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 was 0.979 μm in the pin-hole bearing, the maximum rough contact pressure was 249.406 MPa. 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 0.010 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.