Research On Friction&Lubrication Performance Of Diesel Engine’s Key Friction Pairs

Cylinder liner-piston rings friction pair and main bearings are key friction pairs on a diesel engine, the friction and lubrication properties of which have a great influence on the performance of the engine as a whole. This paper studies the friction and lubrication properties of the cylinder liner-piston rings friction pair and main bearings of diesel engines by combining theoretical analysis, numerical calculation and experimental techniques. The study mainly covers:First, according to the gas flow equation and law of conservation of mass, this paper establishes a leakage calculation model for the piston ring to analyze the leakage of an inline six-cylinder diesel engine; then based on the average Reynolds equation builds a computational model for analysis of the lubrication properties of a cylinder liner-piston ring friction pair, and in the model the effect of roughness on lubrication properties and the effects of variable density and variable viscosity lubricants are considered; this paper derives the difference scheme of discrete and gives the solution to solve the average Reynolds equation; this paper also summarizes the problems that occur in the solving process based on the established lubrication analysis model and gives the matters that need attention. This paper analyzes the lubrication of the given diesel engine. The results show that the film thickness ratio of the cylinder liner-piston ring friction pair at a certain period of crank angle near the top dead center of compression is smaller than4, in the mixed lubrication state. For the rest of the crank angle time, the film thickness ratio is greater than4, in the hydrodynamic lubrication state; the maximum film pressure occurs near the top dead center of compression, more than twice the maximum pressure in the combustion chamber; the maximum frictional force also occurs near the top dead center of compression, mainly caused by the solid contact between two surfaces of the friction pair due to thin film. At other time, the internal frictional force is small. The maximum friction power loss occurs near the top dead center of compression. The maximum adhesive wear depth of the cylinder liner lies at the TDC of the first ring. Second, according to the established cylinder liner-piston ring lubrication computational model, this paper examines the factors that affect the lubrication properties of cylinder liner-piston ring-lubricant viscosity, lubricant temperature, friction pair surface roughness, and piston ring structure parameters; this paper also considers the non-Newtonian of lubricant, derives the power law fluid lubrication model applicable to the calculation of cylinder liner-piston ring lubrication, gives the difference scheme of average Reynolds equation and the solution. The cylinder liner-piston ring lubrication calculation results show that by reducing the surface roughness of cylinder liner-piston ring friction pair, the minimum film thickness tends to increase, the probability of occurrence of the mixed lubrication zone reduces, and the lubrication conditions are improved; the maximum friction and maximum friction power consumption are reduced. The increased lubricant viscosity can improve the lubrication conditions at the top and bottom dead centers, but the friction at other crank angle time slightly increases; with the lubricant temperature decreasing, the probability of the friction pair in the form of mixed lubrication and boundary lubrication decreases, and the probability of dry friction decreases; the maximum friction decreases, but the full-film lubrication friction slightly increases; from the perspective of the entire cycle, except the time when the film thickness is relatively small, the friction power loss increases at other time as the lubricant temperature decreases; therefore, for the purpose of lubrication properties, the lubricant needs to maintain a certain temperature, neither too high nor too low. With the height of piston ring barrel surface reduced, the piston ring’s side profile curve becomes gentle, which is conducive to the formation of the squeeze effect. The minimum film thickness increases at the top and bottom dead centers, and decreases at other crank angle time as the hydrodynamic effect weakens; in the vicinity of the top and bottom dead centers, the friction slightly decreases, and at the rest of crank angle time, the friction slightly increases; therefore, reducing the height of piston ring barrel surface may improve the lubrication properties at the top and bottom dead centers, but diminishes hydrodynamic effect at other crank angle time, and thus the piston ring barrel surface height value should be reasonably selected to achieve the best results. The increase of piston ring height increases the contact area of the friction pair, reduces the carrying capacity of unit area, increases the film thickness, improves the lubrication properties; but at the same time, because of the increased contact area, the fluid resistance may increase and lead to higher friction and friction power loss. So, comprehensive considerations should be made to select the right ring height; when the change of viscosity with pressure is ignored, the calculation results of the minimum film thickness are smaller than the situation when the change in viscosity is not ignored, but the maximum friction and maximum friction power loss are much higher. When considering the non-Newtonian effect of the lubricant, with different power law coefficients, the distribution shape of the oil film pressure is uniform, the locations with the maximum oil film pressure are consistent, and the change trend is consistent. But the value of oil film pressure increases with the increase of the power law coefficient, that is, the greater the power law coefficient is, the higher the carrying capacity of the oil film is. For lubricant with a power law coefficient of0.9, the influence of changing the piston ring height and the friction pair’s surface roughness on the film thickness ratio and oil film pressure is basically the same as the situation when the lubricant is a Newtonian fluid.Experimental analysis has been an important aspect for the study on the friction wear characteristics of cylinder liner-piston ring. For a simulation experiment, the emphasis and difficulty is how to accurately simulate the actual situation. In traditional wear experiments, slices of the material of friction pair part are commonly used as a sample for study. Also, the simulation conditions are somewhat different from the real working environment of the diesel engine. This chapter, from the perspective of traditional analysis of tribology, designs more accurate simulation criteria for the simulation experiment, in which real and complete cylinder liner-piston rings are used for pair, temperature conditions, as well as acidic environment that occurs in reality are included. The simulation experiment of friction wear process is conducted on a dedicated cylinder-piston ring reciprocating friction wear experimenting machine. Great efforts are made to make the wear form and wear conditions closer to the actual working conditions to improve the experiment credibility. By measuring the wear data and real-time friction data in the friction wear process of cylinder liner and piston ring, the comparative analysis of friction characteristics of cylinder liner-piston rings made of four different materials is performed. The comparative analysis is performed through the friction and wear experiment. The analysis results show that combining the experiment results, the best match for the diesel engine cylinder liner-piston ring is austempering cylinder liner-sprayed molybdenum piston ring.In analyzing the lubrication characteristics of the main bearings, the elastic deformation cannot be ignored. The elastic deformation equation should be included in the lubrication equation set to seek solution. In this paper, the multi-body dynamics and main bearings EHL are coupled to study the lubrication properties of the diesel engine. The main parameters obtained for the calculation include the main bearing loads, main bearing axis center orbit, minimum film thickness, maximum film pressure, and friction power loss. The study results show that:in the main bearing loads, the load in the direction perpendicular to the cylinder (Z-direction) is greater than the load in the main and auxiliary thrust surface direction (Y-direction); the peak loads in the Z-direction of2nd,3rd,5th,6th main bearings are huge; the peak loads in the Z-direction of1st,4th,7th main bearings are small; the axis orbits of1st,7th main bearings are similar, mainly in circular motion, rarely in high-speed centripetal motion, and the occurrence probability of cavitation is low; the occurrence probability of axis centripetal motion of the2nd main bearing is not high, thus the lubrication properties are good; most of the axis orbit of the4th main bearing falls in the60℃A-240℃A zone, resulting in a small oil film thickness and high vulnerability to wear; the axis orbits of the3rd,5th and6th main bearings show a significant centripetal motion, prone to cause cavitation. The minimum film thickness of the1st main bearing is the smallest, which needs special attention; the position where the maximum film pressure occurs is basically consistent with the position where the Z-direction bearing peak load occurs. While the position of minimum film thickness is not necessarily consistent with the bearing peak load position; the friction power loss trend is basically the same as that of the maximum film pressure, and the position of peak power friction power loss is basically consistent with the bearing·peak load position. The main bearing lubrication properties when the single cylinder misfires are studied also.Engine block, as the skeleton of an engine, has a very complex structure. The loads it is subject to are also very complex, including static loads such as pre-tightening loads applied by cylinder head and bearing cap bolts, and interference force of bearing pad, and dynamic loads such as cylinder detonation pressure, inertia force of crank linkage mechanism and piston group, and striking force of piston. With the continuous improvement of the performance of internal combustion engine, the operating conditions of crankshaft are increasingly severe and the requirements on strength are more stringent. To consider the lubrication effect on the block&crankshft strength analysis, based on the coupled analysis of the engine block-crankshaft system’s dynamics and tribology, this paper obtains the dynamic stress distribution of engine block and crankshaft in a complete cycle; analyzes the dynamic fatigue strength of engine block and crankshaft, acquires the fatigue safety factors of various parts, and examines the dynamic strength of engine block and crankshaft. The dynamic strength analysis of the main bearing wall is also carried out. The analysis results show that when viewed from the transverse section of the engine block, the smaller dynamic fatigue safety factors for the diesel engine block are located at the edge of the mounting holes of the engine block and cylinder liner on the top surface of the engine block, and the upper end surface of the bearing saddle bore rib on the lateral bulkhead at the engine block bottom; when viewed from longitudinal section of the engine block, the smaller safety factors are generally located at the lateral bulkhead between cylinders, basically consistent with the occurrence positions of smaller safety factors according to analysis results of static fatigue strength; all the smaller safety factors on the crankshaft occur on the fillets of the crank pin and main journal. The zone of smaller fatigue safety factors is basically consistent with the analysis results of static fatigue strength. The minimum safety factor of the main bearing wall lies at the transition fillet of the incision transition fillet below the main bearing cover. The fillets of crank pin and main journal have a relatively weak strength. In establishing the grid model for crankshaft, these fillets need to be refined.