EHD-Contact in Engine Simulations using a Modal Force Approach in Commercial Multi-Body Simulation Software 2010-32-0034
The formation of a hydrodynamic lubrication is the essential condition for any friction bearings in combustion engines for optimum performance. The shaft and the bearing are separated by a thin sustainable lubrication film, which prevents a direct metal to metal contact. The resulting fluid friction allows for a low frictional loss and the prevention of wear under normal operating conditions. The hydrodynamic load capacity of the bearings is a result of an oil flow, caused by the rotating shaft and/or bearing within the viscous medium. Depending on the gap geometry (shaft eccentricity) a hydrodynamic pressure is built up in the lubrication film. This pressure separates the surfaces that define the lubrication film and thus equilibrates the external bearing load.
However, the local pressure inside the lubrication film will cause local deformations of the shaft and bearing, which again will affect the pressure build-up due to a change of the gap geometry. The resulting pressure distribution inside the lubrication film is a result of the coupling between the local stiffness and the hydrodynamic properties of the bearing. Consequently, in engine simulations such elasto hydrodynamic (EHD) interactions between crank drive components (e.g. main bearings, con-rod bearing) play an essential role. Therefore it is necessary to consider the above discussed effects as accurately as possible to obtain high quality results in a virtual product development process.
Current engine simulation software, while capable of simulating the interactions on a highly local level, require high computational resources to accurately representing the complete engine set-up. These software solutions do not allow the simulation of an engine run-up in acceptable time.
An integrated simulation process is presented, which demonstrates the applicability of commercial flexible multi body simulation (MBS) software like ADAMS to perform such complex simulations in engine design. A finite element code is used to compute the elastic bodies with local pressure loadings (load shape functions) by means of a component mode synthesis. The complete dynamic simulation of the engine assembly is performed using ADAMS. The EHD oil film formulation is implemented as a user-written subroutine. The computed pressure can thus be directly applied on the bearing surface by means of a modal force approach.
Additionally, this integrated simulation process enables for friction loss optimization, taking into account the structural interaction and oil properties as well. The presented simulation process is a fully dynamic approach considering local damping and resonance excitations.
Examples of con-rod, crankcase and crankshaft simulations are shown.