The tribology properties of TiO₂/POE, SiO₂/POE and TiO₂-SiO₂/POE nanolubricants were investigated for an automotive air-conditioning system with an electrically-driven compressor (EDC). A two-step preparation method was used in dispersing TiO₂ and SiO₂ nanoparticles into Polyol-ester (POE)-based lubricant at different volume concentrations of 0.01 to 0.1%. The coefficient of friction (COF) and wear scar diameter (WSD) were investigated using a Koehler four-ball tribo tester and microscopes. For the TiO₂/POE, SiO₂/POE and TiO₂-SiO₂/POE nanolubricants, respectively, the lowest COFs with maximum reduction were attained at 37.5%, 33.5% and 31.6% each at volume concentrations of 0.05%, 0.01% and 0.03%. The highest WSD reduction for the TiO₂/POE and SiO₂/POE mono nanolubricants were attained at 12.5% and 26.4%, respectively, at the same volume concentration of 0.01%. Meanwhile, the maximum reduction of WSD for the TiO₂-SiO₂/POE hybrid nanolubricant was reached at 12.4% at 0.03% volume concentration. As a conclusion, mono and hybrid nanolubricants with volume concentrations of less than 0.05% are suggested for use in air-conditioning systems with EDC because of their outstanding tribology performances. Further performance investigation of nanolubricants in the air-conditioning system is required to extend the present work.

Poor characterisation of nanoparticle suspensions impedes the development of nanolubricants for use in refrigeration and air-conditioning systems. Chemical treatment techniques, such as surfactants, are inappropriate for enhancing the stability of nanolubricants intended for use in vapour-compression refrigeration (VCR) systems. Prior to incorporating nanolubricants into the system, the stability of TiO₂ and SiO₂ nanoparticles dispersed in PVE was therefore investigated. The nanolubricants were prepared by a two-step method with the aid of an ultrasonication bath homogeniser. Visual observation and ultraviolet–visible (UV–Vis) spectrophotometric analysis were used, and zeta potential analysis was then performed to confirm the nanolubricants’ stability condition. The TiO₂/PVE nanolubricant was observed to be maintained at a 95% concentration ratio for up to 30 days of evaluation. The TiO₂/PVE, SiO_2/PVE, and SiO₂-TiO₂/PVE exhibited zeta potential values of 203.1 mV, 224.2 mV, and 105.3 mV, respectively, after 7 h of sonication. A high absolute value of zeta potential indicates that the electrostatic repulsive forces between nanoparticles are exceptionally strong, indicating an excellent stable suspension. The high values of zeta potentials validated the excellent stability conditions determined by UV–Vis analysis and visual observation. It can be concluded that ultrasonication times of 7 h produced the most stable state for mono- and hybrid nanolubricants.

The addition of nanoparticles may have a positive or negative impact on the thermal and tribological properties of base lubricant. The objective of this paper is to investigate the effect of nanoparticle dispersion in lubricant base in relation to its application in refrigeration system compressors. An investigation of tribological and thermal properties of nanolubricants for rolling piston rotary systems was carried out through four-ball tribology tests and thermal conductivity measurements. Nanolubricants dispersed with SiO₂ and TiO₂ nanoparticles were tested at various concentrations and temperatures. The changes in thermal conductivity and coefficient of friction (COF) were analyzed while wear weight loss was also calculated from wear scar size. A regression model of thermal conductivity enhancement was proposed for both types of nanoparticles. Zeta potential results show that nanolubricants have excellent stability. The thermal conductivity increases by the increment of nanoparticle concentration but decreases by temperature. The R-square for the regression model is more than 0.9952 with an average deviation not more than 0.29%. The COF for SiO₂/PVE nanolubricant at 0.003 vol.% reduced 15% from the baseline. The COF for nanolubricants exceeds the result for base lubricants when the concentration is more than the threshold value. The optimum concentration of SiO₂ and TiO₂ nanoparticles improved the thermal and tribological properties of PVE lubricant and may offer an advantage when applied to refrigeration systems.

The enhancement of nanolubricant rheological properties can improve the performance of automotive air-conditioning systems. The rheological properties of the TiO₂/POE nanolubricant were investigated in this study at 0.01 to 0.1% volume concentrations and temperatures ranging from 0 to 100°C. TiO₂ nanoparticles were dispersed in the base lubricant of Polyol-ester (POE RL68H) lubricant in two steps. The dynamic viscosity was measured with an Anton-Paar Rotational Rheometer. According to the findings, the TiO₂/POE nanolubricant behaved as Newtonian fluids at all volume concentrations and temperatures. The dynamic viscosity increment of nanolubricants up to 1.75% only occurred for 0.1% volume concentration and temperature of 90 to 100°C. Meanwhile, when compared to POE lubricant, nanolubricants with volume concentrations of 0.01 and 0.05% showed a decrement trend in dynamic viscosity of up to 1.8%. Finally, the TiO₂/POE nanolubricant improved the rheological properties of the POE lubricant for use in automotive air-conditioning systems.

Proper preparation and stability evaluation of the nanolubricant shall be established when applying the nanoparticle dispersion technique in a two-phase system. The stability of the nanolubricant ensures the maximum benefit gained from the dispersion of nanoparticles in specified Polyvinyl ether (PVE). In this study, TiO₂/PVE nanolubricant was prepared using two methods of physical homogenization: high-speed homogenizer (HSH) and ultrasonication bath. The HSH used a preparation time of up to 300 s in the stability assessment. Meanwhile, the ultrasonication bath had a preparation time of 1, 3, 5, and 7 h. The stability condition of the nanolubricant was evaluated using photo capturing, ultraviolet-visible (UV-Vis) spectrophotometer, zeta potential, and zeta sizer. A sample with 180 s of preparation time shows the best stability condition from HSH. The nanolubricant with ultrasonication offers excellent stability at 5 h of homogenizing time with a concentration ratio of more than 90% for up to 30 days of observation. In conclusion, ultrasonication homogenizing methods show better results than HSH with a zeta potential of more than 60 mV. In addition, HSH can be recommended as an optional method to produce nanolubricant with a low preparation time for immediate use.