| Piston ring cylinder liner is one of the key friction pairs of marine low-speed twostroke diesel engine.Its lubrication performance directly affects the economy,reliability and service life of the whole engine.Near TDC,the piston ring is in a harsh service environment with high temperature,heavy load and low sliding speed.It is usually in a boundary lubrication state.It is a typical friction interface with high load and low shear,and is prone to scratch,pitting,scratch,cylinder pulling and other failures with the cylinder liner.The research shows that when the friction pair is in the boundary lubrication state,the polar molecules in the common additive ZDDP for engine lubricating oil are easy to react with specific metal surfaces,forming a kind of solid like film with high hardness,strong toughness,high melting point,and firmly connected to the contact surface,which is commonly called ZDDP tribofilm.The tribochemical reaction is mainly excited by the mechanical stress and thermal effect between the interfaces.The generated tribochemical film can prevent the direct contact of the surface asperities in the coverage area and reduce the surface adhesion wear.Due to the chemical composition and constitutive characteristics of ZDDP tribofilm,its surface is relatively rough and its friction coefficient is relatively high,which affects the lubrication performance of friction pairs.To analyze the lubrication and contact characteristics of piston ring cylinder liner of low-speed engine,it is necessary to consider not only the typical low shear effect interface lubrication characteristics under reciprocating motion,but also the structural characteristics of low-speed engine piston components,as well as the temperature,load and lubricating oil characteristics of piston ring cylinder liner friction pair in low-speed engine.In this paper,the piston ring cylinder liner friction pair of a 400 mm low-speed diesel engine is taken as the research object,considering the influence of the tribofilm,the following research is carried out:(1)In view of the limitations of the current simplified calculation method for the piston ring back pressure of internal combustion engines,considering the influence of the ring groove space structure on the ring groove pressure,based on the gas flow model,the ring groove pressure analysis model was established and its effectiveness was verified.The sensitivity analysis of annular groove axial height,annular bank radius and annular groove depth to annular groove pressure was carried out,and it was determined that the axial height of annular groove was a significant factor.Pearson correlation coefficient is introduced to quantify the influence degree of the axial height of the ring groove and obtain its critical value,and then the optimization algorithm of the ring groove pressure is proposed,forming a calculation method suitable for the ring groove pressure of low-speed engines,improving the calculation accuracy and applicability,and providing a load analysis method for the analysis of the characteristics of the tribofilm between piston rings and cylinder liners of low-speed engines.(2)Based on the Greenwood Williamson classical contact model,Wen Huang boundary lubrication contact model is introduced,and the calculation method of contact pressure of micro convex body under boundary lubrication is modified.The multi lubrication state analysis model of low-speed engine piston ring is obtained,which can reasonably locate the boundary lubrication area and accurately calculate the contact stress of interface micro convex body under different lubrication conditions,and can effectively describe the transformation process of lubrication state,It provides a more real interface mechanical stress activation condition for the follow-up study of the effect of ZDDP tribofilm on the lubrication performance of piston ring cylinder liner of lowspeed engine.(3)Based on Arrhenius model,taking the contact stress of micro convex body under boundary lubrication as the mechanical stress activation condition,the growth and wear rate model of tribofilm is introduced to build the functional relationship between the thickness of ZDDP tribofilm and the comprehensive roughness of friction pair surface,and establish the coupling analysis model of lubrication characteristics of piston ring cylinder liner friction pair of low-speed engine considering ZDDP tribofilm under constant temperature.The results show that compared with the friction test results of the piston ring of the 400 mm principal prototype,the calculated results of the piston ring lubrication model of the low-speed engine considering the friction chemical reaction have smaller deviation and higher accuracy.(4)Using MFT-3000 reciprocating friction and wear tester,the friction coefficient of ZDDP tribofilm was measured by multi factor tests of load,frequency and ZDDP mass fraction of lubricating oil additive.Through comparison and analysis with blank experiments,the average friction coefficient of ZDDP tribofilm was 0.1324.The friction calculation model of piston ring cylinder liner friction pair is modified by using the measured friction coefficient.The deviation between the calculated friction value of the modified model and the test value is reduced by 9.17%,which improves the calculation accuracy of the friction calculation model of low-speed engine piston ring.(5)Temperature is an important factor affecting the process of friction chemical reaction and the viscosity of lubricating oil.Taking the 400 mm cylinder bore low-speed engine as the research object,an analysis model for the temperature distribution inside the cylinder liner was established,the temperature distribution law of the inner wall of the cylinder liner was calculated,and the thermal stress activation conditions of friction chemical reaction were obtained.The influence of temperature on the growth rate of ZDDP tribofilm and the viscosity of lubricating oil was introduced.Based on the established coupling analysis model,The influence of heat effect on the tribochemical reaction and lubrication characteristics of piston ring cylinder liner friction pair of lowspeed engine was quantitatively analyzed. |
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