Energy & Fuels 2009, 23, 2229–2234
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Investigation of Lubricity Characteristics of Biodiesel in Petroleum and Synthetic Fuel Kapila Wadumesthrige, Mahbuba Ara, Steven O. Salley, and K. Y. Simon Ng* Department of Chemical Engineering and Material Science, Wayne State UniVersity, 5050 Anthony Wayne DriVe, Detroit, Michigan 48202 ReceiVed October 14, 2008. ReVised Manuscript ReceiVed January 14, 2009
The lubricity of ultra-low sulfur diesel (ULSD) and synthetic fuel (S8) blended with different levels of cotton seed oil, soybean oil, or poultry fat based biodiesel was evaluated using a high-frequency reciprocating rig (HFRR). The lubricity of ULSD and S8 blends increases sharply as the biodiesel blending level increases and then levels off at ∼2 vol %. The effects of individual minor components of biodiesel, free fatty acids (FFAs), glycerol, antioxidants, phospholipids, and water, on lubricity enhancement of ULSD were investigated. Among the minor components, polar compounds achieved better lubricity improvement. The order of the effect was FFA > soy biodiesel > phospholipids > antioxidant > glycerol > distilled soy biodiesel > individual FAME. Lubricity of the residues of distilled cotton seed oil and yellow grease biodiesel was also compared to the distillates. The effect of the temperature with 2 vol % soybean oil (SBO) blend with ULSD was examined. This biodiesel mixture shows better lubricity, attributed to boundary film formation, at temperatures greater than 70 °C.
1. Introduction The U.S. Environmental Protection Agency has adopted a set of diesel emission standards aimed at drastically reducing the sulfur content of diesel fuel. To meet those standards, petroleum refiners are producing ultra-low sulfur diesel (ULSD) or S15, a cleaner diesel fuel that has a maximum sulfur content of 15 ppm. When the full retail phase-in is complete, this fuel will be a direct replacement for low-sulfur diesel or S500 (which has a sulfur content of 500 ppm). However, the poor lubricity of ULSD has led to the failure of engine parts, such as fuel injectors and pumps, because they are lubricated by the fuel itself.1 The lubricity of diesel fuel largely depends upon the trace levels of its nonsulfur- and sulfur-containing polar compounds, which are responsible for the boundary lubrication, producing a protecting layer on the metal surface to prevent wear of moving parts. The various processing techniques used to achieve low-sulfur fuel remove not only sulfur but also polyaromatic and nitrogen- and oxygen-containing compounds. The removal of these components might be responsible for the low lubricity of ULSD and equipment wear.1-4 Aviation fuel F-34 (JP-8) is replacing distillate diesel fuel in many applications, but it also causes unacceptable wear because of poor lubricity. The synthetic-fuel compound S8 FT (Fischer-Tropsch) fuel, which is used as a substitute for JP-8, is under testing by the Air Force. Several major problems arise from the usage of low-lubricity diesel fuels, such as accelerated wear, injection nozzle erosion or corrosion, engine speed instability, hard * To whom correspondence should be addressed. Fax: 1-313-577-3810. E-mail:
[email protected]. (1) Lacey, P. I.; Lestz, S. J. Effect of low lubricity fuels on diesel injection pumpssPart I: Field performance. SAE Tech. Pap. 920823, 1992. (2) Wei, D. S.; Spikes, H. A. The lubricity of diesel fuels. Wear 1986, (111), 217. (3) Nikanjam, M.; Henderson, P. T. Lubricity of low aromatics diesel fuels. SAE Tech. Pap. 920825, 1992. (4) NikanjamM.; Henderson, P. T. Lubricity of low sulfur diesel fuels. SAE Tech. Pap. 932740, 1993.
starting, low power, and engine smoke. Diesel fuel lubricity can be enhanced by adding lubricity additives, which comprise a range of surface-active chemicals that have an affinity for metal surfaces. ASTM D975-05 (petrodiesel) now requires 0.52 mm, and EN 590 requires 0.46 mm maximum wear scar diameter as measured by ASTM D6079. Engine Manufacturers Association EMA-FQP-1A specifications require 0.45 mm maximum wear scar diameter as measured by ASTM D6079. Biodiesel, which is derived from renewable biological sources, such as vegetable oils or animal fats, can be used as an additive to improve the lubricity of petroleum fuels.5,6 The major components (98%) of biodiesel are methyl esters of longchain fatty acids (FAME) and minor components, including free glycerin, mono-, di-, and triglycerides, antioxidants, sterols, phospholipids, and water. Several studies have investigated the effects of biodiesel components on petrodiesel lubricity. Geller and Goodrum7 examined the effects of individual component fatty acid methyl esters (FAMEs). They found that methyl esters derived from vegetable oils composed of a mixture of several fatty acids had a larger effect than the individual fatty acid esters, and hydroxylated FAMEs may have enhanced lubricating properties as compared to their nonhydroxylated analogues because of the plasticization effect of the hydroxyl group. Knothe and Steidley8 found that fatty compounds possess better lubricity than hydrocarbons found in diesel, because of their polarity-imparting oxygen atoms. Additionally, pure free fatty acids, monoacylglycerols, and glycerol possess better (5) Stoldt, S. H.; Harshida, D. Esters derived from vegetable oils used as additives for fuels, 1998. (6) Ball, K. F.; Bostick, J. G.; Brennan, T. J. Fuel lubricity from blends of a diethanolamine derivative and biodiesel, 1999. (7) Geller, D. P.; Goodrum, J. W. Effects of specific fatty acid methyl esters on diesel fuel lubricity. Fuel 2004, 83 (17-18), 2351–2356. (8) Knothe, G. S.; Kevin, R. Lubricity of components of biodiesel and petrodiesel. The origin of biodiesel lubricity. Energy Fuels 2005, 19, 1192– 1200.
10.1021/ef800887y CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
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Figure 1. Lubricity versus composition of biodiesel on both ULSD and S8. Table 1. FAME Compositions of SBO-, CSO-, and PF-Based Biodiesel FAME composition (wt %) FAME
SBO
CSO
PF
YG
C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C22:1 ∑SFA (%) ∑UFA (%)
0.0 14.1 0.7 5.2 25.3 48.7 6.1 0.0 0.0 19.3 80.8
0.8 24.7 0.4 2.7 18.5 53.0 0.0 0.0 0.0 28.2 71.8
1.0 21.8 3.7 7.6 36.6 27.0 1.8 0.0 0.4 30.5 69.1
0.1 16.0 0.0 3.9 32.7 45.0 2.1 0.3 0.0 20.0 79.8
lubricity than pure esters. An order of oxygenated moieties enhancing lubricity (COOH > CHO > OH > COOCH3 > C-O > C-O-C) was obtained from studying various oxygenated 10 carbon atom (C10) compounds. Additional work with pure 3 carbon atom (C3) compounds containing OH, NH2, and SH groups showed that oxygen enhances lubricity more than nitrogen and sulfur. The addition of biodiesel improves the lubricity of low-sulfur diesel more than pure fatty esters. The addition of polar compounds, such as free fatty acids or monoacylglycerols (MGs), improves the lubricity of low-level blends of esters in low-lubricity diesel. A similar study published by Hu et al.9 showed that methyl esters and MGs determine the lubricity of biodiesel. Free fatty acids (FFAs) and diacylglycerols (DGs) can also affect the lubricity of biodiesel but not as much as monoacylglycerols. Triacylglycerols (TGs) almost have no effect on the lubricity of biodiesel. These studies suggest that the FAMEs are the main components influencing the lubricity-enhancing properties of biodiesel. Among the minor components of biodiesel, only the effects of mono- (MGs), di- (DGs), and triglycerides (TGs) and FFAs were (9) Hu, J.; Du, Z.; Li, C.; Min, E. Study on the lubrication properties of biodiesel as fuel lubricity enhancers. Fuel 2005, 84 (12-13), 1601–1606.
evaluated. The work presented here sought to examine the effects of biodiesel blend levels and different sources of feedstocks of biodiesel on diesel fuel and S8 lubricity as well as to investigate the effects of all of the other biodiesel minor components, FFAs, glycerol, antioxidants, phospholipids, and water, on ULSD lubricity. Lubricity effects of the residues of distilled yellow grease (YG)- and cotton seed oil (CSO)-biodiesel on ULSD were also examined. Finally, the temperature effect of 2 vol % soybean oil (SBO)-biodiesel blend on ULSD lubricity was evaluated. 2. Experimental Procedure 2.1. Blend Preparation. SBO-, CSO-, poultry fat (PF)-, and YG-based biodiesel were obtained from Biodiesel Industries (Denton, TX). Certification #2 ULSD was obtained from Haltermann Products (Channelview, TX). The blends were made on a volume basis and stored in glass bottles at room temperature. Biodiesel is used as neat (B100) or in a blend with petroleum diesel. B2 represents a blend of 2% biodiesel with 98% ULSD, by volume.10 A sample of synthetic aviation fuel (S-8) produced by Syntroleum Corporation (Tulsa, OK) was provided by the National Automotive Center, U.S. Army (Warren, MI). Methyl linoleate (C18:2), which is the major FAME in soy and cotton seed biodiesel; glycerol, a major impurity of biodiesel; palmitic acid, a FFA; asolectin, a phospholipid in soybean oil; and tocopherol, a natural antioxidant in vegetable oil, were used as additives in ULSD with 2% blend. Methyl linoleate and palmitic acid were purchased from Nu-Chek Prep, Inc. (Elysian, MN), and glycerol, asolectin, and tocopherol were purchased from Sigma-Aldrich, Inc. (St. Louis, MO). A total of 2 wt % of the residues of distilled YG- and CSObased biodiesel were mixed with ULSD. A total of 5 wt % of the residues were mixed with distilled YG- and CSO-based biodiesel, and 2% blends of these mixtures were produced with ULSD. Surrogate biodiesel was made up of the mixture of all of the minor components with distilled biodiesel at the level of the natural amounts present in undistilled biodiesel (tocopherol, 500 ppm; (10) American Society for Testing and Materials (ASTM). ASTM D6751-03, Standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels. ASTM International, West Conshohocken, PA, 2003.
Lubricity Characteristics of Biodiesel
Energy & Fuels, Vol. 23, 2009 2231 steel ball was contacted against the stationary disk, with a stroke length of 1 mm at a frequency of 50 Hz for 75 min. The reciprocating motion of the steel ball in contact with the stationary disk resulted in wear scars on both the ball and the disk. The wear scar generated on the test ball, with the average of x and y direction measured under a calibrated optical microscope, was termed as the wear scar diameter (WSD) and was considered to be a measure of the lubricating property of the fuel. Fuels with a WSD of 450 µm or lower at 60 °C are considered acceptable according to ASTM D975.11 Most of the experiments were repeated twice at first, and repeatability was demonstrated to be less than 20 µm, which was within reported repeatability at 60 °C of less than 80 µm.11
3. Results and Discussion
Figure 2. Lubricity of different distilled biodiesel types.
glycerol, 0.145 mass %; FFA (palmitic acid), 0.114 mass %; and phospholipids (asolectin), 0.0005 mass %). 2.2. Distillation of Biodiesel. A total of 500 mL of each type of biodiesel were distilled under reduced pressure (3 × 10-3 torr) at about 130-150 °C. Analysis of the distillates by a gas chromatograph (Perkin-Elmer Clarus 500) equipped with a flame ionization detector (GC-FID) indicated that only FAMEs were present. Once distilled, they were stored at 4 °C. 2.3. FAME Composition. The fatty acid composition of each biodiesel was determined using a Perkin-Elmer Clarus 500 GC-MS with a split automatic injector and a Rtx-WAX (Restek) column (length, 60 m; inner diameter, 0.25 mm; coating, 0.25 µm). The column was held at 120 °C for 1 min and then ramped to 240 °C at 20 °C/min, and it was then held at 240 °C for 13 min. The transfer line between GC and MS was kept at 240 °C. Helium (99.9999%, Cryogenic Gases, Detroit, MI) was used as the carrier gas, with a flow rate of 1.5 mL/min. Total ion count (TIC) was used for the quantification of each component. 2.4. High-Frequency Reciprocating Rig (HFRR) for Lubricity Measurements. A HFRR (PCS Instruments, London, U.K.) was used to measure lubricity based on ASTM D6079. Specifically, a 2 mL sample was placed in the test reservoir at 60 °C, and a steel ball and a steel disk, which were completely submerged in the test fuel, were brought into contact with each other. The steel ball is placed in a vibrator arm, which was loaded with a 200 g mass. The
Figure 3. Lubricity effects of different types of minor components.
3.1. Effect of Biodiesel Feedstocks. The HFRR WSD of ULSD and S8 with various levels of added biodiesel from different sources are shown in Figure 1. The WSD values are 427 ( 4.9 and 670.5 ( 4.2 µm for ULSD and S8, respectively. The composition of ULSD (on the basis of the certificate of analysis) is aromatics, 27.5%; saturated hydrocarbons, 70.8%; and olefins, 1.7% by volume; while S8 contains 100% C7-C18 alkanes. The effects of three different biodiesel blends in the lubricity of ULSD and S8 are very similar to each other. The WSD decreases sharply initially as the biodiesel blending level increases and then levels off at ∼2% biodiesel. When biodiesel blends were used as lubricity enhancers in S8, the HFRR WSD decreased more significantly than ULSD, from 672 to 195 µm. The FAME compositions of different feedstocks are shown in Table 1. Although the FAME composition is different for these three biodiesel types, their effect on the lubricity of ULSD is comparable at levels of 2% and above. This observation can be explained by the development of boundary lubrication. The wear and friction performance of the metal body is determined by the nature of lubricant additives, which form tribofilms on the contacting surfaces through physically and chemically adsorbed molecules or polymer films from the lubricant.12 After complete formation of this tribofilm, lubricity does not increase with the increase of the biodiesel blend over 2%. 3.2. Effect of FAMEs. The main components of biodiesel are FAMEs, which are about 98%. To investigate the effects of
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Figure 4. Lubricity effects of individual minor components in biodiesel.
Figure 5. Structures of the antioxidant and different phospholipids.
FAME chain lengths and degree of unsaturation on lubrication, 2 vol % of distilled SBO-, distilled CSO-, distilled PF-, and distilled YG-based biodiesel were mixed with ULSD. Under these circumstances, it is assumed that the effects of minor components can be eliminated using distilled biodiesel. Figure 2 displays the WSD and percent decrease in WSD for 2 vol % mixtures of various distilled biodiesel in ULSD. Distilled PF-biodiesel exhibit the most lubricity enhancement effect. The higher lubricity effect can be attributed to a higher amount of saturated FAMEs in PF-biodiesel. In saturated compounds, the molecules can align themselves easier in straight chains and are more closely packed on the surface, providing a strong lubricating layer. On the other hand, double bonds prevent rotation and force the chains to bend, making it more difficult to pack closely together and resulting in a weaker lubricating (11) American Society for Testing and Materials (ASTM). ASTM D97596, ASTM standard specification for fuel oils. ASTM International, West Conshohocken, PA, 1997.
layer.12 Increasing unsaturation leads to an increase in the friction coefficient.13 The effectiveness of the boundary lubricant in terms of resistance to wear can be increased with the increase in chain lengths to a threshold value when there are approximately 15 carbon atoms in the molecular chain.14 It should be noted here that the number of carbon atoms in FAMEs (biodiesel) are mainly 16 and 18. Anastopoulos et al. reported that increasing the chain length of several fatty acid esters decreases friction and wear.15 However, in this study, the fatty acid ester chain length was varied by changing the chain length of both the alcohol group and fatty acid part of the ester. Using monosaturated FAMEs with C18-C22, Geller et al. showed that there is no consisting trend relating chain length to lubricity enhancement in this set of FAMEs.7 Considering all of this, it can be suggested that the chain length of FAMEs in biodiesel has no or a minimum effect on the lubrication. 3.3. Effects and Lubrication Mechanism of Minor Components. The minor components of biodiesel are FFAs, glycerol, antioxidants, and phospholipids. In this study, the lubrication effects of individual minor components as an additive to ULSD were examined. Figure 3 illustrates the WSD and percent WSD decrease because of the addition of 2% of each soy biodiesel, distilled soy biodiesel, C18:2 FAME, palmitic acid, glycerol, tocopherol, and asolectin. Undistilled soy biodiesel with all of the major and minor components decreased the WSD by 55%, whereas the distilled biodiesel, which is composed of basically pure FAMEs, decreased the WSD by only 29%. This indicates that the minor components have very (12) Ling, F. E.; Klaus, E. E.; Fein, R. S. Boundary LubricationsAn Appraisal of World Literature; American Society of Mechanical Engineers: New York, 1969; p 87. (13) Kodali, D. R. Ind. Lubr. Tribol. 2002, 54, 165. (14) Bhuyan, S.; Sundararajan, S.; Yao, L.; Hammond, E. G.; Wang, T. Boundary lubrication properties of lipid-based compounds evaluated using microtribological methods. Tribol. Lett. 2006, 22, 2. (15) Anastopoulos, G.; Lois, E.; Karonis, D.; Zanikos, F.; Kalligeros, S. A prelimanary evalution of esters of monocarbixylic fatty acids on the lubricating properties of diesel fuel. Ind. Eng. Chem. Res. 2001, 40, 452– 456.
Lubricity Characteristics of Biodiesel
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Figure 6. Lubricity of residues of different distilled biodiesel types.
Figure 7. Thermal effects of 2% biodiesel blend on ULSD lubricity.
significant effects to increase the lubricity. For individual components, FFA and asolectin demonstrated the greatest effects on enhancing the lubricity of ULSD, with 61 and 44% reductions in WSD, respectively. C18:2 FAME, glycerol, and tocopherol decreased WSD by 12, 23, and 32%, respectively. Figure 4 illustrates the WSD and percent WSD decrease of distilled soy biodiesel of minor components: palmitic acid (as FFA), 1500 ppm; tocopherol as a natural antioxidant in biodiesel, 500 ppm; glycerol, 1450 ppm; distilled (DI) water, 1300 ppm. A total of 2 vol % of each of these additized biodiesel mixtures was mixed with ULSD to measure the lubricity. Surrogate biodiesel was made up of the mixture of distilled biodiesel and all of the minor components at concentrations typically found in undistilled biodiesel. A total of 2 vol % of surrogate biodiesel with ULSD resulted in a WSD of 215.5 µm in comparison to 191 µm for 2 vol % undistilled biodiesel in ULSD. The difference, while not significant, may be due to the presence of other unidentified minor components. On the other hand, the
surrogate biodiesel showed a much improved lubricity enhancement over distilled biodiesel, with a WSD of 301 µm. The significance of the presence of the minor components in biodiesel to increase the lubricity of ULSD is obvious. The effects of individual minor components are as follows: FFA (Palmitic Acid). FFA, as a 2 vol % additive to ULSD, and 1500 ppm FFA added to distilled SBO-biodiesel showed the largest lubricity enhancement effect of a 61% WSD decrease over ULSD alone. The 2 vol % mixture of 1500 ppm FFA to distilled SBO-biodiesel in ULSD results in a percent WSD decrease of 28%. The significant reduction in WSD can be attributed to the polar carboxylic acid moiety. Fatty acids can adsorb on metal surfaces to form metal soaps, which are strongly attached to the surface at fairly low temperature, with the bonding energies in excess of 40 kJ/ mol; however, fatty acids and esters lose their effectiveness as boundary lubricants above 200 °C.15
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Antioxidant (Tocopherol). Tocopherol increased the ULSD lubricity by 32% as an additive and a 35% increase in lubricity as a mixture additive of 500 ppm tocopherol in distilled SBO-biodiesel. Tocopherol is a polar compound (Figure 5), which can be adsorbed on the metal surface to produce a thin film. It has one OH group, a long -CH2- chain, and one aromatic ring, contributing to the surface lubricity.16 Glycerol. Glycerol has very good lubricity because it has a high OH/C ratio (1:1). The OH group tends to have a strong bond with the metal substrate because of ionic interactions and Debye orientation forces compared to those based on dipole (van der Waals) forces.17 For this reason, glycerol showed a good lubricity increase of 23% as an additive and 30% lubricity increase when added to ULSD as a mixture of 1590 ppm glycerol in distilled SBO. Phospholipids. Phospholipids are important surface-active compounds. Soybean oil contains a mixture of lecithins, composed of 20-23% phosphatidylcholines, 16-21% phosphatidylethanolamines, and 12-18% phosphatidylinositides, along with phosphatic acids and other minor phospholipids.18 The chemical formula of phosphatidylcholines and phosphatidylethanolamines are shown in Figure 5.19 It is well-known that phosphorus can plate a film onto the metal surface in the form of phosphorus soaps.16 The phosphorus-containing compounds in phospholipids also have COOH, COOR groups, and a long carbon chain, which are all very good lubricity enhancers. As displayed in Figure 3, as a 2% additive, it increases the lubricity by 44%, which is the second most effective among the minor components. Water. To check the lubricity effect of water present in biodiesel, 1000 ppm water was added to distilled SBO-biodiesel, which contained 245 ppm water. The addition of 1000 ppm water to biodiesel (Figure 4) has no effect on ULSD lubricity. 3.4. Effect of the Residue of Distilled Biodiesel. The residues of distilled YG- and distilled CSO-based biodiesel were mixed with ULSD to examine the lubricity effects. Figure 6 shows the WSD of the 2 vol % residues of distilled YG- and CSO-based biodiesel in ULSD and 2 vol % mixture of 5 vol % of the residues in the corresponding distilled biodiesel in ULSD, because the residue content of biodiesel is 5 vol % of undistilled biodiesel. Both mixtures showed lower lubricity than when only residues were added to ULSD. The addition of only distilled YG- and distilled CSO-based biodiesel has the lowest lubricity effects, which can be attributed to the lack of any minor components. Distillation of YG- and CSO-based biodiesel were carried out under reduced pressure (3 × 10-3 torr) at about 130-150 °C. Hence, the residues contain mainly mono- and diglycerides, sterols, and polymeric FAMEs formed during distillation. Mono- and diglycerides provide very good lubricity improvement on ULSD.9 Polymeric species form viscous (16) Hamrock, B. J.; Schimid, S. R.; Jacobson, B. O. Fundamentals of Fluid Film Lubrication; Marcel Dekker: New York, 2004; pp 7-16. (17) Liang, H.; Totten, G.; Webster, G. Lubrication and Tribology Fundamentals; ASTM International: West Conshohocken, PA, 2003; pp 909-961. (18) Gunstone, F. Fatty Acid and Lipid Chemistry; Aspen Publishers, Inc.: New York, 1999; p 81. (19) Scrimgeour, C. Bailey’s Industrial Oil and Fat Products, 6th ed.; John Wiley and Sons, Inc.: New York, 2005; Vol. 6.
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boundary layers to produce layers of about one polymer coil diameter thickness, which have higher polymer content and higher viscosity than the bulk solution.20,21 3.5. Temperature Effect on Lubricity. Figure 7 shows the temperature effect on 2 vol % of SBO-biodiesel blend in ULSD lubricity. WSD increases as the temperature increases up to 70 °C and then decreases for 80 and 90 °C. At each temperature, four tests were carried out. At lower temperatures, 20-70 °C, the minor polar phase is not yet in optimum contact with the metal surfaces, which are awash with the predominant nonpolar, less lubricating medium because of poor mixing; hence, the increasing WSD trend. However, at the higher temperature range, 80-90 °C, molecular motion for polar components increases enough, enabling these to be more evenly distributed on the metal surfaces and, therefore, enhancing lubricity and decreasing WSD as noted. Also, at temperatures greater than 70 °C, chemical adsorption of the polar compound may take place on the metal surface compared to physical adsorption of the polar compound at lower temperatures and, thus, increase the lubricity. For the formation of the boundary film layers, organic compound can form multilayers on the metal surface.16 To check the reversibility of the boundary film formation, four tests of 2 vol % of SBO in ULSD lubricity were performed at 90 °C without load, then cooled to room temperature, and then tested with the WSD at 60 °C with load in HFRR. The same procedure had been performed at 70 °C. Interestingly, the WSDs observed for these samples were much smaller than the samples without high-temperature treatment (dotted line in Figure 7). This suggests that the boundary films formed at 90 °C were still present on the metal surface to provide good lubrication. 4. Conclusions (1) The lubricity of ULSD and S8 initially increases sharply as the biodiesel blending level increases and then levels off at the ∼2% limit. CSO-, SBO-, and PF-based biodiesels have similar effects on lubricity for both ULSD and S8. (2) FAMEs of SBO-, CSO-, and PF-based distilled biodiesel showed different lubricity effects. When the degree of saturation is increased, the FAMEs lead to an increased lubricity. (3) As additives of 2% blend to ULSD, biodiesel components have lubricity effects of varying contributions. The order of the contribution is FFA > SBO-biodiesel > phospholipids > antioxidant > glycerol > distilled SBO-biodiesel > individual FAME. (4) Residues of distilled biodiesel have much better lubricity than the distilled biodiesel. (5) Temperatures greater than 70 °C have better lubricity attributed to the boundary film formation, which persists partially on the metal surface when cooled from 90 to 60 °C. Acknowledgment. Financial support from the Department of Energy (Grant DE-FG36-05GO85005) for this research is gratefully acknowledged. EF800887Y (20) Smeeth, M.; Gunsel, S.; Spikes, H. A. Boundary film formation by viscosity index improvers. Tribol. Trans. 1996, 39, 726–734. (21) Spikes, H. The borderline of elastohydrodynamics and boundary lubrication. Proc. Inst. Mech. Eng., Part C 2000, 214, 23–37.
