| To meet the needs of modern development,and marine diesel engines have shown a trend of high performance,low emissions and high-intensity operation.The improvement of internal combustion engine performance will inevitably produce higher combustion temperature and more severe mechanical load.The stability of internal combustion engine components is facing a huge challenge.As an important part of the combustion chamber,the exhaust valve has been used in a high-temperature and high-pressure environment for a long time,and it performs high-speed cyclic reciprocating motion under the action of the valve mechanism.The exhaust valve is prone to failure in extremely bad working conditions.According to related studies,the exhaust valve failure rate is as high as 15% of the whole machine.In order to effectively prevent the internal combustion engine from stopping due to this,the reliability analysis of the exhaust valve is particularly important.The main research contents of this thesis are as follows:(1)According to the operating parameters of a certain type of marine four-stroke diesel engine under rated operating conditions,according to the two different working conditions of the exhaust valve opening and seating,the different surfaces of the rod,neck and head are heated and divided,and the heat is released periodically.The process is defined as a steadystate heat transfer process through the third type of boundary conditions.(2)The multi-field coupling model and the equivalent thermal resistance model are used to calculate the thermal boundary conditions of the exhaust valve and perform finite element simulation.The exhaust valve,exhaust port,cylinder head inner wall and valve seat ring are regarded as the exhaust valve component system,and the fluid-structure coupling model is used to calculate the convective heat transfer heat boundary conditions of the exhaust valve head under cooling water and high-temperature gas.The effective thermal resistance model calculates the heat conduction thermal boundary conditions of the exhaust valve,and substitutes the data into the finite element simulation calculation to obtain the temperature field distribution of the exhaust valve.The highest temperature is 669.8℃ at the center of the bottom surface of the exhaust valve,and the second hot spot is the temperature of the exhaust valve neck and the temperature is 645.3°C.(3)Use the overall hardness experiment of the material to draw a standard curve to verify the simulated temperature field.The result shows that the temperature error is within 5%.Based on the thermal load,a single degree of freedom model is used to calculate the equivalent seating force of 435.63 N generated by the exhaust valve when the valve train is operating.The thermo-solid coupling method is used to calculate the equivalent stress of the exhaust valve under both thermal and mechanical load.The simulation results show that the cone surface of the exhaust valve has obvious stress concentration,and the maximum stress value is 214.2Mpa.(4)The material studied in this thesis is Ni Cr20 Ti Al superalloy,and its physical parameters are obtained by tensile tests at different temperatures.The fatigue life of the exhaust valve is predicted by the nominal definition method,and the internal safety factor of the exhaust valve is obtained.It shows that although there is stress concentration in the neck of the exhaust valve,it is far less than the yield limit,and the service life of the cone surface and bottom surface is shorter.Under the continuous and uninterrupted alternating load,the service life of the cone surface of the exhaust valve is 332 h. |
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