Chemistry for Everyone
Using the Relationship between Vehicle Fuel Consumption and CO2 Emissions To Illustrate Chemical Principles Maria T. Oliver-Hoyo* Chemistry Department, North Carolina State University, Raleigh, NC 27606; *
[email protected] Gabriel Pinto Escuela Técnica Superior de Ingenieros Industriales, Universidad Politécnica de Madrid, Madrid, Spain 28006
Because of the increased awareness in global warming effects, carbon dioxide has stood at the forefront as the most prominent of the greenhouse gases. Its contribution to climate changes is regularly mentioned in public media with the accompanying awareness of one of its primary sources, car emissions. In Europe, vehicle fuel consumption and CO2 emissions information must be available to consumers as mandated by law (1), while in the United States information about CO2 emission of vehicles is not regulated. However, CO2’s direct relationship to fuel economy and pollution has made an impact on its tracking methods around the globe. Air pollution measures have prompted changes in engine design, emission control devices, reformulated gasoline options, and vehicle testing among others. Car emissions levels depend on factors such as vehicle operating conditions, fuel characteristics, and ambient conditions (2). The variability in these factors along with the fact that fuel (i.e., gasoline or diesel) is a complex mixture of hydrocarbons can at first glance discourage using car emission data for a chemistry instructional exercise. Remarkably, readily available consumer data can be used to correlate combustion stoichiometry to consumer product information by simplifying the approach to consider only one main fuel component and disregarding all other variables such as driving and climate conditions. Automobile emission data have been used effectively to communicate chemical principles to students at different levels of instruction (3), while global warming has been the foundation of chemistry courses and experiments (4, 5). Information about fuel constituents and combustion engines can be found in several sources (6, 7). In this activity, students utilize basic chemical principles such as stoichiometry, density of liquids, and combustion reactions to calculate theoretical emission rates of CO2, which are then compared to actual consumer product information. Representing graphically the emission of CO2 versus consumption of fuel provides a tangible way of connecting concepts studied in chemistry classes to everyday life. Exercise Students are instructed to gather car CO2 emission and fuel consumption data from a specific source. In different countries this information may come from different sources. For example, in Spain students have gathered these data from auto supplements that appear weekly in Sunday papers. Car manufacturer data in the United Kingdom list this information online. In the United States, since CO2 emissions are not regulated by law, only fuel consumption is provided in car sales labels. A Web site that
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provides the information required in this activity for a variety of carmakers and models is freely accessible (8). In addition, the data in Europe are reported in units of g∙km (emission of CO2) and L∙(100 km) (fuel consumption) while in the United States fuel consumption is in miles per gallon (mpg). This provides an opportunity for unit conversion and verification of calculations. Students might encounter the subtle differences in units such as the gallon in the United States (1 gal = 3.78 L) versus the Imperial or British gallon (1 gal = 4.54 L). Students graph these data, as emission of CO2 versus fuel consumption, to obtain a straight line. The slope of that line is then compared to the theoretical stoichiometric calculations for CO2 production in a combustion reaction for a particular fuel component. Students practice linear regression analysis, which is the method used by the Environmental Protection Agency in the United States to quantify emitted pollutants in vehicles (2). Since two major types of fuel are used in the automobile industry, gasoline and diesel, doing this exercise for the two types of fuels shows differences between them in terms of CO2 emission levels and fuel consumption. Two assumptions for the theoretical calculations must be explicitly stated to the students: only the major component of the fuel is taken into account and all driving or climatic conditions are disregarded. Octane, C8H18, is considered as gasoline’s primary ingredient and dodecane, C12H26, for diesel (6). The strength of this exercise is that even after such simplification in its treatment, theoretical values are comparable to released consumer data. This activity has been used in different ways for first-year undergraduate chemistry students. It has been implemented as a cooperative learning activity in the classroom, as a research group report, and as an individual homework assignment. Students are expected to utilize stoichiometric calculations, unit conversion, and graphing skills while framing this exercise into real-life context. For example, this activity can be extended to calculate annual average CO2 emission per vehicle type. This information is absent in car sale labels in the United States. An average of 13,000 pounds is emitted annually per car driven 15,000 miles per year (9). The magnitude of these numbers may be used to promote awareness of environmental issues in the classroom as well as an opportunity to discuss other current related topics such as the Kyoto Protocol. Results Stoichiometric relationships and the density of the fuel provide the necessary conversion factors to calculate the theo-
Journal of Chemical Education • Vol. 85 No. 2 February 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
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CO2 Emissions / (g/km)
CO2 Emissions / (g/km)
Chemistry for Everyone
300
200
y = 23.9x + 0.035 R2 = 0.9989
100
0 0
5
10
15
20
300
y = 26.5x + 1.17 R2 = 0.9992
200
100
0 0
Gasoline Consumption / (L/100 km) Figure 1. CO2 emissions versus gasoline consumption.
Emission/ (g/km)
11.1
266
9.0
6
8
10
Table 2. Diesel Consumption from Diverse Car Models (8)
Car Model
Consumption/ (L/100 km)
Emission/ (g/km)
AudiCabriolet
8.4
223
AudiCabriolet
216
Audi Sportback
12.1
293
BMW X3
6.0
161
Audi Sportback
7.7
187
BMW miniCooper
9.1
243
BMW X3
13.0
310
Chevrolet Corvette
4.8
129
BMW mini
14.6
346
Chevrolet Tahoe
8.7
233
Chevrolet Captiva
8.4
200
Chrysler Sebring
11.0
255
Chrysler 300M
7.7
203
Chrysler Voyager
5.0
118
Daihatsu Sirion
6.9
185
Chrysler PT Cruiser
6.6
157
Lancia Musa
5.1
135
Lancia Musa
5.8
139
Opel Agila
4.9
132
Opel Agila
4.6
109
Peugeot 107
4.1
109
Peugeot 107
13.5
324
Quattro RS4
6.5
154
Skoda Fabia
4.7
127
Skoda Fabia
6.0
144
Volkswagen Polo
4.6
124
Volkswagen Polo
7.7
184
Volvo S40
5.4
142
Volvo S40
retical quantity of CO2 produced per unit volume of fuel. An example calculation considering the density of gasoline as 0.70 kg∙L is shown below: C8H18 12.5 O2 8CO2 9H2O
4
Figure 2. CO2 emissions versus diesel consumption.
Table 1. Gasoline Consumption from Diverse Car Models (8) Consumption/ (L/100 km)
2
Diesel Consumption / (L/100 km)
700 g C8H18 1 mol C 8 H18 8 mol CO 2 t t 1 mol C 8 H18 114.0 g C 8H18 1L 44.0 . g CO2 t 1 mol CO2
2160 g CO2 L
A similar calculation for the upper limit of density of gasoline (0.78 kg/L) gives 2410 g CO2/L fuel. For the range of densities of diesel fuel (0.80–0.99 kg/L) the corresponding range of values is 2480–3070 g CO2/L fuel (6).
Car Model
Tables 1 and 2 provide data from diverse automobile brands and models (8). Students can graph the data for gasoline or diesel fuel and apply regression analysis to obtain a slope that falls within the calculated ranges in the previous paragraph (Figures 1 and 2). The units for the slopes of these graphs may be derived from the units of the quantities plotted on the y and x axes as
g CO2 km
g CO2 L fuel L fuel t 100 100 km
Students may be surprised that the units derived this way include the number 100 as well as typical units. This means that the numeric value of each slope must be multiplied by 100 (as well as by the units g CO2/L fuel) to obtain a result in g CO2/L fuel.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 2 February 2008 • Journal of Chemical Education
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Chemistry for Everyone
These graphs from consumer data show strong correlations to the theoretical values calculated via this activity and provide a quick visual account of the differences in emission and fuel consumption of gasoline versus diesel engines. These graphs show that even though diesel engines consume less fuel, CO2 emissions reach higher levels per liter of diesel consumed. However, only similar engines should be compared and in reality diesel engines consume less fuel and subsequently have lower CO2 emissions per distance traveled (10, 11). Conclusions This activity is an instructional resource that utilizes consumer product information to compare theoretical stoichiometric calculations to available car emission and fuel consumption data. Considerable simplification of an otherwise complex chemistry problem still provides comparable theoretical and actual data. This clearly links chemistry principles to everyday life. Practice with unit conversion and graphing skills enhance this activity in a practical way promoting skills used by professionals to perform emission measurements. Scientific literacy can be approached by incorporating exercises such as this one into classroom activities, which may be used to bring awareness of car emissions issues such as the environmental impact of CO2 emissions and hybrid engines or gasoline versus diesel engines. Students have expressed keen interest in this type of “tangible” chemistry where a concrete example of everyday life puts textbook chemistry in context. Acknowledgments The authors would like to express their appreciation to Luis R. Lluberas, General Engineer Leader at the Environmental Protection Agency for his assistance regarding emission regulations and measurements. Also, we would like to thank the National Science Foundation (CAREER Award No. REC-0346906) and the Universidad Politécnica de Madrid (Proyecto: Nuevas Metodologías para la Mejora del Proceso de Enseñanza y Aprendizaje de la Química) grants for the opportunity given to the authors to collaborate in this article.
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Note 1. By considering density values given at http://www.simetric. co.uk/si_liquids.htm for gasoline vehicle and gas oils (0.737 kg∙L and 0.890 kg∙L) the values obtained for CO2 emissions are 2280 g CO2∙L fuel and 2760 g CO2∙L fuel, respectively. As an alternative, students may measure density values in the laboratory.
Literature Cited 1. Official Journal of the European Community, L 12/16, 18.1.2000. http://eur-lex.europa.eu/LexUriServ/site/en/oj/2000/l_012/ l_01220000118en00160023.pdf (accessed Nov 2007). 2. EPA Procedures for Emission Volume IV: Mobile Sources, Chapter 3. http://www.epa.gov/oms/invntory/r92009.pdf (accessed Nov 2007). 3. Ganske, Jane A. Chemical Educator 2003, 8 (6), 353–357. 4. Dunnivant, F. M.; Moore, A.; Alfano, M. J.; Brzenk, R.; Buckley, P. T.; Newman, M. E. J. Chem. Educ. 2000, 77, 1602–1603. 5. Elrod, Matthew J. J. Chem. Educ. 1999, 76, 1702–1705. 6. Keating, E. L. Applied Combustion; Marcel Dekker: New York, 1993. 7. Borman, G. L.; Ragland, K. W. Combustion Engineering, Mechanical Engineering Series; McGraw-Hill International Editions: New York, 1998. 8. IDEA Consumo de Carburante de Coches Nuevos. http://www. idae.es/coches/index1.asp (accessed Nov 2007). 9. Resources for the Future. http://www.rff.org/rff/News/Features/ Combating-Global-Warming-One-Car-at-a-Time.cfm (accessed Nov 2007). 10. Sullivan, J. L.; Baker, R. E.; Boyer, B. A.; Hammerle, R. H.; Kenney, T. E.; Muniz, L.; Wallington, T. J. Environ. Sci. Technol. 2004, 38 (12), 3217–3223. 11. Schipper, L.; Marie-Lillin, C.; Fulton, L. Journal of Transport Economics and Policy 2002, 36, 305–340.
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