Mechanical Failure Study And Durability Prediction For The Internal Combustion Engine Piston

The aluminum-silicon alloy piston's durability performance has a great effect on internal combustion engine life. The piston system has a major influence on both the low product cost and the advanced mechanical efficiency. With the reasonable piston structure, the design destination can be achieved. The piston structure needs to meet the stringent emission legislative requirements too. There are no detailed technique reports on the piston mechanical durability study in China. Either the theory research of the test research is badly off in domestic, due to the lack of the testing equipments. It is very important to perform the piston durability test and have the research based on the stress theory and the FEA analysis. With the research, we can optimize the design and improve the piston mechanical safety. This paper puts forward the piston mechanical life test approach and evaluating method. With the reasonable design, the structure excessive safety can be avoided, too.The low density of aluminum alloy is such a most attractive property that it widely used in piston product. In this paper, we study the piston durability by using FEA technique and tests. However, the trend towards higher cylinder pressures has made the mechanical stress of the piston reach the limit of material application. By using the Al-Si alloy, the improved piston structure can provide good performance for the piston and the engine.According to the piston fatigue evaluation flow, concentrated to the key zone of the piston, 3D models of piston assembly parts are built by using Unigraphics software, the mesh and the analysis are performed by Ansys software in this dissertation. The key problems such as the piston pin boss durability research based on the pin bore profile design, the piston combustion chamber reliability determined by the pin bore profile design, the gasoline engine piston ring land failure were analyzed in detail. Besides, the hydrodynamics test criterions for the diesel engine piston and the gasoline engine piston are studied and revised, the pistons with different pin bore profile parameters are tested by the hydro-pulse test bench, the engine durability test and the piston temperature distribution test are carried out for evaluating and comparing the piston reliability performance. The engine combustion pressure is tested to impact the mechanical load on the piston. The S-N curve of the aluminum alloy and the Palmgrem-Miner law are used for predicting the piston cycle life under the running conditions.In order to eliminate the residual fluctuation of the last wave to impact the next wave, we use some efficient innovations in the test equipment. The independence of every sinusoidal wave is ensured. We test different frequency for comparing the different influence during the test criterion study. The test standard is studied considering the dimensions after the engine test and the hydro-pulse test. We also think over the maximum cylinder pressure(P_max), the pressure elevated ration((dp/dφ)_msx) and the cylinder pressure standard deviation(S_d(θ)) to determine the maximum pressure amplitude in the hydro-pulse test. Compared the failure specimen of the engine test and the hydro-pulse test, we can say, the parts failure mechanism from the two test methods are similar.The different durability performance of the piston may be caused by the different parameters of the piston pin bore profile. For the gasoline piston, 5 kinds of different pin bore shapes are analyzed and compared by FEA under mechanical load and thermo-mechanical load, the hydrodynamic tests are performed to evaluate the pistons cycle life. The asymmetrical relief hole which relatives to the gudgeon pin center appears the best ability among them for lessening the stress concentration. The steady thermal load benefits to diminish the mechanical stress concentrated on the top of the pin boss, the stress distributions of the pin bore circumference are uniformity under the thermo-mechanical loads. The worse carrying capacity may be derived from the pin bore with tapered profile that is widely used in the gasoline engine piston due to the unreasonable profile parameters. Moreover, the pin bore profile fitted the rated power conditions may be unfitted other running conditions. The inertia force at the maximum speed will not lead to any high stress or piston failure.The lubrication conditions of the pin bore affect the stress distributions and the piston durability. Using the FEA, we analyze the maximum Von Mises stresses and the friction coefficients with different oil film thickness on the pin bore. Under the constant oil film thickness, the maximum Von Mises stresses will enlarge along with the friction coefficients increasing, but under the same friction coefficient conditions, the maximum Von Mises stress will reduce with the oil film thickness increasing. Increasing the lubrication ability and using the low friction coefficient material can benefit the lubrication action and improve the parts durability. When the friction coefficient is equal to 0.1 or even less, the change of the oil thickness will lead to less change of the maximum Von Mises stress. When the oil thickness is equal to 0.0055mm or larger, the change of the friction coefficient will lead to less change of the Von Mises stress. As a result, the fine cooperation between the gudgeon pin and the pin bore should be pursued during the product design for balancing the contact pressure, and keeping the good lubrication performance.On the other hand, the bigger silicon particles and the ferrous particles concentration existed in the piston body should be eliminated during the produce process. One piston failure sample of the particles concentration is discussed in this paper.The pin bore plastic deformation under high temperature is calculated by using the elastoplastic theory. The creep deformation of the pin boss has been analyzed. As a result, due to the higher engine output, it is necessary to consider the plastic deformation under the engine running conditions for the pin bore durability analysis.Piston combustion chamber fracture is another important failure of modern engine piston damage. For the mechanical fracture failure, the breakage usually commences at the chamber rim, which is parallel to the gudgeon pin axis. This paper studies the chamber durability with a modem diesel engine piston. The different pin bore profiles cause the different stress concentration on the chamber rim. The maximum tensile stress and the maximum Von Mises stress occurred on the bottom of the transition radius, which is between the crown and the chamber, this transition radius is parallel to the gudgeon pin axis. But the maximum compressive stress occurs on the transition radius bottom, which is vertical to the gudgeon pin axis. The maximum combustion pressure causes the tensile stresses on the chamber rim, and the tensile stresses act as the main factor to cause the rim mechanical fatigue failure. The severe running conditions such as the sudden change of the engine load from idle condition to the maximum torque or to the rated power cause the unexpected tensile stresses. The mechanical fatigue failure characteristic is similar to the low cycle failure occurred on the chamber rim due to start-stop conditions or sudden change conditions. The good durability performance of the pin boss and the chamber can be achieved by the optimized pin bore profile. Usually, the analysis and evaluation of the piston durability performance are performed under the steady-state conditions. It is useful for the piston project. Additional, the change of the running conditions should be deliberated too, for the stress distribution change may cause the failure. In other words, we can say, it is essential for the engineers to begin the design with harmonizing the thermal load and the mechanical load.For solving the ring land breakage, increasing the ring land height is the normal method. Nonetheless, with the total height limitation, other heights such as the skirt should be reduced. Stress amplitude-mean stress and Palmgrem-Miner law are adopted in the ring bank failure analysis. The material S-N curve is used to predict the ring land cycle times. According to the research, the breakage mainly caused by the larger stresses which concentrate on the gasoline piston ring groove root, and the thermal stress is the predominant which have the larger weight among the working stresses. Increasing the land height, decreasing and equilibrating the groove temperature should be considered as the significant step for improving the land carrying capacity. With bigger root clearance between the ring and groove, the bigger stress at the groove root increase with an exponential growth. Further more, the casting microcrack in the ring land body should be eliminated, the land working temperature should be equilibrious and reasonable, new material with high temperature strength should be used for the modern engine piston. The material creep action at the elevated temperature should be considered in order to improve the piston reliability.In a word, according to the evaluating flow, the piston mechanical durability research has improved the product performance. Since the numerical simulation and experimental results agreed well, numerical simulation can be used for the design and optimization of the piston. On the one hand, the investigating result can be used to improve the piston durability performance continually. On the other hand, it can provide theoretical and experimental foundations for the further research and wide adoption.