The Influence of Fatty Acids and Fatty Acids Mixtures on the Lubricity of Low-Sulfur Diesel Fuels
Published in: SAE TECHNICAL PAPER SERIES 2001-01-1929
ABSTRACTResearch described in this work relates to tribological properties of fatty acids and fatty acids mixtures and aims at providing more information on and better understanding of the wear process in the presence of low-sulfur fractions containing these substances at very low concentration under boundary lubrication conditions in the steel-on-steel system.
Experiments were carried out using the ball-on-disc friction machine. To get detailed information on interaction of diesel fuel components with individual fatty acids and mixtures of acids, low-sulfur diesel fuel cuts as well as their mixtures and two model hydrocarbons were used as the base fluids.
Although the overall picture of the obtained results is very complex, it can be concluded that: (a) Tribological properties of base diesel fuels are determined by the highest fractions and heterogeneous compounds present in these fractions; (b) Fatty acids and their equimolar mixtures - added to the "reformulated diesel fuels" at the adequate concentration - show excellent antiwear properties; (c) Both the chemical structure and viscosity of base fluids influence the wear behavior of fatty acids and their mixtures.
All the findings are challenging from the view-point of their interpretation as well as from their relation to the diesel fuel constitution. Obtained results also evidently confirm the complexity of the boundary lubrication approach.
INTRODUCTIONThere are several different models of how boundary lubrication prevents the high shear stresses associated with the localized seizures of the surfaces. In fuel injection equipment the mechanism by interposing - either by chemisorption or physical adsorption - a layer on the surfaces to reduce the number of points at which true metal to metal contact occurs, has been considered the most important .
In the 1960s and 1970s a number of serious aviation fuel pump problems were encountered in service. After considerable research it was realized that a prime cause of those problems was an increase in the severity of refinement of aviation kerosene by processes such as hydrotreatment and clay treatment refining process . Nitrogen containing impurities such as pyrolles, indoles, carbazoles, basic pyridines and trace quantities of oxygen-containing impurities such as phenols and certain naphthenic acids were removed. That situation reduced the inherent lubricating properties of fuels. Those problems were overcome by incorporating lubricity additives, for example such as a mono-, di- and trimers of C18 fatty acids .
Wei and Spikes  carried out a study on the diesel fuel lubricity in the early 1980s, in anticipation of possible future lubricity problems. They investigated main factors influencing the lubricity of diesel fuels by carrying out tests on diesel fuels, diesel fuel fractions and model fuels. It has been found that diesel fuel lubricity is to a large extent determined by oxygen-containing polar impurities (some oxygen-containing compounds such as hydroxyquinolines and carboxylic acids reduced wear at concentration of a few parts per million (ppm)).
In the late 1980s and early 1990s environmental concern about the toxic and harmful emission from diesel engines led to large reductions in the level of sulfur and aromatic hydrocarbons in diesel fuels and the development of so-called "reformulated diesel fuels". Almost at once, fuel pump failures began to occur in Sweden and regions of the United States and Canada, where major reduction in sulfur levels for diesel fuels had been introduced. Lubricity of "reformulated diesel fuels" was the subject of a number of studies [1, 5-14]; one of the most significant conclusion was as follows: both low viscosity and the lack of sufficient trace components, such as oxygen- and nitrogen-containing compounds and certain aromatic compounds, can be responsible for equipment wear.
Fatty acids are the most thoroughly studied compounds from the view-point of the boundary lubrication. However, the major emphasis has almost always been put on their antifriction properties [15-21]. Moreover, sufficient evidence exists in the literature to indicate that antiwear properties of these additives mostly concern tests carried out with rather higher concentration, usually more than 0,5 % (5000 ppm) [16, 22]. Additionally, the major tested fatty acids relate to higher molecular weight compounds, mostly from lauric acid (C12) to stearic acid (C18) [16, 22-24].
Wei and Spikes  have found that the significant wear reduction was produced by fatty acids that were introduced into hydrotreated fuel at low concentration: caproic acid (C6) (100 ppm) and palmitic acid (C16) (1 - 300 ppm).
Results of the influence of 50 - 1000 ppm fatty acids C6 - C22 solutions in the diesel fuel components, their fractions and other solvents obtained from tribological tests carried out using ball-on-disc tester and HFR2 apparatus allowed to prove that :
AIM OF RESEARCH
EXPERIMENTAL TECHNIQUELUBRICANT MATERIALS
Three low-sulfur diesel base fuels were tested:
Tribological tests were also carried out for distillation fractions obtained by fractional separation of HON (ten equivoluminal fractions were collected). Physical and chemical properties of these fractions are summarized in Table 2. The chemical constitution of selected fractions was analyzed by gas chromatography combined with infrared detection (AutoSystem 2000 GC/FTIR); parameters of this analysis are listed in Table 3. Tribological properties of some mixtures of the narrow distillation fractions were examined as well.
Additionally, to investigate in more detail the influence of fatty acids on the wear behavior of fuels, model fluids were used. They included: one pure paraffin hydrocarbon (n-hexa-decane), one pure aromatic hydrocarbon (1-meth-yl-naphthalene) and mixtures of n-hexadecane and 1-methyl-naphthalene in the following voluminal ratio: 25 : 75, 50 : 50, 75 : 25, 85 : 15.
The following aliphatic fatty acids:
APPARATUS AND EXPERIMENTAL PROCEDURE
Wear tests were carried out with the ball-on-disc contact machine in which a stationary ball was loaded by a dead weight on a rotating disc. Prior to use, the test specimens were cleaned in acetone in an ultrasonic bath for 20 minutes. Sliding velocity and temperature were recorded continuously during the test. The test conditions (summarized in Table 5) were designed to result in boundary lubrication at the sliding interface. Minimum five tests were performed for each fluid.
INTERPRETATION OF OUTPUTS
The volume of material worn from the ball was calculated from the diameter of the spherical segment removed (wear scar diameter); this was measured after unloading the specimens, using a photomicroscope. Wear results obtained for each lubricant - collected in Tables 6 - 9 - include the wear scar diameter and the relative wear. The relative wear is the ratio of the volume ball wear obtained for a lubricant containing an additive and the volume ball wear for the base fluid, multiplied by 100 %. All other details relating to this procedure are described elsewhere .
RESULTS AND DISCUSSIONTHE INFLUENCE OF FATTY ACIDS AND FATTY ACIDS MIXTURES ON WEAR BEHAVIOR OF LOW-SULFUR DIESEL FUELS
The results presented in Figure 1 (all figures are in Appendix B) show the wear performance differences for the tested fuels. Figure 1 reveals that the wear increase relates to both the sulfur content and the viscosity decrease. According to  usually fuels characterized by the higher density and viscosity contain larger amount of heterogeneous compounds, so the effects of the separate factors can not be distinguished.
Tribological tests for 100, 500, 750, 1000 ppm stearic acid solutions in these fuels were performed to determine the influence of the additive on their antiwear performance. Examining the relationship between properties of the base fuels and the effectiveness of stearic acid, one can say that the acid added to the heaviest fuel shows the lowest wear reduction effect as compared to the fuels of the lower density and viscosity (Figure 2).
In our previous work  HON, containing the lowest level of sulfur, was chosen as a base fluid to prepare solutions of C6 - C18 fatty acids. Tests results for those solutions showed that the wear does not decrease regularly with the acid concentration increase and no specific influence of the molecular weight on wear values was found (some minima and maxima were observed).
HON was also used as a base fuel for solutions of fatty acids mixtures at the concentration range of 50 - 1000 ppm. The obtained data reveal that the wear does not decrease regularly when the concentration of the solution increases for each tested mixtures of acids. It is also to note that in this case the wear does not decrease regularly when the average molecular weight of mixture increases for each examined concentration. However, the equimolar mixtures of fatty acids are very effective antiwear additives for the low-sulfur diesel fuel (Table 6). One can not state that the amount of the wear - caused in the presence of the mixture - is always the average of the wear produced in the presence of individual acids creating the mixture. Comparison of the wear value for HON containing fatty acids mixtures and individual acids in the amount of 500 ppm is shown in Figure 3. Typical synergetic effect appears for capric acid and palmitic acid (C10 + C16). On the other hand, the mixture of myristic acid and palmitic acid (C14 + C16) demonstrates an excellent example of an antagonistic effect.
THE INFLUENCE OF FATTY ACIDS ON WEAR BEHAVIOR OF SEPARATED FRACTIONS AND MIXTURES OF THESE FRACTIONS
HON was studied in detail and was separated into ten fractions (fractions I - X) by vacuum distillation. Clear evidence was found that only the last fraction (fraction X) provided lower wear than HON and for lighter fractions the wear did not correlate neither with the sulfur content nor with the viscosity (Figure 4) .
The application of GC-FTIR analysis for fraction I, V, IX demonstrated that most of compounds present in these fractions are n-paraffin and cycloparaffin hydrocarbons. In fraction I, which causes the smallest wear amongst heavier consecutive eight fractions, a kind of oxygen-containing compound was detected. The FTIR spectrum for a signal, obtained during chromatographic separation of fraction I, is presented in Figure 5. It has been known, e. g., that 200 - 370 °C fraction of Californian crude oil contains benzofurans . Hydrogenation of an aromatic ring can lead to the formation of a lacton or an aliphatic ester. Signals 1769 cm -1, 1218 cm -1 and 1053 cm -1 shown in Figure 5 can be assigned to the stretching vibration of C=O and the stretching vibration of C-O (two bands: symmetric and asymmetric). In fraction I, a substance arising from hydrogenation of an aromatic oxygen-containing compound can exist. No heterogeneous compounds were detected in fractions V and IX. It must be underscored that signals characteristic for C-S and S-S bands vibration are very weak, so the presence of a small amount of sulfur-containing compounds can not be excluded.
The comparison of the wear in the system lubricated with fractions I - X and the wear in the system lubricated with 500 ppm solutions of stearic acid in these fractions let notice that addition of small amount of stearic acid into fractions I - IX produced marked wear reduction, but did not influence on tribological properties of fraction X .
Four fractions were chosen to prepare mixtures: fraction I, fraction V, fraction IX and fraction X. Fractions I, V, IX have different sulfur content and viscosity, but cause comparable amount of the wear (Figure 4). The addition of fraction X to fractions I, V, IX in voluminal ratio 1 : 1 brings out significant reduction of wear. The wear in the system lubricated with aforementioned mixtures of fraction is comparable to the wear in the system lubricated with fraction X (Figure 6). It points out that higher fractions determine lubricity of a diesel fuel.
However, results obtained for solutions of stearic acid in mixtures of fractions indicate that influence of higher fractions on antiwear properties of fatty acids is disadvantageous. The wear in the system lubricated with 500 ppm solutions of stearic acid in the mixture of fractions V + X and in the mixture of fractions IX + X is lower than the wear in the system lubricated with neat mixtures of fractions (Table 7), but is larger than the wear in the system lubricated with 500 ppm solutions of stearic acid in individual fractions V and IX (Figure 7). The reduction of the stearic acid effectiveness seems to be influenced by viscosity of base fluids. On the other hand, the wear in the system lubricated with 500 ppm solution of stearic acid in the mixture of fractions I + X is lower than the wear in the system lubricated with the neat mixture of these fractions and is lower than the wear in the system lubricated with 500 ppm solution of stearic acid in the fraction I (Figure 8). Assuredly, heteroorganic constituents from fraction X and added fatty acid work in the lightest mixture better than fatty acid in the lightest fraction.
THE INFLUENCE OF FATTY ACIDS AND MIXTURES OF FATTY ACIDS ON WEAR BEHAVIOR OF MODEL HYDROCARBONS
Previous research  - in which n-hexadecane was used as the base fluid - revealed the existence of a new phenomenon concerning a very specific "prowear" and antiwear behavior of fatty acids at 50 - 1000 ppm concentration range. Two kinds of events were distinguished:
It is interesting that the relationship between the wear and the concentration for some fatty acids mixtures diluted in n-hexadecane and HON is similar (Figure 11-12). This similarity can be related to highly paraffin character of hydrocarbons in HON.
Previous research carried out for 1-methylnaphthalene and equivoluminal mixture of n-hexadecane and 1-methyl-naphthalene containing palmitic acid at 50, 100, 500, 750, 1000 ppm concentration demonstrated that (Figure 13) :
The ball relative wear values plotted against the number of carbon atoms in the molecule of the fatty acid that creates a mixture with palmitic acid for 500 ppm solutions of mixtures in n-hexadecane, HON and 1-methylnaphthalene are presented in Figure 14. The relationship has similar characteristics for n-hexadecane and HON, but is different from that one for aromatic base fluid.
The comparison of the wear values in the system lubricated with solutions of fatty acids in n-hexadecane, 1-methyl-naphthalene, equimolar mixture of these fluids (Figure 13) and mixtures of fatty acids in n-hexadecane, 1-methyl-naphthalene and HON (Figure 14) provides the evidence for the negative influence of pure paraffin hydrocarbons on tribological properties of fatty acids.
Former test results obtained for solutions of palmitic acid in mineral base oil SN 400 and saturated hydrocarbons separated from the SN 400 base oil also indicated that the chemical constitution of base fluids influenced the relationship between the wear and the concentration and decided about the wear value . For high viscous base fluids: saturated paraffin-naphthenic hydrocarbons separated from the mineral base oil SN 400 and the mineral base oil SN 400, at the 100 ppm addition of the acid, a clear maximum wear value was found (Figure 15). When the concentration of palmitic acid was increased to 1000 ppm, the antiwear behavior of both oils was improved (relative wear around 30 %). In that case, the fluid viscosity controls the tribological behavior of fatty acids used in the lowest concentration. On the other hand, the low viscous fluid - n-hexade-cane - also made the antiwear properties of palmitic acid worse. It seemed to be caused by the molecular interaction between fatty acid molecules and the chemistry of base fluids, but not only by the viscosity of the base fluid.
1), 2), 3) ball relative wear calculated referring to the wear in the system lubricated with HON,