Study On 150 Diesel Engine Piston Thermal Load

With the continuous progress of science and technology,the requirements of automobile for engine quality,emission,power,economy and other aspects are constantly improving.The internal combustion engine is gradually developing to the road of light weight,high power,low emission and low fuel consumption.As a very important part of internal combustion engine,piston is always in direct contact with high-temperature gas,bearing a very large heat load and mechanical load.Large load is easy to lead to piston failure.The main way to analyze piston failure is to calculate the distribution of temperature field and stress field by finite element simulation.The main research work of this paper is as follows:1.Determine the boundary conditions of the top surface of the piston and the method of adding the boundary: use the three-dimensional simulation software converge to obtain the instantaneous convection heat transfer coefficient and instantaneous temperature between the top surface of the piston and the gas Load the average heat transfer boundary conditions on the top surface of the piston.The Fourier transform is used to establish a periodic function relationship between the instantaneous boundary conditions and time,and then the boundary conditions are mapped to the piston top surface.The discrete numerical loading method which can theoretically calculate the infinite cyclic thermal load is theoretically analyzed and the calculation method is deduced.2.Determine the side boundary conditions of the piston: For the side boundary conditions of the piston,use the multi-layer flat wall thermal conductivity formula to calculate the heat transfer coefficients of the thermal bank,ring zone,and skirt.The ambient temperature is obtained by experimental measurement;Empirical formulas and simulations are used to verify each other and to correct the heat transfer boundary of the internally cooled oil cavity;the heat transfer relationship between the piston inner cavity and the oil mist is calculated by the empirical formula,and different exchanges are loaded on the inner cavity surface according to experience Thermal boundary conditions.The calculation method of the transient temperature field of the piston is analyzed in detail on the basis of obtaining the boundary conditions.3.Use Pro / E software to build a detailed three-dimensional piston structure model,and complete the pre-processing of the model through the finite element software.The steady temperature field of the piston is calculated,and the simulation model is verified by the experimental measurement.The heat balance condition of the heat exchange boundary is obtained by calculating the steady temperature field.According to the steady-state heat balance conditions,the time required for the piston to reach the stable state from the initial time is calculated.After 150 s,the temperature field of the piston is considered to reach the stable state.Based on the calculation results of the transient temperature field of the piston,the temperature fluctuation of the top surface of the piston at the initial and stable state is calculated.4.Finally,based on the calculation results of the steady and transient temperature fields of the piston,the detailed analysis and calculation of the thermal deformation,thermal stress and thermal flow distribution of the piston under the coupling action of thermal load,mechanical load and thermal engine are carried out.The fatigue life of the area where the piston top surface may be damaged by fatigue is calculated.The results show that the high temperature region of the piston is concentrated in the head of the piston,and the temperature fluctuation of the head gradually reduces from 7K to 5K in the stable state at the initial time.The temperature gradient of the piston is relatively large in the radial and axial directions,and the temperature gradient of the skirt is small because it is far away from the high temperature gas.When analyzing the deformation of the piston,it is found that the maximum deformation of the piston head is 0.59 mm due to the action of thermal stress,and the maximum deformation of the piston head is 0.63 mm under the action of thermal coupling,which is larger than the fit clearance between the piston and the cylinder liner,and the mechanical load has little effect on the deformation of the piston.The distribution law of coupled stress field of piston is close to that of thermal stress field of piston,and the stress field of skirt is stable all the time.The fatigue life of piston top groove edge is 7.24 × 10 times,which is less than the design requirement of fatigue limit life,so it is necessary to optimize and improve the piston head groove.