Applied Energy 211 (2018) 461–478
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Fuel economy in gasoline engines using Al2O3/TiO2 nanomaterials as
nanolubricant additives
T
Mohamed Kamal Ahmed Alia,b,c, Peng Fuminga,b, Hussein A. Younusd,f,
⁎
Mohamed A.A. Abdelkareema,b,c, F.A. Essae, Ahmed Elagouza,b,c, Hou Xianjuna,b,
a
Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China
Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan 430070, China
c
Automotive and Tractors Engineering Department, Faculty of Engineering, Minia University, El-Minia 61111, Egypt
d
Chemistry Department, Faculty of Science, Fayoum University, Fayoum 63514, Egypt
e
Mechanical Engineering Department, Faculty of Engineering, Kafrelsheikh University, Kafrelsheikh 33516, Egypt
f
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
b
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
Fuel economy is improved by Al O /
• TiO
nanolubricants strategy under
2
3
2
NEDC.
vehicle fuel consumption during
• The
NEDC was reduced by 4 L/100 km.
engine brake power and engine
• The
torque improved during urban and
motorway.
frictional power losses of gaso• Total
line engine were reduced by 5–7%.
mechanical efficiency of gasoline
• The
engine improved in the range
1.7–2.5%.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fuel economy
Gasoline engines
Warm-up phase
Nanolubricants
Nanomaterials
NEDC driving cycle
Energy resources are of strategic interest worldwide. Transportation sector is a principal consumer of different
energy resources, therefore reducing the consumption of vital energy resources is critical in automobiles. The
friction and wear issues impact the energy efficiency of engines, therefore it is an important development of the
lubricant for saving energy. The current study supports that goal. This study deals contribution of Al2O3/TiO2
hybrid nanoparticles as nanolubricants to improve gasoline engine efficiency and fuel economy. The gasoline
engine performance characteristics were evaluated experimentally using an AVL dynamometer under different
operating conditions including the New European Driving Cycle (NEDC). Additionally, the engine was tested
under critical operating conditions (warm-up phase). The results showed that using Al2O3/TiO2 nanolubricants
increases the brake power, torque, and mechanical efficiency, while the brake specific fuel consumption (BSFC)
reduced owing to the mechanical efficiency of the engine improved by 1.7–2.5%, as compared to the engine oil
without nanoparticles. Hence, the vehicle fuel consumption during NEDC could be improved up to 4 L per
100 km in the urban. Furthermore, FESEM, EDS line scanning, XPS, and Raman spectroscopy were conducted to
understand the major tribological reasons for improving the engine performance to link tribological tests in the
⁎
Corresponding author at: Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China.
E-mail addresses: eng.m.kamal@mu.edu.eg (M.K.A. Ali), houxj@whut.edu.cn (H. Xianjun).
https://doi.org/10.1016/j.apenergy.2017.11.013
Received 4 September 2017; Received in revised form 8 October 2017; Accepted 2 November 2017
0306-2619/ © 2017 Elsevier Ltd. All rights reserved.
Applied Energy 211 (2018) 461–478
M.K.A. Ali et al.
laboratory with actual engine performance. Eventually, the results suggest that nanolubricants provide economical engines with high efficiency that it may be an appropriate direction for vehicle manufacturers and users
to suppress the engine fuel cost with engine durability under different operating conditions.
1. Introduction
[18]. The results by Ali et al. [19] showed that Al2O3/TiO2 hybrid
nanoparticles provided low kinematic viscosity by 4.5% besides an increase in the viscosity index by ∼2%. Furthermore, thermal conductivity was reinforced by 16% for a temperature between 10–130 °C
for helping to further enhancement of the heat transfer and maintain
engine oil properties, comparing with lube oil (5-W30). Another study
also showed that using Al2O3 nanolubricants improves the thermal
conductivity by 9.52% [20].
The investigations on the decrease in the fuel consumption values in
automotive have become the major research aspect for different
countries. Fundamentally, brake specific fuel consumption (BSFC) of
the engine demonstrates the amount of fuel consumed per unit work
accomplished [21]. BSFC decreases with increasing load until the
minimum BSFC is reached and then increases in a phenomenon called
over-fueling [22]. In addition, engine load has a large effect on engine
efficiency and BSFC [23]. The mechanical efficiency typically increases
with engine load [24]. Nakamura et al. [25] investigated the fuel
economy by optimizing viscosity characteristics of engine oil (5W-30).
The results showed that the increasing of viscosity index of the lube oil
increase the fuel economy improvement rate. TiO2 nanolubricants reveals the increase in brake thermal efficiency by 4–7% as a promising
approach in fuel economy [26].
Basically, the gasoline engines suffer from higher friction and less
efficient combustion resulting in higher fuel consumption during the
warm-up phase [27]. So, the engine heating during the cold start operating is helpful as it limits quenching of the cylinder liners and
therefore enhances combustion quality and reduces the friction [28].
The increase in oil temperature decreases oil viscosity and improves
organic efficiency and mechanical efficiency through engine cold
starting [29]. According to the results by Will et al. [30], the total losses
in the engine during the cold start phase increased approximately 2.5
times greater than those observed when the lube oil is warm. Therefore,
it is imperative a high heating rate of the lubricant during the cold-start
phase. Kunze et al. [31] stated that the fuel consumption declined by
10% when the temperature increased from 25 °C to 90 °C during NEDC
driving cycle. Additionally, the results by Roberto et al. [32] exhibited
that the fuel consumption during NEDC test reduced in the range of
2.8% resulting from heating of the lube oil.
The selection of nanoparticles is a very important step. Through
statistical comparison of the tribological results, Dai et al. [33] exhibited the results of nanoparticles, which worked as nanolubricant
additives. The results confirmed that the majority of nanolubricants
consisted of metal oxides, metals, and sulfides. Furthermore, the majority of the nano-additives morphologies are spherical, followed by
sheet, and nanotube. The spherical morphology of the nanoparticles
showed superior tribological performance more than carbon nanotubes.
The reason is strongly related to the rolling mechanisms between worn
surfaces during the friction process [34].
The Main advantages of Al2O3/TiO2 nanoparticles over carbon nanotubes are that Al2O3/TiO2 nanoparticles are cheaper, more stable in
lubricant oils and eco-friendly [35]. Al2O3 and TiO2 nanoparticles are
most appropriate for many environmental applications in which TiO2
nanoparticles are environmentally friendly and non-toxic substance
that can sometimes be used in food coloring as reported by [36]. Furthermore, Al2O3 and TiO2 nanoparticles offer excellent tribological
properties as a solid lubricant at high temperatures [11]. They also have
been categorized as ceramic materials [2]. Al2O3/TiO2 nanoparticles
can provide properties that do not exist in an individual nanomaterial
as well as the synergistic effect of the nanoparticles [37]. The nanoparticles such as Al2O3, TiO2, ZnO, MnO, CuO, and CeO2 are normally
With increasing concern about energy shortage and environmental
protection, transportation vehicles account for about 19% of the world’s
energy consumption every year [1]. The friction between two worn
surfaces is a principal cause of energy dissipation in automotive engines. Total power generated by the engine is reduced in the range
17–19% because of the frictional losses. The ability of lubricant oils to
improve the fuel economy is critical in worldwide. Therefore, the investigations on frictional power losses reduction have gained tremendous attention as a promising direction in the performance of gasoline engines for fuel economy [2]. Reducing total frictional power
losses in vehicles could save the US economy as much as US$ 120
billion per year [3]. The mechanical interfaces in automotive engines
are usually lubricated by a blend of lube films and solid tribofilms [4].
Current challenges for improving the tribological behavior in automotive engines require lubricants that adapted to different operating
conditions by replenishing mechanisms for reducing the friction and
wear [5]. To solve this problem, we have focused on Nanotribology in
the engines as the main strategy for minimizing frictional power losses,
wear of sliding surfaces and excessive heat generation, in a manner that
will ultimately lead to an improved performance of automotive engines.
Employing nanolubricant additives is considered as an accepted and
attractive oil lubricant modification technique which is widely adopted
since it does not need any major hardware modifications [6]. For engine efficiency improvement, it is desirable exploring new ways to replace the use of environmentally harmful additives which causes adverse emissions (zinc dialkyldithiophosphate (ZDDP)), without
compromising on tribological performance for automotive engines with
environmentally friendly additives such as ionic liquids and nanoparticles [7,8]. The study of the nanomaterials (1–100 nm) has become
one of the fastest growing research areas in a lot of energy related fields
owing to their excellent properties [9].
Nanolubricants have received a particular attention because of their
great potentials such as friction modifiers, anti-wear additives, and
solid lubricants on sliding worn interfaces in tribological applications
[10]. While previous results on using Al2O3, TiO2 and Al2O3/TiO2 hybrid nanoparticles as nano-additives are promising in improving the
engine tribological performance. According to the tribological results
by Ali et al. [11], the Al2O3 and TiO2 nano-additives into lube oil revealed that the friction coefficient decreased by 9–13%, 33–44%,
48–50% for the hydrodynamic, mixed and boundary lubrication regimes, respectively. However, the wear rate of the piston ring was declined by 29–21% for the use of Al2O3 and TiO2 nanolubricants respectively, after a 50 km sliding distance, as compared to engine oil free
of nanoparticles. Furthermore, the frictional power losses of piston ring
assembly were also reduced by 39–53% for the Al2O3/TiO2 hybrid
nanolubricants [2]. Hence, the friction reduction by 10% in automotive
engines has the potential to fuel economy by approximately 1%, although it depends on the vehicle models [12].
Rameshkumar et al. [13] studied the effect of the addition of iron
oxide nanoparticles to lubricating oil (SAE 10W-30). The results revealed that using nanolubricants lead to improve fuel economy. Another study also demonstrated that using molybdenum improves gasoline engine fuel economy by 3–5% under full load [14,15]. A common
route to obtain fuel economy from the lubricant is to reduce its viscosity
and minimizing the boundary friction coefficient [16]. Tseregounis and
McMillan [17] investigated a gasoline engine via shifting from 20W-50
to 5W-20 oil improved fuel economy by 4% due to lower viscosity. The
fuel economy was reduced by 5–10% using the MoS2 nanolubricants
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used as combustion catalysts to promote complete combustion, and also
to reduce the consumption of fuel, and emissions for hydrocarbon fuels
[38]. Hence, Al2O3 and TiO2 were chosen because of their durability in
harsh engine exhaust conditions for reduction of NOx under lean-burn
conditions [39,40]. For these reasons, Al2O3/TiO2 hybrid nanolubricants are most effective and suitable under different lubrication
conditions in engines.
The principal motivation of this study focuses on tribological behavior as a solution for reducing fuel energy consumption and improving the performance of gasoline engines in automotive via nanolubricants, which combine benefits of both solid and liquid lubrication.
Our current work aimed to provide insights into how nanolubricant
additives could contribute towards for saving energy and fuel economy.
This study begins with a discussion of the numerous factors that can
affect the engine performance under different operating conditions for
both of the lube oil (5W-30) and Al2O3/TiO2 hybrid nanolubricants.
Furthermore, we have conducted further tests of FE-SEM, EDS line
scanning, XPS, and Raman spectroscopy (Section 3.5). These tests give a
brief discussion about all the mechanisms activated by nano-additives
to understand the major reasons for improving the gasoline engines
performance to provide the evidence and link tribological tests in the
laboratory with actual engine performance.
Fig. 2. Photograph of the experimental bench of the engine and AVL dynamometer.
2.2. Engine setup and testing procedures
The experiments were performed on gasoline engine (model
HXDG16-BD-TJ, multi-point injection, independent ignition, water
cooled, and a naturally aspirated). The engine is prepared with AVL
dynamometer for measuring the engine performance. Fig. 2 shows a
photograph of the experimental bench of the engine and AVL dynamometer as well. The technical specification of the engine and vehicle is
given in Table 1. Temperatures of the coolant and oil were measured by
K-type thermocouples. The sensors signal attached to the AVL test
bench were recorded for all selected parameters. These settings were
operated through the dedicated AVL PUMA program displaying all
engine parameters via INCA software connected to the Electronic
Control Unit (ECU) integrated. On the other hand, it fulfills functions
such as adjusting the load and speed of the engine automatically. Furthermore, an AVL Dynamic Fuel Meter balance was combined with the
AVL PUMA system in order to measure the fuel consumption. All the
engine control parameters were controlled by a PC interface that was
directly connected to the engine ECU during the experiment. The recorder is used to sample high dynamic data directly at the test bench. It
works independently from a PUMA measurement. A PUMA operator
interface consists of engine/dynamometer control part and measurement part as shown in Fig. 3.
The laboratory experiments in this investigation were performed at
2. Experimental section
2.1. Formulation Al2O3/TiO2 hybrid nanolubricants
Based on the obtained results in our previous study [2], it was observed that the nanolubricant with 0.1 wt% nanoparticles concentration
was the best sample owing to decline in the friction coefficient by half,
as compared with engine oil without nanoparticles. Hence, the compositions of hybrid nanolubricants comprised Al2O3/TiO2 nanoparticles
in a concentration of 0.1 wt% added to an oleic acid having a concentration of 1.9 wt% and 98 wt% engine oil. In the current study,
Castrol EDGE professional A5 5W-30 was utilized as a base lubricant in
experiments of the engine to demonstrate the role of Al2O3/TiO2 hybrid
nanolubricants in improving the engine performance. The Al2O3 and
TiO2 nanoparticles have a size in the range of 8–12 nm and 10 nm,
respectively. Morphology of the nanoparticles are characterized using
TEM as shown in Fig. 1. TEM test confirmed that the nanoparticles were
fairly spherical, which provided a good rolling effect between the
rubbing surfaces. The details of the tested nanolubricants samples and
their characteristics were reported in our previous publications [2,18].
Fig. 1. TEM micrographs for TiO2 nanoparticles (a)
and Al2O3 nanoparticles (b).
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Table 1
Engine and vehicle specifications.
Parameters
Value
Engine
Engine model
Engine type
Number of cylinder
Swept volume
Cylinder bore
Cylinder stroke
Compression ratio
Max. power
Max. torque
Idle speed
HXDG16-BD-T
Gasoline, 4-stroke and a naturally aspirated
4 cylinder, in line
1600 cm3
75 mm
90.5 mm
10.5:1
85 kW @ 5600 rpm
160 Nm @ 4000–4500 rpm
770 ± 50 rpm
Vehicle
Gross vehicle weight
Transmission
Gear ratios
1690 kg
Manual
5.00, 3.417,1.81, 1.27, 0.975
Fig. 4. Speed profile of the NEDC (ECE + EUDC) driving cycle. Reproduced from [41]
with permission from Springer.
the nano-additives, which allowed the evaluation of the lubricants
under different lubricating conditions in the engine. Moreover, the
engine performance comparison was carried out using New European
Driving Cycle (NEDC) to confirm the effect of the nano-additives with
different operating conditions of the engine. The evaluation by the
NEDC driving cycle allowed to mimic of the nano-additives in real
working conditions with emphasis on the fuel consumption. The experimental of the NEDC driving cycle has been done on an AVL dynamometer. Every NEDC test was started at a cold-starting of the engine
(35 °C).
Fig. 4 demonstrates the speed profile of the NEDC (ECE + EUDC)
driving cycle, which is under study in this work. The NEDC cycle
contains four urban driving segments (ECE) described by low vehicle
speed, low engine load, and low exhaust gas temperature, followed by
one extra-urban driving segment (EUDC) to account for higher speed
driving. On other hand, NEDC driving cycle consists of transient phase
(deceleration and acceleration), idle phase and the constant velocity
(steady state) [42].
various engine speeds ranging from 1000 to 4000 rpm, with an interval
of 250 rpm with four throttle valve positions (30, 50, 75 and 100%).
Moreover, in order to evaluate the load characteristics of engine performance, different throttle valve positions were selected from 30% to
100% (full load), with steps of 10% under 1000–4000 rpm. The load
variation was achieved electronically controlled via AVL. Before taking
performance readings, the lube oil was operated for an equivalent
∼250 km to stabilize all tribological contacts and ensure activation of
the oil additive package in lubricants. The sustainability of the additives
in the commercial lubrication oils is one of the important synthetic
lubricant's properties. In order to ensure the performance sustainability
properties for Al2O3/TiO2 nanoparticles, some extensive durability tests
need to be accomplished for long operating cycles for different conditions, which will be scheduled in our future study plan.
The test begins only after the engine reached a thermal steady-state.
The engine was regarded as warm when the lubricant oil and coolant
were kept at range from 70 to 90 °C and 85 °C, respectively. The measurements were carried four times under the same conditions in an
attempt to replicate experimental results and the average values were
considered as final results, which ensure the reliability of the results to
evaluate the engine performance. After finishing the engine experiments using the lube oil (5W-30), we have changed the oil filter with a
new filter with nanolubricant to ensure that there is no residual lubricant from the previous one. Thereafter, we have adjusted the engine
oil level taking into account equally in both cases. This study also focused on the cold-start phase because it’s desirable an increase heating
rate of the lubricant to its operating temperature for reducing the
frictional power losses during the warm-up period.
For the evaluation of the nano-additives, the measurements were
carried out under different operating conditions to confirm the effect of
3. Results and discussion
The results display the operating characteristics of gasoline engine
such as the brake power, torque, specific fuel consumption, lubricant
temperature, total frictional power losses, and mechanical efficiency for
the nanolubricants, compared to the use of nanoparticles-free lubricant
oil (5W-30) under various engine speeds, throttle valve openings and
NEDC driving cycle.
3.1. Brake power and torque behavior
The brake power results using the nanolubricants and engine oil
Fig. 3. AVL PUMA operator interface panel.
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Fig. 5. Brake power as a function of engine speed using nanolubricants and lube oil (5W-30) without nano-additives under different throttle valve openings.
Fig. 6. Engine brake power behavior during NEDC using nanolubricants and lube oil (5W-30) without nano-additives.
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Fig. 7. Engine torque values as a function of engine speed using Al2O3/TiO2 hybrid nanolubricants and engine oil (5W-30) under different throttle valve openings (a, b, c, d).
opening using Al2O3/TiO2 hybrid nanolubricants.
Additionally, the engine performances were compared during NEDC
cycle for both the nano-additives and lube oil (5W-30). Fig. 6 provides
the engine brake power trends in the different operating conditions
during NEDC. It is apparent from the results during urban that the peak
engine brake power reaches to 19 kW and 23 kW when using the lube
oil (5W-30) and nano-additives, respectively. While the peak brake
power for nanolubricants during motorway increased from 45 kW to
50 kW approximately. Hence, the Al2O3/TiO2 nanolubricants provided
the improving of the brake power during the urban and motorway by
17% and 10%, respectively, in comparison with the lube oil without
nano-additives. This is related to the decline of the frictional power
losses for the engine due to the reduction of the friction between rubbing surfaces as shown in Fig. 15.
Engine torque is a good indicator of an engine ability to do work. It
is known that engine torque is a main function of the brake mean effective pressure (BMEP). Hence, BMEP is a function of the volumetric
efficiency, brake thermal efficiency, calorific value, and fuel-air ratio.
Fig. 7 shows engine torque versus engine speed from 1000 to 4000 rpm
(5W-30) as a function of engine speed are displayed in Fig. 5. The results exhibited that the brake power for Al2O3/TiO2 hybrid nanolubricants increased by ∼20% under operating by 30% of throttle valve
opening, compared to engine oil without nanoparticles. Furthermore, it
was remarked that the brake power oscillates with engine speeds with
lower throttle valve position. Although, it is evident from this results
that brake power increase with increasing the engine speed for both
hybrid nanolubricants and without nanoparticles additive with 50, 75,
and 100 % of a throttle valve openings. This is due to the power stroke
increase per unit time [43].
The results explained a slight increase in the brake power for Al2O3/
TiO2 hybrid nanolubricants during 50, 75, and 100% of throttle valve
openings. Interestingly, the peak brake power for each lubricant was
achieved at 3750 rpm during 50% of a throttle valve opening, which is
42.19 kW for Al2O3/TiO2 hybrid nanolubricant and 40.05 kW for lube
oil without nanoparticles and then decreased as shown in Fig. 5(b). This
is because frictional losses increase with speed and become the dominant factor as shown in Fig. 15. Eventually, it was concluded that the
brake power of the engine improved by 5% during half throttle valve
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Fig. 8. Engine brake power (a) and torque (b) as a function of throttle valve openings under 3000 rpm.
increasing engine speed under 75 and 100% of throttle valve opening
for both hybrid nanolubricants and lubricant oil without nanoparticles
as in Fig. 7(c) and (d). As a general trend, the Al2O3/TiO2 hybrid nanolubricants curve for all speeds is higher with different throttle valve
openings.
Fig. 8 illustrate the effect of throttle valve positions on brake power
and engine torque for Al2O3/TiO2 hybrid nanolubricants and engine oil
(5W-30) under engine speed of 3000 rpm. The results illustrates that the
brake power and torque increases with increasing throttle valve openings. The torque and power can be obtained at a wide-open throttle due
to higher volumetric efficiency. At wide-open throttle valve with
medium or high engine speed, the exhaust gases inertia is relatively
high. The exhaust gases inertia makes a vacuum that sucks a new
charge inside the cylinder through valves overlapping. Consequently, a
longer overlapping period leads to a better cylinder filling and increase
in the volumetric efficiency [46]. Apparently, the difference in brake
power through hybrid nanolubricants and engine oil decreased with the
for both Al2O3/TiO2 hybrid nanolubricants and engine oil (5W-30) at
30, 50, 75, and 100% of throttle valve positions. Based on the obtained
results in Fig. 7(a), it is apparent that when the engine speed increases,
the average values of engine torque decreased. The main causes for the
decrease in engine torque with increasing engine speeds specifically at
lower opening (30%) of a throttle valve are the shortening in time of
intake stroke that the engine cylinder cannot be fully charged. Resulting
in reduction of the engine volumetric efficiency as stated by [44].
The torque versus engine speed curve shows a typical behavior of a
naturally aspirated engine [45] under 50% of throttle valve opening.
The torque reaches a maximum at 1500–3500 rpm and then drops as
shown in Fig. 7(b). The torque decline is attributed to the longer
combustion process (in crank-degrees) of the same amount of injected
fuel at lowering engine speed. Moreover, the engine is unable to ingest
full charge of air at higher speeds speed, which causes a reduction of the
engine volumetric efficiency and frictional losses (negative torque) increased. Obviously, the results exhibited an increase in the torque with
Fig. 9. Engine torque behavior during NEDC using nanolubricants and lube oil (5W-30) without nano-additives.
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Fig. 10. Brake specific fuel consumption as a function of engine speed using Al2O3/TiO2 hybrid nanolubricants and engine oil (5W-30) under different throttle valve openings (a, b, c, d).
Fig. 10(a), it is found that BSFC for both the lubricant increased with
increasing engine speed during 30% of throttle valve opening. The
reason for the rapid increase in BSFC with low throttle valve opening is
that the frictional power losses remains essentially constant, while the
indicated power is being reduced. As a result, the brake power drops
faster than fuel consumption, and hence the BSFC rises as shown in
(Fig. 10(a)). Furthermore, it is clear that the BSFC inversely proportional to break power [47].
Fig. 10(b)–(d) present the variation of BSFC with engine speed of
50, 75 and 100% under throttle valve openings, respectively. The results exhibited that the BSFC decreases as engine speed increases until it
reaches a minimum value, and then increases with high engine speed
(over-fueling). This is due to the fact that, at low speeds, the time
available for heat to be transferred to the cylinder walls is relatively
longer per cycle, and allows more heat loss occurs, resulting in poorer
combustion efficiency [44]. Consequently higher fuel consumption is
required per unit power produced. At higher speeds, the BSFC again
increases due to the higher friction power losses and pumping work.
Based on the obtained results in Fig. 10, it is observed that BSFC for
increase of throttle opening. It was observed from Fig. 8 that the results
also indicate that the brake power and torque at using Al2O3/TiO2
hybrid nanolubricant is better than of nanoparticles-free lubricant oil
under all of throttle openings.
Subsequently, Fig. 9 exhibits a comparison between the engine
torque obtained for the lube oil (5W-30) and nanolubricants during
NEDC driving cycle to show the effect of the nano-additives on the
engine performance. Notably, it is observed that the engine torque increased by 7.2% with the use of the nano-additives on the NEDC driving
cycle, compared to nanoparticles-free lube oil. This suggested that the
nano-additives were more effective in improving the engine performance.
3.2. Brake specific fuel consumption
The performance of the gasoline engine is firstly given by brake
specific fuel consumption (BSFC) at various speeds and throttle valve
openings. The variations of BSFC versus engine speed for different
throttle valve openings are shown in Fig. 10. As demonstrated in
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Fig. 11. Effect of the throttle valve openings on brake specific fuel consumption under different engine speed (a, b, c, d).
Fig. 12. Fuel consumption during NEDC using nanolubricants and lube oil (5W-30) without nano-additives.
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Interestingly in the comparison of the fuel consumption results,
Fig. 13 provided the relationship between vehicle speed and fuel consumption in L/100 km for calculation how much is the economical
profit from using nanolubricants in engine automotive. The results
compare the engine fuel consumption per 100 km for different gearbox
shifts using the lube oil (5W-30) and the Al2O3/TiO2 nanolubricant
additives. It is clearly observed that using Al2O3/TiO2 hybrid nanolubricant could save up to 4 L/100 km in a case of low speed and 2.4 L/
100 km in economic speed (70 km/h) while 1.5 L/100 km reductions in
the fuel consumption in case of high-speed corresponding to lower gear
reduction ratios compared to engine oil without nano-additives.
From the previous results, nano-additives are very effective at the
lower throttle valve openings and lower engine speeds (urban) due to
dominance of the boundary or mixed lubrication regimes during operating conditions of the engine. It is known from tribological experiments that the separation distance between the worn surfaces decreases
with reducing sliding speed. This usually results in an increase of the
real contact area of the rubbing surfaces and an increase of the friction
coefficient with reducing sliding speeds, which leads to thermal activation of the surfaces as well as the nanoparticles into the lube oils are
very effective during these conditions (boundary or mixed lubrication)
as stated by [11].
Fig. 13. Fuel consumption versus vehicle speed for both the lube oil (5W-30) and Al2O3/
TiO2 nano-additives.
Al2O3/TiO2 hybrid nanolubricant was constantly less than engine oil
free of nanoparticles throughout the speed range and throttle valve
openings. Notably, the lowest BSFC is attained at 75 and 100% of
throttle valve openings for both Al2O3/TiO2 hybrid nanolubricants and
engine oil in the range of 2500–3500 rpm (economic speed).
Fig. 11 presented the influence of the throttle valve positions with
different speeds on the fuel consumption compared with engine oil. At
low throttle operation, the BSFC increased especially at high engine
speed (Fig. 11(d)) owing to the rapid increase in frictional losses [48].
Thus, there is no significant variation in the BSFC in the range of
50–100% of throttle valve openings. As a general trend, the BSFC were
reduced using Al2O3/TiO2 hybrid nanolubricant, as compared to engine
oil without nanoparticles. Furthermore, we presented the variations of
the fuel consumption with NEDC driving cycle to show the influence of
the nano-additives on the fuel consumption compared with nanoparticles-free lube oil. The maximum fuel consumption for each lubricant was showed in urban, which is 4.8 kg/h for Al2O3/TiO2 hybrid
nanolubricant and 6 kg/h for engine oil without nano-additives and
then increased during motorway roads. Based on Fig. 12, it can said that
the use of nano-lubricants reduced the fuel consumption by approximately 16–20% in urban and motorway. The reason for this fuel consumption reduction is due to the lower viscosity and higher viscosity
index of the nanolubricants and improving the tribological performance
of the engine because of the decline of the friction and wear [2,19].
3.3. Warm-up phase (cold-start)
The gasoline engines suffer from higher friction and less efficient
combustion resulting in higher fuel consumption, making it imperative
an investigation of the fuel consumption for the engine during warm-up
phase to improve fuel economy. The warm-up phase time is considered
as the time taken for the lubricant and coolant to reach the operating
temperature in the ranges 60 °C and 90 °C, respectively. This section
seeks to discuss the effect of the nanolubricants on warm-up period
(cold start), the sensitivity of nanolubricants to temperature and the
fuel consumption during cold-start operating in automotive engines.
Fig. 14(a) displays the effect of Al2O3/TiO2 hybrid nanolubricants
on the warm-up phase (cold-start) when operating under 12 Nm and
1500 rpm. As a general trend, it is apparent from the investigation that
the lubricant temperature increased with rising the warm-up period for
both nanolubricants and nanoparticles-free lube oil. The results elucidated that the temperature for nanolubricants is higher than lube oil
without nanoparticles. This is related to the higher thermal conductivity of the nanolubricant, which leads to higher rate of the heat
transfer. Furthermore, the nanolubricant gives a low viscosity at low
temperature (at 40 °C), as compared to engine oil as presented in our
previous study [19]. From Fig. 14(a), it can be observed that the
(b)
(a)
Fig. 14. The role of nanolubricants in warm-up phase, (a) the sensitivity of the nanolubricants to the temperature, (b) the effect of the lube oil temperature on the fuel consumption.
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Table 2
Comparison of the experimental studies recent and the current study.
No
Year
References
Fuel economy
Approach methodology
1
2
3
4
5
6
7
8
9
10
11
12
13
2017
2016
2016
2017
2017
2002
2015
2016
2008
2016
2016
2017
2017
Sgroi et al. [18]
Lu et al. [51]
Kobayashi et al. [52]
Chao et al. [53]
Sagawa et al. [54]
Benvenutti et al. [15]
Cipollone et al. [32]
Bonatesta et al. [55]
Skjoedt et al. [14]
Jang et al. [56]
Cui et al. [57]
Tormos et al. [58]
Current study
5–10%
8%
3.3%
2.96%
0.5%
2.8–5.2%
2.8%
5%
0.9–2.6%
2%
2%
3.7%
16–20%
MoS2 Nanolubricant
Turbo-compounding
Modified cooling system
Gas Recirculation system
Changing viscosity grade
Changing viscosity grade
Warm-up acceleration
Variable camshaft timing
Friction modifier
Fuel additives
Viscosity modifier design
Changing viscosity grade
Al2O3/TiO2 nanolubricant
resulting low combustion efficiency during the warm-up phase [28].
The results revealed a decrease in the fuel consumption for the use of
nanolubricants in the ranges 4–10%, as compared with the use of lube
oil without nano-additives during the warm-up phase. The main reason
is related to the declining in the frictional power losses due to improving the tribological behavior and thermophysical properties of the
nanolubricants [19]. Ultimately, the nano-additives showed the effective role in the warm-up phase.
Fig. 15. Total frictional power losses versus engine speed at wide throttle opening.
nanolubricants reached the optimum temperature of 60 °C after 670 s
while the lube oil took around 882 s to reach the same point. Hence, the
nanolubricants accelerate the warm-up time by 24% at same operating
conditions. This means that the nanolubricants are most effective in the
sensitivity of the temperature. In addition, the higher sensitivity of
nanolubricants to temperature is paramount importance in declining
the power losses during the warm-up time.
Besides, Fig. 14(b) shows the influence of the lube oil temperature
on the specific fuel consumption during the warm-up period to reach
the thermal stability of the engine for both nanolubricants and lube oil
without nano-additive (5W-30) under 1500 rpm engine speed and
12 Nm engine torque. It is evident from this test that fuel consumption
decreased with increasing the lube oil temperature for both nanolubricants and lube oil without nano-additive. This is due to the lower
rate of the heat transfer to the cylinder liner per operating cycle
3.4. Total frictional power losses and mechanical efficiency
The total frictional power losses included the pumping losses and
frictional losses for mechanical components. In this study, we estimated
the total frictional power losses by the hot motoring test (non-firing
engine) [49]. The engine motored from 1000 to 4000 rpm in 250 rpm
increments with the throttle wide open because of the pumping losses
close to zero [50]. During the hot motoring tests, the lube oil and water
temperatures were maintained at 70–90 °C and 85 °C via heated before
starting the test, respectively, as would be found in the fired engine. In
Fig. 15, the total frictional losses raises with increasing engine speed for
both Al2O3/TiO2 hybrid nanolubricants and oil without nanoparticles.
The reason is that the dominance of hydrodynamic friction at high
speed causes increase in viscous friction due to the shearing resistance
of the oil film. Therefore, engine speed is the major operation parameter that controls the frictional power losses. The Al2O3/TiO2 hybrid
nanolubricants at different engine speed showed that the total frictional
power losses decreased in the range 5–7% due to improving the tribological behavior of the engine, low viscosity and high viscosity index
[19], which could reinforce an increasing in brake power, torque, and
automotive fuel economy as shown in previous results (Figs. 5, 7, 13).
For this reason, Al2O3/TiO2 hybrid nanolubricants are most effective
under different lubrication conditions.
Besides, the mechanical efficiency is an indicator of the total frictional power losses in automotive engines. It depends upon the operating conditions especially brake power, engine speed and lube oil.
Therefore, mechanical efficiency is defined as the ratio of brake power
to indicated power. Fig. 16 compares the mechanical efficiency of
Al2O3/TiO2 hybrid nano-lubricants with engine oil at various engine
speeds. Obviously, the results exhibited that the mechanical efficiency
for Al2O3/TiO2 hybrid nanolubricants improved in the range 1.7–2.5%,
compared to engine oil without nanoparticles. This suggested that the
greatest effect on mechanical efficiency is the frictional power losses
within the engine.
According to the comparison of the recent experimental studies and
the current study, the Al2O3/TiO2 nanolubricant provided good results
with a lower cost for the fuel economy without major hardware modifications of the engine as shown in Table 2.
Fig. 16. Effect of Al2O3/TiO2 hybrid nanolubricants on the mechanical efficiency for
engine.
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M.K.A. Ali et al.
Fig. 17. Engine characteristics curve for both lubricated engine oil (5W-30) and Al2O3/TiO2 hybrid nanolubricants.
250 N and an average sliding speed of 0.5 m s−1. The friction behavior
showed that the negative part of the curve is due to the change in
sliding speed direction during the reciprocating sliding motion. Furthermore, it was remarked that the friction coefficient oscillates significantly with time owing to the stick-slip phenomenon through the
sliding motion as compared with the modeling results [60]. The results
exhibited that the average of the friction coefficient for nanolubricants
decreased by 53%, compared to engine oil (5W-30) without nanoparticles. This significant decrease in the friction coefficient might be
due to the leading role of Al2O3/TiO2 hybrid nanoparticles to act
through different mechanisms. Our objective in this section is to derive
nanoparticles mechanisms on the sliding contact interfaces lubricated
by Al2O3/TiO2 hybrid nanolubricants to link tribological tests in the
laboratory with actual engine performance.
Here we provide the experimental evidences of the tribofilm on
sliding contact interfaces using Al2O3/TiO2 hybrid nanolubricant additives. EDS element maps detected Al and O rich film on the rubbing
liner surface with Al distribution matching well with the bright pads on
the plateaus in the Al Kα1 image (Fig. 19). This clearly indicates that
the Al2O3 tribofilm was formed on the liner surface.
Cross sectional views of the piston ring tribofilms are shown in
Fig. 20. The microstructure and elemental composition in the linescanning region were produced using FE-SEM and EDS, respectively,
lubricated by engine oil (Fig. 20(a)), and lubricated by Al2O3/TiO2
nanolubricants (Fig. 20(b)). The cross-section analysis provides a
complete morphology of a tribofilm on the worn surface of the ring
interface. Fig. 20(b) is the cross-section morphology of the worn ring
surface, showing that the Al2O3/TiO2 tribofilm is uniform and has a
thickness of ∼4.9 μm (Fig. 20(c)). Fig. 20(c)–(f) shows the elemental
distribution in scanning curves of Al, Ti, P, S, Cl, C, Fe, Si and O, respectively. The results showed that the base material elements i.e. Fe
and Si show a steady increase with distance on going from up to down
across the tribofilm, as shown in Fig. 20(e, f). Interestingly, within the
first micrometers (4.9 μm) of the tribofilm, Al, P, C and Cl, the gradient
Fig. 18. Friction behavior of piston ring assembly with and without the use of Al2O3/TiO2
hybrid nanolubricant additives under a contact load of 250 N and average sliding speed of
0.5 m s−1.
3.5. The main tribological reasons for improving the engine performance
Elucidating the main reasons of improving the gasoline engines
performance using Al2O3/TiO2 hybrid nanolubricants as shown in
Fig. 17 is very crucial for understanding the mechanisms responsible for
the tribological events. The tribological study focused of the issue of
frictional losses of the piston ring assembly, which accounts for 1–3% of
the brake specific fuel consumption [59]. The experimental evidences
based on the combined results of tribological tests based on the FESEM,
EDS, XPS and Raman studies. The tribological tests were characterized
after 30 km a sliding distance under 250 N a contact load, 80 ± 2 °C
lubricant temperature and 0.5 m s−1 average sliding speed using a test
rig designed to mimic actual operating conditions of the ring/liner interface according to ASTM G181-11 [2].
Fig. 18 compares the friction coefficient of Al2O3/TiO2 hybrid nanolubricants with engine oil during part of the test duration at a load of
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M.K.A. Ali et al.
Elements
Al
Ti
P
C
S
Si
O
Fe
Total:
(a) , wt%
0.00
0.00
0.10
3.60
0.05
3.20
0.21
92.84
100.00
(c) , wt%
1.95
0.92
0.18
5.05
0.00
2.29
7.41
82.20
100.00
Fig. 19. Characterization of a selected area of the cylinder liner lubricated by engine oil (5W-30) (a), lubricated by Al2O3/TiO2 hybrid nanolubricants (b), distribution of a tribofilm
elements in the map scanning region, and elemental content of a tribofilm lubricated by Al2O3/TiO2 hybrid nanolubricants (c).
In order to get further insights about the chemical structure of a
tribofilm and to probe the binding environment of elements contributing to the film, we analyzed the worn piston ring surface via XPS
spectra lubricated with engine oil and nanolubricants as shown in
Fig. 21. XPS survey scan confirms the existence of Ti, Al, originating
from the nanolubricant, as well as C, from other oil additives.
Moreover, the survey scan approved the presence of Fe in the deposited film that arises from wear debris of the piston ring and actively
contributed in the tribofilm formation. Fe incorporation in the tribofilm
structure suggests that this film is formed as result of a chemical reaction of the nanolubricant particles with the wear debris, and not only
due to physical adsorption of the nanolubricant additives on the
subbing surface [61]. Notably, the tribofilm was free of Zn ions/metals,
which would be expected to appear as result of ZDDP decomposition
and participation of its other components i.e. C in the tribofilm composition. In addition, Ca ions from the overbased detergent are
is obviously steeper showing a boundary tribofilm formation on the
sliding contact interface of the ring (Fig. 20(c) and (d)). This confirms
the synergistic effects between lubricant additive package specifically
ZDDP and Al2O3/TiO2 nanoparticles with the substrate surface to form
a tribofilms on the rubbing piston ring surface.
Interestingly, an elemental content in EDS mapping and line-scanning (Figs. 19 and 20) showed that the Al2O3 nanoparticle depositions
are more than TiO2 nanoparticle on worn surfaces, although the Al2O3
and TiO2 concentrations are originally equal (0.05 wt%) in the added
hybrid nanoparticles mixture into engine oil. This suggested that the
Al2O3 nanoparticles were more productive in a tribofilm formation on
the rubbing interfaces, and that was considered as the main reason in
improvement tribological performance. While, the TiO2 nanoparticles
were favorable in rolling mechanisms due to a majority of the TiO2
nanoparticles remained blended with the lubricant oil causing mode
change of the friction from sliding to rolling friction (rolling effect).
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4.9 μm
Fig. 20. Microstructure of cross-section of the piston ring lubricated by engine oil (a), lubricated by Al2O3/TiO2 hybrid nanolubricants (b), distribution of tribofilm elements in the linescanning region lubricated by Al2O3/TiO2 hybrid nanolubricants (c, d, e, f).
iron from debris particle was oxidized to Fe2O3 under lubricating
conditions. Thus, the Fe exists in common Fe3+ oxidation state. Since
the ionic radius of Fe3+ (0.64A)° and Ti4+ (0.68A)° are similar, then the
Ti4+ might exchange some Fe3+ in the crystal lattice of Fe2O3 to form
TieOeFe bond or TieNeFe. The observed different diffraction peaks of
titanium 2P and the inclusion of N in the tribofilm further support this
interpretation [64].
Interestingly, the high resolution XPS of Al showed a high shift from
the reported values for Al2O3 (Fig. 22(a)). Previous studies proved that
the Al2p spectra can be shifted ∼ one eV to lower binding energy in the
order γ-Al2O3 > θ-Al2O3 > α-Al2O3, reaching a binding energy of
74.25 eV (FWMH: 1.67 eV) in case of α-Al2O3 phase. We initially use γAl2O3 as additives in our experiment and its Al2p normally appear at
higher binding energy. Thus, our results suggest that aluminum oxide
underwent a phase change under the experiment conditions and
observed in the tribofilm structure proposing the substitution of Zn
ions.
The high resolution XPS of titanium (Fig. 22(b)) showed two major
peaks at 463.7 eV and 458.0 eV corresponding to 2P3/2 and 2P1/2, respectively, with split spin-orbit components (Δ) of 5.70 eV which are in
excellent agreement with those observed for Ti(IV) [62]. The absence of
any peaks in the range 453–456 eV exclude the presence of any traces of
titanium metal, Ti2+ or Ti3+. The absence of peak broadening of Ti
2p3/2 signals (FWHM was about 1.0 eV) may also indicate the presence
of Ti4+ species only [63], and the high crystallization of TiO2 nanoparticles in the tribofilm. The peaks are deconvoluted into three diffraction peaks, with binding energies of 457.8, 458.2, and 458.8 eV,
corresponding to three chemically different types of titanium.
The Fe 2p1/2 and Fe 2p3/2 peaks at 724.3 eV, and 710.5 eV confirming the prescience of iron as Fe2O3 (Fig. 22(f)), which mean that
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the peaks at 146, 396 and 518 cm−1 are characteristically defined for
anatase TiO2 and agree well with the reported Raman spectra obtained
by other researchers [68]. It is noteworthy to say that a high Fe2O3
content is observed in cylinder liner tribofilm as reflected by sharp and
high intensity peaks at 225, 292, and 414 cm−1 which match very well
with the reported results of Fe2O3 [69]. Additionally, two broad bands
at ∼1300 and 1600 cm−1, which are more intense in case of cylinder
liner film than worn ring, due to CeO and C]O bonds which strongly
support the contribution of organic content from the oil additives in the
tribofilm. This means better anti-wear performance and delay the
scuffing occurrence for the frictional surfaces, allowing longer life and
superior in use performance (engine durability).
Based on the Raman results, the peaks confirm a tribofilm formation
on the worn ring and liner surface by the chemical reaction between the
worn surfaces and Al2O3/TiO2 nanolubricants additive under boundary
lubrication. The results are in agreement with XPS results as shown in
Fig. 22. As expected, the debris reinforces mechanisms of a tribofilm
formation during oxidative wear [70]. Eventually, we concluded that
the key mechanism for improving the performance of gasoline engines
using Al2O3/TiO2 hybrid nanoparticles as nanolubricants is a combination of rolling effect, tribofilm formation, mending or patching,
surface polishing effect and enhance lube oil properties.
4. Conclusions and recommendations
Through the above experiments, Al2O3/TiO2 hybrid nanolubricants
provided an improvement in the performance of gasoline engines at all
operating points and NEDC driving cycle. These results proved to be
systematic for fuel economy. The main conclusions are as follows:
Fig. 21. XPS survey spectra of the worn surface of the piston ring lubricated by engine oil
(a) and Al2O3/TiO2 hybrid nanolubricants (b).
1. The results exhibited that the brake power and engine torque increased with the use of Al2O3/TiO2 hybrid nanolubricants in all
specific operative conditions, as compared to the lube oil without
nanoparticles (5W-30). The reason is that the total frictional power
losses decreased by 5–7% using hybrid nanolubricants. As a result,
the mechanical efficiency of the engine improved in the range
1.7–2.5%.
2. The fuel consumption corresponding to Al2O3/TiO2 nanolubricants
reduced by approximately 16–20%. Hence, the reduction in fuel
consumption recorded during the NEDC test entails a fuel economy
of about 4 L/100 km in the urban.
3. Al2O3/TiO2 nanolubricant accelerate the warm-up phase by 24% at
same operating conditions due to improving the sensitivity of the
lube oil to temperature. As a consequence, the reduction in fuel
consumption by 4–10% was obtained.
4. The main reasons for improving the tribological performance are the
rolling effect of the nanoparticles and tribofilm formation (∼5 µm
thickness) on the rubbing surfaces as illustrated by EDS mapping
results.
5. The experimental results from EDS, XPS and Raman spectra indicated that the tribofilm contains not only the nanolubricant additives i.e. Al2O3 and TiO2 but Fe2O3 which is formed as a result of
iron debris particles from rubbing surfaces causing strong bonding
of the tribofilm with the sliding contact interfaces.
6. Further investigations are needed to how nanolubricants influence
on the exhaust emissions from the engine at cold start, low load/
speed operation when exhaust temperature may not be adequate for
NOx reduction. Besides, study the nanolubricants effect on the heat
transfer in the engine. This will be carried out in the near future.
predominately exist in α-Al2O3 phase in the tribofilm. Moreover, in the
deconvolution of the Al2p peak, A double band is observed the first is
located at 73.7 eV (FWHM 1.50 eV) corresponding to Al-O state and the
second is located at 72.85 eV (FWHM 2.5 eV) corresponding to Al-metal
state [65].
The C1s core level spectrum displayed several peaks due to different
functionalized carbons (Fig. 22(c)). The spectrum suggests the presence
of Csp2, CSp3, CeN/CeO, and C]O in the tribofilm as observed in the
peaks at 284.6, 285.1, 285.7, and 288.8 eV, respectively [66]. The O 1s
core level spectra of the tribofilm showed a broad asymmetric band in
the range 328.7–534.5 eV and centered around 531.6. The peak deconvolution with best fitting parameters resulted in 4 peaks which
might correspond to three different oxides TiO2, Fe2O3, and Al2O3 along
with O]C from organic sources (Fig. 22(d)). The high resolution XPS of
phosphorus showed its low contribution in the tribofilm formation with
main form as metal phosphates at 133.07 eV as shown in Fig. 22(e).
Tribofilms govern the tribological behaviors of contact interfaces in
automotive engines. For this purpose, Raman spectroscopy was used to
examine the chemical bonding in the tribofilm formed on the worn ring
and liner surfaces using Al2O3/TiO2 nanolubricants. Fig. 23 shows the
Raman peaks produced on the worn ring and liner surfaces by lubricated Al2O3/TiO2 nanolubricants are closely related to those generated by the Al2O3 and TiO2 nanoparticles. In agreement with EDS and
XPS, Raman spectra of the films exhibited characteristic peaks of Al2O3,
Fe2O3, and TiO2. Peaks at 414, 445, and 671 cm−1 match very well
with repotted values for nanocrystalline Al2O3 [67]. On the other hand,
The
475
previous
conclusions
provide
a
proof
of
Al2O3/TiO2
Applied Energy 211 (2018) 461–478
M.K.A. Ali et al.
Fig. 22. High-resolution XPS core level spectra for piston ring lubricated by Al2O3/TiO2 hybrid nanolubricants: (a) Al 2p, (b) Ti2p, (c) C 1s, (d) O 1s, (e) P2p, (f) Fe2p.
Acknowledgments
nanolubricant could lead to achieve economic performance. The users
and production oils companies can save money and provide operational
compatibility with all engine speeds and loads. Al2O3 and TiO2 nanolubricants are inexpensive and environmentally friendly. The current
study results affirmed that using nanolubricants could help manufacturers and users to suppress fuel cost and maintenance costs due to
fuel economy and longer oil drain interval. Moreover, nanolubricants
help in protecting and extending engine parts life, which helps to
minimize the operating expenses in automotive.
The authors would like to express their deep appreciation to the
Hubei Key Laboratory of Advanced Technology for Automotive
Components (Wuhan University of Technology) for continuous support.
The authors wish to express their thanks to Mr. De Fang for assistance in
XPS tests at Material Research and Testing Center of Wuhan University
of Technology. The authors also wish to express their thanks to Mr. Liu
Huagang for assistance in engine tests at Automotive School of Wuhan
University of Technology. We also wish to thank the reviewers and
editors for their helpful and valuable comments.
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Applied Energy 211 (2018) 461–478
M.K.A. Ali et al.
Fig. 23. Raman spectra of the tribofilms produced
on the worn surfaces of the piston ring and cylinder
liner lubricated by Al2O3/TiO2 hybrid nanolubricants.
Conflict of interest
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