Study On Coupling Lubrication Model And Profile Optimization Methods Of Main Bearing Of Highly-Intensified Marine Diesel Engine

Diesel engines have been widely used because of high thermal efficiency,wide power range,strong applicability and economic efficiency in the field of marine propulsion.When a ship is sailing in severe sea states,the crankshaft may have large amplitudes of misalignment and axial movement.At this time,not only the contacts between inner surface of main bearing and crankshaft,but also the collisions between end-face of main bearing and crankshaft may occur.However,at present there is a lack of research on edge wear and end-face ablation of main bearing under dynamic loads.In this thesis,the main bearing of highly-intensified marine diesel engine is taken as the research object,and the coupling lubrication model of main bearing is established with journal misalignment and axial movement,while the formation mechanisms of edge wear and end-face ablation of main bearing are elaborated.Then,the variable thickness bushings and surface textures are used in main bearing to improve its lubrication performance,which enhance the reliability and durability of marine diesel engine.The main work of this thesis are listed as follows:The coupling lubrication model of main bearing of marine diesel engine is established,which can comprehensively consider the effects of axial movement,journal misalignment,surface asperity contact,surface elastic deformation and thermal effects of lubricating oil as well as describing the three-dimensional motions of journal.Then the finite difference and overrelaxation iterative methods are used to numerically solve the coupling lubrication model,meanwhile the corresponding solution flows are also given herein.Based on the coupling lubrication model,the effects of axial movement on lubrication performance of main bearing are analyzed,meanwhile the formation mechanisms of edge wear and end-face ablation of main bearing are elaborated.It is found that the journal center orbits are unclosed space curves with considering axial movement of crankshaft;the journal misalignment can thin the oil film located at bushing edge,and herein the surface asperities will contact with each other,which leads to obvious asperity contact pressure at bushing edge;the effects of small axial velocities on lubrication performance of main bearing can be ignored,while larger axial velocities not only increase asperity contact pressures,which can intensify edge wear of main bearing,but also expand journal center orbits,which lead to collisions and wear between end-face of main bearing and crankshaft,then resulting in end-face ablation of main bearing.In order to reduce edge wear of main bearing,the effects of double conical,paraboloid and corrugated bushings on the lubrication performance of main bearing are researched under dynamic loads.It is found the double conical,paraboloid and corrugated bushings can greatly reduce and even eliminate the asperity contact pressures,which is helpful to reduce edge wear of main bearing.However,the double conical,paraboloid and corrugated surfaces can also reduce load carrying capacities.For the double conical,paraboloid and corrugated surfaces with same radial heights,while the total friction loss of double conical bushings are slightly greater than the paraboloid and corrugated bushings,the double conical bushings can more significantly reduce the maximum asperity contact pressures.Meanwhile,the double conical bushings have slightly larger load carrying capacities and lower total leakage flowrate.Hence,the double conical bushings are suggested to reduce edge wear of main bearing.Aiming at the existing drawbacks of double conical bushings,such as reducing load carrying capacities and increasing total friction loss,the surface textures are applied to improve the lubrication performance of double conical bushings.It is found that compared with the double conical bushings,textured double conical bushings have lower load carrying capacities and smaller minimum film thickness,which may lead to larger maximum asperity contact pressures;however,textured double conical bushings can also reduce the total friction loss in one cycle as well as reducing lubricating oil temperature.The orthogonal tests and grey relational analysis(GRA)show that there is indeed a compromised test scheme which can both minimized the asperity contact pressures and total friction loss in one cycle of textured double conical bushing.Aiming at long computing time and high research costs in surface texture design,some optimization algorithms are adopted to optimize the distributions of surface textures in main bearing to enhance its lubrication performance,as well as reducing computing time and improving design efficiency.The surface texture design of reducing friction coefficients based on PSO algorithm shows that,the optimal distributions of groove textures are approximate to trapeziums under different eccentricity ratios and rotary speeds,and with increasing of eccentricity ratios,the optimal distributions of groove textures become slenderer.Meanwhile,the reductions of friction coefficients by optimal groove textures are more significant under lower eccentricity ratios.The tribological test results of journal bearings show that the friction coefficients of optimal groove textured bearing are smallest,followed by referenced textured and plain bearings,which is basically consistent with the conclusions of surface texture design of reducing friction coefficients.The surface texture design of reducing eccentricity ratios based on PSO algorithm shows that,the optimal distributions of groove textures are also approximate to trapeziums under different external loads and rotary speeds.Meanwhile,the reductions of eccentricity ratios by optimal groove textures are more significant under lower external loads.The surface texture design of reducing friction coefficients and eccentricity ratios based on MOPSO algorithm shows that,the distributions of groove textures corresponding to trade-off solutions in Pareto optimal solution sets are also approximate to trapeziums under different external loads and rotary speeds.The above optimization results indicate that,the groove textures with trapezoid distributions have more benefits on reducing friction coefficients and suppressing journal vibrations,which can give guidance on surface texture design of main bearing in future work.