Energy Fuels 2010, 24, 1374–1379 Published on Web 12/01/2009
: DOI:10.1021/ef901082k
Lubricity of Ethanol-Biodiesel-Diesel Fuel Blends Magı´ n Lapuerta,*,† Reyes Garcı´ a-Contreras,† and John R. Agudelo‡ † University of Castilla La-Mancha, Escuela T ecnica Superior de Ingenieros Industriales, Avda, Camilo Jos e Cela s/n, 13071 Ciudad Real, Spain and ‡University of Antioquia, Group of Efficient Management of Energy-GIMEL, Engineering Faculty, Colombia
Received September 23, 2009. Revised Manuscript Received November 6, 2009
Blending bioethanol with diesel fuels is an alternative to incorporate a renewable fraction in vehicle fuels which is receiving growing attention for its economic and environmental advantages. The generalized practice in Europe of including some biodiesel content in the commercial diesel fuel has even enhanced the interest of the ethanol blends as a consequence of the wider range of stability when biodiesel is added. However, one of the main concerns is related to the loss of lubricity caused by the presence of ethanol. First, this work reviews the previous experiences studying the effect of renewable components on the diesel fuel lubricity. Second, an experimental work, carried out in a high frequency reciprocating rig at different temperatures, is presented trying to distinguish the nonlinear contributions of ethanol, biodiesel, and petroleum diesel fuel to the lubricity of three series of blends (one composed of binary ethanol-diesel blends and two more composed by different ternary blends). The incorporation of ethanol did not result in significant losses of lubricity until the ethanol concentration was close to 100%. Additionally, in this range, increasing temperatures led to improved lubricities as a consequence of the ethanol evaporation from the lubricating layer.
The addition of specific fatty acids or fatty acid esters has proven to enhance diesel fuel lubricity.7 Moreover, the addition of biodiesel improves the lubricity of low sulfur diesel fuel even more than pure fatty esters,8 meaning that different fatty acid esters show synergistic effects when they are mixed. Anastopoulos et al.7 found that the addition of biodiesel, independently of the raw material, improves the lubricity more than other fatty acid derivatives (lauric diethylamide and palmitic dibutylamide) when they were added to two different low sulfur diesel fuels. Additionally, they did not find major differences between different biodiesel fuels in the corrected wear scar. Other studies have found that there are no appreciable differences between fatty acids as lubricity enhancers, except with the hydroxylated ones whose OH group makes them more effective as a wear protector.9-15 Goodrum and Geller11 found that the effect of the oil feedstock was minor when biodiesel fuels were added in concentrations around 5%. However, the addition of biodiesel produced from hydroxilated oils, such as lesquerella and castor oils, reduced the wear scar much more sharply (with
1. Introduction The continuous advances in diesel engine technology and the more stringent emissions standards have led to higher injection pressure and to modifications in some fuel properties. Among the latter ones, diesel fuels need to increase their lubricity to protect the fuel injection system and other engine components. A higher boiling point and aromatic, nitrogen, and sulfur contents appear to improve diesel fuel lubricity.1 Several studies have reported that the key agents are the highly polar compounds (especially those containing oxygen and nitrogen) present in diesel fuels which derive in forming a protective layer on the metal surface.2 However, many of these surface-active polar compounds are eliminated during the desulfurization processes causing loss of lubricity.3,4 To meet the wear scar limits established in diesel fuel standards (460 and 520 μm in European and U.S. regulations, respectively (EN 590:2009,5 ASTM D 9756)), a variety of lubricity additives can be used, which have a high affinity to metallic surfaces forming a thin protective metal-metal contact layer. This lubricant film is formed by the adsorption of the polar molecules of the additives on the metal surface, which is negatively charged.4
(7) Anastopoulos, G.; Lois, E.; Serdari, A.; Zanikos, F.; Stournas, S.; Kalligeros, S. Energy Fuels 2001, 15, 106–112. (8) Knothe, G. SAE technical paper 2005-01-3672, SAE International (www.sae.org), 2005. (9) Kajdas, C.; Majzner, M. SAE technical paper 2001-01-1929, SAE International (www.sae.org), 2001. (10) Geller, D. P.; Goodrum, J. W. Fuel 2004, 83, 2351–2356. (11) Goodrum, J.; Geller, D. Bioresour. Technol. 2005, 96 (7), 851– 855. (12) Knothe, G.; Steidley, K. R. Energy Fuels 2005, 19, 1192–1200. (13) Ribeiro, N; Pinto, A.; Quintella, C.; da Rocha, G.; Teixeira, L.; Guarieiro, L.; do Carmo, M.; Veloso, M.; Rezende, M.; Serpa da Cruz, R.; de Oliveira, A.; Torres, E.; Andrade, J. Energy Fuels 2007, 21, 2433– 2445. (14) Moser, B. R.; Cermak, S. C.; Isbell, T. A. Energy Fuels 2008, 22, 1349–1352. (15) Knothe, G. Energy Fuels 2008, 22, 1358–1364.
*To whom correspondence should be addressed. E-mail: Magin.
[email protected]. Telephone: þ(34) 926295431. Fax: þ(34) 926295361. (1) Wei, P.; Spikes, H. A. Wear 1986, 111, 217–235. (2) Safran, S. Statistical thermodynamics of surfaces, interfaces, and membranes; Westview press: Boulder, CO, 2003. (3) Nikanjam, M.; Henderson, P. T. SAE technical paper 932740, SAE International (www.sae.org), 1993. (4) Barbour, R. H.; Rickeard, D. J.; Elliott, N. G. SAE technical paper 2000-01-1918, SAE International (www.sae.org), 2000. (5) EN-590:2009. Automotive fuels-Diesel-Requirements and test methods. (6) ASTM D 975 Standard specification for diesel fuel oils. r 2009 American Chemical Society
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: DOI:10.1021/ef901082k
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less than 1% concentrations) than in the case of nonhydroxylated oils (rapeseed and soybean). Almost all studies agree that adding 1% to 2% (% v/v) of biodiesel improves fuel lubricity.7,11,16-20 Several studies have found that there is no longer improvement in lubricity when a certain concentration of biodiesel is added into the blend with diesel fuel, this optimal concentration ranging from 2%16,21,22 up to 15%.23 The same was reported when Fischer-Tropsch (FT) fuel was used as a base fuel. In this case, the corrected wear scar diminished from 672 μm (pure FT) to 195 μm when 2% of biodiesels obtained from different feedstock was added.21 Contrary to most of the literature, Bhatnagar et al.24 reported an almost linear decrease in wear scar as biodiesel concentration was increased in the blend (tests were made with four Indian indigenous nonedible vegetable biodiesels in a concentration up to 100%). The impurities of biodiesel, such as monoglycerides and free fatty acids, are among the main variables affecting biodiesel lubricity.8,19,25 On the contrary, triglycerides almost have no effect on lubricity due to its poor solubility with diesel fuel.25 Ethyl esters have better lubricity than methyl esters, as reported by Kulkarni et al.26 Renewable diesel fuel obtained by pyrolysis of soybean oil (pyrodiesel) has proven to be a lubricity enhancer, especially when it is blended in low concentrations (2-5%) with low or high sulfur diesel fuel. For concentrations higher than 5%, biodiesel showed better performance than pyrodiesel.22 A small number of lubricity tests have been published on ethanol-diesel fuel blends (e-diesel) or ethanol-biodiesel-diesel blends (e-b-diesel). However, it is necessary to establish a limit of wear scar for these blends in order to ensure that fuel injection system durability is not compromised. In general, the addition of ethanol to diesel fuel lowers fuel viscosity and lubricity,27-29 but as reported in this work, the lubricity of e-diesel and e-b-diesel is governed by the tribological properties of the fuel and the ethanol evaporation, showing that no linear relationship between those properties can be found. According to Corkwell et al.,29 the addition of 10% (v/v) ethanol (7700 ppm of water content) in diesel fuel does not have a clear effect on lubricity, even if 2000 ppm of water was added to the final blend. They also reported a dramatic
decrease of wear scar when a low dosage of a low acidic lubricity improver is used. Diesel No. 2 (340 ppm of sulfur) blended with 10% ethanol and low acidic improver produced a wear scar diameter of 377 μm according to ASTM D 607930 (60 °C) standard; this was a significant reduction with respect to 431 μm reported to the same blend without the additive. Although this result is below the EN 590 standard requirement of 460 μm,5 this blend produced a failure in the injection pump during the Bosch pump test. In a recent work, Barab as et al.31 reported the corrected wear scar diameters (EN ISO 12156-132 at 60 °C) for some ternary diesel þ ethanol þ biodiesel blends in which the ethanol was added in 5% and 10% (v/v) and biodiesel was added from 5% to 25% (v/v) every 5%. They found that the lubricity of the ternary blends lower when the ethanol content is increased, while it is improved when biodiesel concentration is increased, although with unclear tendency. Nevertheless, the corrected wear scar (WSD 1.4) of all the tested blends remained below that of diesel fuel reference. The aim of this work is to evaluate the effect of adding ethanol over the lubricity of diesel fuel and biodiesel-diesel blends as base fuels, as well as to investigate the effect of temperature on the lubricity of these blended fuels. This work is complementary to other previous studies about blending stability of e-diesel33 and e-b-diesel blends34 and about engine performance and emissions with ethanol blends under laboratory conditions34-36 and work conditions37 and must be continued with further long-term engine studies, such as those performed by Hansen et al.,38,39 to limit the range of usable blends and to confirm their capability to partially substitute diesel fuels with minor failure risk. 2. Fuels and Experimental Schedule The following fuels were used for preparing the blends to be tested: • Anhydrous ethanol (99.7%) provided by Abengoa Bioenergy and made by fermentation of wheat, barley, and corn, fulfilling the European norm EN 15376: 2007.40 • The diesel fuel is a typical low sulfur diesel fuel similar to those available in Spanish petrol stations and supplied by Repsol. This fuel is supplied with lubricity enhancers so as to keep the wear scar far below the limit required by norm EN 590:20095 (460 mm) for commercial reasons.
(16) Karonis, D.; Anastopoulos, G.; Lois, S.; Stournas, S.; Zannikos, F.; Serdari, A. SAE technical paper 1999-01-1471, SAE International (www.sae.org), 1999. (17) Van Herpen, J. H.; Soylu, S.; Tat, M. E.ASAE paper 996134, The American Society of Agricultural Engineers, 1999. (18) Kinast, J. A. Production of biodiesels from multiple feedstocks and properties of biodiesels and biodiesel/diesel blends; NREL/SR-510-31460; National Renewable Energy Laboratory: Golden, CO, 2003. (19) Knothe, G. Fuel Process. Technol. 2005, 86, 1059–1070. (20) Schumacher, L.; Adams, B. T. Appl. Eng. Agric. 2008, 24 (5), 539– 544. (21) Wadumesthrige, K.; Ara, M.; Salley, S. O.; Simon, K. Y. Energy Fuels 2009, 23, 2229–2234. (22) Suarez, P. A.; Moser, B. R.; Sharma, B. K.; Erhan, S. Z. Fuel 2009, 88, 1143–1147. (23) Sulek, M. W.; Kulczycki, A.; Malysa, A. Wear 2010, 268, 104108. (24) Bhatangar, A. K.; Kaul, S.; Chhibber, V. K.; Gupta, A. K. Energy Fuels 2006, 20, 1341–1344. (25) Hu, J.; Du, Z.; Li, C.; Min, E. Fuel 2005, 84, 1601–1606. (26) Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. Bioresource technology 2007, 98, 2027–2033. (27) Hansen, A. C.; Zhang, Q.; Lyne, P. W. L. Bioresour. Technol. 2005, 96, 277–285. (28) Li, D.g.; Zhen, H.; Xingcai, L.; Wu-gao, Z.; Jian-guang, Y. Renewable energy 2005, 30, 967–976. (29) Corkwell, K.; Jackson, M. SAE technical paper 2002-01-2849, SAE International (www.sae.org), 2002.
(30) ASTM D 6079. Standard test method for evaluating lubricity of diesel fuels by the High-Frequency Reciprocating Rig (HFRR). (31) Barabas, I.; Todorut, A. SAE technical paper 2009-01-1810, SAE International (www.sae.org), 2009. (32) EN ISO 12156-1:2006. Diesel fuel-Assessment of lubricity using the high-frequency reciprocating rig (HFRR)-Part 1: Test method. (33) Lapuerta, M.; Armas, O.; Garcı´ a-Contreras, R. Fuel 2007, 86, 1351–1357. (34) Lapuerta, M.; Armas, O.; Garcı´ a-Contreras, R. Energy Fuels 2009, 23, 4343–4354. (35) Lapuerta, M.; Armas, O.; Herreros, J. M. Fuel 2008, 87, 25–31. (36) Armas, O.; Cardenas, M. D.; Mata, C. SAE technical paper 200724-0131, SAE International (www.sae.org), 2007. (37) Armas, O.; Lapuerta, M.; Mata, C.; Perez, D. Energy Fuels 2009, 23, 2989–2996. (38) Hansen, A. C.; Zhang, Q.; Hornbaker, R. H. Engine fuel system durability with ethanol-diesel blends. Proc. of the 10th Biennial Bioenergy Conference, Boise, ID, 2002; pp 10. (39) Hansen, A. C.; Zhang, Q. ASAE paper 0306033, The American Society of Agricultural Engineers, 2003. (40) EN 15376:2007. Automotive fuels-Ethanol as a blending component for petrol-Requirements and test methods.
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: DOI:10.1021/ef901082k
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Table 1. Fuel Properties diesel fuel
soybean/palm oils biodiesel
bioethanol
834.9 2.72 45.54
878 4.33 39.83
792 1.13 28.05
42.58
37.22
25.22
-19 315
7 257
842
0
98.4 0.48
properties density at 15 °C (kg/m ) viscosity at 40 °C (cSt) gross heating value (MJ/kg) lower heating value (MJ/kg) CFPP (°C) lubricity (μm corrected wear scar at 60 °C) ester content (% w/w) acid number sulfur (ppm w/w) water (ppm w/w) C (% w/w) H (% w/w) O (% w/w) molecular formula iodine number molecular weight (g/mol) 3
Table 2. Methyl Ester Profile of Biodiesel, Made of Soybean and Palm Oils
0
33.9 57 86.13 13.87 0 C15.18H29.13
448.23 76.74 12.16 11.09 C18.43H34.82O2
0 2024 52.14 13.13 34.73 C2H6O
211.7
90.74 288.5
46.06
methyl ester
(% w/w)
myristic C14:0 palmitic C16:0 palmitoleic C16:1 stearic C18:0 oleic C18:1 linoleic C18:2 linolenic C18:3 araquic C20:0 gadoleic C20:1 behenic C22:0 erucic C22:1 lignoceric C24:0 nervonic C24:1
0.275 27.453 0.084 5.469 29.649 32.384 3.355 0.507 0.231 0.392 0.010 0.155 0.037
• The biodiesel used in this study was produced from a blend of soybean oil with palm oil (61.3% w and 38.7% w, respectively), accomplishes the EN 14214:2008 standard,41 and was provided by Biotel. Soybean and palm oils are nowadays two of the most widely used feedstocks for biodiesel production. Their blending proportions are often varied throughout the year to fulfill the optimal compromise between minimum prize and acceptable cold-flow and oxidation stability properties. Its rheological behavior can be used as reference for other biodiesel fuels. The main characteristics of these pure fuels are shown in Table 1, and the methyl ester profile of the biodiesel fuel is presented in Table 2. Lubricity tests were made on three series of samples, as shown in the ternary diagram of Figure 1. In the first and second series, the base fuels were diesel fuel and B30 (previous blends with 30% v/v biodiesel and 70% v/v diesel fuel), respectively, and the ethanol proportions in both e-diesel and e-b-diesel blends were 1%, 2.5%, 7.7%, 17%, 50%, 75%, and 90% in volume. In the third series, the ethanol content was fixed in 7.7% v/v and the biodiesel concentrations in the previous biodiesel-diesel blend were 1%, 2%, 10%, 20%, and 30%. The corresponding compositions in volume and mass are listed in Table 3 for the three series. The selection of B30 as a base fuel for the second series (and as a top concentration for the third) is justified because this biodiesel proportion has often been used in transportation fleets and sold in petrol stations in Europe. In fact, some European countries have approved standards for this kind of blend. The selection of 7.7% ethanol is justified because this proportion guarantees blending stability, even in the case of the use of biodiesel-free diesel fuel as a base fuel, without surfactant or cosolvent additives in a range of reasonably expected extreme ambient temperature and water contamination conditions (-5 °C or 0.5% water m/m).33 Among all the tested blends, only the e-diesel blend with 50% ethanol v/v remained unstable at room temperature,34 thus requiring agitation by ultrasound before testing.
Figure 1. Ternary diagram indicating the tested blends.
3. Experimental Equipment and Procedure The lubricity tests were carried out in a high frequency reciprocating rig (HFRR) of PCS Instruments. These tests provide the wear scar in micrometers of a fuel, following either the European EN ISO 12156-1:2006 standard32 or the ASTM D 6079 standard.29 Although the European standard establishes 60 °C as the fuel temperature (this specification is just required for diesel fuels), the ASTM standard also accepts testing at 25 °C, indicating that this temperature is preferred when there may be concerns about loss of fuel because of its volatility or degradation. In this case, the high volatility of ethanol motivates one to compare tests at both temperatures. While the temperature of the engine metallic surfaces will certainly be closer to 60 °C, the comparison between both temperature tests permits one to separate the effect of the loss of ethanol by evaporation. Prior to each test, all the components of the HFRR having contacted the tested fuels were subjected to a cleaning procedure composed of three 10 min immersions in an ultrasonic bath with toluene (the first and the second) and with acetone (the third). All tests were replicated twice, and if differences in the wear scar were higher than 20 μm, then they were repeated once more. During the tests, which lasted 75 min, the samples were shaken at a frequency of 50 Hz. They remained open to the atmosphere, which favored the ethanol losses by evaporation from both the e-diesel and e-b-diesel samples. Afterward, the size of the wear scar was measured in an electronic microscope Leica DM IRM equipped with a 100 magnification lens. The mean diameter
(41) EN 14214:2008. Automotive fuels-Fatty acid methyl esters (FAME) for diesel engines-Requirements and test methods.
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Table 3. Composition in Volume and Mass of the Tree Series of Tested Blends 1st series: e-diesel blends ethanol/diesel (% v/v)
ethanol/diesel (% m/m)
0/100 1/99 2.5/97.5 7.7/92.3 17/83 50/50 75/25 90/10 100/0
0/100 0.95/99.05 2.37/97.63 7.33/92.67 16.27/83.73 48.68/51.32 74.00/26.00 89.52/10.48 100/0
2nd series: e-b-diesel blends (with B30) ethanol/biodiesel/ diesel (% v/v) 0/30/70 1/29.7/69.3 2.5/29.25/68.25 7.7/27.69/64.61 17/24.9/58.1 50/15/35 75/7.5/17.5 90/3/7 100/0/0
3rd series: e-b-diesel blends (with E7.7)
ethanol/biodiesel/ diesel (% m/m)
ethanol/biodiesel/ diesel (% v/v)
0/31.07/68.93 0.93/30.78/68.29 2.34/30.34/67.32 7.23/28.82/63.94 16.06/26.08/57.86 48.30/16.06/35.64 73.70/8.17/18.13 89.37/3.30/7.33 100/0/0
ethanol/biodiesel/ diesel (% m/m)
7.7/0/92.3 7.7/0.92/91.38 7.7/1.85/90.45 7.7/9.23/83.07 7.7/18.46/73.84 7.7/27.69/64.61
7.33/0/92.67 7.33/0.97/91.70 7.33/1.95/90.73 7.30/9.70/83.00 7.26/19.31/73.43 7.23/28.82/63.94
Table 4. Wear Scar and Corrected Wear Scar for E-Diesel Blends (Series 1) tests at 60 °C (EN 12156-1) blend
% ethanol v/v
diesel e-diesel e-diesel e-diesel (E7.7) e-diesel e-diesel e-diesel e-diesel ethanol
Figure 2. Corrected wear scar for e-diesel blends at 25 and 60 °C.
0 1 2.5 7.7 17 50 75 90 100
tests at 25 °C (ASTM D 6907)
MWSD (μm)
WS 1.4 (μm)
MWSD (μm)
285 297 364.5 335
315 328 351 327
188.5 276 274 268
277 304.5 363 372.35 854
304 307 355 417 842
316 405.5 465.5 529.65 632
Table 5. Wear Scar and Corrected Wear Scar for E-B-Diesel Blends (Series 2) tests at 60 °C (EN 12156-1)
Figure 3. Corrected wear scar for e-b-diesel blends (made by adding ethanol into B30 blends) at 25 and 60 °C.
of the scar observed in the HFRR ball (MWSD) was obtained from the maximum and minimum ones as prescribed in the norms. When the tests were made at 60 °C, as required by the EN ISO 12156-1 norm, the resulting scar size was corrected to normalize the vapor pressure to 1.4 kPa (WS 1.4).
tests at 25 °C (ASTM D 6907)
blend
% ethanol v/v
MWSD (μm)
WS 1.4 (μm)
MWSD (μm)
b-diesel (B30) e-b-diesel e-b-diesel e-b-diesel e-b-diesel e-b-diesel e-b-diesel e-b-diesel ethanol
0 1 2.5 7.7 17 50 75 90 100
257 255 278 313 238 255 261 330 854
257 258 253 298 229 224 288 324 842
226 293 295 305 288 317 362 403 632
be observed in the case of the biodiesel blend, although in this case, the effect of temperature is lower (Figure 3). As a consequence of the previous additivation of the diesel fuel, the addition of 30% biodiesel did not provide a large benefit in lubricity, contrary to other studies.7 • Small concentrations of ethanol in both the e-diesel and the e-b-diesel blends provide significant increases in the wear scar (decreases in lubricity) with respect to the ethanol-free fuels at low temperature (25 °C), although such increases are minor at high temperature (60 °C). This can be observed more in detail in Figure 4. In the case of e-b-diesel blends, a certain mass loss of ethanol by evaporation at high temperature leads to even smaller wear scar than at lower temperature, as a consequence of the better tribological properties of the remaining biodiesel-rich fuel. • After the initial peak of wear scar has been reached, as the ethanol concentration in the blends continues increasing, the wear scar remains within a small variation range or even slightly decreases. This effect can be
4. Results and Discussion The first general observation is that the addition of ethanol into both diesel fuel and B30 fuels decreases lubricity (increases the wear scar) in almost all cases, as a consequence of the lower lubricity of ethanol. However, the increase in the size of the wear scar is not linear, as the concentration of ethanol is increased (see Figures 2 and 3 and Tables 4 and 5). This is probably a consequence of the combined effect of the following properties of the blends: tribological properties, volatility, blending stability, and of the different sensitivity of these properties to the fuel temperature. A more detailed description of the lubricity of the tested samples at different temperatures can be explained as follows: • At null ethanol concentration, the diesel fuel decreases its lubricity as the temperature is increased, since the fuel becomes less viscous, thus reducing the metallic contact resistance (Figure 2). A similar decrease in lubricity can 1377
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: DOI:10.1021/ef901082k
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Figure 4. Corrected wear scar for e-diesel (left) and e-b-diesel (right) blends at 25 and 60 °C for small ethanol concentrations.
explained because the expected loss of lubricity (due to the presence of ethanol) is compensated by the increase of the evaporation losses of ethanol from the lubricating layer. In the case of e-diesel blends, this planar range is extended from 3% to 50% ethanol at 60 °C and from 1% to 7% at 25 °C, while in the case of e-b-diesel blends, this compensation range becomes even wider: from 0% to 50% at 60 °C and from 1% to 17% at 25 °C (Figures 2 and 3). At high temperature, the mentioned compensation leads to a certain synergistic effect, since the lubricity of some of the tested blends become even better than those of their components. This can be explained by the improved tribological properties of biodiesel with respect to those of diesel fuel, together with the better miscibility of ethanol and biodiesel with respect to that of ethanol and diesel fuel. Both advantages of e-b-diesel blends contribute to extend the range in which the poor lubricity of ethanol is hidden. • From this point on, further increases in ethanol content lead again to a certain loss of lubricity, this range being extended up to values close to 100% ethanol. During this whole range (and during most of the previously mentioned plateau), the lubricity is improved for high temperature, contrary to what occurs in the case of the pure components (0% and 100% ethanol), because the evaporation from the lubricating layer removes the component with poorest lubricity. The comparison between e-diesel and e-b-diesel blends shows that the latter ones maintain better lubricities and lower increases of wear scar with increasing ethanol content than the former ones, throughout the whole concentration range. • Finally, when the ethanol concentration becomes very close to 100%, the fuel turns back to behave as a pure substance, therefore with decreasing lubricity for increasing temperature. In this extreme range, small concentrations of diesel or biodiesel fuels lead to sharp improvements in lubricity with respect to pure ethanol, especially when the added fuel contains biodiesel. The evaporation losses, although probably not negligible, do not reach to affect lubricity in this case.
Figure 5. Corrected wear scar of e-b-diesel blends with 7.7% v/v ethanol and 92.3% of a base fuel composed of different concentrations of biodiesel. Table 6. Wear Scar and Corrected Wear Scar for E-B-Diesel Blends (Series 3) tests at 60 °C (EN 12156-1) blend e-diesel (E7.7) e-b-diesel e-b-diesel e-b-diesel e-b-diesel e-b-diesel
tests at 25 °C (ASTM D 6907)
% biodiesel v/v in the base fuel
MWSD (μm)
WS 1.4 (μm)
MWSD (μm)
0
335
327
268
1 2 10 20 30
288 257 268 281 313
290 258 275 289 298
243 205 265 287 305
Schedule) is not the optimal in terms of lubricity improvement. In fact, lower biodiesel contents would have led to even better lubricities, as shown in Figure 5 and Table 6, where corrected wear scar results for different biodiesel concentrations with a fixed 7.7% v/v ethanol content are plotted. As observed in this figure, the addition of small biodiesel concentrations (with the optimal concentration being around 2%) improve lubricity much more than adding 30% v/v. This result is in agreement with others shown in the literature.22,23 However, there are more important reasons than lubricity (which, as shown in this work, does not constitute any problem, at least in the range where the blends could guarantee stability and be compatible with current diesel engines) to select the blending proportions, such as the need to keep increasing the renewable share of automotive fuels, in line with the current European policy promoting reductions of greenhouse emissions from vehicles.
In conclusion, e-b-diesel blends lead to smaller wear scars and to flatter lubricity curves (less sensitive to ethanol concentration) than e-diesel blends. In both cases, a wide range of intermedium ethanol content (although even wider in the case of e-b-diesel blends) can be found where the presence of ethanol reverts the usual trend of lubricity with temperature, leading to improved lubricities when the engine operates under hot conditions. It must be remarked that the concentration selected for biodiesel (B30, as justified in Fuels and Experimental
5. Conclusion The addition of ethanol in the currently used diesel fuels (nowadays carrying different contents of biodiesel in Europe) is an effective way to incorporate an additional renewable 1378
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fraction in the vehicle-engine fuels. In the tested blending ranges, based on the addition of different ethanol contents into both a commercial diesel fuel and a B30 biodiesel-diesel blend and on the addition of 7.7% ethanol v/v into different biodiesel-diesel blends, the progressive incorporation of ethanol did not result in significant losses of lubricity (increases in wear scar) until the ethanol concentration was close to 100%. At 60 °C a certain synergistic effect was observed, since the lubricity of some of the tested blends with intermedium ethanol content became even better than those of their components. Additionally, in this range of ethanol concentration, the effect of temperature was opposite to the usual, this meaning improved lubricities under hot conditions. Both effects can be explained because the ethanol evaporation losses compensate the poorer tribological properties of
ethanol. In comparison with e-diesel blends, e-b-diesel blends led to smaller wear scars and to flatter lubricity curves (less sensitive to the ethanol concentration). The results presented prove that blending ethanol in biodiesel-diesel blends nowadays, within the previously studied range of stability, could guarantee the engine preservation to friction wear even better than in the past, when the presence of biodiesel in the commercial diesel fuel was unusual. Acknowledgment. The authors gratefully acknowledge the financial support provided by the Spanish CDTI (research project CENIT 2007-1031, IþDEA). The companies Abengoa Greencell, Repsol, and Biotel Combustibles Ecol ogicos S.L. are also acknowledged for the supply of the ethanol, diesel, and biodiesel fuels, respectively.
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