Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Effects of Ethanol on Vehicle Energy Efficiency and Implications on Ethanol Life-Cycle Greenhouse Gas Analysis”
W
e thank Strogen et al.1 for their valuable and constructive comments in their letter. We agree with some of the points raised by them and offer clarification to the others. 1. We agree that in the sentence “general notion that ethanol decreases the fuel efficiency of vehicles” “fuel efficiency” should be replaced by “fuel economy”, which was what we meant to use to reflect the fact that metrics such as “fuel economy” (most notably miles per gallon) are still widely used in the literature and policy documents as a proxy to vehicle efficiency. We were not suggesting that the “vehicle efficiency should be evaluated based on energy rather than volume” argument was a “novel conclusion” from our work. Most of the studies we reviewed report vehicle efficiency in volumetric units, which, combined with lack of information on fuel properties, makes vehicle energy efficiency comparisons difficult. Therefore, we wanted to highlight the need for researchers and government agencies to report tested vehicle efficiency in km/MJ or MJ/km as did in some of the studies we evaluated.2,3 We believe this would be immensely helpful for policy makers, consumers, and the research community. Strogen et al. are correct that ethanol has long been recognized for its potential to increase SI engine efficiency. And there were studies published 35 years ago pointing out that ethanol’s effect on vehicle efficiency could greatly influence its energy balance (see Section 4.1.5 of our paper). Yet this issue seemed to be lost in discussion in life cycle analysis (LCA) studies over time until recently. But what really worried us and largely motivated our study was the fact that the handful of LCA studies that did consider the effects of ethanol on vehicle efficiency could potentially be misleading due to their reliance on rather differing results from small samples (see Section 4.1.5 of our paper). We agree that mentioning the effective substitution ratio (ESR) could improve the abstract but note that it was challenging to present a reasonable explanation of the ESR given the word limit without affecting the most important conclusions. 2. The reasons that led to our exclusion of Brazilian studies do not only include the focus on both ethanol/gasoline blends and pure gasoline. For example, we were aware of all of the studies suggested by Strogen et al. but believed these were engine tests under a few operating conditions rather than vehicle tests under standard drive cycles4−7 (except for one that was not an experimental study8) and therefore not suitable for ESR calculations due to the inconsistency with other data sources. In addition, the ethanol content of the standard fuel in Brazil has been evolving over time and currently can vary from 18% to 25%,9 which raises the question as to which blend to use as the baseline fuel for ESR calculation. This suggests a test program that evaluates energy efficiency of regular vehicles and flexible fuel vehicles (FFVs) on fuels in the E18-E25 (E represents ethanol and the number represents the volume percentage of ethanol) range might be necessary. Should adequate data © 2014 American Chemical Society
become available in the future, ESR could be contextualised by using the appropriate baseline fuel. We would also like to further clarify the main reason we chose to exclude studies that did not test pure gasoline. We believed it was appropriate to use an ethanol/gasoline blend as the baseline fuel for calculating ESR only when this blend is used as the standard fuel and the vehicles are optimized around this blend. Otherwise the ESR values obtained could be misleading. For example, in the U.S., the car fleet is optimized on pure gasoline and the ethanol content in the total gasoline pool is close to (or already at) 10%. Consider a scenario where the U.S. car fleet has average energy consumption of 1 MJ/km on pure gasoline and that energy consumption reduces by 2% to 0.98 MJ/km with E10 and E15 and by 5% to 0.95 with FFVs on E85 (see Table and note that these values are hypothetical and only for illustration purposes). The ESR values are therefore 1.3, 1.2, and 1.07 for E10, E15, and E85, respectively. This suggests that simply increasing the ethanol content in E10 to E15 will replace more gasoline than switching to FFVs running on E85. However, if the energy consumption on gasoline is not available, ESR values with E10 as the base fuel would be 1 and 1.04 for E15 and E85, respectively. This could lead to the wrong conclusion that switching to FFVs running on E85 will replace more gasoline than increasing the ethanol content in E10 to E15. Therefore, results from studies that did not test pure gasoline should be interpreted with caution. energy consumption gasoline consumption ethanol consumption ESR with E0 as base fuel ESR with E10 as base fuel
MJ/km MJ/km MJ/km
E0
E10
E15
E85-FFV
1 1 0
0.98 0.913 0.067 1.30
0.98 0.878 0.102 1.20 1.00
0.95 0.201 0.749 1.07 1.04
3. Our statistical analysis was meant to be simple and aimed to reflect the imperfect data sets that were available. We do not feel using the statistical techniques suggested by Strogen et al. would yield much additional insights as the test variations and various sources of uncertainties would overwhelm such attempts until better data sets are available. Even then, it is unlikely that the variations and uncertainties will reduce to such an extent that point estimates of ESR, at least for low and medium blends (due to the sensitivity of their ESR to small changes in vehicle energy efficiency), could be used alone and therefore ESR should still be used in any LCA studies through stochastic simulations. We agree that in section 4.1.2 the results could be discussed in relation to the science-based explanations in section 2. However, we believe the ratio issue and the relative performance between technologies are irrelevant here and stand by our argument that Published: August 4, 2014 9953
dx.doi.org/10.1021/es503420y | Environ. Sci. Technol. 2014, 48, 9953−9954
Environmental Science & Technology
Correspondence/Rebuttal
carburettors “better take advantage of” ethanol. We were not suggesting that vehicles should switch from fuel injection to carburettors but rather when the two technologies are both available, greater benefits could potentially be achieved by using the ethanol in carburettors. In fact, Strogen et al. confirmed this point later in their letter: “ethanol may be found to produce a significantly greater ESR and enable greater air quality benefits when consumed in developing regions of the globe where vehicle fleets have a higher prevalence of carbureted engines”. 4. We disagree that implications of our study can be “stated as inconclusive”. The most important implication of our study is that it is no longer appropriate to simply assume either an ESR of 1 or any other point estimates based on limited sets of experimental data in future LCA studies. Moreover, our paper will stimulate further research and debate on this crucial topic. We support the suggestion by Strogen et al. that a globally relevant database and model should be established. We thank Strogen et al. for pointing out another important issue regarding the refinery benefits, which also needs to be better understood along with the vehicle efficiency benefits. Only then could we ensure that opportunities for optimal societal use of ethanol will not be missed.
Spark Ignition Engine, SAE Technical Paper 2005−01−2183; SAE International: Warrendale, PA, 2005. (7) Costa, R. C.; Sodré, J. R. Hydrous ethanol vs. gasoline-ethanol blend: Engine performance and emissions. Fuel 2010, 89, 287−293. (8) Szklo, A.; Schaeffer, R.; Delgado, F. Can one say ethanol is a real threat to gasoline? Energy Policy 2007, 35, 5411−5421. (9) Walter, A.; Galdos, M. V.; Scarpare, F. V.; Leal, M. R. L. V.; Seabra, J. E. A.; da Cunha, M. P.; Picoli, M. C. A.; de Oliveira, C. O. F. Brazilian sugarcane ethanol: Developments so far and challenges for the future. Wiley Interdiscip. Rev. Energy Environ. 2014, 3, 70−92.
Xiaoyu Yan*,†,‡ Oliver R. Inderwildi§ David A. King§ Adam M. Boies‡ †
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Environment and Sustainability Institute, University of Exeter, Penryn, Cornwall TR10 9FE, United Kingdom ‡ Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, United Kingdom § Smith School of Enterprise and the Environment, University of Oxford, Hayes House, 75 George Street, Oxford, OX1 2BQ, United Kingdom
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Strogen et al. (2) Pelkmans, L.; Lenaers, G.; Bruyninx, J.; Scheepers, K.; De Vlieger, I. Impact of biofuel blends on the emissions of modern vehicles. Proc. Inst. Mech. Eng., Part J 2011, 225, 1−17. (3) Martini, G.; ; Manfredi, U.; ; Mellios, G.; ; Mahieu, V.; ; Larsen, B.; ; McArragher, S.; ; Thompson, N.; ; Baro, J.; ; Zemroch, P. J.; ; Rogerson, J.; et al. Joint EUCAR/JRC/CONCAWE Study on: Effects of Gasoline Vapour Pressure and Ethanol Content on Evaporative Emissions from Modern Cars, JRC Scientific and Technical Reports (EUR collection) JRC36839; Institute for Environment and Sustainability, 2007. (4) Melo, T. C. C. de; Machado, G. B.; Belchior, C. R. P.; Colaço, M. J.; Barros, J. E. M.; de Oliveira, E. J.; de Oliveira, D. G. Hydrous ethanol− gasoline blendsCombustion and emission investigations on a flex-fuel engine. Fuel 2012, 97, 796−804. (5) Melo, T. C. C. de; Machado, G. B.; Oliveira, E. J. de; Belchior, C. R. P.; Cola\aco, M. J.; Barros, J. E. M. Experimental Investigation of Different Hydrous Ethanol-Gasoline Blends on a Flex-Fuel Engine, SAE Technical Paper 2010−36−0469; SAE International: Warrendale, PA, 2010. (6) Amorim, R. J.; Baeta, J. G. C.; Valle, R. M.; Barros, J. E. M.; Carvalho, R. D. B. D. Experimental Analyses of Flexible Fuel Systems in 9954
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