Prediction of frictional and thermal efficiency of high performance transmission system

Efficiency and durability are key areas of research and development in modern drivetrains. The fuel economy requirements, due to both fuel cost and adverse environmental impact, are subject to increasingly stringent measures for the modern vehicles. This necessitates the need for transmission systems, which are capable of operating under severe conditions whilst remaining efficient and reliable. Downsizing; increasing the output power- to-weight ratio and modification of gear teeth geometry are used to reduce both the load-dependent and loadindependent losses. However, these approaches can result in reduced structural integrity and component durability. Achieving a balance between system reliability and optimal efficiency requires detailed integrated multi-disciplinary analyses, with the consideration of system thermal efficiency and contact mechanics/tribological considerations for stress analysis and structural integrity.

In recent years, dry sump lubrication system have been increasingly used for efficient transmission design in order to increase the output power- to-weight ratio, and guard against churning losses; as the main contribution to the load-independent losses. One of the most important aspects of any dry sump system is the assessment of its thermal performance and its capability to remove the generated heat from the loaded and unloaded system surfaces. The generated heat in the highly loaded contacts subject to high shear in high performance transmissions should be dissipated through impinging oil jets and in an air-oil mist atmosphere of the transmission casing in an efficient manner.

This thesis incorporates a component level TCA (Tooth Contact Analysis), a mixedelastohydrodynamics tribological model, and a 3D CFD model into a system level finite element model to evaluate the quantity of generated heat in the lubricated gear teeth pair meshing contacts, as well as the heat removal from the rotating gear surfaces. It predicts the system thermal equilibrium. Furthermore, the transient circumferential temperature distribution on gear surfaces is evaluated and used to determine the transient temperature distribution over a driving cycle. Such an approach has not hitherto been reported in the literature and represents the main contribution of this thesis to knowledge.