农用柴油机钢活塞销孔-销的摩擦副润滑特性分析

为解决钢活塞销孔-销摩擦副因同种材料摩擦配副问题以及钢材密度大和导热性能差所带来的润滑特性差的问题，该研究以D25TCIF农用柴油机钢活塞为对象，建立钢活塞连杆组传热模型和热弹性流体动力学模型，并开展钢活塞温度场测试试验与仿真验证。结合单因素扫值法和Box-Behnken多因素优化算法分析了销孔轴承间隙、销孔表面粗糙度和销孔指数型线内外半径增量变化对销孔轴承润滑特性的影响。结果表明：销孔结构优化后销孔轴承内的最小油膜厚度较优化前增加0.705 μm，最大粗糙接触压力降低255.956 MPa，说明销孔结构对轴承的润滑特性有很大影响，销孔指数型线内半径增量的影响最大，而外半径增量的影响较小。最优参数组合为销孔轴承间隙0.021 mm、销孔表面粗糙度0.798 μm、销孔指数型线内半径增量0.008 mm、销孔指数型线外半径增量0.01 mm，此时预测的最小油膜厚度为0.979 μm；最大粗糙接触压力为249.406 MPa，与该方案下仿真值的相对误差小于5%。本文优化方法效果好，且预测准确，可为后续的钢活塞销孔结构设计提供理论依据。

Pin-hole lubrication in steel piston of agricultural diesel engine

Diesel engines are required for the compact structure with high strength, lightweight, and low emission in the Dual Carbon and Stage IV fuel consumption standards. Among them, the combustion temperature and burst pressure of diesel engines have attracted much attention at present. Specifically, the maximum temperature has reached more than 400 ℃ on the top surface of the piston, while the maximum burst pressure has reached more than 20-22 MPa in the cylinder. However, the aluminum alloy piston cannot fully meet the harsh requirements of agricultural diesel engines, due to the low strength of aluminum-silicon alloys. Steel pistons can be expected to replace aluminum alloy ones. However, the friction and wear of steel pistons can be seriously confined to the high density, low thermal conductivity and the coefficient of thermal expansion of steel. This study aims to improve the lubrication performance of pin-hole friction vice with the friction mating of the same materials. Taking the steel piston of the D25TCIF agricultural diesel engine as the object, a heat transfer model was established for the piston connecting rod group. A temperature field test of the piston was carried out to validate the accuracy of the model. An accurate thermal deformation of the piston pin-hole was obtained to construct the thermoelastic hydrodynamic model of the piston connecting rod group. The simulation was also conducted for the dynamic and lubrication properties of the piston pin-hole bearing. The result showed that the piston pin was followed by the connecting rod steering at a small rotational speed. Only the intake phase delayed the steering, due to the large rotational inertia of the piston pin. The angular velocity of the piston pin was basically zero at the moment of burst pressure, leading to the piston pin-hole bearing in a boundary lubrication state. The insufficient supply of oil also led to a large rough contact pressure at the upper end of the inner side of the pinhole. Therefore, the minimum thickness of the oil film and the maximum rough contact pressure were selected as the evaluation indexes for the lubrication characteristics of the pin-hole bearings. A one-factor test was carried out to investigate the effects of the pin-hole bearing clearance, the pin-hole surface roughness, and the increment of the inner and outer radii of the pin-hole index profiles on the performance of the pin-hole bearings. All structural parameters improved the performance of the pin-hole bearing. The best optimization was achieved in the inner radius increment of the pin-hole index profile. There was the stress concentration in the upper end of the inner side of the pin-hole, leading to the increase in the inner increment of the pin-hole profile. A better match was obtained for the bending deformation of the piston pin in the outburst moment of the pressure. The pin-hole pressure-bearing area increased to reduce the pin-hole concentration of the stress. Box-Behnken multifactor optimization was then introduced to clarify the weights between the factors and the structure parameters of the pin-hole after optimal design, where the minimum thickness of the oil film and the maximum rough contact pressure were taken as the response values. The results show that the minimum thickness of oil film increased by 0.705 μm in the pin-hole bearing, whereas, the maximum rough contact pressure decreased by 255.956 MPa after optimization, compared with the preoptimization. The pin-hole structure shared a great influence on the lubrication characteristics of the bearings. The pin-hole index profile had the greatest influence on the inner radius increment, while the smallest influence was the outer radius increment. A combination of optimal parameters was the pin-hole bearing clearance of 0.021 mm, pin-hole surface roughness of 0.798 μm, as well as the pin-hole index profile inner and outer radius increment of 0.008 and 0.01 mm, respectively. The minimum thickness of the oil film was predicted as 0.979 μm; the maximum roughness of the contact pressure was 249.406 MPa, and the relative error with the simulation was less than 5%. The effective and accurate prediction can provide a theoretical basis for the subsequent design of steel piston pin-hole structures.