Enhancing the thermophysical properties and tribological behaviour of engine oils using nano-lubricant additives

Abstract

This paper presents the enhancement of the thermophysical properties (thermal conductivity and viscosity) of engine oil using nano-lubricant additives and a characterization of tribological behaviour in terms of sliding contact interfaces (piston ring assembly) in automotive engines. Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nanoparticles were suspended in commercially available engine oil (5W-30) in a concentration of 0.25 wt% for formulating nano-lubricants. The sizes of Al₂O₃ nanoparticles were within the range 8–12 nm while the TiO₂ nanoparticles used had a size of 10 nm. The tribological experiments were performed using a tribotester to simulate the sliding reciprocating motion of the piston ring/cylinder liner interface in an engine. The performed tribological tests were all carried out under varying speeds, loads and sliding distances. The experimental results showed that nano-lubricant additives enhanced the thermophysical and tribological properties. The thermal conductivity of lube oil was measured by the 3ω-wire method. Nano-lubricants provide low kinematic viscosity and an increase in the viscosity index by 2%. Meanwhile, thermal conductivity was enhanced by a margin of 12–16% for a temperature range of 10–130 °C facilitating the dissipation of frictional heat and maintaining engine oil properties, as compared with commercial lubricants. The tribological tests showed a minimization of the friction coefficient and wear rate of the ring by 40–50% and 20–30%, respectively. According to the results, nano-lubricants can contribute to improving the efficiency of engines and fuel economy in automotive engines.

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1. Introduction

Nano-lubricant additives present a potential for improving engine fuel economy, the achievement of a cleaner environment as well as a reduction of lube oil consumption, ultimately leading to an improvement in the performance of engines via the minimization of frictional power losses. The frictional losses of piston ring/cylinder liner interfaces contribute nearly 40% to 50% to the total frictional power losses for an engine.^1,2 In the past few years, nanomaterials have been studied as nano-lubricant additives in order to improve the tribological performance of engines, oil properties and fuel economy.³ The size of nanoparticles affects physicochemical properties of engine oil and the tribological properties. When the surface roughness is larger than the grain size, the valleys between asperities of the frictional surfaces can be filled leading to the formation of a tribo-boundary film on worn surfaces that enhances the tribological behaviour of an engine.⁴ The spherical morphology of nanoparticles used in nano-lubricant additives is an important parameter that provides a rolling effect between rubbing surfaces.⁵ Moreover, the nanoparticle concentration is an essential matter because they can act negatively if there is a surplus of nanoparticles.⁶

Nano-lubricants could take different forms including Al₂O₃, TiO₂, Cu, Ag, diamond, and graphene. Nano-lubricants play a key role in molecular kinetic energy due to temperature change, which has an influence on the friction behaviour between worn surfaces.⁷ Nanoparticles in suspension in engine oils move by various mechanisms.⁸ Nano-lubricants have higher thermal conductivity and viscosity index than the base oils.^9,10 Reduction in the viscosity helps to minimize the frictional power losses from viscous work and achieves a lowering of fuel consumption by 0.2–2.5%.¹¹ Furthermore, oil viscosity indirectly affects the asperity (boundary) friction via oil film thickness. The low oil viscosity results in thin oil films, which leads to an increase in the wear of sliding surfaces and the asperity contact.^2,12 For this reason, nano-lubricant additives are active under boundary lubrication conditions in forming a tribo-film at the asperity contact locations to improve the tribological properties. Binu et al. studied the effect of TiO₂ nano-lubricant on the load bearing capacity. The results showed an increase in the load carrying capacity of the journal bearing involving the use of the TiO₂ nano-lubricant additive as compared with the use of the lubricant without nanoparticles.¹³ The deposition of TiO₂ nanoparticles on the worn surfaces exhibited a reduction in friction and wear.^14,15 Furthermore, the viscosity of the lubricant increased with a decline in particle size.¹⁶

The thermal conductivity of nano-lubricant plays a vital role in the transfer of heat during the lubrication process in an internal combustion engine. The thermal conductivity of liquids was showed to be less than of metal oxides. Suspending nanoparticles having high thermal conductivities in lube oil facilitates an increase in the thermal conductivity.¹⁷ Nano-fluids were utilized to solve heat transfer problems in engines. The addition of Al₂O₃ and TiO₂ nanoparticles to the engine oil exhibited an improvement in the heat transfer because of an increase in the Nusselt number.¹⁸ Furthermore, the interaction between nanoparticles increases with increasing concentrations leading to enhanced thermal conductivity.¹⁹ In the study conducted by Pakdaman, it was reported that the thermal conductivity of nanofluids containing multi-walled carbon nanotube (0.4% wt) increased by 15% at a temperature of 70 °C.²⁰ Kole and Dey reported the investigation of the thermal conductivity and viscosity via the addition of Cu to gear oil. The results indicated that the oil containing 2 vol% of Cu enhanced the viscosity and thermal conductivity by 71% and 24%, respectively.²¹ The increase in thermal conductivity of nano-lubricants with rising temperature was reported by Aberoumand.²² Vasheghani et al. investigated the effect of Al₂O₃ nanoparticles on engine oil properties. The results showed that the viscosity of the nano-lubricants increased at room temperature. The thermal conductivity also improved by 37.49% using 3 wt% concentrations.²³

Nano-lubricant additives vary depending on the operational environment for which the oil is designed. The momentary friction mechanism depends on the load, speed, actual lubricant viscosity, and the geometry of the worn contact surfaces.²⁴ TiO₂ nanoparticles under low concentration were sufficient for improving tribological characteristics.²⁵ For the formation of tribochemical films to be effective on the worn surfaces the rate of film removal for protecting the surfaces must be less than the rate at which the oil film is formed.²⁶ The lubrication regimes in automotive engines can be divided into three general categories: boundary, mixed, and hydrodynamic lubrication regime. Nano-lubricants are more effective in boundary lubrication regime.²⁷ The addition of Al₂O₃ nanoparticles to engine oil leads to a change from sliding friction to rolling friction causing a decline in the friction coefficient.²⁸ The friction coefficient was reduced by 17.61% for 0.1 wt% concentration and 78 nm grain size of Al₂O₃ nanoparticles addition to the oil.²⁹

The current study examines the effect of Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nanoparticles used as nano-lubricant additives on the tribological behaviour and thermophysical properties of engine oil (thermal conductivity and viscosity). Formation of tribo-films on worn surfaces was observed using EDS and FE-SEM to reveal the friction reduction and anti-wear reduction mechanism for nano-lubricant formulations. In nano-lubricants, the tribo-film can be formed on the frictional surfaces through chemical reaction and a physical mechanism making it important to understand the mechanisms of tribo-film formation on worn surfaces.

2. Experimental and methods

2.1 Characterization of nanoparticles

The morphology of the Al₂O₃ and TiO₂ nanoparticles was investigated with SEM images as shown in Fig. 1. The morphology of the Al₂O₃ and TiO₂ nanoparticles was fairly spherical. The average sizes of the Al₂O₃ and TiO₂ nanoparticles are 8–12 nm and 10 nm, respectively. The XRD pattern of Al₂O₃ and TiO₂ nanoparticles in the current study is shown in Fig. 2. It can be observed that the peak details are 2θ = 25.2°, 36.9°, 48°, 53°, 55° and 62° with strong diffraction peaks at 25° and 48° confirms TiO₂ in the anatase structure phase. However, Al₂O₃ nanoparticles also show the typical diffraction peaks (311), (400) and (441) at 2θ = 37°, 45° and 67°, respectively. From XRD patterns, an intensity of XRD peaks of the Al₂O₃ and TiO₂ reflects that the nanoparticles are broad diffraction peaks (means poor crystallization), which indicates very small size crystallite.³⁰ All the diffraction peaks of the Al₂O₃ show a high degree of broadness due to a less degeneracy in the crystallites. This result suggests that crystal size of the Al₂O₃ nanoparticle smaller than TiO₂ nanoparticle.

Fig. 1 SEM images of Al₂O₃ and TiO₂ nanoparticles.

Fig. 2 XRD pattern of Al₂O₃ and TiO₂ nanoparticles.

2.2 Formulation of nano-lubricants

Castrol EDGE professional A5 5W-30 (a commercial lubricant) was used as base oil for the investigation. The compositions of nano-lubricants comprised Al₂O₃, TiO₂ or Al₂O₃/TiO₂ hybrid nanoparticles in a concentration of 0.25 wt% added to oleic acid having a concentration of 1.75 wt%. Therefore, nano-lubricants comprised 2 wt% additive solution (Al₂O₃, TiO₂ or Al₂O₃/TiO₂ + oleic acid) and 98 wt% engine oil (5W-30) as shown in Table 1. In order to obtain a homogeneous and stable suspension of nanoparticles in the engine oil a magnetic stirrer was used for 4 hours. Oleic acid was added to the engine oil for the purpose of suspension and the reduction of nanoparticle agglomerates.³¹ The nano-lubricants were kept at room temperature after preparation without sedimentation after a storage time of 35 days as shown in Fig. 3a.

Table 1 The compositions of nano-lubricants

Nano-lubricant	Engine oil (5W-30)	Additive solution
Al2O3	98% oil	2% solution (0.25 wt% nanoparticles + 1.75 wt% oleic acid)
TiO2	98% oil	2% solution (0.25 wt% nanoparticles + 1.75 wt% oleic acid)
Al2O3/TiO2	98% oil	2% solution (0.125 wt% Al2O3 + 0.125 wt% TiO2 + 1.75 wt% oleic acid)

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Fig. 3 Dispersion results of nano-lubricants (a) visual analysis and (b) UV-vis spectrum with different time intervals.

To monitor the stability of nano-lubricants, the dispersion of nanoparticles in the engine oil was measured using the UV-visible spectroscopy (at 489 nm wavelength) as shown in Fig. 3b. The nanoparticles showed good suspension stability in the engine oil. The higher absorbance suggests a better dispersion and solubility of the nanoparticles within the engine oil. A reduction in the stability of nano-lubricants was clearly evident after 55 days. The absence of dispersion of nanoparticles in the engine oil can lead abrasive contact between rubbing surfaces and could cancel the beneficial effect of Al₂O₃ and TiO₂ nanoparticles on friction and wear reduction.

2.3 Measuring the thermophysical parameters

The thermophysical parameters (thermal conductivity and viscosity) of the nano-lubricants were measured in the current investigation. The kinematic viscosity of the engine oil (5W-30) without and with Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nanoparticles at two temperatures 40 °C and 100 °C was measured. The kinematic viscosity and viscosity index of nano-lubricants measured was based on GB/T265-1988 and GB/T1995-1998 lubricant testing standards. The 3-ω method is a widely used method to measure the thermophysical properties of bulks³² thin films³³ and liquids³⁴ and was used in the current study.

Fig. 4 shows a schematic and photograph illustration of the experimental setup of the thermal conductivity measurement used in the current research. The platinum wire was welded on a copper rod. The diameter of Pt wire was 20 μm, with the length of the wire being 26.6 mm. AC current from the signal generator passes through the platinum wire and the resistor. The voltage signal of the wire and the resistor are then input into the lock-in amplifier through the differential amplifiers AMP03. The resistor was adjusted to balance the 1ω voltage signal of the platinum wire because the 1ω voltage is thousands of times larger than the 3ω voltage. If the 1ω voltage is not eliminated, the obtained 3-ω voltage may not be accurate. The lock-in amplifier can get the 3ω voltage signal through the differential input A–B (see Fig. 4a). According to literature,^35,36 the relationship between the thermal conductivity, 3ω voltage and frequency is:

where κ is the thermal conductivity of the sample, α is the temperature coefficient of the heater, V_1ω is the voltage of the heater, l is the length of the heater, R is the resistance of the heater before heated, and V_3ω is the 3ω voltage of the heater.

Fig. 4 Experimental setup for the measurement of thermal conductivity by the 3-ω method.

Details of the measurement of thermal conductivity are as follows: an electrical current having a frequency ω and magnitude I₀

I(t) = I₀ cos(ωt) (2)

This was applied to the heater. Due to the fact that the resistance change of the heater is much smaller than its resistance R₀ at ambient temperature, the power dissipated by the heater is given by:

For small temperature changes, the resistance of the wire varies linearly with temperature as given by the following equation:

R(t) = R₀[1 + αΔT cos(2ωt − Φ)] (4)

where α is the temperature coefficient of resistance (TCR), given by:

The resulting voltage across the wire is obtained through the input current multiplied by the heater resistance, and is given by:

In such a structure with an alternating electrical current (AC) passing through the specimen, the heat generation and diffusion into the specimen can be described by the following equation:

Given that the initial and boundary conditions are given by:

r → ∞, U(r) = 0 (8)

So, solving the equation, we can get

where V_1ω is the voltage of the heater, R₀ is the resistance of the heater before heated, l is the length of heater, κ is the thermal conductivity of sample, D is the thermal diffusion of sample, and r is the radius of heater.

From eqn (6), it is possible to obtain the temperature rise by measuring the 3ω voltage, given by:

V_3ω is the 3-ω voltage of the heater, withα representing the temperature coefficient of the heater.

According to eqn (10) and (11), the different 3ω voltage of different frequency was measured to obtain the thermal conductivity.

Fig. 5 illustrates the measured 3ω voltage as a function of ω for the engine oil with and without nanoparticles at a temperature of 292.55 K. As the relationship between 3ω voltage and ω is not linear when the frequency is larger than 15 Hz, only the lower frequency was used for the calculation. The thermal conductivity calculated using eqn (1) was 0.136 W m⁻¹ K⁻¹. For each point, the measurement was taken three times.

Fig. 5 Experimental data for engine oil at a room temperature of 292.55 K. The black squares represent the measured value of 3ω voltage, with the blue solid line representing the linear fitting. At low frequency, V_3ω has a linear relationship with logarithmic frequency.

2.4 Tribotest rig

The tribotest rig is an important experimental tool for investigating the mechanisms of friction reduction, wear and the formation tribo-film by nano-lubricant additives. Tribotest rig was used to simulate the sliding reciprocating motion of the piston ring/cylinder liner interface in an engine according to ASTM G181.³⁷ The friction force was measured by the use of a piezoelectric force transducer (see Fig. 6). Subsequently, the friction force was divided by the normal force to obtain the coefficient of friction. In this set-up, piston ring and cylinder liner samples were used as the friction specimens and were cut from the actually fired engine components in an attempt to ensure that the materials tested are the same as in a real engine. The average hardness of the piston ring surface was 320 VH (Vickers hardness), whilst the hardness for cylinder liner was 413 VH. The hardness was measured according to stipulated standards.³⁸

Fig. 6 Photograph of the bench tribometer for the designed piston ring/cylinder liner interface.

In the current investigation, the tribological properties of lubricants were evaluated for engine oil with and without the addition of nanoparticles under different operating condition (load and speed). The load was varied from 20 to 230 N, which corresponds to contact pressures ranging from 0.5 to 5.7 MPa, respectively. The sliding speed was controlled by the motor rotational speed that varied between 0.5 and 1.2 m s⁻¹. The tribological tests were performed using the nano-lubricants at a temperature of 100 °C, characterizing a top dead centre (TDC) position near the surface of the cylinder liner temperature in an engine.³⁹ The frictional samples (ring and liner) were cleaned using acetone and subsequently dried before the tests.

3. Results and discussion

3.1 Effect of nano-lubricants on thermophysical properties

The kinematic viscosity is the ratio between the dynamic viscosity and the density. Fig. 7 presents both kinematic viscosity and viscosity index for Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants in comparison with the commercially available lubricant (5W-30) under 40 °C and 100 °C temperatures using 0.25 wt% nanoparticles concentration. The kinematic viscosity of nano-lubricants exhibited at the temperatures of 40 °C and 100 °C slightly decreased as shown in Fig. 7a and b. The reason for this viscosity reduction is nanoparticles coming between the lube oil layers leading to the ease of relative movement between nano-lubricant layers, acting as catalysts in a cracking reaction and heat transfer properties.⁴⁰ The kinematic viscosity of engine oil decreased with increasing temperature. This can be related to lower forces between the oil layers. Low viscosity helps to minimize the hydrodynamic (viscous) friction, which is important for reducing the friction force between the piston ring and cylinder liner.

Fig. 7 Effect of Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricant additives on kinematic viscosity and viscosity index of lubricants; (a) viscosity at 40 °C (b) viscosity at 100 °C, (c) viscosity index.

Additionally, the results showed that the viscosity index of the nano-lubricants was increased by 2% as compared with the commercial lubricant (see Fig. 7c). The increase of the viscosity index indicates a more stable kinematic viscosity with temperature change, which provides better resistance to thinning and film strength retention under conditions of heat application for nano-lubricants. Furthermore, the addition of Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants suppressed the rate of reduction in kinematic viscosity with an increase in temperature making the engine oil more suitable for high temperature.

The thermal conductivity of heat transfer plays an imperative role in preserving the engine oil properties for extended periods of the service. Fig. 8 clearly illustrates the variations of the thermal conductivity of nano-lubricants versus temperature as compared with commercial lubricant (5W-30). The results showed that the thermal conductivity for all lubricants increased with an increase in temperature due to weak cohesive forces among the oil layers at high bulk oil temperatures.⁴¹ On the other hand, a nano-lubricant temperature increase results in a more steady dispersion and suspension of nanoparticles in engine oil which leads to rising thermal conductivity. Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants exhibited an increase in the thermal conductivity by approximately 12–16% in the temperature range of 10–130 °C using a nanoparticle concentration of 0.25 wt% in the engine oil. The increase in thermal conductivity in nano-lubricants is mainly due to the Brownian movement of the nanoparticles, which increase with a rise in temperature.⁸ Al₂O₃/TiO₂ hybrid nano-lubricants provided the greatest enhancement in thermal conductivity. This is due to a significant reduction in the thermal interface resistances between nanoparticles, providing a rapid thermal path between nanoparticles.⁴²

Fig. 8 Effect of Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricant additives on thermal conductivity.

Additionally, the TiO₂ nano-lubricant showed higher thermal conductivity than Al₂O₃ nano-lubricant, even though TiO₂ nanoparticle has low thermal conductivity than Al₂O₃ nanoparticle. The reason for this is related to the phenomenon of clustering of TiO₂ nanoparticles in the engine oil as shown in Fig. 3b. TiO₂ nano-lubricant showed a lower dispersion with time than that of Al₂O₃ nano-lubricants due to a slight agglomeration of the TiO₂ nanoparticles in the engine oil. This can easily result in the creation of channels for thermal waves and fast transport of heat resulting in an enhancement in thermal conductivity of the TiO₂ nano-lubricant in comparison with the Al₂O₃ nano-lubricant. The formation of these nanoparticle clusters of nanoparticles tends to enhance the thermal conductivity of the nano-lubricant.⁴³

3.2 Friction and wear tests

Fig. 9 shows the effect of nanoparticles concentration on the friction coefficient for an average sliding speed of 0.7 m s⁻¹ and a contact load of 160 N (the equivalent of 4 MPa contact pressure) over a 25 minute period. Based on the obtained results, it was evident that the nano-lubricant with 0.25 wt% concentration of nanoparticles was the best sample of the nano-lubricants considered. Therefore, the optimum concentration (0.25 wt%) of nanoparticles was used for the investigation of the tribological performance in the current study.

Fig. 9 Effect of nanoparticle concentrations on average friction coefficient.

Illustrated in Fig. 10 is a comparison of the friction coefficient between the ring and the liner against crank angle under 0.75 m s⁻¹ average sliding speed and 100 N contact load, corresponding to a contact pressure 2.5 MPa. Friction coefficient with crank angle shows the negative part due to the change of speed direction during the reciprocating sliding motion. Interestingly, it was observed that the maximum of friction coefficient value is reached near the ends of stroke (TDC and BDC) because of the increase in the metal contact between worn surfaces. On the other hand, the reason is related to the critically low sliding speed attained at dead centers (TDC and BDC), which inhibiting the good access of the lube oil to these locations (boundary or mixed lubrication). However, at mid-stroke low values of friction coefficient were recorded because of the good access of the lube oil (hydrodynamic lubrication) due to the maximum of sliding speed.

Fig. 10 The variation of the friction coefficient during one cycle of crankshaft rotation for nano-lubricants and engine oil without nanoparticles.

From friction coefficient curve, it was observed that the friction coefficient do not remain constant as explained in the modeling results obtained the work done by Ali,² but rather oscillates significantly with crank angle or time owing to stick slip phenomenon during the sliding motion. Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants showed an obvious decrease in the friction coefficient compared with commercial lubricant (5W-30) without nanoparticles. The positive effects of nano-lubricants are more evident under boundary and mixed lubrication regimes as shown in Fig. 10 at TDC and BDC locations when the oil film is so thin such that metal asperities make contact with each other.

Fig. 11 indicates the effect of the contact load and sliding speed on friction coefficient for both lubricants without nanoparticles and Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants. The results revealed that a decrease in friction coefficient with increasing sliding speed and contact load. The high loads applied and sliding velocities lead to surface frictional heating and thus formation of a thin molten layer at the asperity contacts. This could be attributed the mechanism for nano-lubricants, which leads to the formation of a stable tribo-film, reducing the shear strength at the contact resulting in very low friction coefficient. The friction coefficient for the use of nano-lubricants was reduced by 40–50% in comparison with lubricant without nano-particles. The main reasons for the decrease in friction coefficient for nano-lubricants is the ability of the nanoparticles to change pure sliding friction into rolling friction due to reduced interfacial interaction for frictional surfaces and deposition of nanoparticles on rubbing surfaces and formation tribo-film. Furthermore, it can be observed that the oleic acid alone contributes to friction reduction due to chemical reaction on worn surfaces.⁴⁴

Fig. 11 The tribological behaviour of nano-lubricants under different contact loads and sliding speeds; (a) under a sliding speed of 0.95 m s⁻¹ (b) under a contact load of 120 N.

Fig. 12 shows the wear rate of the piston ring for Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants in comparison to commercial lubricant (5W-30) under 230 N contact load. It was evident that the wear rate increased with an increase in the sliding distance. This was because of the frictional surface was no longer able to support the oxide layer and wear changes from tribo-oxidation to adhesion. Based on the obtained results, it was evident that nano-lubricants can significantly reduce the wear rate of the piston ring by 20–30%, in comparison with engine oil without nanoparticles. For the anti-wear mechanism for nano-lubricants, the nanoparticles may carry a portion of the pressure and separate the worn surfaces to prohibit adhesion. However, the formation of a tribo-film on worn surfaces by nano-lubricants enhances the anti-wear characteristic. It was observed that the TiO₂ nano-lubricants provided a minimum wear rate for the ring owing to the deposition of TiO₂ on the surfaces of the ring and liner being more than the Al₂O₃ as shown in Fig. 13 and 14.

Fig. 12 Comparison of wear rate for Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants with commercial lubricant (5W-30) under a contact load of 230 N.

Fig. 13 FE-SEM and EDS patterns with elemental content of the piston ring surface for: (a) commercial lubricant (5W-30), (b) Al₂O₃ nano-lubricant, (c) TiO₂ nano-lubricant, and (d) Al₂O₃/TiO₂ hybrid nano-lubricant.

Fig. 14 FE-SEM and EDS patterns with elemental content of the cylinder liner surface for: (a) commercial lubricant (5W-30), (b) Al₂O₃ nano-lubricant, (c) TiO₂ nano-lubricant, and (d) Al₂O₃/TiO₂ hybrid nano-lubricant.

3.3 Surface analysis

In this section, FE-SEM and EDS chemical analysis on worn surfaces of the piston ring and cylinder liner was performed and results are shown in Fig. 13 and 14, respectively. FE-SEM and EDS patterns provided the explanation of the mechanisms of friction reduction and tribo-film formation as experimental evidence. These patterns showed the elements analysis of the frictional surfaces lubricated with nano-lubricants containing Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid elements on worn surfaces of the ring and liner. Tribo-films can be formed on the piston ring and cylinder liner surfaces through chemical reaction and physical mechanism.

The EDS pattern displays a trace amount of phosphorus (P active element of ZDDP) in tribo-film on worn surfaces (see Fig. 13 and 14). This suggests the involvement of nanoparticles and the oil additive package, specifically zinc dialkyldithiophosphate (ZDDP) in the engine oil (5W-30) with substrate surface for the formation of tribo-film (via chemical reactions). The increase of the thermal activation for nano-lubricant leads to the lubricity of the nanoparticles, which are deposited on the worn surfaces as a solid lubricant. Furthermore, Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nanoparticles in the engine oil (5W-30) can fill scars and grooves of the worn surfaces (physical mechanism) to help the separation, reducing metal-to-metal contact, thus providing a hydrodynamic effect responsible for a minimizing the friction coefficient and making the surfaces smoother. The worn surfaces were studied and analysis of piston ring and cylinder liner by 3D surface profiler to determine the surface roughness. Fig. 15 shows that the surface roughness of the piston ring and cylinder liner lubricated by nano-lubricants decreased by 50–82% and 3–7%, respectively, due to the mending effect¹ by Al₂O₃ and TiO₂ nanoparticles, as compared with the commercial lubricant (5W-30).

Fig. 15 The surface roughness of piston ring and cylinder liner lubricated by Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricants.

Based on the EDS patterns, there was a detected evidence for nano-lubricants in the form of the deposition of TiO₂ more than Al₂O₃ nanoparticles (see Fig. 13 and 14), although its concentration was equal for all nano-lubricants (0.25 wt%). This suggested that part of Al₂O₃ remain dispersed in engine oil or become a third body to help facilitate the change from sliding to rolling friction and hence minimize the friction coefficient. The deposition of TiO₂ nanoparticles on the worn surface of the piston ring and cylinder liner can decrease the wear rate of the ring via delamination of the TiO₂ nanoparticles as shown in Fig. 12, which compensates the mass loss of materials from worn surfaces.

4. Conclusions

The current study presented the possible enhancement of the thermophysical properties and tribological behaviour of engine oils via the addition of Al₂O₃, TiO₂ and Al₂O₃/TiO₂ hybrid nano-lubricant additives. These additives can contribute to improving the efficiency of engines. On the basis of results presented in the preceding section, the following conclusions could be drawn:

(1) The kinematic viscosity of nano-lubricants decreased slightly owing to the presence of nanoparticles between the lube oil layers leading to the ease of relative movement between nano-lubricant layers and acting as catalysts in a cracking reaction. The reduction in viscosity might be attributed to the decrease in the hydrodynamic (viscous) friction. Moreover, the viscosity index increased with the use of nano-lubricants by 2% as compared to engine oil without nanoparticles, which provided better resistance to lubricant thinning and film strength retention.

(2) Nano-lubricants showed an improvement of 12–16% in thermal conductivity in the temperature range of 10–130 °C due to the Brownian movement of nanoparticles. Al₂O₃/TiO₂ hybrid nano-lubricants presented a maximum enhancement of thermal conductivity because of the reduction in thermal interface resistance between the nanoparticles, as compared with engine oil without nanoparticles.

(3) The surface roughness of the piston ring and cylinder liner decreased with the use of nano-lubricant additives by 50–82% and 3–7%, respectively, due to mending effect of the nanoparticles.

(4) The friction coefficient and wear rate of the ring were reduced by 40–50% and 20–30%, respectively, for the use of nano-lubricants due to the rolling effect of nanoparticles together with a tribo-film formation on worn surfaces, as compared to the commercial lubricant.

(5) The analysis of EDS patterns showed that the nanoparticles and the oil additive package, specifically the phosphorus (P) active element of ZDDP involvement in formation of a tribo-film on worn surfaces, assisted in significantly lowering the friction and the wear as a consequence.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

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. M. K. A. Ali and R. F. Turkson acknowledge the Chinese Scholarship Council (CSC) for financial support for their PhD studies in the form of CSC grant Numbers 2014GF032 and 2013GXZ993 respectively. M. K. A. Ali also appreciates the financial support from the Egyptian Government.

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