Dynamic Modeling And Condition Monitoring Of Sliding Bearings For Internal Combustion Engines

Sliding bearings of internal combustion engines are subjected to complex alternating loads and their working conditions are very poor.Without timely and adequate lubrication,sliding bearings inevitably generate various degrees of wear during long-term operation,resulting in serious failures such as deteriorating engine operating conditions,abnormal overall vibration and noise,burning of bearing bushes,or crankshaft fracture.Because the big end bearing of the engine connecting rod is located inside the engine,it is very difficult to dismantle and check.Therefore,it is very significant to study non-intrusive condition monitoring and fault diagnosis methods of the big end bearing of the engine connecting rod without inspection.In the field of engine fault diagnosis based on acoustic emission technology,most of the existing researches are conducted under the conditions of simulating known fault sources and fault types.However,there are relatively few researches on unknown fault sources and types of faults.For the engine,because it contains many types of frictional components and there are a wide variety of faults,it is necessary to study the location of the unknown fault sources and the type of fault analysis.In order to obtain the physical correlation between the internal fault and the external acoustic signal,the modeling and in-depth simulation of the dynamic pressure lubrication process of the large-end bearing of the connecting rod was carried out.According to the engine parameters,the big end bearing force of the connecting rod is calculated,and the big end bearing force diagram of the connecting rod is drawn.The analysis shows that as the speed increases,the maximum load at the big end of the connecting rod gradually decreases,while the load at the other crankshaft corner positions increases with the increase of the rotational speed.Secondly,using the finite difference method to solve the two-dimensional Reynolds equation to calculate the oil film pressure distribution at the big end bearing.Then,a one-dimensional model of the engine lubrication system was established to analyze the lubrication conditions of the engine connecting rod big end bearing under the condition of different oil temperature and different connecting rod big end bearing clearances.At the same engine oil temperature,the flow of oil at the big end bearing of the connecting rod of the engine increases significantly with the increase of the clearance between the big end bearings of the connecting rod,and the oil pressure at the big end bearing of the connecting rod decreases with the increase of the relative clearance.Under the condition that the connecting rod big end bearing shell have the same clearance,the oil flow rate at the big end bearing of the engine connecting rod increases with the increase of the oil temperature,the oil pressure at the big end bearing of the connecting rod decreases as the temperature of the oil increases.This laid the foundation for the analysis of sound and vibration signals resulting from,for example,excessive bearing clearance,insufficient oil supply,abnormal frictional wear,etc.Based on single-cylinder diesel engine,a bench test was conducted to compare the laser displacement sensor signal of the crankshaft position under different rotational speeds and torque conditions,and the axial trajectory of the big end bearing under different working conditions was investigated.Combined with the simulation results,it can be seen that the peak values of the bearing force and crankshaft displacement at the crankshaft rotation angles of 0°CA,90°CA,270°CA,360°CA,450°CA,and 630°CA coincide with each other.This proves that the simulation and test results are consistent.The crankshaft produces displacement changes under the action of the bearing force,and a periodic axis trajectory is formed as the bearing force periodically changes.As the engine speed and torque increase,the oil pressure of the engine lubrication system increases,the oil film thickness at the sliding bearing becomes thinner,the oil film pressure increases,the oil film bearing capacity decreases,and the vibration response at the crankshaft position increases,this causes the crankshaft axis trajectory to shift outward.At last,the acoustic emission fault diagnosis was performed on the engine.By comparing the peak response of abnormal acoustic emission signals at different measuring points,locating the source of the fault,combined with theoretical analysis and disassemble inspection to confirm the diagnostic results of the defected bearing.