Research On Friction And Wear Reduction Mechanism Of Engine Nano Lubricating Oil And Its Influence On Particulate Matter Emission

Lubricating oil is known as the"blood"of engines,and is one of the key factors affecting engine mechanical efficiency,reliability and particulate matter（PM）.In recent years,the urgent demand of high efficiency,energy saving,near zero emission and low fuel consumption of engines has put forward higher requirements for lubricating oil quality,such as low ash,low sulfur phosphorus,low viscosity,long oil change cycle,friction reduction and wear reduction performance.With the development of nano science and technology,nano additives have been applied to the industrial lubrication field,which has excellent friction-reducing effect and has been widely concerned.However,the complex and changeable and extremely harsh working conditions of the engine pose a higher challenge to the dispersion stability,friction and wear reduction efficiency of nano lubricating oil.At the same time,the influence of nano lubricating oil consumption on PM emission and post-treatment reliability has not been proved,which seriously hinders the promotion and application of nano lubricating oil in the engine field.Therefore,in this paper,two nano additives,Cerium Oxide（Ce O₂）and nano graphene,which have the most potential and representative application for reducing friction and wear,are introduced into engine low-viscosity lubricating oil.Based on the molecular dynamics simulation,friction test and engine performance test platform,from both nano and macro perspectives,a series of studies were carried out on the dispersion stability,physical and chemical properties,friction and wear reduction mechanism,PM particle size distribution and physical and chemical properties of nano lubricating oil,which laid a theoretical and scientific basis for the design,development and application of nano lubricating oil.The main research contents of this paper are as follows:（1）Based on the molecular dynamics software,Material Studio（MS）,and according to the characteristics of nano additives and lubricating oil,the grafting modification method of oleic acid and stearic acid on the surface of nano additives was proposed,the molecular model of nano lubricating oil before and after modification was established,and the influence of nano additives modification on the dispersion stability of nano lubricating oil was studied.The results show that the dispersion stability of the modified nano lubricating oil is improved effectively at both ambient temperature and engine operating temperature,and the effect of nano Ce O₂ modification on dispersion stability is stronger than that of nano graphene.This is because the long carbon chain of the oleic acid and stearic acid can stretch in the lubricating oil to form a resistance layer,produce dimensional repulsion potential energy,hindered the reunion of nano additive.And compared with the layered nano graphene,the oleic acid and stearic acid grafted onto the spherical surface of nano Ce O₂ can be more fully extended in all directions to form steric layers.（2）The nano additives were modified by oleic acid and stearic acid through chemical grafting test.Nano lubricating oil with different concentration before and after modification was prepared.Combined with precipitation method and transmittance method,the actual dispersion stability of nano lubricants modified by nano additives and concentration was studied at ambient temperature.The effects of the modification and concentration of nano additives on the dispersion stability of nano lubricating oil at ambient temperature were studied by precipitation method and light transmittance method.Based on relevant standards of lubricating oil testing,the influence of modification and concentration of nano additives on the physicochemical properties of nano lubricating oil were analyzed.The results show that the dispersion stability of the modified nano additives in lubricating oil is improved,which is consistent with the simulation results.With the increase of nano additive concentration,the dispersion stability decreases slightly.The dispersion stability of Ce O₂ in lubricating oil is slightly lower than that of graphene in lubricating oil.This is due to the fact that the steric hindrance of oleic acid and stearic acid and the effect of gravity should be considered comprehensively in the nano additive,while Ce O₂ has a higher density and is easy to precipitate.With the increase of Ce O₂concentration,the lubricating oil density increases slightly and oxidation performance improves.With the increase of graphene concentration,the viscosity of lubricating oil fluctuates slightly,decreasing first and then increasing.Nano additives have little effect on other physical and chemical properties of lubricating oil（pour point,flash point,total acid value）.（3）Based on MS software,the intermolecular force model between friction pairs and different lubricating oils was established from the molecular level,and the contact behavior at molecular level between nanoparticles and lubricating oil and its friction and wear reduction mechanism were revealed.The results show that when the nano additive is near the surface of the friction pair,the solid-like layer attached to the surface of the friction pair overlaps with the adsorption layer on the surface of the nano additive,resulting in zero displacement of the nano additive relative to the surface of the friction pair at a certain shear speed.As the nano additives are far away from the surface of the friction pair,the shear stress decreases,indicating that the nano lubricating oil with better dispersion has better friction reduction effect.Compared with pure lubricating oil,nano lubricating oil has lower compression rate under different pressures,which helps to reduce the contact of micro-convex body and reduce wear.It can be seen from the radial distribution function of lubricating oil molecule pairs that this is due to the liquid-solid two-phase characteristics of nano lubricating oil.Nano lubricating oil can better demonstrate shear resistance under high pressure,which is due to the liquid-solid transformation of the lubricating oil under high pressure,and the transformation pressure of nano lubricating oil is higher.（4）Based on the friction test and related characterization test platform,the friction and wear properties of nano lubricating oil under different engine working conditions were studied,and the interaction mechanism between nano additives and the surface of friction pair was revealed.The friction reduction effect of nano lubricating oil on the engine was studied through engine motored test.The results show that nano additives can reduce friction in different degrees,especially at high temperature.This is because the viscosity of lubricating oil at high temperature is lower,the molecular movement is more intense,the nano additive effect plays a dominant role,the self-lubrication effect of graphene interlayer sliding and the microbearing effect of Ce O₂ is more prominent.With the addition of nano additives,the anti-wear performance improves first and then decreases.This is due to the attachment and deposition of nano additives on the surface of the friction pair.Excessive nano graphene will fold or agglomerate in the process of friction and shear,and excessive Ce O₂ will cause accumulation and increase the wear.The results of tribology test are verified by the reduction of friction loss work lubricated with nano lubricating oil.（5）The engine performance test platform was set up,the effects of nano lubricating oil on particle size distribution and physical and chemical characteristics of PM were studied by means of blending and burning diesel with lubricating oil,and the risk of nano lubricating oil on reducing the reliability of the post-treatment system was assessed.The results show that,compared with pure lubricating oil,the number of nuclear PM and accumulated PM corresponding to nano lubricating oil increase significantly,and the increase of accumulated PM is more obvious,and this phenomenon is more obvious when the load increases.This is because when nano lubricating oil is involved in combustion,nano additives will nucleate themselves,increasing the risk of generating nuclear PM.Moreover,these self-nucleated PM and their aggregates will be directly adsorbed on the carbon surface or further agglomerate with carbon particles to form accumulated PM,resulting in increased risk of accumulated PM.And with the increase of load,temperature increases,resulting in the increase of agglomeration and accumulated PM.It is revealed that the use of nano additives leads to the change of PM micro-nano structure,surface functional group composition and the catalytic performance of Ce O₂ itself,which are the fundamental reasons for the change of PM oxidation characteristics（PM oxidation performance of graphene lubricating oil decreases,while PM oxidation performance of Ce O₂ lubricating oil improves）.PM produced by nano lubricants has some negative effects.