MODELING SHEAR HEATING IN THE HYDRODYNAMIC LUBRICATION OF CRANKSHAFT JOURNAL BEARING OF A HIGH-TORQUE DIESEL ENGINE OPERATING AT A LOW SPEED

摘要

Journal bearing plays a critical role in carrying the extensive transient hydrodynamic loads to prevent adhesive wear of crankshaft of a high-torque low-speed diesel engine. The nominal clearance between the shaft-pin and the bearing journal invites viscous shearing of the lubricant on the initiation of rotation at the time of low speed engine start up. Shear heating adversely affects the load carrying ability of the bearing by reducing its viscosity as a function of time. It invites physical contact and wear of bearing and the crankshaft compromising their designed life. In this work the 2-D Reynolds equation is used to model hydrodynamic lubrication phenomenon of crankshaft covering the steady state wedging and transient squeeze which are modeled under the lubricant flooding conditions. The viscous shear heating is modeled by solving energy equation encompassing 2-D convection and 1-D conduction phenomena. The lateral displacements are incorporated in the lubrication model to analyze the effects of secondary dynamics of crankshaft on viscous shearing and friction. The relationships between temperature, viscosity and density are defined to ascertain their effects on bearing lubrication at low engine speed. The numerical simulation results are analyzed for the complete 720-degree 4-stroke engine cycle at a low operating speed. The results show that viscous heating adversely affects the lubrication of journal bearing by significantly reducing the viscosity of lubricant film at low transient loads and speed. The study determines hydrodynamic pressures, temperature, density, viscosity and thermal conductivity of lubricant suitable to minimize the possibility of rupture and adhesive wear due to shear heating under the flooding conditions at a low initial engine speed. It will facilitate towards enhancing the life of crankshaft of a heavy-duty diesel engine.