Investigation On In-Cylinder Excitation And Shaft Torsional Vibration Characteristics Of Marine Dual-Fuel Low-Speed Engine

Regulations on green emission are becoming increasingly stringent,and the International Maritime Organization(IMO)has set a higher level of requirements for ship emissions in the newly proposed Tier III emission standards.Low-speed dual-fuel engines with high power and low emissions have gradually become the main power plant for ocean-going vessels.Such engine is large with high power output generated from each cycle,and the requirements for the design and strength of the components are also increased.Excitation force analysis in the internal working process and the torsional vibration characteristics of power shaft system are all important issues to be concerned.For this purpose,the following research is carried out in this thesis.Firstly,the working characteristics of the dual-fuel engine are clarified,and the one-dimensional model of the engine working process simulation in both fuel and gas modes is completed in the software based on modular concept.The trend of the in-cylinder pressure variation under the full-load condition of the two modes is compared with the in-cylinder pressure given by the manufacturer,and the correction and verification of the established model are carried out.Furthermore,based on the above simulation model,the in-cylinder pressure curve of single-cylinder flameout failure conditions is also obtained,solving the cost issue of in-cylinder pressure test under different working conditions.Secondly,the torsional vibration characteristics of propulsion shaft system is analyzed.Based on the lumped parameter method and finite element method,the finite element model and the concentrated inertia model of the crankshaft and shafting parts are established respectively.Since the inertia model precision is the basis of all subsequent calculations,after the simulation results of the shafting system inherent properties are compared with the file provided by the manufacturer,and the results are in agreement with the each other.Combined with the structural characteristics of the low-speed and high-power engine,with the excitation torque generated by the reciprocating components gravity under consideration,which is neglected in calculation of the medium and high-speed engines,the forced vibration analysis of propulsion shafting is conducted.The combined simulation of multiple excitation forces of dual-fuel engine is completed,and the variation law of shafting torsional vibration of the under the two combustion modes is obtained.Finally,in order to further explore the torsional vibration characteristics of the engine propulsion shaft system,simulation results under abnormal conditions and the variable parameters of torsional vibration system were analyzed.The calculation mainly includes the comparison of vibration characteristics for different conditions,such as the condition of uneven firing of adjacent cylinders or spaced cylinders,the fault condition of single cylinder flameout,the situation of counterclockwise ignition and the change of firing order,and the selection of damper and before and after damper replacement.It is found that the fault condition of single-cylinder flameout has a great influence on the shafting torsional vibration.The damper matching should be considered from multiple factors such as inertia,damping parameters and vibration torque of different speeds.Therefore,in the operation of dual-fuel engine,attention should be paid to the working condition in engine cylinder and the torsional vibration state to avoid the fatigue caused by excessive vibration.