The use of active lubrication in journal bearings helps to enhance the thin fluid films by increasing the fluid film thickness and consequently reducing viscous friction losses and vibrations. One refers to active lubrication when conventional hydrodynamic lubrication is combined with dynamically modified hydrostatic lubrication. In this case, the hydrostatic lubrication is modified by injecting oil at controllable pressures, through orifices circumferentially located around the bearing surface. In order to study the performance of journal bearings of reciprocating machines, operating under conventional lubrication conditions, a mathematical model of a reciprocating mechanism connected to a rigid / flexible rotor via thin fluid films was developed. The mathematical model involves the use of multibody dynamics theory for the modelling of the reciprocating mechanism (rigid bodies), finite elements method for the modelling of the flexible rotor (crankshaft) and hydrodynamic fluid film theory for describing the dynamics of the thin fluid films. When active lubrication is introduced to modify conventional hydrodynamic lubrication, by means of aplying radial oil injection at controllable oil pressures, the Reynolds equation is modified to accomodate the terms related to the controllable oil injection pressures and orifice distribution on the bearing surface. The active bearing forces and the dynamics of the oil injection system are coupled to the set of nonlinear equations that describes the dynamics of the reciprocating engine, obtained with the help of multibody dynamics (rigid components) and finite elements method (flexible components), and the global system of equations is numerically solved. The analysis of the results was carried out with focus on the behaviour of the journal orbits, maximum fluid film pressure minimum fluid film thickness. The reduction in the cyclic averaged power consumption due to viscous friction forces is also studied. The modelling of two oil injection systems is presented in the work. The main governing equations of the dynamics of a piezo-actuated oil injection system and a mechanical-actuated unit injector are developed. It is shown how the dynamics of the oil injection system is coupled to the dynamics of the bearing fluid film through equations. Applying controllable radial oil injection to dynamically loaded journal bearings helps: a) to reduce friction losses by increasing the fluid film thickness; b) to reduce vibrations (i.e., smaller journal centre orbits); and c) to increase the effective carrying load area by modifying the pressure distribution profile, which can make it possible to use bearings of smaller dimensions with similar load-carrying capacity.
Reciprocating Machinery; Active Lubrication; Mechatronics; Fluid Film Lubrication; Journal Bearings