Contents

The Influence of Fatty Acids and Fatty Acids Mixtures on the Lubricity of Low-Sulfur Diesel Fuels

Czeslaw Kajdas
Central Petroleum Laboratory in Warsaw, Poland
Warsaw University of Technology, Institute of Chemistry, Poland

Marzena Majzner
Warsaw University of Technology, Institute of Chemistry, Poland

ABSTRACT

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.

INTRODUCTION

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 [2]. 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 [3].

Wei and Spikes [4] 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 [4] 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 [25]:

• fatty acids added to low-sulfur diesel fuel components at the concentration range of 50 - 1000 ppm show excellent antiwear properties;
• the tribological wear does not decrease with the increase of fatty acid concentration in the tested diesel fuel component;
• for 500 ppm solutions of C₆ - C₂₂ acids in the tested diesel fuel component the wear does not decrease with growth of fatty acid molecular weight;
• antiwear behavior of fatty acids is the result of complex interaction between molecules of the acid and particular types of hydrocarbons; such thesis has been confirmed by the results obtained for palmitic acid solutions in n-hexadecane, 1-methylnaphthalene and the mixture of these hydrocarbons at equivoluminal ratio;
• antiwear behavior of fatty acids under boundary lubrication conditions is influenced by the rate of acid molecule diffusion to the friction surface; this conclusion was drawn from the test results obtained for systems lubricated with palmitic acid solutions in synthetic oils (polyalphaolefins) possessing various viscosity; it has also been found, that in some cases structure of the solvent molecule has a predominant influence on the wear (e. g., interaction between molecules of n-hexa-de-cane and fatty acid).

AIM OF RESEARCH

The objective of this paper is:
1. to present tribological properties of low-sulfur diesel fuels and determine the influence of fatty acids and fatty acids mixtures on their tribological properties;
2. to describe the influence of fatty acids on the wear of systems lubricated with (a) fractions obtained by fractional distillation of a diesel fuel and (b) mixtures of these fractions;
3. to demonstrate how the chemical structure and the viscosity of base fluids influence the antiwear and "prowear" behavior of fatty acids and mixtures of fatty acids.

EXPERIMENTAL TECHNIQUE

Base fluids

Three low-sulfur diesel base fuels were tested:

• low-sulfur fraction from Installation of Diesel Fuel Desulfurisation containing 76 ppm of sulfur (denoted in this work as HON);

• diesel base fuel containing 390 ppm of sulfur (denoted in this work as diesel base fuel S 390);

• diesel base fuel containing 420 ppm of sulfur (denoted in this work as diesel base fuel S 420).

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.

Additives

The following aliphatic fatty acids:

• caproic acid (hexanoic acid);
• caprylic acid (octanoic acid);
• capric acid (decanoic acid);
• lauric acid (dodecanoic acid);
• myristic acid (tetradecanoic acid);
• palmitic acid (hexadecanoic acid);
• stearic acid (octadecanoic acid);
• arachidic acid (eicosanoic acid);
• behenic acid (docosanoic acid)

  and equimolar mixtures of:

• caproic acid + palmitic acid;
• caprylic acid + palmitic acid;
• capric acid + palmitic acid;
• lauric acid + palmitic acid;
• myristic acid + palmitic acid;
• stearic acid + palmitic acid;
• arachidic acid + palmitic acid;
• behenic acid + palmitic acid

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 [27].

RESULTS AND DISCUSSION

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 [4] 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 [25] 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) [25].

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 [26]. 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 [25].

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 [27] - 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:

• a clear "prowear" effect of myristic acid, palmitic acid and stearic acid at 750 ppm concentration;
• a distinct antiwear effect of caproic acid at 500 ppm concentration.

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) [25]:

• 100 - 500 ppm concentration of palmitic acid reduced the ball wear for both lubricants very significantly;
• for the solution of palmitic acid in the mixture of aromatic and paraffin hydrocarbons the maximum wear appeared at the same concentration (750 ppm) as for n-hexadecane;

  no wear maximum existed for 1-methylnaphthalene.

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 [28]. 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.

CONCLUSIONS

1. Tribological properties of base diesel fuels are determined by their highest fractions and heterogeneous compounds existing in these fractions.
2. Fatty acids and their equimolar mixtures, added to the "reformulated diesel fuels" at the adequate concentration, provide excellent antiwear properties.
3. The following factors seem to control wear behavior of fatty acids:
   • viscosity of the base fluids;
   • chemical composition of the base fluids.
4. Tests carried out with fatty acids mixtures dissolved in n-hexadecane point out the existence of the phenomenon concerning a very specific "prowear" and antiwear behavior of these compounds; this phenomenon was discovered for solutions of acids earlier.
5. All the above discussed findings are challenging from the view-point of their interpretation as well as from their relation to the diesel fuel constitution.
6. Obtained results evidently confirm the complexity of the boundary lubrication approach.

REFERENCES

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15. Bowden, F. P., Tabor, D., "Friction and Lubrication of Solids", Clarendon Press, Oxford, 1950.
16. Kajdas, C., Luczkiewicz, J., Ozimina, D., Wawak-Pardyka, E., "The Influence of Concentration of Alcohols and Acids on Tribological Properties of Paraffin Oil", Tribologia, 1980, 11, 6, 22-24 (in Polish).
17. Jahanmir, S., "Chain Length Effect in Boundary Lubrication", Wear, 1985, 102, 331-349.
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19. Beltzer, M., Jahanmir, S., "Role of Dispersion Interactions Between Hydrocarbon Chain in Boundary Lubrication", ASLE Transactions, 1987, 30, 1, 47-54.
20. Nakayama, K., Studt, P., "The Adsorption of Polar Cyclic Compounds on Iron Surfaces from Hydrocarbon Solutions and Their Lubricating Properties", Wear, 1987, 116, 107-118.
21. Minami, I., Kikuta, S., Masuka, M., Okabe, H., "Lubricating Functions of Dicarboxylic Acids in Polar Base Oils", Japanese Journal of Tribology, 1990, 35, 4, 505-511.
22. Wachal, A., "The Influence of Chemical Structure of Additives on Antiseizure and Antiwear Properties of Oils", Tribologia, 1977, 8, 5-6, 161-163 (in Polish).
23. Hironaka, S., Yahagi, Y., Sakurai, T., "Effect of Adsorption of Some Surfactants on Antiwear Properties", ASLE Transactions, 1978, 21, 3, 231-235.
24. Masuko, M., Ohmori, T., Okabe, H., "Antiwear Properties of Hydroxycarboxylic Acid with Straight Alkyl Chains", Tribology International, 1988, 21, 4, 199-203.
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27. Kajdas C., Majzner M., Konopka M., "The Effect of Fatty Acids at Low Concentrations in n-Hexadecane on Wear of Steel in Boundary Lubrication", Tribologia, 1997, 153, 3, 221-237 (in Polish).
28. Kajdas C., Majzner M., "Influence of Fatty Acids Solutions in Hydrocarbons and Petroleum Fractions on the Antiwear Properties of the Steel-on-Steel System", Tribologia, 1998, 159, 3, 285-317 (in Polish).

APPENDIX A

Table 2. Selected properties of distillation fractions obtained from HON
Fraction	Initial boiling point, °C	Final boiling point, °C	Kinematic viscosity at 40 °C, mm2/s	Sulfur content, ppm
Fraction I	173	201	1,08	46
Fraction II	201	216	1,18	58
Fraction III	216	221	1,30	46
Fraction IV	221	227	1,40	46
Fraction V	227	238	1,53	50
Fraction VI	238	242	1,71	56
Fraction VII	242	256	1,79	44
Fraction VIII	256	270	2,07	78
Fraction IX	270	287	2,44	99
Fraction X	287	-	3,34	153

Table 3. Parameters of the GC/FTIR analysis performed to determine chemical constitution of fractions I, V, IX
Parameter	Fraction I	Fraction V	Fraction IX
	Characteristics of parameters
Parameters of furnace work	150 °C (60 s) ® 210 °C (60 s) 5 °C/60 s	150 °C (60 s) ® 220 °C (60 s) 5 °C/60 s	190 °C (60 s) ® 270 °C (60 s) 5 °C/60 s
Injector temperature, °C	250	250	300
Sample volume, ml	0,5	0,5	0,5
Column type Column length, m	Rtx-5 30	Rtx-5 30	Rtx-5 30
Carrier gas type Carrier gas pressure, kPa	Helium 5,5 40	Helium 5,5 40	Helium 5,5 40
Detector type Detector temperature, °C	FTIR (light pipe) 250 °C	FTIR (light pipe) 250 °C	FTIR (light pipe) 250 °C

Table 4. Characteristics of fatty acids
Fatty acid	Chemical structure	Molecular weight	Purity, %
Caproic acid	CH3-(CH2)4-COOH	116,16	99
Caprylic acid	CH3-(CH2)6-COOH	144,21	98
Capric acid	CH3-(CH2)8-COOH	172,27	~ 99
Lauric acid	CH3-(CH2)10-COOH	200,32	> 99
Myristic acid	CH3-(CH2)12-COOH	228,38	≥ 99
Palmitic acid	CH3-(CH2)14-COOH	256,43	> 97
Stearic acid	CH3-(CH2)16-COOH	284,49	≥ 90
Arachidic acid	CH3-(CH2)18-COOH	312,54	98
Behenic acid	CH3-(CH2)20-COOH	340,59	-

Table 5. The ball-on-disc experimental set-up and test conditions
Material system	52100 steel-on-52100 steel
Geometry	Sphere-on-flat (the fixed ball on the rotating disc)
Specimens - ball - disc	Diameter: 3,18 mm; Ra = 0,3 - 0,35 mm; hardness: 63 HRC Diameter: 25,4 mm; Ra = 0,2 mm; hardness: 58 - 63 HRC
Wear track radius, mm	8
Applied load, N	9,81
Sliding velocity, m/s	0,25
Sliding distance, m	500
Temperature, °C	25

Table 6. The tribological wear in the system lubricated with 50 - 1000 ppm solutions of equimolar mixtures of fatty acids C6 - C22 and palmitic acid C16 in HON
Mixture constitution	Mixture concentration in HON	Wear scar diameter		Volume ball wear	Ball relative wear
	ppm	µm		10-13 m3	%
		Average value	Standard deviation
HON	0	476	39	16,01	100,0
Caproic acid + palmitic acid	50	276	21	1,79	11,2
	100	205	36	0,55	3,4
	500	219	17	0,71	4,4
	750	239	30	1,00	6,3
	1000	211	31	0,62	3,8
Caprylic + palmitic acid	500	267	46	1,58	9,9
Capric acid + palmitic acid	50	210	14	0,60	3,8
	100	179	34	0,32	2,0
	500	155	15	0,18	1,1
	750	181	8	0,34	2,1
	1000	168	14	0,25	1,6
Lauric acid + palmitic acid	50	260	32	1,41	8,8
	100	211	28	0,62	3,8
	500	160	31	0,20	1,3
	750	208	37	0,58	3,6
	1000	171	36	0,27	1,7
Myristic acid + palmitic acid	500	293	76	2,28	14,2
	1000	250	64	0,56	7,6
Stearic acid + palmitic acid	50	232	22	1,21	5,6
	100	178	16	0,31	1,9
	500	191	41	0,41	2,6
	750	196	31	0,46	2,8
	1000	167	20	0,24	1,5
Arachidic acid + palmitic acid	500	274	89	1,75	10,9
Behenic acid + palmitic acid	500	220	64	0,72	4,5
	1000	237	48	0,98	6,1

Table 7. The influence of the addition of 500 ppm stearic acid C18 on tribological properties of mixtures of fractions I + X, V + X, IX + X
Lubricating substance	Wear scar diameter, mm		Volume ball wear, 10 -13 m3	Ball relative wear, %
	Average value	Standard deviation
HON	476	39	16,01	100,0
Fractions I + X	265	49	1,54	9,61)
Fractions V + X	244	56	1,10	6,92)
Fractions IX + X	243	54	1,08	6,73)
500 ppm stearic acid solution in the mixture of fractions I + X	194	8	0,44	28,44)
500 ppm stearic acid solution in the mixture of fractions V+ X	202	32	0,51	46,65)
500 ppm stearic acid solution in the mixture of fractions IX+ X	191	32	0,41	38,16)

¹⁾, ²⁾, ³⁾ ball relative wear calculated referring to the wear in the system lubricated with HON,
⁴⁾, ⁵⁾, ⁶⁾ ball relative wear calculated referring to the wear in the system lubricated with the neat mixture of fractions.

Table 8. The tribological wear in the system lubricated with 50 - 1000 ppm solutions of equimolar mixtures of fatty acids C6 - C22 and palmitic acid C16 in n-hexadecane
Mixture constitution	Mixture concentration in n-hexadecane	Wear scar diameter		Volume ball wear	Ball relative wear
	ppm	µm		10-13 m3	%
		Average value	Standard deviation
n-Hexadecane	0	292	8	2,25	100,0
Caproic acid + palmitic acid	100	267	3	1,57	69,7
	300	243	1	1,08	47,8
	500	244	4	1,09	48,5
	600	258	4	1,37	60,8
	700	313	8	2,98	132,0
	800	293	45	2,27	100,6
	900	311	64	2,90	128,5
	1000	340	42	4,15	184,1
Caprylic acid + palmitic acid	100	239	29	1,02	45,1
	300	269	78	1,61	71,4
	500	306	62	2,72	120,8
	600	342	15	4,23	187,5
	700	259	4	1,40	62,2
	800	288	38	2,12	94,2
	900	289	52	2,15	95,3
	1000	278	47	1,86	82,6
Capric acid + palmitic acid	100	262	8	1,46	64,9
	300	252	3	1,25	55,6
	500	265	5	1,53	67,8
	600	272	8	1,69	74,8
	700	325	33	3,44	152,6
	800	350	35	4,63	205,2
	900	350	34	4,67	207,2
	1000	290	52	2,18	96,8
Lauric acid + palmitic acid	100	237	13	0,97	43,1
	300	229	22	0,85	37,5
	500	206	15	0,56	24,8
	600	253	4	1,26	56,1
	700	299	15	2,46	109,1
	800	284	21	20,1	89,0
	900	266	32	1,55	68,7
	1000	220	30	0,72	31,9
Myristic acid + palmitic acid	100	307	17	2,76	122,6
	300	293	17	2,27	100,7
	500	355	4	4,91	217,6
	600	383	2	6,64	294,6
	700	418	2	9,47	420,1
	800	398	4	7,79	345,7
	900	377	4	6,26	277,5
	1000	401	8	8,04	356,5
Stearic acid + palmitic acid	100	272	1	1,69	74,8
	300	231	1	0,88	38,8
	500	193	1	0,43	19,2
	600	225	9	0,79	35,1
	700	240	2	1,03	45,6
	800	308	2	2,77	122,9
	900	362	3	5,31	235,6
	1000	278	4	1,86	82,3
Arachidic acid + palmitic acid	100	268	4	1,61	71,2
	300	302	6	2,59	114,8
	500	353	3	4,80	213,0
	600	329	2	3,64	161,5
	700	353	12	4,80	212,8
	800	374	16	6,06	268,9
	900	406	16	8,41	373,2
	1000	371	25	5,88	260,7
Behenic acid + palmitic acid	100	202	1	0,52	23,0
	300	247	25	1,16	51,3
	500	306	10	2,71	120,4
	600	287	20	2,09	92,6
	700	358	10	5,12	227,1
	800	256	9	1,32	58,6
	900	352	6	4,78	211,9
	1000	379	7	6,38	283,1

Table 9. The tribological wear in the system lubricated with 50 - 1000 ppm solutions of palmitic acid C16 in 1-methylnaphthalene, n-hexadecane and mixtures of these hydrocarbons
Mixture of 1-methylnaphthalene and n-hexadecane		Palmitic acid concentration in base fluid	Wear scar diameter		Volume ball wear	Ball relative wear
n-Hexadecane content, % (V/V)	1-Methylnaphthalene content, % (V/V)		µm		10-13 m3	%
		ppm	Average value	Standard deviation
0	100	0	385	13	6,84	100,0
		50	279	22	1,87	27,4
		100	208	37	0,58	8,4
		500	215	16	0,66	9,6
		750	192	7	0,42	6,1
		1000	174	9	0,29	4,2
25	75	0	329	12	3,64	100,0
		750	208	8	0,57	15,8
50	50	0	364	21	4,43	100,0
		50	268	47	1,60	36,0
		100	178	11	0,31	7,0
		500	178	7	0,31	29,5
		750	255	52	1,31	23,4
		1000	241	63	1,04	23,4
75	25	0	283	5	1,99	100,0
		750	265	57	1,53	77,1
85	15	0	261	2	1,44	100,0
		750	261	8	1,43	99,1
100	0	0	261	10	1,43	100,0
		750	272	32	1,70	118,7

APPENDIX B

APPENDIX C

Contents

• Czeslaw Kajdas, Marzena Majzner; The Influence of Fatty Acids and Fatty Acids Mixtures on the Lubricity of Low-Sulfur Diesel Fuels
• Abstract
• Introduction
• Aim of Research
• Experimental Technique
• Results and Discussion
• Conclusions
• References
• Appendix A
• Appendix B
• Appendix C