Research On The Thermophysical Mechanisms Of Nano-Lubricants For Improvement Of The Lubrication And Friction Properties Of Piston Set-Cylinder Liner Of Internal Combustion Engine

The lubrication between the piston and cylinder liner directly affects the power performance, economical efficiency and reliability of internal combustion engine. The friction characteristic between the piston and cylinder liner is extremely complex, such as high temperature, heavy load and speed change. These behaviors consequently lead to the coexistence lubrication states of hydrodynamic lubrication, thin film lubrication and boundary lubrication. The nano-lubricant, which is composed of nanoparticles and lubricant, can not only improve the tribology characteristics but also enhance heat transfer effectively. Therefore, the nano-lubricant shows great potential in the lubrication and cooling of piston set-cylinder liner. The mechanisms of nanoparticles for improvement of tribology properties under different lubrication state are not fully known yet. The several lubrication states cannot be analyzed separately by traditional research methods. Fortunately, the molecular dynamics (MD) simulation can be used to simulate different lubrication states and working conditions, and to effectively reveal the mechanisms of lubrication and friction from the microscopic view. The structural features of nanofluids also can be accurately described by this method. Therefore, the deep researches on the thermophysical mechanisms responsible for the favorable friction properties of nanoparticles under different lubrication states can be performed with the MD method.The simulated systems under hydrodynamic lubrication, thin film lubrication and boundary lubrication were built, respectively, according to the lubrication states of piston set-cylinder liner. This paper investigated the tribology behaviors of nano-lubricants with the MD method. By the analyzing of the differences in the tribology properties between the base oil and nano-lubricant, the roles of nanoparticles in the improvement of tribology properties under different lubrication states were given. At last, the coupling interaction between the heat transfer enhancement and the tribology properties of nanofluids was further investigated. The main research contents of this paper were described as follows:(1) Under the conditions of hydrodynamic lubrication, the effect of nanoparticles on the shear flow properties of lubrication film for the simulated systems either with smooth surface or with rough surface was studied. Results show that the load-carrying capacity of lubrication film is increased by the influence of nanoparticles on the microstructures of fluid. Nanoparticles deposited in the pits play a role of filling friction surfaces, reducing the friction force. However, nanoparticles except deposited in pits increase the friction force by increasing the viscosity of fluid. No matter the base fluid and nanofluids, the load-carrying capacity of lubrication film is reduced with the temperature increasing.(2) Under the thin film lubrication, the tribology behavior differences between the base fluid and nanofluids were mainly analyzed. With the increase of load, liquid-solid transitions take place for both base fluids and nanofluids. Specifically, the transition pressure for nanofluids is higher than that of the base fluid. The higher transition pressure of nanofluids can keep the friction force at very low level in a wide range of normal force. Because of the higher transition pressure as well as the volume effect and the micro-motions of nanoparticles, the friction force of nanofluids is lower than that of base fluid when the load is high.(3) Under the boundary lubrication, the influences of Cu and diamond nanoparticles on the load-carrying capacity of boundary film were studied. Results indicate that the load-carrying capacity of lubricant films can be improved by the addition of nanoparticles. The mechanisms of nanoparticles in improving load-carrying capacity were analyzed in terms of three aspects. Firstly, because of the compact adsorption layer around nanoparticles, the nano-lubricant molecules become more organized and compact compared with the base oil, which contributes to improving the boundary film strength. Secondly, with the increase of load the nano-lubricant film structure composition is changed to the special form:adsorption layer on the upper wall-adsorption layer around the nanoparticle-nanoparticle-adsorption layer on the lower wall. The nano-lubricant strength therefore increases compared with the base oil. Thirdly, with the further increase of load, nanoparticles begin to provide excellent support effect for the boundary film.(4) The effects of Cu, diamond and SiO2 nanoparticles on the tribology behaviors of asperity contact were investigated, respectively. The synergy between nanoparticles and friction pair and the changes in mechanical properties of the friction system were mainly discussed. Results confirm that a solid lubricating film is formed by the soft Cu nanoparticle on the friction surfaces, which accommodates the velocity gradient. Because of this, the plastic deformation, defect structures and temperature distribution in friction pair are all reduced greatly. The diamond and SiO2 nanoparticles separate the two blocks from each other, act as ball-bearings and have polishing effect on the friction surfaces. Due to the action of the two hard nanoparticles, the plastic deformation and temperature distribution in friction pair are reduced significantly. With the increase of the temperature of friction pair, both the improvement of soft and hard nanoparticle in the tribology properties of asperity contact becomes more significant. Under high velocity conditions, a transfer layer appears adjacent to the interface of friction pair. The nanoparticles are trapped in the transfer layer, which leading to the disappearance of the friction reduction and anti-wear properties.(5) By studying the effect of nanoparticles on the flow behaviors as well as the coupling action between the flow and heat transfer properties, the mechanism of heat transfer enhancement in nanofluids was further clarified. The coupling interaction between the heat transfer enhancement and the tribology properties of nanofluids was also expounded. The results show that the velocity fluctuation in nanofluids is enhanced compared with the base fluid. There is slip velocity between nanoparticles and base fluid. The contributions of chaotic movements of nanoparticle and increased thermal conductivity to the heat transfer enhancement of nanofluids are equal. The heat transfer enhancement of nanofluids is benefit to the improvement of friction and wear properties of piston set-cylinder liner.