| As one of the core components of the engine,the piston is becoming more and more severe as the engine performance continues to be strengthened.Because the piston works under the environment of high thermal load and mechanical load for a long time,problems such as top ablation and cracks,wear and tear of the ring groove and ring bank,and pin hole cracking often occur.These problems are often highly unpredictable.Once it appears,it may have a serious impact on production and life.Therefore,how to correctly predict the fatigue life of the piston and ensure the working reliability of the piston under high strength is a very important part in the design of the current piston.To this end,taking a heavy-duty diesel engine piston as the research object,by establishing a three-dimensional model,the temperature field and thermal stress of the piston and the coupling stress and deformation of the heat engine were calculated and analyzed.Based on the piston temperature field and coupled stress results,the fatigue analysis software Fe safe was used to analyze the fatigue life of the piston.At the same time,the effects of piston structure parameters on piston temperature,coupling stress and fatigue life were studied around the three piston structure parameters of internal cooling oil cavity position,throat chamfer radius and pin seat length.Finally,through the thermal fatigue test,the fatigue reliability of the piston was tested to provide a test basis for the reliability and life prediction of the piston.The main findings are as follows:(1)The highest temperature of the piston under steady state is 359.18?,located at the throat of the exhaust side,and the lowest temperature is 131.75?,located at the skirt of the piston.In the piston combustion chamber,there is a certain correlation between the temperature change and the geometry of the combustion chamber.Within a certain range,adjusting the length of the pin seat has the smallest effect on the temperature field of the piston.The temperature changes of 16.32? and 23.47? in the center and combustion chamber and the top of inner chamber can be produced by adjusting the position of inner cooling oil chamber.(2)The overall thermal stress distribution of the piston is uneven.The maximum thermal stress appears on the lower surface of the first ring groove,which is 170.47 MPa,and the minimum thermal stress appears on the gap of the nozzle of the piston skirt,which is 0.46 MPa.The maximum value of the coupling stress of the piston and the thermal stress appear at the same position.It can be seen from the comparison that the thermal stress is the main reason for the larger coupling stress at this position.Judging from the coupling deformation results of the piston,the coupling deformation of the top outer ring area is more serious,with a maximum deformation of 0.58 mm.Within a certain range,changes in structural parameters have little effect on the overall distribution of piston coupling stress.Compared with changing the position of the inner cooling oil chamber and the length of the pin seat,changing the chamfering radius of the piston throat has the smallest effect on the coupling stress of the piston.By changing the position of the inner cold oil chamber and the length of the pin seat,the maximum coupling stress of the piston will produce a change of 36 MPa?23.1MPa,respectively,and the stress of other positions of the piston will also have different degrees of influence.(3)The fatigue life distribution of the piston is highly correlated with the coupling stress distribution,and the fatigue life of the inner and upper area of the piston pin hole is relatively short.As the surface roughness value increases,the piston life will be shortened accordingly.Within a certain range,changes in structural parameters have little effect on the distribution of overall piston life.Compared with changing the position of the inner cooling oil chamber and the length of the pin seat,changing the chamfer radius of the piston throat has the least impact on the life of the piston.By adjusting the position structure of the inner cooling oil chamber,the life of the piston is the longest when the distance between the bottom of the inner cooling oil chamber and the top surface of the piston is 34 mm.By adjusting the length of the pin seat,when the length is 95 mm,the life of the piston is the shortest,and when the length of the piston pin seat is 97 mm,the life of the piston is the longest.(4)The thermal fatigue test shows that the method of punching the piston and the location of the hole will directly affect the thermal fatigue test results of the piston.In the test,the temperature rise at different positions on the top surface of the piston was different at the same time.The temperature at the throat rose fastest,followed by the center of the piston,and the temperature at the bottom of the piston rose relatively slowly.When the temperature is close to 400?,the temperature difference between the bottom ring of the piston and the throat is about 20-22?,and the temperature difference between the center position of the piston and the throat is about 17?.After improving the drilling method and position,the effect of drilling on the test results was effectively avoided.After 5000 test cycles,no cracks appeared on the top surface of the piston,indicating that the material and structure design of the piston can meet the requirements of thermal fatigue test assessment. |
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