Contents

Boundary Lubrication of Low-Sulphur
Diesel Fuel in the Presence of Fatty Acids

Czeslaw Kajdas and Marzena Majzner
Warsaw University of Technology, Institute of Chemistry, Plock, Poland

Abstract

This paper presents an overview of the effect of aliphatic acids on the tribological properties of selected hydrocarbons and petroleum fractions. The major experimental part of the work focuses on improvements to the lubricity of low-sulphur diesel fuel. Experiments were carried out using a pin-on-disc friction machine and HFRR test rig. The wear results obtained clearly show a specific effect of the test acids dissolved in hydrocarbons in the concentration range 0.005-0.1% (50-1000 ppm). Although the overall picture presented by these results is very complex, it can be concluded that a very small amount of the selected acids dramatically improves low-sulphur diesel fuel lubricity.

Keywords: lubricity, diesel fuel, boundary lubrication, aliphatic acids, HFRR, concentrations


Published in : Lubrication Science 14-1 p. 84-107

INTRODUCTION

Boundary lubrication plays a key role in lubrication of diesel fuels. However, there is considerable evidence that viscosity still has some influence, because many pump contacts operate in the mixed lubrication regime, where the hydrodynamic film formed is comparable to the surface roughness. Unfortunately, fuels are inherently poor lubricants from both the hydrodynamic and boundary lubrication point of view [1].
The first line of defence of any liquid lubricant is to form a hydrodynamic film between rubbing solid surfaces. The thickness of such films increases with the dynamic viscosity of the fluid. Since the viscosity of fuels is far lower than that of most liquid lubricants, it is rarely possible to generate a full hydrodynamic film in fuel systems, and some contact load must be borne by solid-solid interactions and thus boundary lubrication.
In fuel injection equipment, the mechanism of interposing a layer on the surfaces, either by chemisorption or physical adsorption, to reduce the number of points at which true metal-to-metal contact occurs has been considered most important. It can be achieved through adsorption of polar species on to the metal surface, giving a low shear stress layer at the majority of interacting points [2]. Onion and Suppeah [3] suggested that thick, reacted films are produced on the rubbed metal surfaces of diesel fuel injection equipment. Wei and Spikes [4] carried out a study of diesel fuel lubricity in anticipation of possible future lubricity problems. They investigated the main factors influencing the lubricity of diesel fuels by performing tests on diesel fuels, diesel fuel fractions, and model fuels using a high-frequency reciprocating machine (HFRR). It was found that: (a) diesel fuel lubricity is to a large extent determined by polyaromatics and oxygen-containing polar impurities; (b) monoaromatic and diaromatic hydrocarbons have little influence on wear, and (c) most sulphur impurities are pro-wear.
In the late 1980s and early 1990s environmental concern about the toxic and harmful emissions from diesel engines led to large reductions in the levels of sulphur and aromatics in diesel fuels, and the development of so-called 'reformulated diesel fuels'. Almost at once, fuel pump failures began to occur in Sweden and some regions of the USA and Canada, where major reductions in sulphur levels had been introduced [5].
The lubricity of 'reformulated diesel fuels' has been the subject of a number of studies. Wang and Cusamo [6] employed scuffing ball on cylinder lubricity evaluator (SL BOCLE) to measure the lubricity of low-sulphur diesel fuels and found that some low-sulphur diesel fuels can have a lubricity as high as high-sulphur fuels, and that lubricity is thus likely to be independent of sulphur levels. They suggested that: (a) the lubricity of low-sulphur diesel fuels is determined primarily by the viscosity and the diaromatic compound content; and (b) both hydrodynamic lubrication and boundary lubrication are important in preventing the wear of the fuel system.
Test results obtained by Lacey and Westbrook using SL BOCLE confirmed that: (a) no direct correlation exists between sulphur content and scuffing load capacity; (b) good direct correlation is observed between fuel lubricity and diaromatic and polyaromatic compound concentration; (c) none of the model sulphur and aromatic compounds introduced to jet fuel increases scuffing capacity to the extent predicted by consideration of the total sulphur and aromatics content in real fuels, and it is likely that sulphur and aromatic compounds simply reflect the effects of severity of refinement on other surfaceactive compounds in the fuel; and (d) low-viscosity fuels tend to have a poor
Table 1. Properties of low-sulphur diesel fuel components
PropertyHON componentHK component
Density at 20C, g/cm30.8130.835
Kinematic viscosity at 40C, mm2/s1.73.0
Sulphur content, ppm761
CFPP,C<-30-18
Cetane index5152
Distillation
    IBP,C
    T10,C
    T50,C
    T90, C

173
201
238
287

198
245
277
305

scuffing load capacity, partly due to hydrodynamic lift between the specimens during testing that is always present during lubricated sliding.
It has been found [8] that correlations between HFRR test results and various properties, such as cloud point, viscosity, sulphur content, and diaromatic and polyaromatic content shows similar correlation coefficient values, with no single inspection property correlating well with the measured lubricity. The influence of other more subtle and difficult-to-measure characteristics, such as the presence of small amounts of natural antiwear agents, may be important.
The general field of transportation fuel lubricity has recently been reviewed by Wei and Spikes [9] with the conclusion that increases in the severity of refinement over the last forty years, driven initially by the search for fuel stability, but more recently by environmental concerns, have led to a progressive reduction in fuel lubricity. It is also emphasised that the origin of the natural lubricity of fuels is not yet clear, largely because it probably resides primarily in very low concentration, and thus difficult-to-identify, polar impurities within the fuel.
Fatty acids are the most thoroughly studied compounds from the viewpoint of boundary lubrication. However, most of the detailed information concerning the tribological properties of carboxylic acids relates to the friction coefficient [10-13]. The same is also true of papers published in recent decades [14, 15].
Moreover, the literature [16] indicates that evidence of the antiwear properties of these additives mostly comes from tests carried out with rather higher concentrations, usually 0.4-2.0%. Additionally, the major fatty acids tested relate to higher molecular weight compounds, mostly from lauric acid (C12) to stearic acid (C18).
Table 2. Characteristics of distillation fractions of HON component
FractionIBP, CFBP, Cat 40C, mm2/s Sulphur content ppm
I1732011.146
II2012161.258
III2162211.346
IV2212271.446
V2272381.550
VI2382421.756
VI!2422561.844
VIII2562702.178
IX2702872.499
X287-3.3153
Wei and Spikes [4] found that significant wear reduction was produced by the fatty acids caproic acid (C6) and palmitic acid (C) that were introduced into hydrotreated diesel fuel at low concentration.
All the research described in the present paper relates to the tribological properties of fatty acids, and aims at providing more information on, and a better understanding of, the wear process in the presence of real and model fluids containing these substances at very low concentration, under boundary lubrication conditions, in a steel-on-steel system.

AIM OF RESEARCH

The objectives of this paper are:
  • to present the tribological properties of fractions used to produce lowsulphur diesel fuels and to determine the influence of fatty acids on the tribological properties of these components
  • to describe the influence of fatty acids on the wear of systems lubricated with fractions obtained by fractional separation of components
  • to demonstrate how the chemical structure and viscosity of base fluids influence the antiwear and pro-wear behaviour of fatty acids.
    Table 3. Kinematic viscosity of base fluids
    Base fluidKinematic viscosity at 40C, mm2/s 1-Methylnaphthalene2.1
    Mixture of n-hexadecane and 1 -methylnaphthalene (volume ratio 1:1)2.2
    n-Hexadecane 2.9
    PA0416.6
    PAO 628.9
    Saturated hydrocarbon mixture from SN 40050.4
    Mineral base oil SN 40076.7

    EXPERIMENTAL

    Lubricant materials

    Base fluids Two major components of low-sulphur diesel fuel were tested: the HON component and the HK component. The HON component is the low-sulphur fraction from diesel fuel desulphurisation. The HK component is the ultralow-sulphur fraction from hydrocracking. The properties of low-sulphur diesel fuel components are given in Table 1.
    Additionally, tribological tests were carried out for distillation fractions obtained by fractional separation of the HON component (properties of fraction I-X are summarised in Table 2).
    Seven other base lubricants with a wide range of viscosities (Table 3) were used to find out the influence of fatty acids on the wear behaviour of some model saturated and aromatic hydrocarbons along with the typical mineral base oil. These fluids include:
    • two pure model hydrocarbons: n-hexadecane and 1 -methylnaphthalene (used as the reference paraffin and aromatic base fluid)
    • mixture of n-hexadecane and 1-methylnaphthalene (a low-viscosity hydrocarbon reference base fluid)
    • two synthetic hydrocarbon base oils: PAO 4 and PAO 6
    • saturated hydrocarbons separated from SN 400 base oil
    • mineral base oil SN 400.
    Table 4. Characteristics of fatty acids
    Fatty acidChemical structureMolecular weightPurity, %
    Caproic acidCH3-(CH2)4-COOH116.1699
    Capric acidCH3-(CH2)8-COOH172.27~99
    Lauric acidCH3-(CH2)10-COOH200.32>99
    Palmitic acidCH3-(CH2)14-COOH256.43>97
    Stearic acidCH3-(CH2)16-COOH284.49≥90
    Behenic acidCH3-(CH2)18-COOH340.61-

    Table 5. Solutions of fatty acids in different base fluids investigated
    Base fluidFatty acidFatty acid concentration, ppmt
    HON componentCaproic acid50, 100, 500, 750, 1000
    Capric acid50, 100, 500, 750, 1000
    Lauric acid50, 100, 500, 750, 1000
    Palmitic acid50, 100, 500, 750, 1000
    Stearic acid50, 100,500,750, 1000
    Behenic acid500
    HK componentStearic acid100, 500, 1000
    Fraction I-X of HON componentStearic acid500
    1 -MethylnaphthalenePalmitic acid50, 100, 500, 750, 1000
    Mixture of n-hexadecane and 1-methylnaphthalene (volume ratio 1:1)Palmitic acid50, 100,500, 750, 1000
    n-HexadecanePalmitic acid50, 100, 500, 750, 1000
    PA04Palmitic acid50, 100 ,500, 750, 1000
    PA06Palmitic acid50, 100, 500, 750, 1000
    Saturated hydrocarbon mixture from SN 400Palmitic acid50, 100, 500, 750, 1000
    Mineral base oil SN 400Palmitic acid50, 100,500,750, 1000

    Fatty acid solutions Table 4 presents the aliphatic fatty acids used as additives in the present work and the molecular weight and purity of these substances. All the compounds were applied as received, without further purification. Fatty acids were added to base fluids in the concentration range 50-1000) ppm. The solutions of fatty acids tested are listed in Table 5.

    Table 6. Pin-on-disc experimental set-up and test conditions
    Material system52100 steel-on-52100 steel
    GeometrySphere-on-flat (fixed ball on rotating disc)
    Specimens
    ball
    disc

    Diameter: 3.18 mm; Ra = 0.3-0.35 m; hardness: 63 HRC
    Diameter: 25.4 mm; Ra = 0.2 m; hardness: 58-63 HRC
    Wear track radius, mm 8
    Applied load, N9.81
    Sliding velocity, m/s0.25
    Sliding distance, m500
    Temperature, C25

    Apparatus and test conditions

    The wear tests were carried out using:
    • HFR2 apparatus (made by PCS Instruments, UK)
    • pin-on-disc contact machine (Tester T-01 M made at the Terotechnology Institute, Radom, Poland).
    HFR2 apparatus The HFR2 apparatus used an electromagnetic vibrator to oscillate a moving specimen (a ball) over small amplitude while pressing it against a fixed specimen (a disc). The lower, fixed specimen was held in a small bath, which contained the test lubricant. During the test three values were logged continuously: the friction coefficient, the fuel temperature (programmed temperature 60C), and the electrical contact resistance.
    The test conditions were chosen according to CEC F-06-A-96 'Measurement of diesel fuel lubricity - approved test method. HFRR fuel lubricity test'.

    Pin-on-disc contact machine The specimens (a stationary ball and a rotating disc) were clamped in place with holders. The disc holder contained the test material, fully flooding the contact region. A dead-weight loading system applied normal force. The sliding velocity and temperature (programmed temperature 25C) were recorded continuously during the test.

    The test conditions (summarised in Table 6) were designed to result in boundary lubrication at the sliding interface.

    Procedure

    Prior to use, the test specimens were ultrasonically cleaned in toluene for 20 min and in acetone for 20 min. The same cleaning procedure was applied to the holders before testing each lubricant. Tribological tests using the HFR2 apparatus were carried out under test conditions in accordance with CEC F-06-A-96: 17.0C < temperature < 23.5C; absolute humidity > 0.8 kPa, relative humidity <: 70%; 23.5C < tem- perature ^ 28.0C; and 0.8 kPa < absolute humidity < 2.0 kPa. The same con- ditions were selected for tests with the pin-on-disc contact machine. A minimum of two tests was done using the HFR2 apparatus and a min- imum of five tests was carried out by the pin-on-disc contact machine for a test material.

    Interpretation

    HFR2 apparatus test outputs The wear-scar diameter was measured in both the X and the Y directions using a photomicroscope to an accuracy of 1 m, and the mean wear-scar diameter (MWSD) was evaluated:

    MWSD =(X + Y)/2(1)

    In accordance with CEC F-06-A-96, the corrected wear-scar diameter was calculated in um as follows:

    WS1.4 = MWSD + HFC x (1.4 -AVR)(2)

    where HFC is the humidity correction factor in m/kPa (for unknown fuel samples, ?HCF = 60 m/kPa); and AVP is the mean vapour pressure during the test in kPa, given by

    AVP = (ACT1 + AVP2)/2(3)

    AVP1 is the absolute vapour pressure at the start of the test in kPa, given by
    AVP2 = (%RH2 x 10n)/750(4)

    where %RH1, is the relative humidity at the start of the test, and n is given by

    n = 8.017352- [1705.984/(231.8636 + T1)](5)

    where T1, is the air temperature at the start of the test in C. AVP2 is the absolute vapour pressure at the end of the test in kPa, given by

    AVP2 = (%RH2 x 10 n)/750(6)

    where %RH2> is the relative humidity at the end of the test, and

    v = 8.017352 - [1705.984/(231.8636 + T2)](7)

    where T2 is the air temperature at the end of the test in C.

    The volume of worn material from the ball (volume wear, VW) was calculated from the corrected wear-scar diameter as follows:

    (8)

    where D is the diameter of the test ball (D = 6.00 mm).

    The relative wear (RW) was calculated as the ratio of the volume of ball wear obtained for the solution of fatty acid additive in the base fluid (VWfa) and the volume of ball wear for the base fluid (VWbf):
    RW = (VWfa/VWfb) x 100%(9)

    The force transducer attached to the lower specimen housing measured the friction force throughout a test. An A/D converter captured the friction force signal from the force transducer and the centre-line average value of the signal was calculated. That value was divided by the applied load to give the friction coefficient. The limiting value of the friction coefficient is not defined by CEC F-06-A-96.
    The electrical contact resistance (a semi-quantitative indicator of the formation of a separating film in the contact) was monitored throughout a test. The contact resistance circuit applied a 15 mV potential across the specimen contact and a balance resistor in series. The contact and the balance resistor thus formed a potential divider circuit. The series resistance was set by the control software and was set to 10 W by default. The potential drop across the contact was thus a measure of the contact resistance, as compared to the balance resistor. A low or zero film reading meant that the potential dropped across the contact, and hence the contact resistance was very low, i.e., there was a significant metal-to-metal contact taking place between the test specimens. This is usually associated with a high friction force and with high wear. A high film reading meant that the metal surfaces were apart. The limiting value of the film is not defined in CEC F-06-A-96.

    Table 7. Tribological properties of tested diesel fuel components (HFR2 apparatus)
    Diesel fuel componentTribological wearFilm %
    Corrected wear-scar
    diameter, m
    Relative ball wearAverage valueStandard deviation
    Average valueStandard deviation
    HON component 6916100.0214
    HK component7033100.0132

    Pin-on-disc test outputs Wear-scar diameter was measured in both the X and Y directions using a photomicroscope to an accuracy of 1 m, and the MWSD was evaluated. The influence of the absolute humidity was not taken into account, and the corrected wear-scar diameter was not determined. VW was calculated from the MWSD as follows:

    (10)

    where D is the diameter of the test ball (D = 3.18 mm).

    The relative wear (RW) was defined in the same way as for the HFR2 test outputs.

    RESULTS AND DISCUSSION

    Test results

    Influence of fatty acids on the trihological properties of low-sulphur diesel fuel components The tribological properties of two low-sulphur diesel fuel components were compared. The results of tests carried out using the HFR2 apparatus show that the HK component, containing a lower amount of sulphur, has slightly worse antiwear behaviour than the HON component (Table 7).
    Tribological tests of 100, 500, and 1000 ppm stearic acid solutions in both components were performed using the HFR2 apparatus to determine the influence of small fatty acid additive amounts on the tribological properties of the components. The largest difference in wear value for these solvents appears at a concentration of 500 ppm. Furthermore, a 500 ppm solution of stearic acid has better antiwear characteristics in the HK component (Table 8).
    Table 8. Effect of stearic acid on tribological properties of HON component and HK component (HFR2 apparatus)
    Stearic acid concentration, ppmTribological wearFilm %
    Corrected wear-scar
    diameter, m
    Relative ball wearAverage valueStandard deviation
    Average valueStandard deviation
    HON component
    100493325.8625
    500409212.2872
    100031984.5903
    HK component
    1004701519.7444
    50033635.1811
    100033174.8891

    The HON component was chosen as a base fluid for preparing C6-C16 acid solutions in the concentration range 50-1000 ppm. Tests carried out for those solutions using the pin-on-disc tester show that the wear does not decrease with increased solution concentration for each acid tested, and the wear does not decrease with increased molecular weight of the acid for each concentration (Table 9).
    The test results obtained from the HFR2 apparatus for 500 ppm solutions of C6-C22 acids (Table 10 ) confirm the untypical relationship between wear and the number of carbon atoms in the fatty acid molecule (molecular weight) (Figure 1). Additionally, it is found (Figure 2) that there is a correlation between wear and film thickness for capric, lauric, palmitic, stearic, and behenic acids (C10-C22 fatty acids). However, for caproic acid, in spite of a thin film, the wear-scar diameter is relatively small.

    Influence of fatty acids on tribological properties of separated fractions The HON component was studied in detail and separated into fractions (fraction I-X) by vacuum distillation. These fractions were tested using pin-on-disc and
    Table 9. Tribological wear in systems lubricated with 50-1000 ppm C6-C18 fatty acid solutions in HON component (pin-on-disc apparatus)
    Fatty acidFatty acid concentration, ppmWear-scar diameter, mRelative ball wear %
    Average valueStandard deviation
    Caproic acid502992715.4
    100240246.4
    5002722610.6
    750237356.0
    10002993715.5
    Capric acid503051716.8
    100183252.2
    5003002315.6
    750234395.8
    1000232365.6
    Lauric acid5020333.3
    10017391.7
    500210343.8
    750238356.2
    1000197322.9
    Palmitic acid50216134.2
    100217334.3
    500197152.9
    750200213.1
    1000207283.6
    Stearic acid50191152.6
    100206213.5
    500221374.6
    750196222.8
    1000187192.3

    Table 10. Tribological properties of 500 ppm C6-C18 fatty acid solutions in HON component (HFR2 apparatus)
    Fatty acidTribological wearFilm %
    Corrected wear-scar
    diameter, m
    Relative ball wearAverage valueStandard deviation
    Average valueStandard deviation
    Caproic acid4171313.2467
    Capric acid446317.3604
    Lauric acid438116.1633
    Palmitic acid3891510.0893
    Stearic acid409212.2872
    Behenic acid4611219.8561

    Figure 1: Tribological wear in systems lubricated with 500 ppm solutions of fatty acids in the HON component under different test conditions

    Figure 2: Tribological wear and film thickness in systems lubricated with 500 ppm solutions of C6-C18 fatty acids in the HON component (HFR2 apparatus)

    Table 11. Tribological wear in systems lubricated with distillation fractions of HON component (pin-on-disc apparatus)
    FractionTribological wear
    Wear-scar diameter, mRelative ball wear %
    Average valueStandard deviation
    I52233144.2
    II60937267.9
    III58236224.4
    IV61972286.3
    V53464158.0
    VI59446243.7
    VII56752201.7
    VIII54771174.2
    IX52755150.4
    X237286.1

    Table 12 Tribological wear in systems lubricated with distillation fractions of HON component (HFR2 apparatus)
    FractionTribological wear
    Corrected wear-scar diameter, mRelative ball wear %
    Average valueStandard deviation
    I75714144.0
    II78024162.6
    III77030154.1
    IV7459135.3
    V74316133.6
    VI72322119.7
    VII69925104.8
    VIII6911499.8
    IX6591682.3
    X5331535.3

    Figure 3: Influence of sulphur content on tribological wear in systems lubricated with distillation fractions of the HON component under different test conditions

    Figure 4: Influence of viscosity on tribological wear in systems lubricated with distillation fractions of the HON component under different test conditions

    Figure 5: Tribological wear in systems lubricated with distillation fractions of the HON component and 500 ppm solutions of stearic acid in these fractions (pin-on-disc apparatus)

    HFR2 apparatus. On the basis of the pin-on-disc results, clear evidence was found that only the last fraction (fraction X) gives lower wear than the HON component (Table 11). However, the HFR2 test results show that fraction IX also has antiwear properties (Table 12).

    In order to study the factors influencing the tribological properties of fractions, the sulphur content and viscosity of fractions I-X were determined (Table 2). The relative ball wear obtained from tests carried out using the HFR2 apparatus decreases with increasing viscosity and correlates with sulphur content for all the fractions except fraction VII (Figures 3 and 4). The pin-on-disc test results indicate that for lighter fractions wear does not depend on sulphur content and viscosity; the difference in the wear values for fractions I-VI may be caused by the tribological properties of particular groups of hydrocarbons in individual fractions under these test conditions (Figures 3 and 4).
    Figure 5 shows wear in the systems lubricated with fractions I-X, and wear in the systems lubricated with 500 ppm solutions of stearic acid in these fractions plotted against the initial boiling point of the fractions. The addition of 500 ppm of stearic acid into fractions I-IX produces marked wear reduction but does not influence the tribological properties of fraction X (Table 13).

    Influence of fatty acids on the tribological properties of different hydrocarbon base fluids Figure 6 (see p. 312) shows the relative change in wear for n-hexadecane, 1-methylnaphthalene, and their mixture containing palmitic acid
    Table 13 Tribological wear in systems lubricated with 500 ppm stearic acid solutions in distillation fractions of HON component (pin-on-disc apparatus)
    FractionTribological wear
    Wear-scar diameter, mRelative ball wear %
    Average valueStandard deviation
    I236674.1
    II212641.5
    III155150.5
    IV188360.8
    V184321.4
    VI225352.0
    VII200471.5
    VIII166170.8
    IX189131.7
    X24921122.4

    *The ratio of the volume of wear of the ball obtained for a fraction containing 500 ppm stearic acid additive and the volume of wear of the ball obtained for the fraction, multiplied by 100%.

    at different concentrations. In considering the influence of palmitic acid concentration on both 1-methylnaphthalene and its 1:1 ratio by volume mixture with n-hexadecane, three effects can be seen:

    • a 100 ppm concentration of palmitic acid reduces the ball wear for both lubricants very significantly
    • for the mixture of aromatic and paraffin hydrocarbons at a 750 ppm concentration of palmitic acid, a maximum value of wear appears, as in the case of n-hexadecane containing the same amount of acid (note that in Figure 6, this latter value is multiplied by 10 -1)
    • no wear maximum appeared for 1-methylnaphthalene.
    The effect of viscosity on the tribological behaviour of fatty acids was studied using two polyalphaolefin base oils. Figure 7 shows that an
    Figure 6: Relative ball wear in systems lubricated with 50-1000 ppm solutions of palmitic acid in n-hexadecane, 1-methylnaphthalene, and equivolume mixture of n-hexadecane and 1-methylnaphthalene (pin-on-disc apparatus)

    increase in the viscosity of the base oils decreases the wear reduction at all palmitic acid concentrations in the range 100-1000 ppm. Figure 7 demonstrates also that a 100 ppm addition of acid to synthetic base fluids PAO 4 and PAO 6 reduces the wear significantly, and at this acid concentration a minimum wear value appears.

    Interestingly, for high-viscosity base fluids with 100 ppm addition of acid, a clear maximum wear value was found (Table 14). Additionally, a 50 ppm concentration of palmitic acid in these base fluids also increased the wear. These trends are true for the saturated hydrocarbons (paraffin and naphthenic) separated from mineral base oil SN 400 and for the mineral base oil SN 400.

    Discussion

    Oxygen-containing impurities (e.g., naphthenic acids, phenols) are usually present in diesel fuels. Over the past few years a range of hydrogenation treatments of varying severity and hydrocracking processes have become
    Figure 7: Relative ball wear in systems lubricated with 50-1000 ppm solutions of palmitic acid PAO 4 and PAO 6 (pin-on-disc apparatus)

    common. These are especially effective at reducing the concentration of polar compounds and are potentially responsible for a large decrease in fuel lubricity.

    Wei and Spikes [4] considered that the significant wear reduction was produced by oxygen compounds with phenolic-type or carboxylic acid groups and occurred at low concentrations (Table 15). Our studies confirm that fatty acids added to low-sulphur diesel fuel components in a concentration range of 50-1000 ppm produced excellent antiwear properties (Tables 8-10).
    Fatty acids of higher molecular weight, starting from lauric acid (C12), appear to be superior friction-reducing agents. A wide variety of early systematic studies [11, 13, 14] indicate that friction is reduced by the existence of a thin layer of a fatty acid material different from that of the contacting surfaces and which adheres strongly to the solid surface. Depending on the substrate surface, physical adsorption or chemisorption or both play an important part in friction-reducing film formation. At this point a question arises: what is the relation between friction and wear? According to Bowden and Tabor,13 although friction shows a strong correlation with wear, there is no simple general relation between the two. The fact that low friction does not necessarily mean low wear
    Table 14 Wear data for model and real base fluids containing palmitic acid in the concentration range 0-1000 ppm (pin-on-disc apparatus)
    Base fluidPalmitic acid concentration, ppmWear-scar diameter, mRelative ball wear %
    Average valueStandard deviation
    1 -Methylnaphthalene038513100.0
    502792227.4
    100208378.4
    500215169.6
    75019276.1
    100017494.2
    Mixture of 1-methylnaphthalene and n-hexadecane034621100.0
    502684736.0
    100178117.0
    50017877.0
    7502554229.5
    10002414323.4
    PA04028135100.0
    502523764.9
    100181917.0
    5001994125.2
    7502055928.1
    10001925621.7
    PA06021316100.0
    501943968.1
    1001632134.0
    5001722242.0
    750175845.9
    1000174743.9
    Saturated hydrocarbon mixture from SN 400021223100.0
    5023217145.2
    10023814159.3
    5002051688.3
    7501902665.4
    10001561029.6
    Mineral base oil SN 400022218100.0
    5026410202.3
    10027018219.0
    5002151388.7
    75022514107.0
    10001671532.4

    Table 15 Influence of oxygen-containing compounds on tribological properties of hydrotreated diesel fuel [4]
    FluidOxygen from added compound, ppmWear-scar diameter, m
    Hydrotreated diesel fuel0350
    Solution of 8-hydroxyquinoline in hydrotreated diesel fuel10320
    Solution of 8-hydroxyquinoline in hydrotreated diesel fuel100180
    Solution of 1,4-hydroquinoline in hydrotreated diesel fuel100220
    Solution of caproic acid in hydrotreated diesel fuel100230
    Solution of palmitic acid in hydrotreated diesel fuel1330
    Solution of palmitic acid in hydrotreated diesel fuel10220
    Solution of palmitic acid in hydrotreated diesel fuel100180
    Solution of palmitic acid in hydrotreated diesel fuel300170

    was emphasised over fifty years ago [19]. However, this fact is often overlooked and it has also contributed somehow to the lack of systematic and detailed studies on the influence of fatty acids on the wear of tribological mating elements. It has been shown, however, that although there is no simple, universally applicable relation between friction and wear, for specific systems, where effects such as ploughing and corrosion are negligible, a relation can be established [20]. It has been shown (Figure 2) that there is a correlation between wear and film thickness for capric, lauric, palmitic, stearic, and behenic acids (C10C22) fatty acids). For caproic acid, in spite of a thinner film, the corrected wear-scar diameter is relatively small. For all fatty acids tested, the film thickness is greater than for neat diesel fuel components. Figure 8 shows wear and friction coefficient against number of carbon atoms and it can be seen that there is no correlation between wear and friction coefficient for C6C22 fatty acids.

    In our opinion, two points seem to be of importance:
    • a kind of molecular interaction exists between the fatty acid additive and the chemistry of the base fluids
    • the viscosity of the base fluids seems to be a factor in the boundary lubrication process.
    Figure 8: Tribological wear and friction coefficient in systems lubricated with 500 ppm solutions of C6-C22 fatty acids in the HON component (HFR2 apparatus)

    Our previous research [21], in which n-hexadecane was used as the base fluid, indicated a new phenomenon concerning a very specific prowear and antiwear behaviour of fatty acids in the 50-1000 ppm concentration range. For this specific behaviour of fatty acids, two kinds of effects could be distinguished.
    • a clear corrosive effect at a concentration of 750 ppm of myristic acid (C14), palmitic acid (C16), and stearic acid (C18)
    • a distinct antiwear effect at a concentration of 500 ppm of caproic acid (C6).
    Results concerning 1-methylnaphthalene and its mixture with n-hexadecane (1:1 by volume) confirm that at a 100 ppm concentration of palmitic acid, a significant wear reduction of the steel-on-steel system can be achieved. For the mixture, the low-wear region extends to 500 ppm. At a 750 ppm concentration a maximum wear value appears (Figure 6); however, the maximum is located in the low-wear region (relative wear around 30%). In the case of 1-methylnaphthalene the increasing acid concentration slightly decreases the wear. Figure 7 demonstrates that for the synthetic base fluids, a clear minimum wear value exists at a 100 ppm concentration of palmitic acid; however, at a 750 ppm concentration of the acid, only a slight maximum is observed. In comparison with the wear data for n-hexadecane-based lubricants, both PAO 4- and PAO 6-based lubricants have these maxima at a relative wear reduction below 50%. Surprisingly, it was found that the addition of 50 and 100 ppm of palmitic acid to SN 400-saturated hydrocarbons increases wear by around 50% (Table 14). The same is true for the mineral base oil SN 400. However, in this case the corrosive effect is even larger; the wear is increased by a factor of two. When the concentration of palmitic acid was increased to 1000 ppm, the antiwear behaviour of both oils was improved (relative wear around 30%). The viscosity can influence the wear behaviour of fatty acids used at the lowest concentrations. For polyalphaolefin base oils, an increase in viscosity decreases wear reduction at all palmitic acid concentrations of 100-1000 ppm (Figure 7).
    All the above-discussed findings are challenging from the viewpoint of their interpretation as well as from their relation to the lubricating oil formulation. The results obtained evidently confirm the complexity of the boundary lubrication approach.

    CONCLUSIONS

    The results of the influence of 50-1000 ppm C6-C22 fatty acid solutions in diesel fuel components, their fractions, and other real and model solvents obtained from tribological tests carried out using a pin-on-disc tester and HFR2 apparatus have been described and discussed.
    One of the most interesting conclusions is that fatty acids added to low-sulphur diesel fuel components in the concentration range 50-1000 ppm produce excellent antiwear properties.
    Interestingly, tribological wear does not decrease with increased concentration of any fatty acid in a diesel fuel component. It was also found that the fatty acid chain length was not a controlling factor in the wear process for either the pin-on-disc apparatus or the HFR2 apparatus.
    Palmitic acid solutions in real and model solvents show that the antiwear behaviour of fatty acids is the result of complex interactions between molecules of the acid and particular types of hydrocarbons. This has been confirmed by results obtained for palmitic acid solutions ofn-hexadecane, 1-methylnaphthalene, and mixtures of these hydrocarbons.
    The antiwear behaviour of fatty acids under boundary lubrication conditions is influenced by the rate of acid molecule diffusion to the friction surface. This conclusion is drawn from results of experiments obtained for systems lubricated with palmitic acid solutions in synthetic oils (polyalphaolefins) of various viscosities. It has also been found that in some cases the structure of the solvent molecule has a predominant influence on wear.

    References

    1. Spikes, H.A., and Wei, D.-P., 'Fuel lubricity - fundamentals and review', Proc. 1st International Colloquium on Fuels, Technische Akademie Esslingen, Germany, 1997, pp. 249-58.
    2. Spikes, H.A., and Cameron, A., 'A comparison of adsorption and boundary lubricant failure', Proc. Roy. Soc. London Ser. A, 336 (1974) 407-19.
    3. Onion, G., and Suppeah, J.K., 'The generation and properties of boundary films formed by diesel fuel at a steel/steel conjunction', Trib. Int., 17 (1984) 277-87.
    4. Wei, D., and Spikes, H.A., 'The lubricity ofdiesel fuels'. Wear, 111 (1986) 217-35.
    5. Nikanjam, M.. and Burk, E., 'Diesel fuel additive study', SAE paper 942014, 1994.
    6. Wang, J.C., and Cusamo, C.M., 'Predicting lubricity of low sulfur diesel fuels', SAE paper 952564, 1995.
    7. Lacey, P.I., and Westbrook, S.R., 'Diesel fuel lubricity', SAE paper 950248, 1995.
    8. Mitchell, K.J., 'Diesel fuel lubricity - a survey of 1994/95 Canadian winter diesel fuels', SAE paper 961181, 1996.
    9. Wei, D.-P, and Spikes, H.A., 'Fuel lubricity - fundamentals and review'. Fuels International, 1, 1 (2000) 45-65.
    10. Hardy, W.B., Collected Works, Cambridge University Press, Cambridge, UK, 1936.
    11. Dorrison, A., and Ludema, K.C., Mechanics and Chemistry in Lubrication, Tribology Series 9. Elsevier, Amsterdam, 1985.
    12. Campbell, W.E., 'Boundary lubrication', in Boundary Lubrication - An Appraisal of World Literature, ed. F.F. Ling, E.E. Klaus, and R.S. Fein, ASME, New York, 1969, pp. 87-117.
    13. Bowden, P.P., and Tabor, D., Friction and Lubrication of Solids, Clarendon Press, Oxford, 1950.
    14. Jahanmir, S., 'Chain length effect in boundary lubrication'. Wear, 102 (1985) 331-49.
    15. Bellzer, M., and Jahanmir, S., 'An adsorption model for friction in boundary lubrication', ASLE Trans. 29, 3 (1986) 423-30.
    16. Wachal, A., 'The influence of chemical structure of additives on antiseizure and antiwear properties of oils', Tribologia, 8, 5-6 (1977) 161-3 (in Polish).
    17. Kajdas, C., Luczkiewicz, J., Ozimina, D., and Wawak-Pardyka, E., 'The influence of concentration of alcohols and acids on tribological properties of paraffin oil', Tribologia, 11, 6 (1980) 22 (in Polish).
    18. Wills, J.G., Lubrication Fundamentals, Marcel Dekker, New York, 1980.
    19. Larsen, R.G., and Perry, G.L., 'Investigation of friction and wear under quasi-hydrodynamic conditions', Trans. ASME, 67 (1945) 45-50.
    20. Rabinowicz, E., 'The friction on boundary-lubricated surface', in Proc. 2nd Nat. Cong. on Applied Mechanics, 1954, pp. 595-9.
    21. Kajdas, C., Majzner, M., and Konopka, M., 'The effect of fatty acids at low concentrations in n-hexadecane on wear of steel in boundary lubrication', Tribologia, 153, 3 (1997) 221-37 (in Polish).

    A revised version of a paper first presented at the 2nd International Colloquium on Fuels, Technische Akademie Esslingen, Germany.