commonly known as tetraethyl lead. With the need to remove lead because of its detrimental effect on catalytic aftertreatment (and the negative environmental and human impacts of lead), knock resistance was provided by further changing the organic composition of the fuel and initially by reducing the compression ratio and hence the octane requirement of the engine. Subsequently, a better understanding of engine combustion and better engine design and control allowed increasing the compression ratios back to and eventually higher than the pre-lead-removal levels. The recent reduction of fuel sulfur levels to less than 15 parts per million (ppm) levels enabled more effective and durable exhaust aftertreatment devices on both SI and CI engines.
The main properties that affect fuel consumption in engines are shown in Table 2.1 . The table shows that, on a volume basis, diesel has a higher energy content, called heat of combustion, and higher carbon content than gasoline; thus, on a per gallon basis diesel produces almost 15 percent more CO2. However, on a weight basis the heat of combustion of diesel and gasoline is about the same, and so is the carbon content. One needs to keep in mind that this difference in energy content is one of the reasons why CI engines have lower fuel consumption when measured in terms of gallons rather than in terms of weight. Processing crude oil into fuels for vehicles is a complex process that uses hydrogen to break
TABLE 2.1 Properties of Fuels
Lower Heat of Combustion (Btu/gal)
Lower Heat of Combustion (Btu/lb)
Carbon Content (g/gal)
Carbon Content (g/lb)
SOURCE: After GREET Program, Argonne National Laboratory, http://www.transportation.anl.gov/modeling_simulation/GREET/ .
down heavy hydrocarbons into lighter fractions. This is commonly called cracking. Diesel fuel requires less “molecular manipulation” for the conversion of crude oil into useful fuel. So if one wants to minimize the barrels of crude oil used per 100 miles, diesel would be a better choice than gasoline.
Ethanol as a fuel for SI engines is receiving much attention as a means of reducing dependence on imported petroleum and also of producing less greenhouse gas (GHG). Today ethanol is blended with gasoline at about 10 percent. Proponents of ethanol would like to see the greater availability of a fuel called E85, which is a blend of 85 percent ethanol and 15 percent gasoline. The use of 100 percent ethanol is widespread in Brazil, but it is unlikely to be used in the United States because engines have difficulty starting in cold weather with this fuel.
The effectiveness of ethanol in reducing GHG is a controversial subject that is not addressed here, since it generally does not affect the technologies discussed in this report. It is interesting to note that in a very early period of gasoline shortage, it was touted as a fuel of the future (Foljambe, 1916).
Ethanol has about 65 percent of the heat of combustion of gasoline, so the fuel consumption is roughly 50 percent higher as measured in gallons per 100 miles. Ethanol has a higher octane rating than that of gasoline, and this is often cited as an advantage. Normally high octane enables increases in the compression ratio and hence efficiency. To take advantage of this form of efficiency increase, the engine would need to be redesigned to accommodate an increased combustion ratio. For technical reasons the improvement with ethanol is very small. Also, during any transition period, vehicles that run on 85 to 100 percent ethanol must also run on gasoline, and since the compression ratio cannot be changed after the engine is built, the higher octane rating of ethanol fuel has not led to gains in efficiency. A way to enable this efficiency increase is to modify the SI engine so that selective ethanol injection is allowed. This technology is being developed and is further discussed in Chapter 4 of this report.
FUEL ECONOMY TESTING AND REGULATIONS
The regulation of vehicle fuel economy requires a reproducible test standard. The test currently uses a driving cycle or test schedule originally developed for emissions regulation, which simulated urban-commute driving in Los Angeles in the late 1960s and the early 1970s. This cycle is variously referred to as the LA-4, the urban dynamometer driving schedule (UDDS), and the city cycle. The U.S. Environmental Protection Agency (EPA) later added a second cycle to better capture somewhat higher-speed driving: this cycle is known as the highway fuel economy test (HWFET) driving schedule, or the highway cycle. The combination of these two test cycles (weighted using a 55 percent city cycle and 45 percent highway cycle split) is known as the Federal Test Procedure (FTP). This report focuses on fuel consumption data that reflect legal compliance with the CAFE requirements and thus do not include EPA’s adjustments for its labeling program, as described below. Also discussed below are some technologies—such as those that reduce air- conditioning power demands or requirements—that improve on-road fuel economy but are not directly captured in the FTP.
Compliance with the NHTSA’s CAFE regulation depends on the city and highway vehicle dynamometer tests developed and conducted by the EPA for its exhaust emission regulatory program. The results of the two tests are combined (harmonic mean) with a weighting of 55 percent city and 45 percent highway driving. Manufacturers self-certify their vehicles using preproduction prototypes representative of classes of vehicles and engines. The EPA then conducts tests in its laboratories of 10 to 15 percent of the vehicles to verify what the manufacturers report. For its labeling program, the EPA adjusts the compliance values of fuel economy in an attempt to better reflect what vehicle owners actually experience. The certification tests yield fuel consumption (gallons per 100 miles) that is about 25 percent better (less than) EPA- estimated real-world fuel economy. Analysis of the 2009 EPA fuel economy data set for more than 1,000 vehicle models yields a model-averaged difference of about 30 percent.
The certification test fails to capture the full array of driving conditions encountered during vehicle operations. Box 2.1 provides some of the reasons why the certification test does not reflect actual driving. Beginning with model year 2008, the EPA began collecting data on three additional test cycles to capture the effect of higher speed and acceleration, air-conditioner use, and cold weather. These data are part of air pollution emission compliance testing but not fuel economy or proposed greenhouse gas compliance. However, the results from these three test cycles will be used with the two FTP cycles to report the fuel economy on the vehicle label. Table 2.2 summarizes the characteristics of the five test schedules. This additional information guides the selection of a correction factor, but an understanding of fuel consumption based on actual in-use measurement is lacking.
The unfortunate consequence of the disparity between the official CAFE (and proposed greenhouse gas regulation) certification tests and how vehicles are driven in use is that manufacturers have a diminished incentive to design vehicles to deliver real-world improvements in fuel economy if such improvements are not captured by the official test. Some examples of vehicle design improvements that are not completely represented in the official CAFE test are more efficient air conditioning; cabin heat load reduction through heat-resistant glazing and heat-reflective paints; more efficient power steering; efficient engine and drive train operation at all speeds, accelerations, and road grades; and reduced drag to include the effect of wind. The certification tests give no incentive to provide information to the driver that would improve operational efficiency or to reward control strategies that compensate for driver characteristics that increase fuel consumption.
Shortcomings of Fuel Economy Certification Test
The measurement of the fuel economy of hybrid, plug-in hybrid, and battery electric vehicles presents additional difficulties in that their performance on the city versus highway driving cycles differs from that of conventional vehicles. Regenerative braking provides a greater gain in city driving than in highway driving. Plug-in hybrids present an additional complexity in measuring fuel economy since this requires accounting of the energy derived from the grid. The Society of Automotive Engineers (SAE) is currently developing recommendations for measuring the emissions and fuel economy of hybrid-electric vehicles, including plug-in and battery electric vehicles. General Motors Company recently claimed that its Chevrolet Volt extended-range electric vehicle achieved city fuel economy of at least 230 miles per gallon, based on development testing using a draft EPA federal fuel economy methodology for the labeling of plug-in electric vehicles (General Motors Company press release, August 11, 2009).
The objective of this study is to evaluate technologies that reduce fuel consumption without significantly reducing customer satisfaction. Although each vehicle manufacturer has a proprietary way of defining very precisely how its vehicle must perform, it is assumed here that the following parameters will remain essentially constant as the technologies that reduce fuel consumption are considered:
Interior passenger volume;
Trunk space, except for hybrids, where trunk space may be compromised;
Acceleration, which is measured in a variety of tests, such as time to accelerate from 0 to 60 mph, 0 to 30, 55 to 65 (passing), 30 to 45, entrance ramp to highway, etc.;
TABLE 2.2 Test Schedules Used in the United States for Mileage Certification
Driving Schedule Attributes
High Speed (US06)
Air Conditioning (SC03)
Cold Temperature UDDS
Low speeds in stop-and-go urban traffic
Free-flow traffic at highway speeds
Higher speeds; harder acceleration and braking
Air conditioning use under hot ambient conditions
City test with colder outside temperature
18% of time
7% of time
19% of time
18% of time
Vehicle air conditioning
SOURCE: After http://www.fueleconomy.gov/feg/fe_test_schedules.shtml .
TABLE 2.3 Average Characteristics of Light-Duty Vehicles for Four Model Years
Adjusted fuel economy (mpg)
0 to 60 acceleration time (sec)
SOURCE: EPA (2008).
Safety and crashworthiness; and
Noise and vibration.
These assumptions are very important. It is obvious that reducing vehicle size will reduce fuel consumption. Also, the reduction of vehicle acceleration capability allows the use of a smaller, lower-power engine that operates closer to its best efficiency. These are not options that will be considered.
As shown in Table 2.3 , in the past 20 or so years, the net result of improvements in engines and fuels has been increased vehicle mass and greater acceleration capability while fuel economy has remained constant (EPA, 2008). Presumably this tradeoff between mass, acceleration, and fuel consumption was driven by customer demand. Mass increases are directly related to increased size, the shift from passenger cars to trucks, the addition of safety equipment such as airbags, and the increased accessory content. Note that although the CAFE standards for light-duty passenger cars have been for 27.5 mpg since 1990, the fleet average remains much lower through 2008 due to lower CAFE standards for light-duty pickup trucks, sport utility vehicles (SUVs), and passenger vans.
TRACTIVE FORCE AND TRACTIVE ENERGY
The mechanical work produced by the power plant is used to propel the vehicle and to power the accessories. As discussed by Sovran and Blaser (2006), the concepts of tractive force and tractive energy are useful for understanding the role of vehicle mass, rolling resistance, and aerodynamic drag. These concepts also help evaluate the effectiveness of regenerative braking in reducing the power plant energy that is required. The analysis focuses on test schedules and neglects the effects of wind and hill climbing. The instantaneous tractive force (FTR) required to propel a vehicle is
where R is the rolling resistance, D is the aerodynamic drag with CD representing the aerodynamic drag coefficient, M is the vehicle mass, V is the velocity, dV/dt is the rate of change of velocity (i.e., acceleration or deceleration), A is the frontal area, ro is the tire rolling resistance coefficient, g is the gravitational constant, Iw is the polar moment of inertia of the four tire/wheel/axle rotating assemblies, rw is its effective rolling radius, and ρ is the density of air. This form of the tractive force is calculated at the wheels of the vehicle and therefore does not consider the components within the vehicle system such as the power train (i.e., rotational inertia of engine components and internal friction).
The tractive energy required to travel an incremental distance dS is FTRVdt, and its integral over all portions of a driving schedule in which FTR > 0 (i.e., constant-speed driving and accelerations) is the total tractive-energy requirement, ETR. For each of the EPA driving schedules, Sovran and Blaser (2006) calculated tractive energy for a large number of vehicles covering a broad range of parameter sets (r0, CD, A, M) representing the spectrum of current vehicles. They then fitted the data with a linear equation of the following form:
where S is the total distance traveled in a driving schedule, and α, β, and γ are specific but different constants for the UDDS and HWFET schedules. Sovran and Blaser (2006) also identified that a combination of five UDDS and three HWFET schedules very closely reproduces the EPA combined fuel consumption of 55 percent UDDS plus 45 percent HWFET, and provided its values of α, β, and γ.
The same approach was used for those portions of a driving schedule in which FTR < 0 (i.e., decelerations), where the power plant is not required to provide energy for propulsion. In this case the rolling resistance and aerodynamic drag retard vehicle motion, but their effect is not sufficient to follow the driving cycle deceleration, and so some form of wheel braking is required. When a vehicle reaches the end of a schedule and becomes stationary, all the kinetic energy of its mass that was acquired when FTR > 0 has to have been removed. Consequently the decrease in kinetic energy produced by wheel braking is
The coefficients α′ and β′ are also specific to the test schedule and are given in the reference. Two observations are of interest: (1) γ is the same for both motoring and braking as it relates to the kinetic energy of the vehicle; (2) since the energy used in rolling resistance is r0M g S, the sum of α and α′ is equal to g.
Sovran and Blaser (2006) considered 2,500 vehicles from the EPA database for 2004 and found that their equations fitted the tractive energy for both the UDDS and HWFET schedules with an r = 0.999, and the braking energy with an
r = 0.99, where r represents the correlation coefficient based on least squares fit of the data.
To illustrate the dependence of tractive and braking energy on vehicle parameters, Sovran and Blaser (2006) used the following three sets of parameters. Fundamentally the energy needed by the vehicle is a function of the rolling resistance, the mass, and the aerodynamic drag times frontal area. By combining the last three into the results shown in Table 2.4 , Sovran and Blaser (2006) covered the entire fleet in 2004. The “high” vehicle has a high rolling resistance, and high aerodynamic drag relative to its mass. This would be typical of a truck or an SUV. The “low” vehicle requires low tractive energy and would be typical for a future vehicle. These three vehicles cover the entire spectrum in vehicle design.
The data shown in Table 2.5 were calculated using these values. The low vehicle has a tractive energy requirement that is roughly two-thirds that of the high vehicle. It should also be noted that as the vehicle design becomes more efficient (i.e., the low vehicle), the fraction of energy required to overcome the inertia increases. As expected, for both driving schedules the normalized tractive energy, ETR /MS, decreases with reduced rolling and aerodynamic resistances. What is more significant, however, is that at each level, the actual tractive energy is strongly dependent on vehicle mass, through its influence on the rolling and inertia components. This gives mass reduction high priority in efforts to reduce vehicle fuel consumption.
TABLE 2.4 Vehicle Characteristics
SOURCE: Based on Sovran and Blaser (2006).
TABLE 2.5 Estimated Energy Requirements for the Three Sovran and Blaser (2006)Vehicles in Table 2.4 for the UDDS and HWFET Schedules
Rolling Resistance (%)
Aerodynamic Drag (%)
Effect of Driving Schedule
It is evident from Table 2.5 that inertia is the dominant component on the UDDS schedule, while aerodynamic drag is dominant on the HWFET. The larger any component, the greater the impact of its reduction on tractive energy.
On the UDDS schedule, the magnitude of required braking energy relative to tractive energy is large at all three vehicle levels, increasing as the magnitude of the rolling and aerodynamic resistances decreases. The high values are due to the many decelerations that the schedule contains. The braking energy magnitudes for HWFET are small because of its limited number of decelerations.
In vehicles with conventional power trains, the wheel-braking force is frictional in nature, and so all the vehicle kinetic energy removed is dissipated as heat. However, in hybrid vehicles with regenerative-braking capability, some of the braking energy can be captured and then recycled for propulsion in segments of a schedule where FTR > 0. This reduces the power plant energy required to provide the ETR necessary for propulsion, thereby reducing fuel consumption. The significant increase in normalized tractive energy (ETR/MS) with decreasing rolling and aerodynamic resistances makes reduction of these resistances even more effective in reducing fuel consumption in hybrids with regenerative braking than in conventional vehicles. The relatively small values of braking-to-tractive energy on the HWFET indicate that the fuel consumption reduction capability of regenerative braking is minimal on that schedule. As a result, hybrid power trains only offer significant fuel consumption reductions on the UDDS cycle. However, as pointed out in Chapter 6 , hybridization permits engine downsizing and engine operation in more efficient regions, and this applies to the HWFET schedule also.
Effect of Drive Train
Given the tractive energy requirements (plus idling and accessories), the next step is to represent the efficiency of the power train. The power delivered to the output shaft of the engine is termed the brake output power, and should not be confused with the braking energy mentioned in the previous section. The brake output power, Pb, of an engine is the difference between its indicated power, Pi, and power required for pumping, Pp; friction, Pf; and engine auxiliaries, Pa (e.g., fuel, oil, and water pumps).
Brake thermal efficiency is the ratio of brake power output to the energy rate into the system (the mass flow rate of fuel times its energy density).
The brake thermal efficiency is ηb, while ηi is the indicated thermal efficiency, and Hf is the lower heating value of the fuel. This equation provides the means for relating pumping losses, engine friction, and auxiliary load to the overall engine efficiency. Equations for fuel use during braking and idling are not shown here but can be found in Sovran and Blaser (2003), as can the equations for average schedule and maximum engine efficiency.
Ultimately the fuel consumption is given by Equation 2.6:
where in addition to the terms defined earlier, g* is the fuel consumption over the driving schedule, and represent the fuel consumed during idling and braking, Hf is the fuel density of fuel, is the average drive train efficiency for the schedule, ηb,max is the maximum engine brake thermal efficiency, is the average engine brake thermal efficiency, and EAccessories is the energy to power the accessories. The term ηb,max is repeated in the denominator to show that to minimize fuel consumption the fraction in the denominator should be as large as possible. Thus things should be arranged so that the average engine efficiency be as close to the maximum.
The principal term in Equation 2.6 is the bracketed term. Clearly fuel consumption can be reduced by reducing ETR and EAccessories. It can also be reduced by increasing . As stated earlier, this can be done by down sizing the engine or by increasing the number of gears in the transmission so that average engine brake thermal efficiency, , is increased. Equation 2.6 explains why reducing rolling resistance or aerodynamic drag without changes in engine or transmission may not maximize the benefit, since it may move farther from its optimum point. In other words, changing to lower-rolling-resistance tires without modifying the power train will not give the full benefit.
The tractive energy ETR can be precisely determined given just three parameters, rolling resistance r0, the product of aero coefficient and frontal area CDA, and vehicle mass M. However, many of the other terms in Equation 2.6 are difficult to evaluate analytically. This is especially true of the engine efficiencies, which require detailed engine maps. Thus converting the tractive energy into fuel consumption is best done using a detailed step-by-step simulation. This simulation is usually carried out by breaking down the test schedule into 1-second intervals, computing the ETR for each interval using detailed engine maps along with transmission characterizations, and adding up the interval values to get the totals for the drive cycle analyzed. Such a simulation is frequently called a full system simulation, FSS.
The discussion above on tractive energy highlights the fact that the effects of the three principal aspects of vehicle design—vehicle mass, rolling resistance, and aerodynamic drag—can be used to calculate precisely the amount of energy needed to propel the vehicle for any kind of drive schedule. Further, the equations developed highlight both the effect of the various parameters involved and at the same time demonstrate the complexity of the problem. Although the equations provide understanding, in the end estimating the fuel consumption of a future vehicle must be determined by FSS modeling and ultimately by constructing a demonstration vehicle.
DETAILED VEHICLE SIMULATION
The committee obtained results of a study by Ricardo, Inc. (2008) for a complete simulation for a 2007 Camry passenger car. This FSS is discussed further in Chapter 8 ; one set of results is used here for illustration. Table 2.6 gives the specifications of the vehicle in terms of the parameters used in the simulation.
First, the tractive energy and its components for this vehicle were calculated to illustrate how these vary with different test schedules. Although the US06 cycle described in Table 2.2 is not yet used for fuel economy certification, it is interesting to note how it affects the energy distribution. Table 2.7 shows the results. Energy to the wheels and rolling resistance increase from the UDDS to the US06, with the total tractive energy requirement being almost double that of the UDDS. The aero energy requirement increases from the UDDS to the HWFET, but it is not much increased in going to the US06, in spite of the higher peak speed. What is somewhat surprising is the amount of braking energy for the UDDS and the US06 compared to the HWFET. This is where hybrids excel.
For the highway, rolling resistance and aero dominate, and very little energy is dissipated in the brakes. As expected, the aero is dominant for the US06, where it is more than
TABLE 2.6 Specifications of Vehicle Simulated by Ricardo, Inc. (2008)
TABLE 2.7 Energy Distribution for Various Schedules (in kilowatt-hours)
Total Tractive Energy
Total Rolling Resistance
Total Aerodynamic Drag
half the total tractive energy. Note, though, that the US06 has a significant amount of energy dissipated in the brakes.
As discussed earlier, some people will drive in a UDDS environment and some on the highway. A vehicle optimized for one type of driving will not perform as well for the other, and it is not possible to derive a schedule that fits all driving conditions. Table 2.7 shows the impractically of developing a test that duplicates the actual driving patterns.
Note that the data in Table 2.7 show the actual energy in kilowatt-hours used to drive each schedule. The unit of total energy is used to allow for an easier comparison between the schedules on the basis of energy distribution. Since as shown in Table 2.2 , the distances are 7.45 miles for the UDDS, 10.3 miles for the HWFET, and 8 miles for the US06, the energies should be divided by distance to provide the energy required per mile.
An FSS provides a detailed breakdown of where the energy goes, something that is not practical to do with real vehicles during a test schedule. Figure 2.4 illustrates the total energy distribution in the midsize car, visually identifying where the energy goes.
Table 2.8 shows the fuel consumed for this vehicle for the UDDS, HWFET, and US06 schedules. Efficiency is the ratio of tractive energy divided by “fuel energy input.” Clearly this gives a more succinct picture of the efficiency of an internal combustion engine power train in converting fuel to propel a vehicle and to power the accessories. Depending on the drive schedule, it varies from 15 to 25 percent (including the energy to power accessories). This range is significantly less than the peak efficiency ηb,max discussed earlier.
In addition to the specific operating characteristics of the particular components, the computation of engine fuel consumption depends on the following inputs: (1) the transmission gear at each instant during the driving schedule and (2) the engine fuel consumption rate during braking and idling. None of these details is available, so the data in Table 2.8 should be considered as an illustrative example of the energy distribution in 2007 model-year vehicles with conventional SI power trains.
FINDINGS AND RECOMMENDATIONS
Finding 2.1: Fuel consumption has been shown to be the fundamental metric to judge fuel efficiency improvements from both an engineering and a regulatory viewpoint. Fuel economy data cause consumers to undervalue small increases (1-4 mpg) in fuel economy for vehicles in the 15- to 30-mpg range, where large decreases in fuel consumption can be realized with small increases in fuel economy. For example, consider the comparison of increasing the mpg rating from 40 mpg to 50 mpg, where the total fuel saved in driving 10,000 miles is only 50 gallons, compared to 500 gallons for a change from 10 mpg to 20 mpg.
FIGURE 2.4 Energy distribution obtained through full-system simulation for UDDS (top), HWFET (middle), and US06 (bottom). SOURCE: Ricardo, Inc. (2008).
TABLE 2.8 Results of Full System Simulation (energy values in kilowatt-hours)
Total Tractive Energy
Fuel Input Energy
Power Train Efficiency (%)
Recommendation 2.1: Because differences in the fuel consumption of vehicles relate directly to fuel savings, the labeling on new cars and light-duty trucks should include information on the gallons of fuel consumed per 100 miles traveled in addition to the already-supplied data on fuel economy so that consumers can become familiar with fuel consumption as a fundamental metric for calculating fuel savings.
Finding 2.2: Fuel consumption in this report is evaluated by means of the two EPA schedules: UDDS and HWFET. In the opinion of the committee, the schedules used to compute CAFE should be modified so that vehicle test data better reflect actual fuel consumption. Excluding some driving conditions and accessory loads in determining CAFE discourages the introduction of certain technologies into the vehicle fleet. The three additional schedules recently adopted by the EPA for vehicle labeling purposes—ones that capture the effects of higher speed and acceleration, air-conditioner use, and cold weather—represent a positive step forward, but further study is needed to assess to what degree the new test procedures can fully characterize changes in in-use vehicle fuel consumption.
Recommendation 2.2: The NHTSA and the EPA should review and revise fuel economy test procedures so that they better reflect in-use vehicle operating conditions and also better provide the proper incentives to manufacturers to produce vehicles that reduce fuel consumption.
EPA (U.S. Environmental Protection Agency). 2008. Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through 2008. EPA420-R-08-015. September. Washington, D.C.
Fischer, C. 2009. Let’s turn CAFE regulation on its head. Issue Brief No. 09-06. May. Resources for the Future, Washington, D.C.
Foljambe, E.S. 1916. The automobile fuel situation. SAE Transactions, Vol. 11, Pt. I.
General Motors Company. 2009. Chevy Volt gets 230 mpg city EPA rating. Press release. August 11.
Heywood, J.B., 1988. Internal Combustion Engine Fundamentals. McGraw-Hill, New York.
Kettering, C.F. 1918. Modern aeronautic engines. SAE Transactions, Vol. 13, Pt. II.
Larrick, R., and J. Soll. 2008. The mpg illusion. Science 320(5883):1593-1594.
Ricardo, Inc. 2008. A Study of Potential Effectiveness of Carbon Dioxide Reducing Vehicle Technologies. Prepared for the U.S. Environmental Protection Agency. EPA420-R-08-004. Contract No. EP-C-06-003. Work Assignment No. 1-14. Ann Arbor, Michigan.
Sovran, G., and D. Blaser. 2003. A contribution to understanding automotive fuel economy and its limits. SAE Paper 2003-01-2070. SAE International, Warrendale, Pa.
Sovran, G., and D. Blaser. 2006. Quantifying the potential impacts of regenerative braking on a vehicle’s tractive-fuel consumption for the US, European and Japanese driving schedules. SAE Paper 2006-01-0664. SAE International, Warrendale, Pa.
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Assessment of Fuel Economy Technologies for Light-Duty Vehicles
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Various combinations of commercially available technologies could greatly reduce fuel consumption in passenger cars, sport-utility vehicles, minivans, and other light-duty vehicles without compromising vehicle performance or safety. Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy estimates the potential fuel savings and costs to consumers of available technology combinations for three types of engines: spark-ignition gasoline, compression-ignition diesel, and hybrid.
According to its estimates, adopting the full combination of improved technologies in medium and large cars and pickup trucks with spark-ignition engines could reduce fuel consumption by 29 percent at an additional cost of $2,200 to the consumer. Replacing spark-ignition engines with diesel engines and components would yield fuel savings of about 37 percent at an added cost of approximately $5,900 per vehicle, and replacing spark-ignition engines with hybrid engines and components would reduce fuel consumption by 43 percent at an increase of $6,000 per vehicle.
The book focuses on fuel consumption–the amount of fuel consumed in a given driving distance–because energy savings are directly related to the amount of fuel used. In contrast, fuel economy measures how far a vehicle will travel with a gallon of fuel. Because fuel consumption data indicate money saved on fuel purchases and reductions in carbon dioxide emissions, the book finds that vehicle stickers should provide consumers with fuel consumption data in addition to fuel economy information.
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