Numerical investigation of the piston skirt lubrication in heavy duty diesel engines

Abstract

Friction reduction in the power cylinder system of internal combustion engines has been undergoing broad and intense study in an industry-wide effort to reduce CO2 from the engines. As a major source of frictional loss in the system, the piston skirt-liner interface, specifically in heavy duty diesel engines, is investigated in this thesis work using a model developed in-house. Prior to the calculation of various cases for parametric study, improvements were made to the existing model in order to incorporate the characteristics of heavy duty diesel engines, and to enhance the robustness and accuracy of the model in general. These improvements include enabling arbitrary distribution oil supply to the system, more efficient way to incorporate the shear-thinning effect of multigrade lubricants, and new scheme at the start of the simulation to resolve the situation with large overlap between piston skirt and liner. The first part of the analysis from application focuses on the geometric parameters of the system such as installation clearance, deformation of the components, and surface roughness of piston skirt. The effects of each individual parameter are discussed and summarized. The second part of the analysis is focused on the sensitivities of the system to the amount and distribution of oil supply. It was found that more lubricant can help reduce friction on thrust side during expansion stroke and on anti-thrust side during compression stroke. However, due to the rapid loss of oil at the piston-liner interface during early compression stroke, there is a limit to the advantage of more oil addition. It has also been suggested that with a certain amount of oil supply, it is more beneficial to add the lubricant higher on the liner. This thesis work is the first effort with the model to systematically study the piston skirt lubrication in heavy duty diesel engines. It is expected to be facilitated by the measurement and observation from experiments in the future.