CO2 Emission Benefit of Diesel (versus Gasoline) Powered Vehicles

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Policy Analysis CO2 Emission Benefit of Diesel (versus Gasoline) Powered Vehicles J. L. SULLIVAN, R. E. BAKER, B. A. BOYER, R. H. HAMMERLE, T. E. KENNEY, L. MUNIZ, AND T. J. WALLINGTON* Scientific Research Laboratory, SRL-3083, Ford Motor Company, Dearborn, Michigan 48121-2053

Concerns regarding global warming have increased the pressure on automobile manufacturers to decrease emissions of CO2 from vehicles. Diesel vehicles have higher fuel economy and lower CO2 emissions than their gasoline counterparts. Increased penetration of diesel powered vehicles into the market is a possible transition strategy toward a more sustainable transportation system. To facilitate discussions regarding the relative merits of diesel vehicles it is important to have a clear understanding of their CO2 emission benefits. Based on European diesel and gasoline certification data, this report quantifies such CO2 reduction opportunities for cars and light duty trucks in today’s vehicles and those in the year 2015. Overall, on a wellto-wheels per vehicle per mile basis, the CO2 reduction opportunity for today’s vehicles is approximately 24-33%. We anticipate that the gap between diesel and gasoline wellto-wheel vehicle CO2 emissions will decrease to approximately 14-27% by the year 2015.

The approach used is to compare the CO2 emission data for 2001 MY (model year) gasoline and diesel vehicles reported by automobile manufacturers to the German government. When comparing diesel vehicles with their gasoline “equivalents” one or more bases for “equivalence” need to be specified. Possible parameters that could be used to determine equivalence include the following: performance (i.e., acceleration, maximum torque, horsepower, etc.), engine displacement, manufacturing cost, retail price, interior space, cargo capacity, and vehicle weight. For convenience, in the present work we chose vehicle inertia weight as the basis for comparison of diesel and gasoline cars and trucks. Simple physical laws dictate that the energy needed to move a vehicle is related to the vehicle weight. Hence, to assess the CO2 benefit of diesel vehicles it seems reasonable to compare vehicles of similar weight. As discussed below, we believe that inertia weight provides a reasonable and robust basis for comparison between the performance of diesel and gasoline powered vehicles covered in the 2001 MY database. For future vehicles, the likely impact of powertrain efficiency improvements and their partial degradation due to the addition of aftertreatment technology were considered. We report herein a quantitative analysis of the CO2 emission benefits of diesel vehicles versus their gasoline equivalents for 2001 MY and 2015 MY in European and North American markets.

2. Methodology 2.1. Relationship between CO2 Emissions, Fuel Economy, and Fuel Consumption. The fuel economy (FE) and fuel consumption (FC) of diesel and gasoline vehicles are related to CO2 emissions by the following expressions:

1. Introduction Recognition of the importance of global climate change has led to pressure on automobile manufacturers to reduce CO2 emissions from new vehicles. Diesel and gasoline engines have fundamentally different modes of operation. In diesel engines, the fuel is ignited in the combustion chamber by the high temperature generated in the compression stroke. A spark is not required to ignite diesel fuel, which is a heavier, less volatile mixture of hydrocarbons than gasoline. Relative to gasoline engines, diesel engines have higher compression ratios, more rapid combustion, and lower throttling losses and operate leaner. As a result, diesel engines have a greater thermodynamic efficiency and hence a higher fuel economy (lower fuel consumption) than gasoline engines. A transition from gasoline to diesel powered vehicles offers a possible approach for reducing CO2 emissions from vehicles. An assessment of the utility of this approach requires consideration of many factors such as consumer acceptance, vehicle performance, fuel infrastructure, fuel pricing, fuel formulation, NOx and particulate emission regulations, manufacturing capacity, and refining capacity. However, before these issues are discussed there needs to be a clear understanding of the current, and likely future, CO2 emissions benefit associated with the use of diesel powered vehicles. The goal of the present work is to provide such a quantitative analysis of the CO2 emission benefits of diesel vehicles. * Corresponding author phone: (313)390-5574; fax: (313)322-7044; e-mail: [email protected]. 10.1021/es034928d CCC: $27.50 Published on Web 05/13/2004

 2004 American Chemical Society

[CO2]d ) 6231/FEd ) 26.5 * FCd

(1)

[CO2]g ) 5550/FEg ) 23.6 * FCg

(2)

The subscripts d and g denote diesel and gasoline, respectively. The units for the terms in eqs 1 and 2 are as follows: g/km for [CO2], miles per gallon for FE, and L/100 km for FC. The different constants in eqs 1 and 2 reflect the fact that diesel and gasoline fuels have different densities and carbon contents. The derivations of these equations and a listing of relevant fuel properties are provided in the Supporting Information. The range of gasoline and diesel densities specified for use in North America and Europe are similar; therefore, one set of fuel properties for each fuel was used for all the calculations in this study (1, 2). The variation in the average fuel properties between North America and Europe is small compared to the seasonal and regional fluctuations of specific fuel formulations within North America and Europe. The following equation (derived from eqs A-3 and A-4 in the Supporting Information) shows the relative change in CO2 emissions of diesel versus gasoline-fueled vehicles:

∆[CO2] [CO2]g

)

[CO2]d - [CO2]g [CO2]g

) 1.12 *

FCd -1) FCg FEg 1.12 * - 1 (3) FEd

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distance) and the different energy and carbon contents of diesel and gasoline can be a source of confusion when comparing the relative performance of diesel and gasoline vehicles. A given % change in fuel economy is not equivalent to the same % change in fuel consumption and neither quantity is the same as the % change in CO2 emissions. For example, let us consider an average diesel C-class vehicle with its gasoline counterpart. Assuming FCd ) 5.8 and FCg ) 8.9 (L per 100 km) and using expressions A-1 and 3, we can calculate that when compared to its gasoline counterpart the diesel vehicle has 35% lower fuel consumption, 53% higher fuel economy, and 27% lower CO2 emissions. 2.2. CO2 Emission Record. Manufacturers of vehicles sold in Germany have a statutory duty to provide certified vehicle emission data to the government. The data include CO2 emissions and is a matter of public record (3). We have used the 2001 model year (MY) vehicle certification data reported to the German government to provide a picture of the CO2 emissions per kilometer for both gasoline and diesel passenger cars and trucks in Europe. Vehicles in the United States are defined as either “cars” or “trucks”. There is no such designation in the German certification data. For simplicity, we have applied the United States definitions of passenger car and light truck fleet to the German fleet where convertibles, hatchbacks, sedans, and station wagons are considered “cars” and minivans, sport utility vehicles, trucks, cargo vans, and passenger vans are considered “trucks”. It should be noted that our use of the word “trucks” refers to vehicles < 2300 kg and does not include heavy duty commercial trucks. 2.3. Representativeness of German Data. Due to its size and comprehensiveness, German Certification data were chosen as the basis of comparison for this report. While it is possible that these data are not representative of the European Union as a whole, this factor is unlikely to have a significant impact on the conclusions of the present analysis (see section 3.1). Vehicle technologies available in Germany are available elsewhere in Europe. Availability is the key point, because the average CO2 emission values generated for each weight class included all available technology and vehicle offerings in the market place and are not sales weighted averages. The same applies to our treatment of North American data. 2.4. Direct versus Indirect Injection. During the 1990s indirect injection (IDI) diesel technology began to be replaced by more efficient direct injection (DI) technology. By 2001 more than 90% of the diesel passenger vehicles sold in German employed DI technology. No attempt was made to separate indirect injection diesel from direct injection diesel technology in data analyzed in the present work. Consequently, the results cited herein for the diesel CO2 reduction benefit over gasoline are conservative and tend to underestimate the actual benefit of the current DI fleet. Given that (i) by 2001 less than 10% of the new vehicles employed the old IDI technology and (ii) DI vehicles typically have 1015% better fuel economy than IDI vehicles, we conclude that the magnitude of the underestimation is small (no more than a couple of percentage points) and will not impact the conclusions of this report.

3. Results 3.1. CO2 Emission Data for German Gasoline and Diesel Passenger Vehicles. In Figure 1 we plot the CO2 emissions reported on the New European Drive Cycle (NEDC) for the German passenger car fleet (3) as a function of the vehicle inertia weight. It should be noted that the data in Figure 1 describe the range of vehicles for sale in the German market and are not sales weighted averages. As expected there is a trend of increased CO2 emissions with increasing vehicle weight. Clearly there is considerable variation in the amount 3218

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FIGURE 1. 2001 MY German certification data for gasoline (blue crosses) and diesel (red triangles) passenger cars. For clarity, the diesel data are offset by 15 kg.

FIGURE 2. Weight class-averaged 2001 MY German certification data: passenger car. of CO2 emitted at each inertial weight class. Further inspection of the data in Figure 1 shows a considerable difference between the average emissions for diesel and gasoline fueled passenger cars. To make this distinction a little clearer, average values of the data at each inertia weight class for each fleet are replotted in Figure 2; one-sigma standard deviation bars have also been included as well as linear regression lines representing the averages of each fleet as a function of vehicle mass. To reduce undue weighting on individual vehicles, averages in Figures 2 and 3 are only provided for data sets consisting of at least 5 vehicles. As seen in Figure 2, the linear regressions provide reasonable estimates of the average CO2 emission at each weight for both the diesel and gasoline car fleets. Such lines are used as interpolation functions for our analysis. Also shown in Figure 2 are percent reductions of diesel over gasoline [CO2] emissions as a function of vehicle weight. The CO2 reduction ranges from 29 to 25%. Although there appears to be a small trend of decreasing reduction with increasing weight, this is

TABLE 1. Average CO2 Emission Rates for 2001 MY Gasoline and Diesel Vehicles av C-class (3125 lb, 1420 kg)

N. Americaa Germanb

gasoline diesel % ∆ (D-G)/G gasoline diesel % ∆ (D-G)/G

av light truck (4500 lb, 2040 kg)

fuel economy [mpg]

fuel consumption [L/100 km]

CO2 emissions [gCO2/km]

fuel economy [mpg]

25.2 40.5 60 26.1 39.9 53

9.3 5.8 -38 9.0 5.9 -35

220 154 -30 213 156 -27

17.4

13.5

319

19.5 27.7 42

12.1 8.5 -30

285 225 -21

fuel consumption [L/100 km]

CO2 emissions [gCO2/km]

a North American data represent combined city and highway label fuel economy from U.S. certification results taken on the Federal Test Procedure (FTP) cycle. b German data represent combined city and highway CO2 emissions from German certification results taken on the New European Drive Cycle (NEDC).

FIGURE 3. Weight class-averaged 2001 MY German certification data: truck. not significant given the uncertainties associated with the individual data points shown in Figure 2. Figure 3 shows a plot of the average CO2 emissions of the German truck fleet versus vehicle inertia weight. The error bars are one standard deviation bars. The solid lines are linear least-squares fits to the gasoline truck (circles) and diesel truck (triangles) data. The dotted line shows the percent CO2 reduction of diesel over gasoline (see right-hand y-axis for scale). The CO2 reduction varies from 24 to 18%. Although there appears to be a small trend of decreasing reduction with increasing weight, this trend is not significant given the uncertainties associated with the individual data points shown in Figure 3. 3.2. CO2 Emission Data for North American Gasoline and Diesel Passenger Vehicles. We now turn to North American CO2 performance and examine the 2001 MY U.S. certification data (4). Unlike the gasoline vehicle fleet, there is very little certification data available for diesels in North America. However, as seen in Figure 4, there is enough data to infer that the potential CO2 reduction opportunity for diesel over gasoline cars is about the same in the United States as in Europe. Figure 4 shows three types of information: (i) average inertial class [CO2] emissions for the gasoline passenger car and light truck fleet with one-sigma standard deviation bars, (ii) linear regressions representing the weight dependence of the averages, and (iii) limited diesel passenger car data from “C-class” vehicles. As a point of reference for the reader, VW Golf, Ford Focus, and Toyota Matrix are examples of C-class vehicles (although they are not necessarily available in the United States with diesel engines). The CO2 emissions from the limited diesel car data in Figure 4 lie 30% below the emissions from the average

FIGURE 4. Weight class-averaged 2001 MY U.S. certification data for cars (triangles) and trucks (circles). For clarity of presentation, the truck data have been plotted with a 15 kg offset. gasoline car data. Table 1 provides a summary of North American and German results. Notice the consistency between European and North American results. It seems reasonable to conclude that the potential CO2 reduction opportunity for diesels in North America is approximately the same as in Europe, at this time considering current emissions standards. It should be noted that because of higher particulate matter emissions from diesels relative to gasoline and the more stringent tailpipe emission regulations in North America, diesels will become increasingly difficult to certify in North America. Unlike the German data, which are measured and reported CO2 results, the U.S. CO2 emission values used herein are instead calculated from combined city and highway label fuel economy. The city label fuel economy is 0.9 times the fuel economy obtained on the city cycle of the U.S. Federal Test Procedure (FTP). The highway label fuel economy is 0.78 times the fuel economy measured on the highway cycle of the U.S. FTP. The following relation gives the combined fuel economy which was used with eqs 1 and 2 to calculate the data shown in Figure 4:

FECmb ) 1/(0.55/FECity + 0.45/FEHwy)

(4)

From the least-squares fits to the data in Figures 2-4 we derive the CO2 emission rates for an average C-class car (1420 kg) and an average light truck (2040 kg) shown in Table 1. Fuel economy and fuel consumption data were calculated from the CO2 data using eqs 1 and 2. 3.3. Effect of Transmission Type. It is important to consider the impact of differences in the distribution of transmission types (automatic vs manual) available in the VOL. 38, NO. 12, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. CO2 Emissions (g/km), Calculated from Linear Fits Shown in Figures 2-6, for German Fleet by Vehicle and Transmission Type for Selected Vehicle Weights vehicle

weight [kg]

car

1420

truck

2040

transmission

diesel

gasoline

% difference

manual automatic combined manual automatic combined

154 164 156 230 239 225

207 219 213 278 295 285

26 25 27 17 19 21

FIGURE 5. CO2 emissions for German cars by transmission and fuel type.

FIGURE 7. CO2 emissions for American cars by transmission and fuel type.

FIGURE 6. CO2 emissions for German trucks by transmission and fuel type. German and North American vehicle fleets. European drivers prefer manual transmissions, while North American motorists prefer automatic transmissions. Vehicles equipped with manual transmissions generally have better fuel efficiency and hence lower CO2 emissions than those with automatic transmissions. The objective of the present study was to characterize the CO2 emission differences between gasoline and diesel fueled cars and trucks within the North American and European vehicle fleets and not between the fleets. The availability of transmission types for gasoline and diesel fueled vehicles is expected to be the same within each fleet, and transmission effects are not expected to be present in our results. Indeed, this is what is found for the German fleet; insufficient data are available to confirm this for the North American fleet. Results are given below. German CO2 emissions data by vehicle transmission type are displayed in Figures 5 and 6. These figures along with Table 2 show that the CO2 emission reduction benefit of diesel over gasoline-fueled vehicles is essentially the same 3220

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as the overall average for each vehicle type, irrespective of transmission type. The combined data in Table 2 was obtained from the regressions in Figures 1 and 2. The effect of transmission type on average CO2 emissions for the U.S. passenger car and truck fleets is shown in Figure 7. Unfortunately, due to a lack of North American diesel fueled vehicle data, this figure, with the exception of a pair of dieselCO2 emission data points, shows only emission data for gasoline fueled cars and trucks. Nevertheless, based on the limited North American diesel-CO2 results and their consistency with German results (discussed above) and based on the extensive German diesel vs gasoline vehicle record, it seems reasonable to conclude that the infusion of dieselfueled vehicles into the American vehicle fleet would have the same CO2 benefit in North America as in Germany. 3.4. Projected CO2 Emissions from 2015 MY Vehicles. Fuel consumption and economy and CO2 emissions for gasoline and diesel vehicles are expected to change in the future due to two main factors. First, improvements in fuel efficiency are likely to occur in both gasoline and diesel vehicles, but since diesel engines are inherently more efficient, gasoline engines have more potential for improvement by becoming more diesel-like in their operation. Second, the future trend toward more stringent NOx and particulate matter emission standards in both North America and Europe and the addition of new emissions technology necessary to achieve the standards is expected to reduce diesel efficiencies. Therefore, anticipated increases in gasoline vehicle efficiencies and decreases in diesel vehicle efficiencies will tend to decrease the CO2 reduction opportunity of diesel vehicles over gasoline vehicles in the 2015 time frame.

TABLE 3. Technologies Affecting Future Fuel Economy Relative to that of Current Vehicles

vehicle diesel

factor

approach

particulate emissionsa NOx emissionsa

filter traps

NOx trap or urea SCRb engine efficiency friction red gasoline aftertreatmenta 3-way cat. engine efficiency DI-VCTc engine efficiency friction red engine efficiency lean burn NA transmission - car CVTd NA transmission - truck 6-speed ATe Europe transmission - both 6-speed ASMf

% FE change [%]

fi

-3

0.97

(-5) -1 +3 0 +3 +2 +2 +4 +4 +4

(0.95) 0.99 1.03 1.00 1.03 1.02 1.02 1.04 1.04 1.04

a Meeting U.S. Tier II, bin 5 (or CA LEVII, ULEV) standard expected to be in effect in 2015. b Selective catalytic reduction. c Variable cam timing. d Continuous variable transmission. e Automatic transmission. f Automatic shift manual.

Mathematically, changes in vehicle fuel economy can be represented as a current fuel economy, FEo, times a number of adjustment factors that represent changes in technology anticipated to be added to future vehicles. More specifically, this is written as M

FE ) FEo

∏f

(5)

i

i)1

where the fi are factors that either enhance (fi > 1) or diminish (fi < 1) the fuel efficiency of the current vehicle, “i” is the factor index, and M is the number of factors. Table 3 provides a summary of the likely technology changes and estimates of their impact on fuel economy. The 3% penalty assumed for filter traps is taken from recent estimates in the literature (5-7). The 5% penalty assumed for NOx trap technology is based upon an EPA assessment in 2002 (8) and recent laboratory work (7). The 1% penalty associated with use of urea SCR and the relative merits of NOx trap versus SCR technology are discussed in detail elsewhere (7). Single values for the FE potential for advanced gasoline engine and transmission technologies were selected from the National Research Council (9) ranges, considering use of the technologies in combination. The potential fuel economy benefit for reducing friction in diesel engines was estimated to be higher than that on gasoline engines by 1% because the improved cold start performance of advanced combustion and common rail injection systems would permit

diesel engine compression ratio optimization at lower levels. The values in Table 3 are estimates, not precise determinations, of the likely impact of future technology on fuel economy. While these may change with time, we believe they are reasonable estimates for use in the present discussion. The effects of using widely available 2015 powertrain and aftertreatment technologies were estimated by appropriately combining the projected vehicle fuel economy benefits listed in Table 3 and applying them to the current vehicles in Table 1. The resulting effects of these technologies on fuel economy, fuel consumption, and tank-to-wheels CO2 are shown in Table 4. The fuel economies of gasoline-powered vehicles are projected to increase by more than 11%, while that of diesel engine powered vehicles will make essentially zero (for NOx trap) to modest 3% gains (for urea SCR). Consequently, the advantages of diesel powertrains over gasoline powertrains will diminish. The tank-to-wheels CO2 advantage of diesel engines is reduced from the 21%-30% range in 2001 (Table 1) to 11%-24% in 2015 (Table 4). The diesel fuel economy advantage drops from 42%-60% to between 26% and 48%, and the fuel consumption benefit declines from 30%-38% to 21%-32%. Clearly, projections of 2015 powertrain technologies and emissions standards are made with considerable uncertainty. There are several research technologies, e.g., direct injection, homogeneous charge compression ignition (HCCI), and electromechanical valve trains, that have the potential of producing significantly larger FE gains for gasoline engines. Higher pressure fuel injection systems, diesel HCCI combustion, and improved lean aftertreatment systems could produce modest diesel engine FE improvement opportunities. However, more stringent tailpipe NOx emissions standards are likely to have a greater negative impact on diesel engines, further reducing the advantages of future diesels relative to gasoline engines. In a scenario where the best gasoline engine technologies are deployed and NOx standards approach that of partial zero emission vehicles (PZEV) (10), the diesel tank-to-wheels CO2 benefit could fall to 5-10%. Finally, it should be noted that the advantages of DI diesel technology were not fully exploited in the 2001 model year fleet but will be fully exploited in 2015. As a result of this factor, the diesel advantage given in Table 4 is understated slightly (by perhaps 1-2 percentage points) see section 2.4 for more details. 3.5. Well-to-Wheels Analysis. Diesel and gasoline fuels are products and energy is consumed and emissions generated during their production. Quantifying the production and use burdens of a product is generally known as life cycle inventory assessment (LCI); it is usually called a well-to-

TABLE 4. Estimated Average CO2 Emission Rates for 2015 MY Gasoline and Diesel Vehicles av C-class (3125 lb, 1420 kg)

N. America

gasoline diesela %∆ (D-G)/G

Europe

gasoline diesela % ∆ (D-G)/G

av light truck (4500 lb, 2040 kg)

fuel economy [mpg]

fuel consumption [L/100 km]

CO2 emissions [gCO2/km]

fuel economy [mpg]

fuel consumption [L/100 km]

CO2 emissions [gCO2/km]

28.1 (39.9) 41.6 (42) 48 29.0 (39.4) 41.1 (36) 41

8.4 (5.9) 5.7 (-30) -32 8.1 (6.0) 5.7 (-26) -29

197 (156) 150 (-21) -24 191 (158) 152 (-17) -21

19.4 b

12.1 b

286 b

b

b

b

21.7 (27.3) 28.5 (26) 31

10.8 (8.6) 8.3 (-21) -24

256 (228) 219 (-11) -14

a Two diesel aftertreatment systems are considered: (1) particulate filter with NO trap - values in parentheses and (2) particulate filter with x urea selective catalytic reduction. b Insufficient data (see section 3.2).

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TABLE 5. Comparison of Tank-to-Wheels (TTW) and Well-to-Wheels (WTW) CO2 Emissions for 2001 MY Vehicles av C-class (3125 lb, 1420 kg)

av light truck (4500 lb, 2040 kg)

TTWa WTW TTWa WTW CO2 CO2 CO2 CO2 gCO2/km gCO2/km gCO2/km gCO2/km N. America gasoline diesel % (D-G)/G German gasoline diesel % (D-G)/G a

220 154 -30 213 156 -27

273 183 -33 247 175 -29

319

395

285 225 -21

331 252 -24

From Table 1.

TABLE 6. WTW CO2 Emission Rates for 2015 MY Gasoline and Diesel Vehicles av C-class (3125 lb, 1420 kg)

av light truck (4500 lb, 2040 kg)

TTW WTW TTW WTW CO2a CO2 CO2a CO2 gCO2/km gCO2/km gCO2/km gCO2/km N. America gasoline dieselb %(D-G)/G Europe

gasoline dieselb %(D-G)/G

197 (156) 150 (-21) -24 191 (158) 152 (-17) -21

245 (185) 178 (-24) -27 222 (177) 170 (-20) -23

286 c

354 c

c

c

256 (228) 219 (-11) -14

297 (256) 246 (-14) -17

a From Table 4. b Two diesel aftertreatment systems are considered: (1) particulate filter with NOx trap - values in parentheses and (2) particulate filter with urea selective catalytic reduction. c Insufficient data to make projection (see section 3.2).

wheels (WTW) analysis when LCI is applied to automotive fuels. The total CO2 generation for the production and use of a unit of fuel in a vehicle is

[CO2]WTW ) [CO2]WTT + [CO2]TTW

(6)

where WTT denotes well-to-tank and TTW stands for tankto-wheels. In the preceding analyses we have considered just tailpipe (i.e., TTW) emissions. A number of WTW analyses for the production of various fuel have been conducted over the past decade. A comprehensive study (11) has been recently conducted by General Motors and several oil companies. While that study focused primarily on the production of fuels in Europe, a range of values including those of a comparatively recent North American Study are cited therein. For our WTW analysis, we employ [CO2]WTW/[CO2]TTW ) 1.162 and 1.121 for gasoline and diesel in the EU and [CO2]WTW/[CO2]TTW ) 1.239 and 1.186 for gasoline and diesel in the United States, respectively, taken from Figures 3.1-6 and 3.1-7 in ref 11. The different emission ratios for the EU and United States reflect differences in fuel production practices between the two continents, including the relative fractions of product that goes to diesel versus gasoline. Applying these ratios to the tailpipe CO2 emissions given in Table 1 gives the WTW CO2 emissions given in Table 5. For the German tailpipe results, we use EU emission ratios, and for the North American tailpipe results we use U.S. emission ratios. 3222

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On a WTW basis, diesel cars in North America and Germany emit approximately one-third less CO2 than their gasoline counterparts. Diesel light duty trucks in Germany emit approximately one-quarter less CO2 than their gasoline counterparts. Although there may be incremental changes in feedstocks, refinery operations, and fuel specifications in the coming decade, it seems reasonable to assume that the WTT advantage of diesel in 2015 will not be substantially different from those at present. With this assumption we arrive at the WTW projections for the year 2015 given in Table 6.

4. Comparison with Previous Work and Climate Change Implications We find that 2001 MY German fleet diesel passenger cars emit on average 29 to 25% less gCO2/km than gasoline passenger cars for vehicle weights between 1100 and 2100 kg. For the light truck fleet, the CO2 reduction varies from 24 to 18%, diesel over gasoline, for vehicle weights of 1800 to 2300 kg. For a 2040 kg truck the reduction is 21%. Based on very limited diesel data in the United States, 2001 MY diesel passenger cars weighing 1420 kg emit on average 30% less CO2 than their gasoline counterparts; a value which is in reasonable agreement with the German results. We anticipate that in 2015, the tailpipe CO2 benefit of diesel over gasoline vehicles will decrease by approximately 5-10 percentage points (depending on the aftertreatment system used to achieve the lower NOx and PM emission standards) compared to the 2001 values in both the U.S. and European markets. Taken collectively, in 2015 U.S. and German diesel C-class cars are expected to provide a 1724% reduction in tailpipe CO2 benefit; European diesel trucks weighing 2040 kg will offer a 11-14% reduction. A well-to-wheels analysis (where CO2 emissions associated with fuel production are included with tailpipe emissions) indicates that diesel vehicles have an additional approximately 2-3 percentage points CO2 emissions benefit. Our results are consistent with a recent well-to-wheels CO2 emissions study in which it was found that a current technology 1.7 L direct injection diesel vehicle had 25% lower emissions than the baseline gasoline vehicle (12). Schipper et al. (13) have reported an analysis of the characteristics, usage, energy savings, and CO2 emission implications of diesel vehicles in Europe during the 1970s-1990s. Where direct comparison is possible, the results from the present study are consistent with the findings of Schipper et al. (13). For example, Schipper et al. (13) considered pairs of diesel and gasolines vehicles from 1998 or 1999 model years matched on the basis of performance. For the 11 direct injection pairs the average fuel economy benefit was 33%. This value can be compared to the 35% benefit calculated in the present work for an average C-class vehicle (see Table 1). The diesel vehicles considered by Schipper et al. (13) were on average 8.2% heavier than their gasoline equivalents. Assuming that an average C-class vehicle is representative of the vehicles considered by Schipper et al. (13) and correcting for the effect of weight (see Figure 2) we predict a 29% fuel economy benefit for the diesel vehicles used by Schipper et al. (13). This prediction is consistent with the value of 33% reported by Schipper et al. (13). The quantitative analysis presented here shows that increased use of diesel powered vehicles offers a possible approach to reducing CO2 emissions associated with vehicles and hence addressing climate change.

Supporting Information Available Derivation of the expressions given in section 2.1 and other supporting material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw Hill: New York, 1988; p 915. (2) Owen, K.; Corey, T. Automotive Fuels Reference Book, 2nd ed.; Society of Automotive Engineers: 1995; p 801. (3) Deutsche Kraftfahrt-Bundesamt (German Federal Motor Transport Authority). http://www.kba.de/. (4) U.S. Environmental Protection Agency, Vehicle Certification Test Results and Data, 2001. http://www.epa.gov/otaq/crttst.htm. (5) Stamatelos, A. M. Energy Convers. Mgmt. 1997, 38, 83. (6) Johnson, T. V. Soc. Automot. Eng. Paper Ser. 2003, 2003-010039. (7) Lambert, C.; Hammerle, R.; McGill, R.; Khair, M.; Sharp, C. Soc. Automot. Eng. Paper Ser. 2004, 2004-01-1292. (8) USEPA report Highway Diesel Progress Review. www.epa.gov/ air/caaac/clean_diesel.html, June 2002. (9) National Research Council. Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards; National Academy Press: Washington, DC, 2002.

(10) State of California Environmental Protection Agency Air Resources Board, Amendments to the California Zero Emission Vehicle Program Regulations, Final Statement of Reasons, December 2001. http://www.arb.ca.gov/regact/zev2001/ fsor.pdf. (11) Choudhury, R. GM Well-to-Wheels Analysis of Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems A European Study, L-B Systemtechnik GmbH, Daimlerstrasse 15, 85521 Ottobrunn, Germany, September 2002. http:// www.lbst.de/gm-wtw. (12) Atkins, M. J.; Koch, R. Soc. Automot. Eng. Paper Ser. 2003, 200301-0081. (13) Schipper, L.; Marie-Lilliu, C.; Fulton, L. J. Transport Econ. Pol. 2002, 36, 305.

Received for review August 23, 2003. Revised manuscript received February 27, 2004. Accepted April 6, 2004. ES034928D

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