Thermohydrodynamic analysis of lubricated piston rings of internal combustion engine

Abstract

Petroleum and automotive industries are facing tough time across the globe due to steep rise in petroleum based fuels’ prices and ever increasing governments regulations related to improving the fuel economy and lower emissions from the fuel and lubricant system of IC engines. Therefore, worldwide substantial efforts in the design of reciprocating internal combustion (IC) engines are being made by the researchers for improving the fuel economy and reducing the exhaust emissions. Nowadays, great attention is being given on the reduction of friction at the various interfaces formed between the mating components of the IC engines. Effective lubrication at the interfaces of cylinder liner/piston rings, piston/piston rings, and piston skirt/cylinder liner play vital role in achieving high power efficiency, ensuring long operational life of the interfaces, limiting the consumptions of lubricating oil and fuel, providing a good dynamic sealing at various interfaces. Thus, efficient design of contacts formed between piston rings and counter surfaces in piston assembly is a great desirable task in the emerging scenario. It is worth mentioning here that dearth of studies can be seen in literature on the design and development of smart piston rings. Therefore, the main objective of this thesis is to study mathematically hydrodynamic lubrication of the interfaces formed between the cylinder liner and various surface profiles of the piston rings for reducing the frictional losses at the interfacial contacts. In the investigation reported herein, four single continuous surface profiles (Catenoidal, Cubic, Exponential, and Parabolic) on the face (surface in contact with cylinder liner) of piston rings have been considered for arriving on the best efficient face profile among these. These surface profiles for face of piston rings have been chosen in this investigation looking their potential performance as faces of pads in thrust bearing. The additional objective of this thesis is to perform the experiments on a commercial IC engine by manufacturing the best face profile (arrived based o mathematical modeling) on all the compression piston rings for accessing the fuel saving and exhaust emissions. Coupled solution of governing equations (Reynolds equation, Energy equation, Film thickness relation, and Rheological relation) has been achieved using finite difference method. Gauss-Seidel iterative method is employed in the solution of discretized equations. Using appropriate boundary conditions and convergence criterion, performance parameters of a single interface (cylinder liner and face of piston ring) have been thoroughly investigated. Based on the numerical study, it has been observed that exponential face profile of the piston ring yields lesser friction than other three profiles (Catenoidal, Cubic, and Polynomial). Thus, exponential face profile has been tried to fabricate on the faces of compression piston rings. However, due to manufacturing constraint an approximate exponential face profile only could be realized. Using approximate exponential face profile on compression piston rings, experiments have been carried out on a commercial diesel engine fueled with diesel and Jatropha based biodiesel (B100) at various loads. This exponential face profile of piston ring has considerable impact on engine’s brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), and mass flow rate, irrespective of fuels used. BTE of engine fueled with diesel increases 2 - 8% with exponential face profile design (III) of piston ring in comparison to standard (conventional) piston ring. BTE enhances 8 - 16% when engine is fueled with biodiesel using exponential face profile design (III) on piston rings. Corresponding to increase in BTE, the reduction in BSFC (biodiesel) is about 28 - 34%. Moreover, significant reductions in exhaust emissions are also recorded with exponential face profile on compression piston rings. Moreover, friction reduction due to dimpling has been also explored. In this direction, some experiments have been conducted using a pin-on-disc machine. The investigations carried out with micro-dimpling on generic tribo-contact give very significant reduction at interfacial friction. Thus, in future dimpling may be tried on face of piston rings for development of smart piston rings.