Factors Affecting Diesel Engine Piston Structural Strength And Heat Transfer

Along with the increasingly strict emission regulations, high speed, high power, low fuel consumption and low exhaust has gradually become the mainstream direction of the development of internal combustion engine. In order to increase its power and rotational speed, the engine’s demands on temperature and the intensity must be considered. As the main heated parts of diesel engine, piston often works in high temperature, high speed and high cyclic heat load environment, which leads to the difficulty in piston cooling. Moreover, piston’s structure strength and heat transfer characteristics is the key to affect engine durability, reliability and economy. Therefore, the improvement of piston structure design is crucial to increase engine intensity. In order to investigate the influence rules of piston structure parameters on its heat transfer characteristics and structural strength, series of study on a turbocharged diesel engine were presented through finite element method (FEM) in this paper.(1) The finite element analysis and calculation for piston thermo-mechanical coupled stress and deformationAt first, the outerside heat-transfer boundary conditions of piston were estimated by empirical formula, and the heat-transfer boundary conditions of each point on the surface of inner cooling oil cavity were obtained by mapping its CFD calculation which was carried out through AVL Fire software. Then, according to the measured temperature of some feature points on piston surface, the heat-transfer boundary conditions of all piston outerside surfaces and the final temperature fields were obtained by way of combination FEM and trial method. Finally, piston’s thermo-mechanical coupled stress and deformation were calculated on the premise that the temperature field is a thermal preload.(2) The influence of the inner cooling oil cavity structure parameters on the piston heat-transfer characteristics and structure strength.Different inner cooling oil cavity structure parameters’effect on the piston heat-transfer and structure strength was studied by finite element analysis and numerical simulation method. Results show that:the existence of inner cooling oil cavity has some influence on the piston heat-transfer and strength. The piston heat-transfer characteristics can be improved when the oil cavity exists in piston head, but the structure strength of the whole piston was weakened. The heat distribution of piston top surface and the thermo-mechanical coupled stress and deformation of the location on inner cooling oil cavity, oil drain hole and pin boss can be improved by decreasing the surface area of oil cavity appropriately. When the oil cavity was moved to piston top surface properly, the heat-transfer characteristic of piston top surface and the structural strength of piston top surface, oil drain holes and pin boss were enhanced. Through the orthogonal experiment analysis, some conclusions were found. What matters most to the piston temperature and deformation is the surface area of inner cooling oil cavity, and the distance between the centerline of oil cavity and piston top surface has influenced piston most in its stress distribution. The orthogonal experiment’s optimal combination scheme is that:the cross section shape is uniform section, the distance between the centerline of oil cavity and piston top surface is19mm and the surface area of oil cavity is6627mm.(3) The influence of the other piston’s main structure parameters on the piston heat-transfer characteristics and structure strength.Through FEM and numerical simulation method, the other main structure parameters of piston effect on its heat-transfer and structure strength was studied. Analysis results indicate that:the length of pin boss has little influence on the piston heat-transfer, but has a greater influence on the strength of pin boss. The increase of pin boss length can reduce its stress. The pin boss diameter has relatively significant impact on its heat-transfer and strength. Along with the increase of pin boss diameter, its temperature went up and the stress was decreased at the same time, but the stress on oil drain hole was increased. The piston junk thickness has greater influences on piston head heat-transfer characteristics and the structural strength of pin boss and inner cooling oil cavity. The piston head temperature went up and its deformation was decreased, but the stress on pin boss and the surface of oil cavity was increased as the increase of piston junk thickness. The relative position of oil drain hole on the same side has some influence on the strength of piston top surface, oil drain hole and inner cooling oil cavity. When the distance between oil drain holes on the same side was increased properly, the stress of oil drain holes and inner cooling oil cavity was decreased, but the stress of piston top surface was increased. Some conclusions were found through the research for orthogonal experiment analysis. What has the most impact on piston temperature and deformation is piston junk thickness, and the length of pin boss has greater influenced on piston stress distribution. The orthogonal experiment optimal combination scheme is that:the length and the diameter of pin boss respectively are80.5mm and35mm, piston junk thickness is8mm and the relative position of oil drain hole on the same side is6627mm.