Research On The Mechanisms Of Nano-Lubricants For Improvement Of The Heat Transfer And Lubrication Property Under The Condition Of Hydrodynamic Lubrication Of Piston Set-cylinder Liner

The piston acts as a combustion chamber component and is subjected to a direct effect of the heat load from the high temperature gas in the cylinder.Most of the heat is passed through the piston ring group to the cooling water jacket which will increase the lubricating oil temperature.High temperature will make the lubricating oil film viscosity drops sharply and decrease the thickness of lubricating oil film which cause the condition of lubrication difficulty and the abrasion becomes more serious.Ignoring these problems will worsen the wear in the internal combustion engine and some severe accidents such as coking and abrasions.Consequently,mechanical loss and material waste increased.Fortunately,nano-lubricant has the advantages of improving lubrication friction property and strengthening heat transfer ability.If apply nano-lubricant to lubrication and heat transfer processes between piston set-cylinder liner of internal combustion engine can not only improve its tribological performance and reduce the friction loss,but also improve the efficiency of the internal combustion engine and reduce the exhaust emission.The heat transfer process through the oil film between the piston ring and cylinder liner to the cooling water jacket will be intensified and decrease the oil film temperature,further improving the lubrication property in the piston ring-cylinder liner friction pair.The majority of the lubricating oil film between the piston ring and the cylinder liner is under the condition of hydrodynamic lubrication.The computational fluid dynamics method can be used to study the heat transfer and lubrication properties of lubricating oil film which is under the condition of hydrodynamic lubrication.The CFD method can investigate the flow and heat transfer characteristics of nano-lubricant from the perspective of the momentum and energy transfer between the nanoparticles and the basic lubricants.The effects of the addition of nanoparticles on the convective heat transfer and lubricating friction coupling of the lubricating oil were analyzed from the perspective of the momentum transfer and energy transfer between the nanoparticles and the basic lubricant.In this dissertation,the actual working conditions of the diesel engine are simulated,and the CFD method is used to study the physical mechanism of the piston-ring liner nano-lubricating oil to improve the fluid lubrication convection heat transfer and lubrication friction coupling.First of all,we obtained the physical parameters contained the piston ring and cylinder liner temperature,oil film thickness,pressure and oil viscosity of the nano-lubricant in piston ring-cylinder liner friction pair by single-phase model.And then we took these parameters as the boundary condition of two-phase model and researched the heat transfer and lubrication performances of nano-lubricant corresponding to every single crank angle.Finally,the influence of the addition of nanoparticles on the temperature field and velocity field of the basic lubricating oil was considered to study the talent of the heat transfer and lubrication performances of nano-lubricant.The following is the content of this paper:(1)We used the parameters of BF6M1013 diesel engine as the conditions of piston ring and set liner unsteady heat and mixture lubrication model.Then the results acquired from this model,such as the piston ring and cylinder liner temperature,oil film thickness,pressure and oil viscosity of the nano-lubricant,had been the boundary conditions for the following CFD numerical simulations.(2)During the operation of the internal combustion engine,the physical parameters of the lubricating oil film change with time and are unstable.In this paper,the unsteady state is converted to quasi-steady state,for each crank angle corresponding to the state of the oil film to carry out two-phase flow study.The CFD method was used to study the convective heat transfer and lubrication friction characteristics of nano-lubricating oil during the working process of the internal combustion engine.Compared with the basic lubricating oil,it was found that the introduced lubricating oil film in the internal combustion engine had improved the properties of lubrication friction and heat transfer.(3)The lubricating oil corresponding to a crank angle had been the research object to analyze the lubrication and heat transfer performances.Firstly,the effects of nanoparticles volume fraction,diameter and species on the convective heat transfer and lubrication friction were discussed.From the view of the interaction between lubricant and nanoparticles,we observed the variations of temperature and velocity field when nano-particles were added.We study the acting force between the particles and base oil and investigated the physical mechanism of the outstanding performances of nano-lubricant between piston ring and cylinder liner.