Improving the tribological characteristics of piston ring assembly in automotive engines using Al 2 O 3 and TiO 2 nanomaterials as nano-lubricant additives

What is it about?

To minimize the frictional power losses in automotive engines, it is imperative to improve the tribological characteristics of the piston ring assembly. This study examined the tribological behavior of the piston ring assembly using nanoparticles as nano-lubricant additives. The average size of Al2O3 and TiO2 nanoparticles were 8-12 nm and 10 nm, respectively. The nanoparticles were suspended using oleic acid in four different concentrations in the engine oil (0.05, 0.1, 0.25 and 0.5 wt.%). The tribological behavior of nano-lubricants was evaluated using a tribometer under different operating conditions to mimic the ring/liner interface. The results showed a decrease in the friction coefficient, power losses and wear. The study provides insights into how nano-lubricant additives could contribute towards energy saving and improved fuel economy in automotive engines.

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Why is it important?

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%, respectively, and could be attributed to the chemical reactions on the frictional surfaces. 2. The kinematic viscosity of Al2O3 and TiO2 nano-lubricants decreased slightly due to the presence of nanoparticles between the lubricant layers leading to an ease of relative movement with the nanoparticles acting as catalysts. On the other hand, the viscosity index increased with the use of nano-lubricants by 1.86% which could lead to an improved fuel economy in automotive engines. 3. The friction coefficient decreased by 48-50%, 33-44% and 9-13% for the boundary, mixed and hydrodynamic lubrication regimes, respectively, as compared with the use of the engine oil without nanoparticles. This implied that nano-lubricants are most effective in reducing the friction coefficient in the boundary lubrication regime. 4. The frictional power losses were also reduced by 45% and 50% for the Al2O3 and TiO2 nano-lubricants, respectively. The reduction in friction and power losses could be attributed to the conversion of sliding into rolling friction and the formation of tribo-films on the worn surfaces. 5. The wear rate of the piston ring was reduced by 21-29% for the use of TiO2 and Al2O3 nano-lubricants respectively, after a 50 km sliding distance, as compared to engine oil without nanoparticles. The anti-wear mechanism was generated by tribo-film through a chemical reaction and a physical mechanism. 6. The surface morphology of the piston ring revealed that nano-lubricant additives resulted in smoother worn surfaces. It is noteworthy that the Al2O3 nano-lubricant was more effective in improving the anti-wear and scuffing resistance via the formation of self-laminating protective films. These protective films could take the form of a solid lubricant or an ultra-thin lubricating coating. Whereas the TiO2 nano-lubricant was more effective in reducing the friction coefficient. This was because a majority of the TiO2 nanoparticles remained blended with the engine oil to produce the rolling effect.