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Effect of gasoline–bioethanol blends on the
properties and lubrication characteristics of
commercial engine oil
L. S. Khuong,*
a
H. H. Masjuki,*
a
N. W. M. Zulkifli,
a
E. Niza Mohamad,
a
M. A. Kalam,
a
Abdullah Alabdulkarem,
b
A. Arslan,
a
M. H. Mosarof,
a
A. Z. Syahir
a
and M. Jamshaid
a
Concerns over depleting fossil fuel reserves, energy security, and climate change have resulted in
stringent legislation demanding that automobiles use more renewable fuels. Bioethanol is being given
significant attention on a global scale and is being considered as a long-term gasoline replacement
that helps reduce exhaust emissions. The piston ring and cylinder wall interface is generally the largest
contributor to engine friction and these regions of the engine also suffer the highest levels of fuel
dilution into the lubricant from unburned fuel, especially for bioethanol as it has a high heat of
vaporization, which enhances the tendency of the fuel to enter the oil sump. As bioethanol is being
blended with gasoline at increasingly higher concentrations and the accumulation of fuel in the
crankcase is significant, it is crucial to study the effect of various bioethanol blends on the degradation
of engine oil's properties and the friction and wear characteristics of engine oil. A fully synthetic oil
was homogenously mixed with five formulated fuels such as gasoline blend (E0), gasoline–10%
ethanol (E10), gasoline–20% ethanol (E20), gasoline–30% ethanol (E30), and gasoline–85% ethanol
(E85). These mixtures were then tested in a four-ball wear tester according to the ASTM D4172
standard test. Under selected operating conditions, the results show that the addition of a gasoline–
bioethanol blend decreases the oil viscosity, whereas the acid number increases because bioethanol is
more reactive compared to gasoline, which enhances oil degradation and oxidation. Fuel dilution
reduces the lubricating efficiency and the wear protection of the engine oil. All fuel-diluted oil samples
have higher friction and wear losses, compared to the fresh synthetic oil. E10 has slight effects on the
friction and wear behaviors of the engine oil. Thus, it still has a high potential to be widely used as
a transportation fuel for existing gasoline engines.
1. Introduction
Fossil fuels such as diesel and gasoline which are non-
renewable energy resources, are being depleted day by day. It
is believed that these conventional fuels will become scarce
during next several decades.
1,2
To reduce the concern over this
undesirable issue, researchers have been putting in much effort
to reduce the use of fossil fuels by using alternative biofuels, in
particular biodiesel and bioethanol fuels, which are being
blended with diesel and gasoline at increasingly higher
concentrations. USA is now the leading producer of bioethanol
in the world, whereby 18.3 billion liters of bioethanol was
produced by the USA in 2006.
3
Nowadays most vehicles are
fuelled with 10–15% ethanol, and the so-called ‘biofuel’engines
are typically designed to run on pure hydrated ethanol (93%
ethanol, 7% water) or E85 blends which are made up of 85%
ethanol and 15% gasoline. Interestingly, in Brazil, ex fuel
vehicles (FFV) have been manufactured to use with any bio-
ethanol–gasoline blends.
4
This can enhance energy indepen-
dence whilst combating fundamental issues such as fossil
depletion, oil price escalation, carbon emission, and particulate
mass concentrations in the vehicle exhaust.
5,6
According to the
National Association of Automotive Vehicle Manufacturers,
more than 85% of ex-fuel vehicles are manufactured in Brazil.
7
Both Sweden and Belgium aim to increase the use of bioethanol
as a transportation fuel.
8,9
More importantly, ethanol has
become one of the alternative fuels of interest in Sweden ever
since the 1990s. The annual use of ethanol was 65 000 m
3
in
2001. About 3% of the public service bus eet (more than 400
buses) use ethanol as fuel, and about 4450 ex-fuel vehicles run
on E85 blends.
10
However, the bioethanol employment also poses some
undesirable issues. Firstly, bioethanol is miscible with water
which can cause a corrosive effect on engine components
a
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya,
50603 Kuala Lumpur, Malaysia. E-mail: sokhuongum@gmail.com; masjuki@um.edu.
my; Fax: +60 3 79675317; Tel: +60 173238647
b
Mechanical Engineering Department, College of Engineering, King Saud University,
11421 Riyadh, Saudi Arabia
Cite this: RSC Adv.,2017,7, 15005
Received 10th January 2017
Accepted 27th February 2017
DOI: 10.1039/c7ra00357a
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such as fuel injector and electric fuel pump.
4,11
Moreover,
bioethanol attracts more water from the environment
because it is hygroscopic in nature.
12
Secondly, vehicle start-
up problem can happen in cold weather when the engine is
fuelled with pure ethanol which is hard to vaporize.
13
Thirdly,
when bioethanol is used to fuel the engine, the tribological
effect on lubricant properties and performance resulting
from fuel dilution always appears. During the combustion
process, some amount of unburned fuel will impinge on the
cold wall of the combustion chamber and then be scrapped
into the crankcase of engine oil through cylinder liner.
14
It
shall be notied that the impact of bioethanol on lubricating
oil's properties and performance is completely different from
that of gasoline due to the fact that bioethanol has a higher
tendency to enter the oil sump of an engine due to its high
heat of evaporation compared to gasoline.
11
The amount of
bioethanol inside lubricant can degrade the properties and
performance of engine oil signicantly. As mentioned earlier,
bioethanol is miscible with water but immiscible with oil, so
that there would be the formation of emulsions inside bio-
ethanol–water–oil mixture, which leads to serious engine
wear and catastrophic engine failure.
15
As a consequence,
engine oil needs to be drained on a frequent basis. It has been
observed that even a small amount of fuel dilution is possible
to degrade the physicochemical properties of lubricant
(viscosity, total base number, and total acid number) which
play an important role in the lubricating system.
16,17
In gasoline engines, various sliding components produce
more friction between metal-to-metal contact surfaces,
especially the piston assembly consisting of the piston ring
and cylinder liner which contribute to engine friction to
approximately 40–60%.
18
This friction, in turn, affects engine
durability as well as fuel consumption. Therefore, various
methods are used to improve the performance of automotive
enginessuchascoatings,
19
laser texturing,
20–23
reducing the
weight of components and modifying the composition of
automotive lubricants.
24
Thus engine oil having high lubricity
is needed to reduce friction and wear in order to extend
engine life. Synthetic oil is widely used as lubricating oil in
most vehicle engines due to its high thermal and oxidative
stability, alkalinity, and viscosity index. It has a strong lm
strength which can help improve its frictions behavior in long
operation.
25–27
1.1. Objective of this study
Although bioethanol has been widely used as an alternative
fuel, it is deemed important to look at its effects on the
tribological characteristics of engine oil. This current work
aims to investigate the oil degradation, and lubricating effi-
ciency (friction and wear losses) of fully-synthetic oil diluted
with various bioethanol–gasoline blends. It is believed that
this paper will provide useful insights of the current work
pertaining to fuel dilution so that practitioners and
researchers can select the optimum gasoline–bioethanol
blend which has a slight lubricating effect on the engine and
engine oil.
2. Literature review
The study of bioethanol dilution is limited and its effect on
lubricating oil is not clearly established. However, there is
some literature in this eld as described in Table 1. Mostly, it
has been observed that the use of ethanol with fossil fuel can
cause a signicant effect on automotive lubricant's properties
such a drop in engine oil viscosity, and total base number
(TBN), as well as an increase in total acid number (TAN), and
an effect on oil performance such as an increase in friction,
wear losses and deposit formation resulting from oxidation
and corrosion of bioethanol.
28
The addition of bioethanol to
gasoline enhances the tendency of the fuel to reach the
crankcase of engine oil due to its higher heat of vaporization,
thus increasing the rate of fuel dilution.
11,29
Signicant
amounts of diluted ethanol fuel, in particular, E85 (between
6% and 25%) have been found in the engine oil crankcase aer
eld tests
12
and bench sequence tests.
11,30
Astudyoffuel
dilutionwasdonebyT.Huet al.
31
They explained that turbo-
charged gasoline direct-injection (TGDI) engine operated at
high torque had a signicant inuence on fuel dilution (gasoline)
which was found to be up to 9%. This percentage caused
a decrease in oil viscosity, an increase in fuel consumption and
the formation of carbon on the piston ring area. Tippayawong
and Sooksarn
32
investigated into the lubricating performance of
lubricant diluted with ethanol and gasoline. They prepared four
combinations of fuels and lubricants to be tested with motor-
cycle engine: (1) gasoline and mineral oil, (2) gasoline and
synthetic oil, (3) 5% ethanol and mineral oil and (4) 5% ethanol
and synthetic oil. The results indicated that viscosity of all oil
samples decreased with operating time. Aer 3000 km mileage,
the viscosity changed about 20% and 45% for synthetic-based
and mineral-based oil, respectively. Mineral oil produced larger
scar areas, compared to synthetic oil. For the same type of engine
oil, the wear scar areas from the gasohol-run engine were about
10% larger than those from the gasoline-run engine. It also
explains that lubricant properties changed with operating time
because of additive depletion, oxidation, thermal degradation,
reaction with sliding surface and engine blow-by contamination.
Another experiment on friction and wear characteristics was
conducted by Ajayi et al.
33
They diluted lubricant with three
different fuels: gasoline (E0), gasoline–10% ethanol (E10), gaso-
line–16% isobutanol (i-B16) and used with a marine machine
which was operated under start-and-stop and on-water condi-
tions. It was shown that ethanol showed the least amount of fuel
dilution due to its high evaporation while i-B16 showed the
highest rate of fuel dilution. It is also notied that high engine
speed enhanced fuel dilution, leading to a signicant decrease in
oil viscosity as well as producing more wear. However, friction
was slightly reduced probably due to the drop in viscosity. Costa
and Spikes
17
investigated the effect of temperature on the lubri-
cating performance of fully-formulated oil contaminated with
5% of both anhydrous and hydrated ethanol fuels and a solution
of ZDDP antiwear additive. They found that this addition
destroyed the pre-formed antiwear tribolm and deteriorated the
rubbing surface.
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3. Methodology
3.1. Material used
Fully-synthetic oil (SO, SAE 5W40) was purchased from the
market in Malaysia. This lubricant is mostly used under the
most heavy-duty condition. Table 2 shows the properties of the
engine oil. Gasoline and pure bioethanol which were also
collected from Malaysian market were added to the fully-
synthetic oil. The properties of gasoline and bioethanol are
shown in Table 3.
3.2. Test sample preparation
Formulated fuels such as gasoline (E0), gasoline–10% v/v bio-
ethanol (E10), gasoline–20% v/v bioethanol (E20), gasoline–30%
v/v bioethanol (E30), and gasoline–85% v/v bioethanol (E85)
were blended at a xed volume of 100 ml using shaking
machine. The properties of each gasoline–bioethanol blends
were measured and shown in Table 4. Each fuel blend was then
added to fully-synthetic oil at the dilution rate of 6% v/v in order
to prepare fuel–oil samples for the test (i.e. E10–SO ¼10% v/v
bioethanol fuel + 90% v/v synthetic oil). 6% v/v concentration
is selected because the permissible dilution rate of fuel in
Table 1 Previous studies of lubricant diluted with fuel contaminant
Ref. Methods/material Conditions Results
32 Engine testing Mileage: 3000 km Viscosity falls up to 45%
Bench wear tester, mineral oil,
synthetic oil, gasoline/gasohol
Load: 33 N, speed: 500 rpm, Temp:
25 C
Gasohol engine has 10% higher
wear, mineral oil: high wear,
synthetic oil: wear increased,
friction (unknown)
33 Engine testing Load: 15 N Approx. 3.7–6% fuel dilution
Unidirectional sliding,
reciprocating sliding, four-ball wear
test, gasoline, ethanol, isobutanol,
SAE 10W-30
Velocity: 0.1–20.0 cm s
1
, speed: 10–
300 rpm, frequency: 0.1–5 Hz, four-
ball load: 15 kg, speed: 1200 rpm,
Temp: 70 C, time: 1 h
E0: viscosity 30–43% decreased,
E10: viscosity 25–42% decreased, i-
B16: viscosity 28–46% decreased, all
fuels have slight effect on friction,
wear protection of the oil reduced
with fuel dilution
15 Plint TE77 Duration: 2 h Friction reduction when tested with
Shell Helix HX7, SAE 5W-30, no
friction modier, ethanol 95%
purity, water
Liner T:70–110 C, test Temp: 25–
40 C, speed: 2000 rpm, load: 150 N
(4 MPa), Stroke length: 5 mm, cold-
start/short-journey
Separated phase of ethanol–water–
oil mixture, viscosity decreased,
implication: fuel economy in a red
gasoline engine
34 Tribotest (ball on discs) Duration: 20 min No effect on friction signicantly
Ball: AISI 52100, discs: AISI H13,
engine oil: SAE 5W30, E22, (E100,
7% water)
Test sample: 3 ml, Temp: 40 C,
load: 35 N, Stroke length: 5 mm,
frequency: 10 Hz, Max. speed: 0.159
ms
1
Aged oil showed FC value similar to
those obtained from base oil.
Viscosity and TAN analysis show
that E22 is more oxidized and may
cause oxidative wear
35 Ball on disk wear test Dilution rate: 5 wt% Tribolm thickness was reduced by
the addition of ethanol to both oils.
Anhydrous ethanol caused more
lm thickness reduction than
hydrated ethanol. Tribolm was
allowed to form during rubbing
using an ethanol-free oil. Hydrated
destroyed pre-formed antiwear
tribolm and rubbed surface
A sliding/rolling contact of ball and
disc, ball: AISI 52100 steel, disc: AISI
52100 steel, fully formulated oil
with 0.08 wt% ZDDP, hydrated
ethanol & anhydrous ethanol
Speed: 3500 mm s
1
, reduce 25
steps down to 7 mm s
1
, load: 31 N,
Hertz contact pressure: 0.95 GPa,
contact diameter: 250 mm, slide-to-
roll ratio: 50%
36 Modied Plint TE77 Cold-start: dilution (5 wt% ethanol
+ 8 wt% water)
Interaction between ethanol and
water contributes signicantly to
friction, independently. Ethanol
does not signicantly impact on
friction reduction. Interaction
(water & temperature) has a small
impact on friction reduction
Taguchi method, engine oil: SAE
5W30 Shell Helix HX7, ethanol: 95%
purity, distilled water, real piston
ring and cylinder liner
Temp: 25 C; load: 100 N, time: 2 h;
speed: 1500 rpm, warm-up: dilution
(10 wt% ethanol + 16 wt% water),
Temp: 40 C; load: 150 N, time: 2 h;
speed: 2000 rpm
37 Minitraction machine ball-on-disc
test, base oil (additive), fully
formulated oil without additive,
hydrated ethanol and anhydrous
ethanol
Dilution rate: 5 wt%, speed: 1000
mm s
1
, reduce 31 steps down to 1
mm s
1
, load: 20 N; Temp: 40 C,
70 C, 100 C, Hertz pressure: 0.82
GPa
At low speed, the addition of
ethanol produces a boundary lm,
which was not present in the base
oils, but the addition of ethanol to
formulated oil reduced lm
thickness in all lubrication regimes,
and ethanol reduces friction at
higher speed
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engine oil is limited to 4% for a gasoline engine. This study
aims to investigate the effect on lubrication performance of
engine oil at this 6% v/v dilution. To obtain well-blended test
samples, each fuel–oil sample was mixed using magnetic stirrer
at an appropriate volume of 100 ml. The shaker and stirrer were
continuously blending the fuel–oil mixtures for 30 min, at the
speed of 400 rpm and at ambient temperature.
3.3. Properties of fuel–oil blends
In order to observe oil degradation by fuel dilution, the physi-
cochemical properties of fuel diluted lubricant such as density,
viscosity, viscosity index (VI), acid number were measured by
using instruments mentioned in Table 5. All measurements of
the properties were done according to ASTM standard methods.
Table 6 indicates the properties of synthetic oil diluted with 6% v/
v of gasoline–bioethanol blends. Stabinger viscometer was used
to measure viscosity index, density at 15 C and kinematic
viscosity at 40 C and 100 C. The acid number were measured
using TAN analyzers. Flash temperature parameter was deter-
mined by using the formula mentioned in Section 3.6. The values
of ash temperature parameter (FTP) of each sample at 40 kg and
80 kg are also shown in Table 6.
3.4. Friction test and procedure
Fig. 1 illustrates the schematic diagram of the four-ball wear
tester. Four-ball wear tester (TR-30H) was used to investigate
friction and wear characteristics of fuel diluted synthetic oil.
This test is one of the ASTM standards (D4172) for evaluating
the performance of lubricant under relatively severe sliding
contact under boundary condition.
33
Referring to the Stribeck
curve, there are a few main sliding engine components (piston
ring/cylinder liner, valve train, and camsha) which mainly or
partly experience boundary lubrication regime.
38
The balls used
in this experiment were chrome alloy steel balls, AISI 52-100
standard, 64-66 Rc hardness, diameter (12.7 mm) and surface
roughness (0.1 mm, C.L.A: Center Line Average). The 52100 steel
Table 4 The physicochemical properties of bioethanol–gasoline
blended fuels
Properties Unit E10 E20 E30 E85
Density at 15 Ckgm
3
0.754 0.758 0.763 0.788
Kinematic viscosity at 15 Cmm
2
s
1
0.576 0.647 0.748 1.411
Kinematic viscosity at 20 Cmm
2
s
1
0.546 0.614 0.705 1.292
Table 5 List of equipment for fuel–oil properties measurement
Properties Equipment Manufacturer Accuracy
Density Stabinger viscometer Anton Paar, UK 0.1 kg m
3
Kinematic viscosity Stabinger viscometer Anton Paar, UK 0.1 mm
2
s
1
Viscosity index Stabinger viscometer Anton Paar, UK 0.1
Acid number TAN analyzer Metrohm 0.001 mg KOH per
g
Table 6 The physicochemical properties of each fuel–oil blend, NM ¼not measurable
Test sample
Viscosity (mm
2
s
1
)
VI
Density
(g cm
3
)15C
Acid number
(mg KOH per g)
FTP
40 C 100 C 40 kg 80 kg
SO only 82.03 13.2 171 0.8540 1.4 178.19 33.96
E0–SO 58.52 10.99 183 0.8517 2.11 155.00 33.10
E10–SO 56.63 10.86 182 0.8517 2.17 155.58 29.38
E20–SO 57.14 N.M. N.M. 0.8517 2.28 156.74 31.57
E30–SO 57.69 N.M. N.M. 0.8523 2.36 152.76 30.74
E85–SO 62.45 N.M. N.M. 0.8524 2.89 145.29 28.41
Table 3 The properties of gasoline and ethanol
Properties Units
Gasoline (RON
95) Ethanol
Density 15 Cgcm
3
0.750 0.799
Kinematic viscosity 15 Cmm
2
s
1
0.542 1.713
Kinematic viscosity 20 Cmm
2
s
1
0.529 1.478
Boiling point C35 78
Table 2 The physicochemical properties of fully-synthetic oil (SO, SAE
5W40)
Oil parameter Units Value ASTM method
Density 15 Ckgl
1
0.854 D4052
Viscosity 40 Cmm
2
s
1
82.03 D445
Viscosity 100 Cmm
2
s
1
13.2 D445
Viscosity index, VI —171 D2270
Total base number, TBN mg KOH per kg 9.1 D2896
Total acid number, TAN mg KOH per kg 1.4 D664
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is a common material used for internal combustion engine
(ICE) piston rings. To set up each test, the test balls were
cleaned by using toluene and wiped by using a neat tissue to
make them completely dry. Three test balls were placed into the
oil cup and a fuel–oil sample was poured into the cup until it
fully covered the balls. In this four-ball test, three balls in the oil
cup were kept stationary and another ball was rotating in
contact with these stationary balls. Four-ball machine was
performed under conditions (time: 60 min, temperature: 75 C,
load: 40 kg and 80 kg, and rotational speed: 1200 rpm). The
reason for choosing these conditions is due to the fact that the
parameters closely response to speed, load, and temperature of
the piston-ring assembly conditions.
39,40
The normal tempera-
ture of engine oil during warm-up and short-journey driving
condition is 40–80 C. Therefore, to prevent frictional heating
and to reduce the evaporation of bioethanol which boils off
quickly at the temperature above the boiling point (78 C), the
test was performed at 75 C. Aer completing the test, frictional
data, and the stationary balls were collected for friction coeffi-
cient and wear scar diameter (WSD) evaluations, respectively.
Aer collecting all the data, the test was started over again using
the same procedure.
3.5. Friction and wear evaluations
The friction coefficient (FC) can be evaluated from frictional
torque and spring constant.
41
In four-ball wear test, the lower
balls create the maximum torque which is determined by a load
cell. The eqn (1) is used to calculate the friction coefficient:
Friction coefficient ðmÞ¼ frictional torque ðkg mmÞ ffiffiffi
6
p
3applied load ðkgÞdistance ðmmÞ
¼Tffiffiffi
6
p
3Wr
(1)
where, T,W, and rcan be expressed as friction torque (kg
mm), applied load (kg) and distance (mm), respectively. The
distance (r) measured from the center of the lower ball contact
surface to the rotating axis is 3.67 mm.
According to the ASTM D4172 method, wear scar diameter
(WSD) produced on the three tested balls was measured by
optical microscope model C2000 (IKA, UK) with 0.01 mm
accuracy. To obtain a clear wear scar image, the magnifying lens
was adjusted to a better position on the wear scar surface of the
ball. Aer measuring the wear scar diameter of those three
balls, the arithmetic average result was calculated between
them.
3.6. Flash temperature parameter (FTP) calculation
Flash temperature parameter (FTP) is a number used to express
the critical temperature above which an engine oil will fail
under selected conditions.
42
The lubricating oil has failed at the
critical ash temperature. The FTP also refers to the possibility
of lubricant breakdown.
43
The higher the FTP indicates the less
possibility of lubricant lm to breakdown. This ash tempera-
ture can be measured by the thermocouple and expressed by
a solitary number of FTP. The discussion of FTP number
prediction model was discussed in ref. 44. For conditions
existing in four-ball machine, the FTP was calculated using the
following relationship:
45
FTP ¼applied load
ðwear scar diameterÞ1:4¼W
d1:4(2)
where, Wexpresses applied load (kg), dexpresses wear scar
diameter (mm), FTP is the maximum value of the ratio W/d
1.4
obtained from various run. It shall be notied that the FTP
(kg mm
1.4
) relationship is expressed as a number (maximum
value) in most previous studies
42,46
and therefore, the FTP
(kg mm
1.4
) is also considered as unit less in this study.
3.7. Surface analysis
The wear morphology of the tested ball was analyzed accordingly
using a scanning electron microscope (SEM) which is the most
known instrument for the surface analytical techniques. High-
resolution images of surface topography, with excellent depth
of eld, were taken using a highly-focused, scanning (primary)
electron beam. Three areas of wear scar surfaces were captured
and discussed respectively to prove friction and wear behaviors of
each fuel diluted synthetic oil.
3.8. Error analysis
Error analysis was calculated to determine the level of accuracy
and uncertainty of the collected data due to the fact that, during
the experiment, there might be errors arising from instrument
selection, testing condition, environmental condition, obser-
vation, calibration and reading and collecting data.
47
The test
was repeated three times, and the collected data was averaged
and performed through graph plotting and precision
measuring. The accuracy for WSD and FC are 0.01 mm and
0.05, respectively. The acceptable uncertainty is less than or
equal to 5%. Appendix (A) shows the statistical and error anal-
yses associated with each measurement and Appendix (B) shows
the summary of relative uncertainty and accuracy of this
Fig. 1 The schematic diagram of the four-ball wear machine [1:
stationary balls, 2: steel cup, 3: heating zone, 4: rotating arm, 5:
thermocouple, 6: torque arm].
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experiment. The overall uncertainty of this study was calculated
by using eqn (3).
Overall uncertainty ¼square root of hðuncertainty of FCÞ2
þðuncertainty of WSDÞ2i
OU ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
UFCL402þUFCL80 2þUWSDL40 2þUWSDL802
q
¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2:5012þ1:7732þ2:5562þ2:65692
p¼4:80%
(3)
where OU ¼overall uncertainty; UFC ¼uncertainty of FC;
UWSD ¼uncertainty of WSD.
4. Result and discussion
4.1. Viscosity
Viscosity is an essential property of automotive engine oil,
which should be high enough to resist the internal ow and low
enough to prevent substantial energy losses.
48
Viscosity is high
when used oil has been deteriorated by oxidation or by solid
contaminants while it is low when used oil is diluted with lower
viscosity oil or by fuel. Change in the engine oil viscosity as
a result of fuel dilution is undesirable in the lubrication system
of an engine as it inuences the lubricating efficiency and oil
lm thickness. Inadequate oil viscosity affects lubricating lm
and load bearing capacity leading to excessive wear of bearings,
journals and other moving components, low oil pressure and
poor oil economy.
49
Fig. 2 shows the degradation of the viscosity of synthetic oil
(SO) diluted with different types of fuel blends, measured at
40 C. It is investigated that the viscosity decreased signicantly
when synthetic oil was diluted with bioethanol blends. This is
due to the lower viscosity of the diluted fuels. The variations in
the viscosity of each diluted oil sample were almost identical.
Synthetic oil diluted with E85 fuel had slightly higher viscosity
compared to other fuel diluted oil samples. The viscosity of
E85–SO was dropped to 62.45 mm
2
s
1
from the base oil value of
82.03 mm
2
s
1
. E85–SO sample also contains a large amount of
ethanol, which may cause more interference with the formation
of the boundary layer as well as corrosive wear, leading to
a thinner boundary lm (low lubricating performance) and also
to higher friction, compared to fresh oil because the decrease in
viscosity may also cause more contact between the asperities of
the surface.
37
However, during full hydrodynamic lubrication,
the reduction in viscosity may contribute to a decrease in fric-
tion due to the fact that low viscosity plays a very important role
in reducing friction in this full lm lubrication regime. The
drop in viscosity indicates that the amount of fuel deteriorated
the lubricating efficiency of the oil. E0–SO had the viscosity of
58.52 mm
2
s
1
which was slightly higher than the viscosities of
E10–SO, E20–SO, and E30–SO. This indicates that oil contami-
nated with gasoline which is the hydrocarbon compound has
lower lubricity and undergo more severe degradation compared
to the oil contaminated with a small amount of bioethanol. In
Table 6, it can be seen that some viscosities of the oil samples
could not be determined at 100 C. This was due to the evapo-
ration of bioethanol at high temperature. It can be stated that
fuel dilution has a signicant effect on the oil viscosity. It can
result in two opposing effects: one benecial effect and one
detrimental effect. A decrease in viscosity is expected to lower
the viscous losses of the lubricating oil, which reduce the overall
engine friction and increase the efficiency of the engine.
However, in boundary lubrication, the difference in viscosity
has no signicant effect on friction. The reduction in lubri-
cating oil viscosity is also known to reduce engine oil lm
thickness. As a result, it will push the contact surface more into
the boundary lubrication regime.
33
This will result in high
friction and wear aer a long period of four-ball testing.
4.2. Acid number
The acid number is the quantity of the higher organic acid or
acid-like derivatives in the lubricant and is the indicator of oil
serviceability and higher susceptibility of the contact surface to
corrosion.
50
At higher temperature, fuel molecules and other
organic acids inside lubricant are decomposed during opera-
tion and hence, increase the acidity. The acid number results
are shown in Table 6. Since bioethanol is highly evaporative at
high temperature, the change in the acid number of each fuel–
Fig. 2 Variation of viscosity of synthetic oil diluted with bioethanol blends measured at 40 C.
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oil sample was determined before conducting four-ball wear
tests. Generally, the acid value of the new oil is not necessarily
nil since oil additives can be acidic in nature. An increase in
acid number indicates that lubricant is contaminated with
bioethanol and when acid number increase, it makes the metal
surface more susceptible to corrosion.
4,50
It can be seen that
synthetic oil samples diluted with 6% of bioethanol–gasoline
fuels show higher acid number, compared to fresh SO. This, in
turn, may provoke undesirable corrosive wear on engine
components.
16,51
Bioethanol is more chemically reactive than
gasoline due to the presence of oxygen. This possibly interacts
with lubricant base uid and dissolved additives such as
detergent and dispersants containing in the commercial engine
oil, thus leading to oil degradation. The degradation of ethanol
in the lubricant creates ethanoic (acetic) acid, which attacks so
metals and increases oxidation of the oil.
52
As a consequence, it
is possible that 6% dilution rate may cause serious effects such
as oxidation and corrosion of engine oil, especially for synthetic
oil diluted with fuel containing 85% v/v of bioethanol (E85–SO).
Lubricant diluted with bioethanol–gasoline mixtures becomes
degraded and produces higher wear compared to fresh engine
oil.
4.3. Flash temperature parameter (FTP)
As mention in Section 3.6, ash temperature parameter (FTP) is
a single number that is used to express the critical ash
temperature at which an engine oil will fail under selected
conditions. The FTP also refers to the possibility of lubricant
breakdown.
43
Generally, a greater FTP number indicates a better
lubricating efficiency of the lubricant while a smaller FTP
number indicates the reduction in lubricating lm thickness.
41
As seen in eqn (2), load and wear scar diameter (WSD) predict
FTP number. Fig. 3 illustrates the variations of the FTP of fresh
synthetic oil and diluted synthetic oils tested at both condi-
tions. At the load of 40 kg, it can be seen that fresh synthetic oil
(SO) had the highest FTP (178.19) indicating that fresh SO
provided better lubricating performance and had less possi-
bility of lubricating lm breakdown, while all fuel diluted
synthetic oils showed lower FTP numbers. The FTP values of
E0–SO, E10–SO, E20–SO, E30–SO, and E85–SO were 155, 155.60,
156.74, 152.76 and 145.29 which meant that their lubricating
performance is worse compared non-diluted oil. The least FTP
was found in E85–SO which contained a higher amount of
bioethanol inside the synthetic oil while E10–SO showed lower
FTP value compared to other fuel diluted synthetic oils. The FTP
number dropped signicantly when the load was increased to
80 kg, which can be explained that at higher load and temper-
ature the degradation of tested lubricating oil may occur and
thus push the rubbing surface close to each other, which
eventually increase the friction.
53
The FTP values of lubricating
oils were slightly different. SO, E0–SO, E10–SO, E20–SO, E30–
SO, and E85–SO had FTP values of 34, 33.10, 29.38, 31.57, 30.74
and 28.41, respectively. The fresh synthetic oil still had higher
FTP compared to the fuel diluted oils which can be explained
that it may have better lubricating performance during high
load, compared to fuel–oil mixtures. However, fuel contaminant
particularly bioethanol inside synthetic oil deteriorated lubri-
cating properties and may cause lubricating lm breakdown at
high load.
4.4. Effect of bioethanol fuels on friction
Fig. 4 describes the variation of FC of synthetic oil (SO)
contaminated with bioethanol blended fuels at the load of 40
kg. There were slight differences of FC between synthetic oil and
bioethanol diluted synthetic oil samples. It was due to the
evaporation of bioethanol at high temperature or their slight
differences in viscosity which caused almost identical friction
behaviors. The average FC values of fresh oil and fuel-diluted
oils tested at 40 kg and 80 kg are shown in Fig. 5. At 40 kg,
the friction coefficients yielded by diluted synthetic oils were
slightly higher than that yielded by the fresh synthetic oil. Based
on the data averaged from the three tests, it is shown that E0–
Fig. 3 Flash temperature parameter of diluted synthetic oil at different loads.
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SO, E10–SO, E20–SO, E30–SO, and E85–SO had FC values of
0.111, 0.108, 0.106, 0.109 and 0.112, respectively. It can be seen
that the average FC of SO was not the lowest value (0.108) while
E20–SO had the lowest FC value (0.106). However, synthetic oil
had the lowest frictional behavior as shown in Fig. 4. At rst
30 min, SO performed higher friction compared to E10–SO,
E20–SO, and E30–SO while its friction decreased to the
minimum value during the remaining time. It might be due to
the fact that oil additive has been activated to create lubricating
lm aer half an hour. All fuel diluted oils had increasing or
stable frictional characteristics, compared to fresh synthetic oil.
It is observed that all bioethanol diluted oils yielded lower
friction at rst 30 min, which was probably because of the
presence of oxygen and polarity of bioethanol that could
enhance the lubricity of the oils.
54
Moreover, there might be
a stronger physical adsorption on surfaces of the rotating balls
which reduce interference between asperities and increase
contact resistance, thus stabilizing the friction behavior in the
lubricant wear test.
55
However, the synthetic oil contains base
uid and oil additive concentrations which possibly react with
bioethanol under the presence of oxygen, causing oil degrada-
tion and oxidation, thus increasing friction.
4
The more fuel
dilutes into the lubricant, the more interactive effects on
lubricant additive occurs aer a particular operating time; as
a result, no lubricating lm may generate. E85–SO yielded lower
friction compared to SO, but aer 15 min it yielded the highest
friction value due to the higher amount of bioethanol. It shall be
notied that gasoline would result in higher friction compared
to intermediate-level bioethanol fuels because gasoline (the
mixture of hydrocarbon) has less lubricity than a polar molecule
such as bioethanol.
54
The variation of FC of diluted synthetic
oils tested under the load of 80 kg is shown in Fig. 6. At this high
load, it can be seen that all fuel diluted synthetic oils showed
higher FC, compared to fresh synthetic oil. The average FC
Fig. 5 Average friction coefficient of each fuel diluted synthetic oil sample.
Fig. 4 Friction coefficient of synthetic oil contaminated with different blended fuels at 6% dilution at 75 C and L40 kg.
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values of E0–SO, E10–SO, E20–SO, E30–SO, and E85–SO were
0.129, 0.113, 0.132, 0.115 and 0.118, respectively while SO had
FC value of 0.111 which was slightly lower (Fig. 5). Synthetic oil
performed the lowest frictional characteristics and had the
minimum value during the whole testing time. As mentioned
earlier, when synthetic oil was not diluted by a mixture of
gasoline and bioethanol, it has better lubricating efficiency
compared to fuel-diluted oils because the oil additive plays
a role in reducing friction by creating the lubricating lm. It can
be seen that E0–SO and E20–SO had signicantly higher FC
values, respectively due to the increase of load (80 kg). At high
load, the thermal vibration of fuel–oil molecules may occur due
to additional frictional heating, causing a more decrease in the
viscosity of these tested oils mixtures. The combination of lower
oil viscosity and high loading, the lm thickness is very low,
resulting in boundary lubrication.
56
Therefore, the high contact
stress on surfaces caused plastic deformation of the contact
surface, thus reducing the stress in the contact region and
increasing the contact area with a higher friction coefficient.
55
However, at this 80 kg load, the high bioethanol containing
blends such as E85–SO and E30–SO yielded lower frictions than
E0–SO and E20–SO samples. During high load test, the fric-
tional heat can be induced, so the test temperature may exceed
the boiling point of the bioethanol (78 C) and cause bioethanol
to become more chemically reactive and enhance corrosive
reactions. The high bioethanol containing oils also have higher
acidity which can cause more corrosion to the contact surface,
especially at elevated test temperature. This eventually reduces
the friction because the corroded surface is plowed with low
friction force, whereas the amount of wear increases due to
corrosive wear resulting from the high amount of bioethanol.
57
Overall, it can be stated that a small amount of ethanol in
lubricant may help increase the lubricity of the lubricant
temporarily, but increase friction when it interacts with lubri-
cant and oil additives. There might be also oxidation between
oxygenated component (bioethanol) and lubricant (SO) addi-
tives, which can destroy the oil lubricating performance.
4.5. Effect of bioethanol fuels on wear behavior
Fig. 7 illustrates the variation of wear scar diameter (WSD)
of the ball tested with synthetic oil diluted with various
Fig. 7 Wear scar diameter of synthetic oil diluted with 6% gasoline–bioethanol blends.
Fig. 6 Friction coefficient of synthetic oil contaminated with different blended fuels at 6% dilution at 75 C and load 80 kg.
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gasoline–bioethanol fuels at 40 kg and 80 kg. Although
gasoline–bioethanol fuel dilution has a little effect on fric-
tional behavior, it is observed that the WSD seems to increase
signicantly with an increase in bioethanol amount inside
engine oil. The WSD of the three stationary balls were
measuredandaveragedforgraphplotting.Generally,when
the scar is large, it indicates a severe deterioration of material
on the contact surfaces. At 40 kg load, it can be seen that all
fuel diluted synthetic oils had higher wear scar diameter,
compared to their fresh oil (SO). The WSD of E0–SO, E10–SO,
E20–SO, E30–SO, and E85–SO were 0.380 mm, 0.379 mm, 0.377
mm, 0.384 mm and 0.398 mm, respectively while synthetic oil
yielded WSD of 0.344 mm. The increase in the values of WSD
for fuel diluted oils is due to the fact that fuel dilution reduces
wear performance of the engine oil. The amount of bioethanol
fuel dilution degrades the engine oil and reduce the perfor-
mance of the oil, especially in the case when SO is contami-
nated with higher percentages of gasoline–bioethanol
mixtures. The addition of bioethanol is known to reduce the
lm thickness of the lubricating oil.
37
Fresh synthetic oil which
was not contaminated with fuel mixture had a better lubri-
cating performance due to its oil additive concentration which
can generate the lubricating lm.However,inthecaseoffuel
dilution of engine oil, it affects lubricating properties (weak-
ening the lubricant detergency, forming acid and corrosion,
and reducing of the oil lm causing metal asperities to contact
each other promoting engine wear), thus increasing in friction
and wear losses. Moreover, fuel dilution reduces oil viscosity,
and the concentration of engine oil additives, potentially
compromising lubricant's performance.
58
Inadequate oil
viscosity affects oil lm formation and load-bearing capacity.
This can potentially lead to excessive wear of bearing, journals
and other moving components.
58
The effectofloadonwear
scar is very signicant.ThelargeamountofWSDcouldcause
more wear. At 80 kg load, the contact area is very big; as
a result, the WSD of each test lubricant is extremely large,
compared to the ball tested at 40 kg. All fuel diluted oil
samplesnamelyE0–SO, E10–SO, E20–SO, E30–SO and E85–SO
had WSD of 1.879 mm, 2.045 mm, 1.943 mm, 1.980 mm and
2.095 mm respectively while the WSD of SO was 1.844 mm.
Upon increasing of operation time, the oil additive will be
depleted gradually and bioethanol may react with base oil
and lubricant additive under the presence of oxygen, and
causes oil oxidation of some of the lubricant constituents and
subsequent formation of carboxylic acid and eventually
destroys lubricating performance.
59
E0–SO, E20–SO, E30–SO,
and E85–SO had higher WSD, respectively with an increase in
the amount of ethanol. These fuel-diluted oil samples also
caused additional corrosive wear due to their higher acidity,
compared to fresh synthetic oil. These samples are more
chemically reactive and yield corrosive wear when there is
elevated test temperature from frictional heat at this higher
load (80 kg).
57
E10–SO had higher WSD compared to other oils
except this E85–SO while still remaining low friction. The
increase in WSD of this oil was probably due to the interac-
tion between asperities and contact surface which increase
the contact area, thus increasing the amount of wear. It might
be also due to the chemical attack on the surface of the ball by
bioethanol presented in this oil.
55
It shall be notied that the
less effectonWSDisduetoalargeevaporatedamountof
bioethanol from the lubricant. It is believed that the WSD will
be more severe when 6% of bioethanol fuel diluted into the
oil sump in the vehicle engine where fuel is always diluted
during engine operation, suggesting that ethanol-resistant
engine oil should be produced in order to avoid serious
wear from bioethanol, which can lead to engine catastrophic
failure.
4.6. SEM analysis of worn surfaces
Fig. 8 shows the worn surface morphologies of stationary
balls tested in SO, E0–SO, E10–SO, E20–SO, E30–SO and E85–
SO at 40 kg. For fresh synthetic oil, the ball surface indicated
mild abrasive wear. The micro grooves along the sliding
direction were presented on the worn surface (Fig. 8a). It
shall be notied that the adhesive wear is indicated on the
worn surface when it is greater than 20 mm.
60
In Fig. 8b–f,
more severe surface deteriorations were found on the balls
tested with lubricant diluted with gasoline–bioethanol
mixtures. The balls tested with gasoline oil (E0–SO) showed
moderate wear mechanism including abrasive wear as shown
in Fig. 8b. The grooves along the surface were slightly bigger,
compared to the fresh SO. The ball surface of bioethanol oils
looked smoother, compared to that of gasoline oil because
bioethanol is a polar molecule which has a better lubricity
compared to gasoline (the mixture of hydrocarbon). However,
the worn surface of the balls tested with bioethanol-diluted
synthetic oils indicated more surface deterioration from
abrasive wear and chemical wears (Fig. 8c–f). E10–SO shows
fracture and delamination on the surface. E20–SO and E30–
SO indicate light abrasive wear as small grooves and pits can
be observed on the surfaces. The surface of E85–SO shows
smoother surface and almost no groove indicated. This may
be due to higher acid in this oil which corrodes the surface
and the corroded surface is eventually plowed away by its
counterpart. These fuel-diluted oils seem to undergo chem-
ical reaction between lubricant (base uid and oil additives),
and bioethanol which may cause chemical wears such as
oxidative and corrosive wear, thus increasing wear losses.
Oxidative wear occurs due to the higher oxygen content of
bioethanol inside the oil, which enhances the oxidation of
fuel–oil mixture (Fig. 8c–f). This wear caused material
removal by oxidative chemical reaction of the metal surface.
Corrosive wear is the gradual deterioration of unprotected
metal surfaces, caused by atmosphere and acid, and then
creates pits and small cracks. This corrosive wear may also
dissolve metal parts and cause plowing on the surfaces. It
can also be seen that there were presences of the corrosive
product of black colors due to the fact that oxidization
and corrosion occurred and corrosive acids enhanced
corrosive wear.
61,62
Moreover, during the test at elevated
temperature, the oxidation of fuel–oil mixture may generate
peroxide in the present of oxygen, and this peroxide will
undergo further reaction to form carboxylic acid, ketones,
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aldehydes, and alcohols; as a result, it may increase the
acidity of the lubricant
63,64
(Fig. 8d–f).FromthisSEM,itcan
be deduced that bioethanol blend inside synthetic oil
resulting from fuel dilution is possible to degrade lubricating
performance and also affect the friction and wear behaviors
of lubricant.
Fig. 8 Worn surface of steel balls tested with fresh and diluted synthetic oils (test condition: load ¼40 kg; speed ¼1200 rpm; duration ¼1h;
temperature ¼75 C).
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5. Conclusion
This experimental study was conducted to assess the impact
of bioethanol–gasoline blends, on the tribological perfor-
mance of engine oil under selected conditions using four-ball
wear tester. The results of the degradation of properties,
friction, and wear losses of the contaminated engine oil have
been thoroughly discussed. The conclusions can be drawn as
the followings:
The 6% addition of all bioethanol–gasoline blends to
fully-synthetic oil signicantly decreased the viscosity of the
fresh oil to about 30%, compared to fresh synthetic oil. The
decrease in viscosity from ethanol dilution may result in the
thinner boundary lm and higher wear because viscosity
reduction causes more contact between asperities of the
surface.
Bioethanol–gasoline diluted oils show slightly higher the
acid number, compared to fresh synthetic oil. The engine oil
needs to be more alkaline to prevent the metal surface from
corrosion. However, bioethanol is more chemically reactive
compared to gasoline, which enhances the degradation of the
fuel–oil mixture. Therefore, the fuel diluted lubricants contain
more acidity making the surface more susceptible to corrosion,
thus increasing wear losses.
The addition of ethanol–gasoline fuels to synthetic oil
shows that there is no clear trend or conclusive indication of
each fuel–oil mixture is worse for friction at both loads, but
during the test, it was shown that the ethanol rapidly and fully
evaporated from the lubricant at the temperature of 75 C. This
minimal effect on the friction behavior is also due to the fact
that the tests were conducted under boundary lubrication
regime.
Although bioethanol fuels have slight impact on the
frictional characteristic of the oil, it has signicant differ-
ences in the amount of wear. At both 40 kg and 80 kg loads,
fuel–oil mixtures increase wear losses more or less with the
increased amount of bioethanol, compared to their fresh oil.
E30–SO and E85–SO have higher wear losses but low friction
because acid corrodes the surface and causes additional
corrosive wear. From the SEM of the worn surface of tested
ball, it can be seen that the worn surface of the fuel–oil
samples has more surface deterioration than fresh oil.
Additional damage and chemical wears (oxidative wear and
corrosive wear) can be investigated on the surface of the ball
tested with these oils.
Overall, it can be concluded that 6% of bioethanol fuel
dilution can degrade properties performance (viscosity
and alkalinity) and tribological behaviors of the synthetic
oil. Ethanol fuel dilution may reduce friction, compared
to pure gasoline due to its polarity and acidity. However,
high acid in the oil causes high wear due to its corrosion on
the surface, suggesting that ethanol-resistant engine oil
should be produced in order to avoid serious wear from
bioethanol.
Appendix A and B
Tables 7–13
Table 8 Uncertainty analysis for FTP at 80 kg
Samples FTP 1 FTP 2 FTP 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 33.36 34.33 34.22 33.36 34.33 34.338 33.345 33.841 1.438 1.438
E0–SO 31.86 34.86 32.62 31.86 34.86 34.871 31.85 33.362 4.490 4.490
E10–SO 29.22 28.73 30.25 28.73 30.25 30.260 28.72 29.492 2.572 2.572
E20–SO 31.54 31.63 31.54 31.54 31.63 31.644 31.53 31.589 0.144 0.144
E30–SO 30.00 30.00 32.33 30.00 32.33 32.342 29.99 31.165 3.744 3.744
E85–SO 28.50 27.67 29.09 27.67 29.09 29.095 27.66 28.375 2.502 2.502
Uncertainty level 2.482 2.482
Table 7 Uncertainty analysis for FTP at 40 kg
Samples FTP 1 FTP 2 FTP 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 180.39 181.88 173.93 173.93 181.88 181.888 173.916 177.902 2.234 2.234
E0–SO 156.74 160.30 148.41 148.41 160.30 160.311 148.398 154.354 3.852 3.852
E10–SO 162.75 155.01 148.94 148.94 162.75 162.762 148.930 155.846 4.431 4.431
E20–SO 155.01 164.00 152.20 152.20 155.01 155.021 152.190 153.605 0.915 0.915
E30–SO 149.48 162.75 151.65 149.48 162.75 162.762 149.465 156.113 4.252 4.252
E85–SO 148.41 139.37 145.29 139.37 148.41 148.418 139.357 143.888 3.141 3.141
Uncertainty level 3.137 3.137
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Table 12 Uncertainty analysis for WSD at 80 kg
Samples WSD 1 WSD 2 WSD 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 1868 1830 1834 1830 1868 1868.01 1829.99 1849 1.02758 1.02758
E0–SO 1930 1810 1898 1810 1930 1930.01 1809.99 1870 3.20855 3.20855
E10–SO 2053 2078 2003 2003 2078 2078.01 2002.99 2040.5 1.83778 1.83778
E20–SO 1944 1940 1944 1940 1944 1944.01 1939.99 1942 0.10298 0.10298
E30–SO 2015 2015 1910 1910 2015 2015.01 1909.99 1962.5 2.67515 2.67515
E85–SO 2090 2135 2060 2060 2135 2135.01 2059.99 2097.5 1.78784 1.78784
Uncertainty level 1.773318 1.77331
Table 11 Uncertainty analysis for WSD at 40 kg
Samples WSD 1 WSD 2 WSD 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 341 339 350 339 350 350.01 338.99 344.5 1.59651 1.59651
E0–SO 377 371 392 371 392 392.01 370.99 381.5 2.75229 2.75229
E10–SO 367 380 391 367 391 390.53 366.71 378.62 3.14299 3.14299
E20–SO 380 365 385 365 385 385.01 364.99 375 2.66666 2.66666
E30–SO 390 3674 386 374 390 390.01 373.99 382 2.09424 2.09424
E85–SO 388 410 398 388 410 410.01 387.99 399 2.75689 2.75689
Uncertainty level 2.50160 2.50160
Table 10 Uncertainty analysis for FC at 80 kg
Samples FC 1 FC 2 FC 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 0.1093 0.1148 0.1077 0.1077 0.1148 0.1248 0.0977 0.1112 3.19101 3.19101
E0–SO 0.1314 0.1252 0.1292 0.1252 0.1314 0.1414 0.1152 0.1283 2.41621 2.41621
E10–SO 0.1110 0.1154 0.1125 0.1110 0.1154 0.1254 0.1010 0.1132 1.94346 1.94346
E20–SO 0.1323 0.1284 0.1343 0.1284 0.1343 0.1443 0.1184 0.1313 2.24590 2.24590
E30–SO 0.1115 0.1165 0.1157 0.1115 0.1165 0.1265 0.1015 0.1140 2.19298 2.19298
E85–SO 0.1176 0.1223 0.1130 0.1130 0.1223 0.1323 0.1030 0.1176 3.95240 3.95240
Uncertainty level 2.65699 2.65699
Table 9 Uncertainty analysis for FC at 40 kg
Samples FC 1 FC 2 FC 3 Min. Max.
Accuracy
Avg.
Uncertainty%
0.01 0.01 +
Pure SO 0.109 0.110 0.115 0.109 0.115 0.125 0.0994 0.1122 2.49919 2.49919
E0–SO 0.114 0.114 0.109 0.109 0.114 0.124 0.0990 0.1115 2.24215 2.24215
E10–SO 0.103 0.111 0.103 0.103 0.111 0.1208 0.0931 0.1070 3.5998 3.5998
E20–SO 0.112 0.109 0.109 0.109 0.112 0.122 0.0987 0.1104 1.49524 1.49524
E30–SO 0.106 0.112 0.103 0.103 0.112 0.1219 0.0927 0.1073 4.28704 4.28704
E85–SO 0.110 0.113 0.1125 0.110 0.113 0.1228 0.1001 0.1115 1.21130 1.21130
Uncertainty level 2.55579 2.55579
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Acknowledgements
The rst author is greatly indebted to the Japan International
Cooperation Agency (JICA) and AUN/Seed-Net for funding the
author's postgraduate study as well as providing the research
grant under the ‘Collaborative Research’programme. The
authors amiably thank the Ministry of Higher Education
Malaysia and University of Malaya, Malaysia for funding this
study through the Grand Challenge Project (GC001-14AET),
UMRG Grant (RP016-2012B), and FRGS Grant (FP063-2015A).
Authors also appreciate International Scientic Partnership
Program (Ref no. ISPP#0092) at Kind Saud University (KSU),
Saudi Arabia for funding this research work.
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Friction coefficient (FC) 0.05 At load 40 kg: 2.501
At load 80 kg: 1.773
Wear scar diameter (WSD) 0.01 At load 40 kg: 2.556
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