Jurnal Tribologi 5 (2015) 1-11
Received 12 March 2015; received in revised form 5 May 2015; accepted 28 May 2015.
To cite this article: Binfa et al. (2015). Comparison of lubricant properties of Castor oil and commercial engine oil.
Jurnal Tribologi 5, pp.1-10.
Comparison of lubricant properties of castor oil and commercial engine oil
Binfa Bongfaa,*, Peter A. Atabora, Atuci Barnabasb, M.O. Adeotic
a
Mechanical Engineering Department, Federal Polytechnic, PMB 1037, Idah, Nigeria.
Mubeco Petroleum Company Limited, Kaduna, Nigeria.
c
Mechanical Engineering Department, Federal Polytechnic, PMB 55, Bida, Nigeria.
*
Corresponding author: [email protected]
b
HIGHLIGHTS
 Unrefined castor oil possesses better "oiliness" and higher weld load, than a formulated foreign
commercial 20W-50 crankcase oil.
 Vegetable oils will require a strong antiwear agent to protect against wear under tarrying load.
 Castor oil can reduce friction and improve fuel conservation and mechanical efficiency in engines
more than the commercial mineral-based oil.
ABSTRACT
The tribological performance of crude Nigeria-based castor oil has been investigated and compared with
that of a foreign, 20W-50 high quality crankcase oil, to see its suitability as base oil for lubricating oils in
indigenous vehicle and power plants engines. The experiment was conducted using a four ball tester. The
results showed that unrefined castor oil has superior friction reduction and load bearing capability in an
unformulated form than the commercial oil; can compete favorably with the commercial oil in wear
protection when formulated with suitable antiwear agent, hence can be a good alternative base stock for
crankcase oils suitable for Nigeria serviced vehicles, and plants engines from tribological, environmental,
and non-food competitive points of view.
Keywords:
| Unrefined castor oil | Engine oil | Antiwear | Extreme pressure | Friction reduction |
© 2015 Malaysian Tribology Society (MYTRIBOS). All rights reserved.
Jurnal Tribologi 5 (2015) 1-11
1.0
INTRODUCTION
Adequate protection of contacting surfaces in a tribosystem against wear and
scuffing are key requirements for the selection of a lubricating oil, as well as base oil for
formulating a lubricating oil that is suitable for any tribological design. To meet these
requirements, most mineral based lubricating oils formulators involve reasonable dose of
heavy metal, sulphur and phosphorus additives compounds such as zinc
dialkyldithiophosphate (ZDDP) (Gangopadhyay et al., 2008; Morina and Neville, 2007),
and the like, which aggravate the environmental hazardous nature of the formulated oils
(Bartz, 2006; McDonald, 2003).
Giving the level of indigenous technology in Nigeria, locally fabricated
tribological components for internal combustion engines such as piston rings, and journal
bearings are of assorted specifications in terms of dimensional tolerance, material
compositions, strength, surface textures and so on, as against imported foreign
technology. These indigenous products enjoy more patronage by users and maintenance
shops for vehicles and plants because of their readily lower costs. The products however,
may be short of tribological performance specifications of the original imported rightly
specified parts which are used to replace worn out ones during repairs. These often result
in higher fuel energy losses due to higher friction generation, high deposit contaminants
in sumps and pollution of exhaust gas emission (from contamination of fuel with
lubricating oils and its products of combustion) owing to increased wears of the rings,
valve trains, and bearings, and possible resultant increased breakdown of engines. The
salvation of engines from these losses and failures, and the environment from pollution
can only come from high tribological performing and environmentally friendly crankcase
oils. Premium oils from indigenous as well as foreign manufacturers could offer marginal
solution to the tribological challenges, but at exorbitant prices, and not so much to
environmental issues. The challenge is getting alternative crankcase oil source(s) that can
proffer solutions to all the attendant problems.
Vegetable oils for some decades had been identified to be environmentally benign
lubricant base stocks (Baumgart et al., 2010), having some attractive lubricating
properties in addition to their non-toxic composition, wholesome biodegradable qualities,
and renewability (Baumgart et al., 2010; Salih et al., 2013). One of such advantageous
properties is high lubricity, due to the atoms of oxygen present in the ester molecules,
causing the molecules to form a monolayer over the metal surfaces (Silva et al., 2013).
The attachment of these ester molecules are oriented with hydrocarbon chains almost
perpendicular, while the adherent to the metal surfaces is by the polar heads (Ajithkumar,
2009). The attachments are so strong that they are not easily eroded by water, mechanical
and or thermal stresses.
The competition noticed between the food sectors and industrial lubricants sectors,
as vegetable oils are being engaged for industrial lubricants usage, have prompted the
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Jurnal Tribologi 5 (2015) 1-11
search for non-edible oils as suitable means of resolving the problem. Castor oil, a nonedible vegetable oil produced from the seeds of castor plant is one of the largely
investigated oils for industrial and automotive lubrication. Its high viscosity due to the
dominant ricinoleic acids, around 90% (Silva et al., 2013), suggests the possibility of its
application as engine oils. Although, the weakness of vegetable oils in terms of oxidation
and thermal stability have placed some limitations on their use as engine oils, they still
enjoy being engaged as full or components of total lost lubricants base stocks
(Cheenkachorn & Udornthep, 2006). Moreover, those limitations can be assuaged by the
introduction of the relevant additives. The world production of castor beans is around
1.4Mt. Africa, including Nigeria, has a contribution of around 0.08Mt (FernandezMartinez & Velasco, 2011). Castor plant is very productive in most of the agricultural
lands within Nigeria. The yield of castor seed in Nigeria is rated at between 950-1,500
kg/ha (Gana et al., 2014), and virtually every part of the country is suitable for castor
plantation. The average oil content of a castor seed on the basis of dry weight is 50%
(Brigham, 1993). This has attracted the investigation of the lubrication properties of this
oil.
In this work, the antiwear and extreme pressure performance of unrefined,
mechanically extracted castor oil, from a local extractor in Kishi village of Oyo state,
Nigeria is compared with those of high quality commercial engine oil from a foreign
manufacturer, using a four ball tester.
2.0
EXPERIMENTAL
2.1
Measurement of Friction and Wear
This test was carried out based on ASTM D4172 method. A steel ball
(comparable to EN 31, 64–66 HRC type), 12.7mm in diameter is thoroughly washed
using acetone, wipe dried using industrial tissue paper, mounted on the motor spindle of a
four ball tester, and pressed into the cavity of three balls (of the same material, cleaned by
the same procedure) clamped in a ball cup filled with lubricant to at least 3mm above the
three balls. The ball in the spindle, refer to as the rotating ball normally makes a point
contact with each of the three balls in this arrangement (Figure 1). The setting was loaded
to 392 N - static load (indicated by the controller), then the lubricating oil heated to 75°C
using an inbuilt heating device, after which the motor spindle was set rotating at 1200 ±
60 rpm for 60 minutes, 10 seconds. The loading, temperature, speed and time were set on
a controller, interfacing the four ball machine and a Winducom 2010 software installed
on a PC, for the purpose of extracting the experimental data. After the 60 minutes 10
seconds, the three lower balls were removed, cleaned with acetone and the scar diameters
made on them owing to friction between their contacting surfaces with the top rotating
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Jurnal Tribologi 5 (2015) 1-11
ball were measured using a metallurgical microscope. All relevant data from the software
and microscope were recorded and analyzed.
2.2
Measurement of Extreme Pressure
The extreme pressure load for the experimented oils was determined based on IP 239
method, similar to ASTM D 2783. The procedure is the same as in ASTM D4172(ASTM,
2010), except that the speed, the time, the temperature in this case were 1760 ± 60 rpm,
10 seconds, and 32 ± 5°C respectively, and the experiment was performed for a load
of491N. This procedure was repeated with higher loads in a logarithmic increment as in
IP 239 (618N, 785N, 981N, 1030N, 1236N, 1373N …), until welding occurred. The
minimum load at which the four balls get welded together was reported as the weld load
or load bearing capacity of the oil sample.
Figure 1: Scheme of the four ball arrangement
3.0
RESULTS AND DISCUSSION
3.1
Friction and Wear
The molecular structure of the oil and the thin lubricating film formation ability
on the surfaces of contacting mechanical components, determine the friction reduction
efficiency of the oil. The adherence of the polar heads and consequent formation of
monolayer by vegetable oil molecules, the near vertical orientation of their hydrocarbon
chains to contacting metal surfaces, generally have been discussed by biobased lubricant
researchers as the bases of their superiority over mineral oils of similar viscosities, as far
as lubricity is concerned. Figure 2, clearly confirmed these concepts, as it can be seen that
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Jurnal Tribologi 5 (2015) 1-11
CoF
the friction reduction ability of castor oil is quite better than that of the formulated
commercial mineral oil. The lower coefficient of friction demonstrated by castor oil
means that it has superior "oiliness" and is able to adhere to surfaces of high temperature
components of high performance engines (Gana et al., 2013). This good lubrication
property can also be due to high presence of free fatty acids (Baumgart et al., 2010). If
this oil is used as an engine oil, it will reduce energy lost due to friction more than the
referenced commercial oil, giving such an engine lower fuel consumption, higher
mechanical power output and efficiency (Tung & McMillan, 2004).
Although, vegetable oils have such good friction reduction abilities, their
protection of asperities and the surfaces in contact within a tribological system is limited,
especially under tarrying load condition. The protection offered to surfaces against wear
by the natural oil is not up to that of the mineral oil because the mineral oil is supported
by strong antiwear additives that could form two layers, physical layer (physiosorbed),
and chemical layer (chemosorbed). After the physiosorbed layer is worn out, the
chemosorbed layer remains, and it is not easily eroded from the contacting surfaces by
mechanical shear even under high load condition (Waara et al., 2001), so making the
commercial oil to permit lower occurrence of wear scar on the contacting surfaces of the
balls than the natural oil. Remembering that the wear test was conducted at a high
temperature of 75°C, the lower wear protection of the natural oil in the tribosystem
confirmed the fact that, though, their molecules have good adherence to high temperature
components, their antiwear properties are sensitive to high temperatures (Ajithkumar,
2009). It is however, very possible for the castor oil to compete favourably with the
commercial mineral-based oil when treated with a suitable antiwear supplement.
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0.0763
0.0607
Castor oil
Engine oil
Lubricant type
Figure 2: Coefficient of frictions of the investigated oils under wear test
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Jurnal Tribologi 5 (2015) 1-11
0.8
0.7
0.68625
WSD (mm)
0.6
0.5
0.40322
0.4
0.3
0.2
0.1
0
Castor oil
Engine oil
Lubricant type
Figure 3: Average wear scar diameters of steel balls in the oils
Figure 4(a): Morphology of the three worn balls in commercial oil (scale of 100 µm)
Figure 4(b): Morphology of the three worn balls in castor oil (scale of 100µm)
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Jurnal Tribologi 5 (2015) 1-11
The morphology of the surfaces of the three lower balls in each of the oils
examined under high magnification factor (500×) of the microscope are shown in Figures
4 (a) and 4(b). Figure 4(a) shows the worn surfaces in the mineral engine oil. The
surfaces have more uniform wear. This should be due to the effectiveness of the antiwear
additives. The surfaces revealed the strength of the tribochemical reaction between the
additives and the surfaces of the balls which made the surfaces very resistant to
mechanical shear and abrasive wear. The furrows on the surfaces of the balls in castor oil
(Figure 4b) are indications of the absence of antiwear additives. The bonding between the
oil molecules and the ball surfaces are more of physical with weaker chemical layer,
which is not a very good arrangement for this tribosystem, being more a boundary
lubrication system. Once the physical bonding is sheared by constantly attacking
mechanical shearing due to the top rotating ball, the chemical bonding are soon overcome
and the direct metal-to-metal contact are ensured. This results in the tearing of particles
from each surface. The torn out particles will in-turn aggravate the wear by acting as
abrasive substances in the system. That informed the nature of the deep groves on the
scarred surfaces of the balls. The uniformity of the groves showed how uniform the
vegetable oil molecules oriented themselves on the contacting surfaces of the balls.
3.2
Extreme Pressure (EP)
The CoF data plotted in Figure 5 shows that castor oil has better lubricity
performance for almost all the regime of loading under EP test. Even at higher loads, it
has manifestly demonstrated lower CoF than the commercial mineral-based formulated
oil. This proves why wax ester extracts from vegetable oils are proposed for used as
friction modifiers in industrial and automotive lubricants (Bart et al., 2012).
From Figure 6, point A to point B represents the compensation line of the
tribosystem with engine oil as the lubricating oil, where the loading on the lower three
balls neither cause seizure nor welding. Point B is the last non-seizure load. The average
wear scar diameters within the stated load regime in engine oil are quite interestingly low.
B to C is incipient seizure region, where the lubricating film experiences a temporal
break-down. Point C to D is region with immediate seizure, indicating increasing wear
scar, leading to the welding of the four balls together at point E (ASTM, 2009) under the
lubrication of the commercial oil.
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Jurnal Tribologi 5 (2015) 1-11
1.8
Castor oil
Engine oil
1.6
1.4
CoF
1.2
1
0.8
0.6
0.4
0.2
0
0
500
1000
1500
Load (N)
2000
2500
Figure 5: Coefficient of friction (CoF) against Load (N) in EP test
Figure 6: Average Wear Scar Diameter verses the Applied Load
The corresponding regions and locations with Castor oil as the lubricant in the
four ball cup tribo-setting are Q to R (compensation line), R (non-seizure load point), R
to S (incipient seizure region), S to T (immediate seizure region), and U as the weld
point. Points E and U have indeterminate average wear scars diameters, as the four balls
were actually welded together in each case. The immediate seizure region in castor oil is
quite longer than in engine oil, implying a more durable resistance to wear at higher load.
The attachment of the polar heads of the hydrocarbon chains of these oil molecules to the
contacting surfaces of the balls were so strong for the intermediate mechanical loads to
brush them off. Moreover, the hydrocarbon chains must be the long type, which immune
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Jurnal Tribologi 5 (2015) 1-11
the surfaces from severe wear at these loads contrary to the case in engine oil. Castor oil
demonstrated a higher weld load (2158N) compare to engine oil (1766N). In the internal
combustion engine, the lubrication regime is hydrodynamic at the thrust and journal
bearings, where metal-to-metal contacts are rear, except with low-viscosity lubricating
oils, and at high loads and low speeds, but mixed and boundary lubrication take place at
the piston rings assembly and the valve trains (Tung & McMillan, 2004). These may
suggest that extreme pressure is not a very critical subject as friction and wear. Hence,
the oil may just need to be sufficiently equipped against friction and wear. This may be
the reason for the quite lower EP load of the commercial oil. However, with the modern
engines, very high loading could occur during post firing expansion strokes, making it
possible to have at the piston rings, elastohydrodynamic lubrication regime (Priest &
Taylor, 2000; Tung & McMillan, 2004; Xu, 1999). Moreover, the prediction of minimum
film thickness in today’s passengers cars engine bearings is 0.5 to 1.0 µm, meaning that
for some positions of engine cycle, there are possibilities of metal-to-metal contacts
between the bearing and the journal, and the cam and followers(Priest & Taylor, 2000;
Tung & McMillan, 2004). Putting these together, and considering the lower quality spare
parts, in terms of materials and dimensional tolerance, we have in our country, welding of
components due to excessive wears in internal combustion engines can be rampant.
Giving a serious consideration to weld load in our crankcase lubricant selection and or
formulation is not and overdesign. This will give preference to castor oil which had
demonstrated higher weld load. The picture of the welded steel balls in the two lubricants
are shown in Figure 7.
Figure 7: Welded balls in the experimented oils
CONCLUSION
The superior tribological performance of castor oil to the foreign high quality
crankcase oil shows that, if other properties of the oil are improved on to make it meet the
requirements for internal combustion engines, it can be a better lubricating oil than the
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Jurnal Tribologi 5 (2015) 1-11
mineral oil-based crankcase oils in the market. Castor oil as engine oil base stock,
blended with effective antiwear/extreme pressure additive(s) and friction modifier(s) will
significantly reduce the lost energy of the fuel due to friction, thereby combating the low
mechanical efficiency, being one of the drawbacks in internal combustion engines. This
will surely be an environmentally benign lubricating oil and its waste products, and a
potential cost effective product, suitable as crankcase oil for vehicles and small-scale
power generating plants use in Nigeria.
ACKNOWLEDGEMENT
The authors wish to thank the staff of National Cereals Research Institute, P.M.B.
8 Bida, Niger State, Nigeria, for their kind assistance towards this work.
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