Influence of fin structure on the strength and heat transfer performance of the cylinder for aviation piston air-cooled engine

Abstract: An aviation piston air-cooled engine has been widely used in modern agriculture, plant protection, and unmanned aerial vehicles, because of its compact structure with small size, light weight, high thermal efficiency, simple manufacturing process, and low maintenance cost, as well as the low fuel consumption rate. However, the high thermal load has posed a great threat to the aviation piston air-cooled engine. In this study, a novel one-dimensional simulation model was established to determine the influence of fin structure on the strength and heat transfer of cylinder in an aviation piston air-cooled engine using GT-Power software. The aviation air-cooled engine was taken as the horizontally opposed four-cylinder with the four-stroke piston in the gasoline system. The cylinder pressure test was also carried out to evaluate the working pressure in the engine cylinder under calibrated power conditions. The structure and heat transfer were then characterized for the air-cooled engine cylinder. A systematic calculation was made on the boundary conditions of the gas side in the cylinder. A thermocouple was used to measure the working temperature of the key area in the cylinder under the calibrated power condition. After that, a coupled three-dimensional model was established to combine the temperature and heat transfer coefficient of the combustion chamber in the cylinder and each position of the cylinder. A fluid-structure coupling model was achieved in the heat transfer and strength, including the cylinder head, cylinder, crankshaft, crankcase, and cooling air. The temperature and stress distribution of the cylinder were then obtained from the temperature and heat transfer coefficient distribution of the air-cooling field. The average error of 4.8% was calculated to compare the simulation with the in-cylinder pressure test, particularly fully meeting the engineering needs and the subsequent calculation requirements. The average temperature error of each temperature measurement point was less than 5%, when comparing the measured and simulated cylinder temperature. It indicates that the fluid-structure coupling model fully met the calculation requirements. The fin thickness, spacing, and length were selected to analyze the influence of their heat dissipation on the structural strength and heat transfer performance of the cylinder. The results show that different variations were found in the three structural parameters. Specifically, the temperature of the cylinder decreased by 14.5 ℃, whereas, the thermal stress increased by 23.9 MPa, with the increase of fin length. The temperature and thermal stress of the cylinder decreased by 19.8 ℃ and 33.0 MPa, respectively, with the increase in fin thickness. The temperature of the cylinder decreased by 13.5 ℃, whereas, the thermal stress increased by 6.9 MPa, with the increase of fin spacing. Furthermore, the three-factor five-level orthogonal experiment was designed to take the fin thickness, spacing, and length as the factors. The calculation results show that the greatest influence on the cylinder temperature was found from the fin length, followed by the fin thickness, and the fin spacing was the least. The cylinder body temperature of the machine decreased by 18.2 ℃ after optimization, compared with the original. The greatest influence on the structure strength of the cylinder was from the fin thickness, followed by the fin length, and the fin spacing was the least. The maximum thermal stress of the machine decreased by 50.1 MPa after optimization, compared with the original. The findings can provide a strong reference to design and optimize the heat fin of aviation piston air-cooled engines.