Improving the tribological characteristics of piston ring assembly in automotive engines using Al2O3 and TiO2 nanomaterials as nano-lubricant additives
Graphical abstract
Introduction
Existing and future automotive engines would require more efficient engine oils, a situation that presents a new challenge for researchers and designers with regard to finding ways of enhancing the tribological characteristics of internal combustion engines while achieving a reduction in fuel and lube oil consumption. Most designers and researchers have focused on Nano tribology in internal combustion engines as the key strategy for minimizing frictional power losses, excessive heat generation and the wear of contact surfaces, in a manner that will ultimately lead to an improved performance of automotive engines. The piston ring assembly makes a significant contribution towards the total frictional power losses. The power losses in automotive engines vary between 17% and 19% of the total energy generated [1]. An improvement in the tribological performance of the lubricating oils and the piston ring assembly leads to an improved efficiency and fuel economy of engines because the friction between the piston ring and the liner accounts for almost40–50% of the power losses [2], [3]. Furthermore, controlling friction via the use of nano-lubricants leads to a decline in the level of wear and an increase in the service intervals for which an oil change is required. This ultimately translates into minimized maintenance costs.
The lubrication is classified into three general regimes: boundary, mixed, and elastohydro-dynamic/ hydrodynamic [4]. During one stroke, the different lubrication regimes can occur over the stroke depending on running conditions. Boundary or mixed lubrication regimes occur near the top and bottom dead center of stroke (TDC and BDC), with the hydrodynamic lubrication regime occurring at mid-stroke [2]. Nevertheless, hydrodynamic friction increases under the conditions of high speeds and low loading [5]. The total friction of the piston ring assembly comprises boundary friction at asperity contact locations (TDC and BDC) and viscous friction due to shearing of lubricant. The boundary lubrication can occur if the oil film becomes thin enough. The load is carried on the surface peaks and not by the lubricant film. For this reason, nano-lubricant additives are most effective under boundary lubrication conditions as it forms a tribofilm at the asperity contact locations to separate the sliding surfaces [6].
Recently, nanoparticles have attracted increasing interest and have been used in a lot of energy related fields owing to their unusual electrical, mechanical and piezoelectric properties [7]. The nanoparticles could be used as nano-lubricant additives in automotive engines to improve the tribological performance, oil properties, exhaust emission, combustion, saving fuel and enhance heat transfer rate. The addition of nanomaterials to lubricating oils can effectively improve the tribological properties through the formation of a protective film on surfaces and creating a rolling effect between friction surfaces [8], [9]. Nanomaterials are different from traditional bulk materials because they possess high specific surface areas and extremely small sizes. The selection of nanomaterials is a very important step towards the improvement of the tribological performance and enhancing the properties of engine oils [10]. Re-formation of solid-like tribo-films by the nanoparticles on the friction surfaces is not possible at high speeds when the lubricant film thickness is larger than the nanoparticles diameter [11]. The bearing and sliding between the nanoparticles play the main role in lowering the friction and wear between worn surfaces [12]. Furthermore, the lower elastic modulus and the comparatively higher magnitude of hardness possessed by nanomaterials can be considered as the main reasons for the excellent lubricating properties [13].
Aluminum and titanium oxides (Al2O3 and TiO2) nanoparticles are the most appropriate for many environmental applications due to their excellent tribological, chemical and thermal properties [14]. The addition of TiO2 nanoparticles with lubricant oil showed stable friction due to the formation of protective films on worn surfaces [15]. Shenoy et al. [16] investigated the influence of TiO2 nanoparticles additives in lube oil. The results exhibited a higher load bearing capacity by a margin of 35% compared to the use the lube oil without nanoparticle addition. Friction and wear experiments were conducted by Kao and Lin [17] using a reciprocating sliding tester. The average diameter of TiO2 was 50 nm and 5 wt% particle concentration in the rapeseed oil. The results revealed that there was an 80.84% reduction in mean surface roughness. The coefficient of friction and the wear scars decreased approximately by 15.2% and 11%, respectively [18]. Mohan et al. [19] studied the influence of the addition of Al2O3 nanoparticles to lubricating oil (SAE 20W40). The results illustrated that using 20 nm of grain size for a 0.5% wt% could reduce the friction by 49.1% and 21.6% under flooded and starved conditions, as compared to the lubricant without nanoparticles. Another study also demonstrated that using 40–80 nm of grain size Al2O3 nanoparticles at 5 wt% concentration reduced of the wear rate [20].
The agglomeration of the nanoparticles in engine oil inhibits their free motion and eliminates the mechanism of nanoparticles (the transfer nanoparticles from engine oil to rubbing surfaces to form tribofilm and produce a rolling effect). Therefore, the nanoparticles were added to the engine oil with different solvents to disperse the particles to form stable suspensions for a more effective operation [21], [22]. Gulzar et al. [23] added 1 wt% of oleic acid to nanoparticles for the purpose of suspension and the reduction of agglomerates. The results suggested that adding oleic acid reduced of the wear. The results from the experimental work carried out by Luo et al. [24] exhibited that the average reduction of the friction coefficient was 17.61% for the four-ball test and 23.92% for the thrust-ring test, for a 0.1 wt% concentration and 78 nm grain size of Al2O3 nanoparticle addition to the oil. Using a low concentration of TiO2 nanoparticles is enough for improving the tribological characteristics [25]. Vasheghani et al. [26] investigated the effect of tiny Al2O3 nanoparticles on engine oil properties. The results presented that the thermal conductivity improved by 37.49% using 3 wt% concentrations. Nanoparticles can easily enter into small gaps between sliding surfaces because of their ultrafine sizes, whereas the micron-scale traditional additives cannot [27].
The objectives of the current study include the formulation of Al2O3 and TiO2 nano-lubricants and the investigation of the tribological characteristics of a piston ring assembly under different running conditions such as contact loads, reciprocating sliding speeds, sliding distance and concentration of nanoparticles. In addition, more investigations were performed using field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and noncontact 3D surface profiler for analyzing the worn surfaces to understand the major mechanisms leading to improve the tribological behavior of piston ring assembly in automotive engines using Al2O3 and TiO2 nano-lubricant additives.
Section snippets
Materials
Against the background that Al2O3 and TiO2 are the most appropriate for many tribological applications (including solid lubricants) because of their excellent tribological behavior, these nanoparticles were chosen for the current investigation. Al2O3 and TiO2 powders were purchased from Nanjing XFNANO Materials Tech Co., Ltd. The average sizes of the Al2O3 and TiO2 nanoparticles were 8–12 nm and 10 nm, respectively. The varying concentrations of Al2O3 and TiO2 used were 0.05, 0.1, 0.25 and 0.5 wt%
Characterization of Al2O3 and TiO2 nanoparticles
The FE-SEM images in Fig. 4 illustrate the morphology of the nanostructures for the Al2O3 and TiO2 nanoparticles. The morphology of the Al2O3 and TiO2 nanoparticles was fairly spherical, which provided a very good rolling medium within the engine oil. The XRD pattern of TiO2 nanoparticles in the current investigation is shown in Fig. 5. 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°. The most pronounced peaks are
Conclusions
In the current study, the tribological performance of a piston ring assembly involving the use of Al2O3 and TiO2 nano-lubricants has been investigated. On the basis of the results presented above, it can be concluded that:
- 1.
The optimum concentration of Al2O3 and TiO2 nanoparticles blended with the engine oil was 0.25 wt%. Moreover, the addition of oleic acid as a solvent not only aided nanoparticle suspension but also reduced the friction coefficient and wear rate of the ring by 11% and 2.6%,
Notes
The authors declare no competing financial or potential conflict of interest.
Acknowledgments
The authors would like to express their deep appreciation to the Hubei Key Laboratory of Advanced Technology for Automotive Components and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, (Wuhan University of Technology) for finacial 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
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