Research On Fault Mechanism And Online Detection Method Of Key Moving Parts Of Piston Engine

The piston engines are core power take-off devices widely used in vehicles,ships,construction machinery,and emergency power generation.Piston engines have a complex structure,many parts,and bad operating conditions,which leads to frequent mechanical faults;typical failure modes include valve wear,connecting rod bearing wear,shafting misalignment,and piston scraping.Due to the relatively low level of current monitoring and diagnosis technology of piston engines,it is difficult to detect their mechanical faults effectively,which is easy to cause malignant accidents,such as connecting rod and crankshaft fractures.It will seriously affect the equipment stability and safety in production and bring serious direct and indirect economic losses.Therefore,the research on the mechanisms and online detection methods of typical mechanical faults of piston engines is of great significance to guarantee reliability and avert malignant accidents.This thesis takes the three key and vulnerable moving parts of the piston engine as the research object: valve,connecting rod bearing,and crankshaft shafting.Aiming at the characteristics of these parts bearing alternating and large impact load for a long time under the action of combustion impact force and reciprocating and centrifugal inertia forces,based on the principles of machinery dynamics and hydrodynamic lubrication,fault mechanisms,signal analysis,and detection methods of typical mechanical faults of these parts are studied.The main research works are as follows:Firstly,aiming at the valve clearance fault,a dynamic model of the valve mechanism is constructed,and the fault mechanism simulation is carried out.The dynamic simulation and experimental results show that the starting position of valve seating impact is sensitive to the change of valve clearance and is not affected by working conditions.Further according to this fault characteristic,an adaptive and accurate extraction method for vibration impact starting position based on the gradient neighborhood of energy operator is proposed,which can accurately detect the state of valve clearance and track the wear evolution of clearance online,and overcomes the problems of poor self-adaptability and robustness of the traditional threshold judgment method.Secondly,aiming at the wear fault of connecting rod bearing,the construction and optimization of the lubrication model are carried out.Aiming to the time lag characteristics of viscoelastic deformation of connecting rod bearing material under strong impact load,the temporal correlation deformation equation of bearing is constructed based on viscoelastic constitutive equation(Kelvin or standard linear solid model)and finite element method.It is used to correct oil film thickness and micro asperity contact pressure.Therefore,a new mixed-visco-elastohydrodynamic lubrication model is proposed.Compared with the existing mixed elastohydrodynamic lubrication(MEHD)model,the new model breaks the limit of complete elastic assumption and can more accurately analyze the complex coupling between the time lag deformation and the three effects of bearing micro lubrication flow field(hydrodynamic,extrusion,expand and contract),and can identify the impact characteristics such as asperity impact and rapid change of film pressure in advance.Then,the new mixed-visco-elastohydrodynamic lubrication model is applied to study the wear evolution mechanism of connecting rod bearing in piston engine.Aiming at different lubrication states of big end bearing,coupling the dynamic model of crank train,a multi-space and time scale adaptive lubrication analysis model for connecting rod big end bearing is constructed,and the simulation research for its lubrication characteristics and wear fault is carried out;The results show that vibration impact occurs in bottom dead center(BDC)phases after big end bearing wearing.The fault simulation experiment verifies the accuracy of the simulation conclusion and provides a basis for online detection of wear fault of connecting rod big end bearing.Finally,aiming at the misalignment fault of crankshaft shafting,based on the morphological characteristics of the coupling between the shafting and the bearing structure,the mapping mechanism model between the average value of shafting radial vibration and the misalignment value is constructed.Further,based on this model and considering the compact structure and the strong interference characteristics of vibration signal of piston engine,an online quantitative detection method for shaft misalignment is proposed.It can calculate the value and direction of misalignment in real-time,and its accuracy is not affected by working conditions.It overcomes the shortcomings of traditional detection methods based on vibration frequency characteristics in quantitative diagnosis and direction judgment.Further,an experimental device of target self-recovery regulation in six degrees of freedom for the diesel engine is built;a self-recovery regulation method for misalignment is proposed,and its effectiveness is verified by co-simulation.The studies of this thesis have a clear engineering background and urgent current demand.The research results further reveal the typical mechanical fault mechanism of piston engines and form the detection method of common faults of key moving parts,which lays a solid foundation for the practical application of piston engine fault monitoring and diagnosis technology.