Regression analysis of fatigue property of internal combustion engine piston with bionic stripe structure

Abstract: Piston is the main moving part of internal combustion engine, and works in the worst environment. During the reciprocating motion in the air cylinder piston endures the mechanical loads from multiple directions and the thermal load with high periodical variation. There are also contact friction between piston and air cylinder, poor heat dissipation and high temperature corrosion. All of these can make piston fatigue failure easy. First, this article made the dynamics analysis based on actual working conditions of internal combustion engine, and put forward to using the third thermal boundary condition to compute the temperature of temperature zone. So the boundary conditions of coupled thermo-mechanical analysis were got. Then we could observe the stress, strain and deformation of piston from coupled thermo-mechanical analysis results. Second, most animals have evolved and formed non-smooth surface, which can reduce drag and resist wear. The soil animal earthworm is one of the perfect animals, which has non-smooth surface. It shuttles back and forth in the soil, which was just like piston doing reciprocating motion in air cylinder. We copied stripes and holes from earthworm and enlarged them. The enlarging scale was decided according to piston size. Then the stripes and holes on the piston skirt were processed. The bionic piston could resist attrition and heat dissipation, and increase fatigue life. The coupled thermo-mechanical analysis results of standard piston showed that the stress of perfective aspect of piston was non-uniform. The stress was concentrated on the piston top and the third ring groove, and the maximum deformation was at the bottom of piston skirt. So this design made depth, width and space of stripe variable size. For the stripe, the closer to the top of piston, the larger the depth, width and space between columns. There were 8 stripes on the skirt from top to bottom, and every adjacent 2 stripes had the same size (1 and 2, 3 and 4, 5 and 6, 7 and 8). Third, the three-level and three-factor orthogonal array was used, and 9 testing programs of bionic pistons were made. The first factor was stripe distribution pattern, and it included 3 levels which were stripe distribution, drilling in stripe and a line of stripe with a line of bore. The second factor was stripe depth, and it included 3 levels which were A (0.8, 0.7, 0.6 and 0.5 mm), B (0.9, 0.8, 0.7 and 0.6 mm) and C (1, 0.9, 0.8 and 0.7 mm). The third factor was stripe width, and it included 3 levels which were I (0.8, 0.7, 0.6 and 0.5 mm), II (0.9, 0.8, 0.7 and 0.6 mm) and III (1, 0.9, 0.8 and 0.7 mm). The coupled thermo-mechanical analysis was carried out on 9 bionic pistons. Three typical indices that were the maximum stress of piston top, the maximum stress of the third stripe groove and the deformation of piston, were extracted from the analysis results. Then the cycle index of starting-operation-stopping was computed based on the deformation of piston. The maximum stress of piston top, the maximum stress of the third stripe groove and the fatigue life were optimized by the part orthogonal polynomial regression design, which could find out the inherent law between 3 typical indices and 3 factors. Forth, the piston with optimum performance from the coupled thermo-mechanical analysis was the bionic piston 8, and that from the regression design was the bionic piston 1 and 3. The bench test was carried out with 3 bionic pistons and standard piston. In the bench test, the tested items included the abrasion loss of every piston, the observed wear pattern of piston skirt after experiment and the temperature of piston top under the normal condition. The experiment results could farther verify the simulation results. Ultimately, the fatigue lives of bionic piston averagely increased by 8.8% compared to standard piston. The influence of 3 factors on the performance of bionic piston from high to low was stripe width, stripe depth and stripe distribution pattern. It confirmed that the stripe distribution of a line of stripe with a line of hole was good at increasing fatigue life of piston, and the stripe depth of "B" and the stripe width of "I" and "III" were good at unloading concentrated stress. Abrasion loss of bionic piston averagely decreased by 90% and its top temperature averagely reduced by 5% compared to standard piston.