ENGINE BEARING TRIBOLOGY

Overview

The design of journal bearings is important for the development of internal combustion

engines. They experience load that varies both in magnitude and direction, the load being

caused by the pressure forces and inertial forces of the crank–slider mechanism. It has been

found that the most common causes of premature failure in internal combustion engine

bearings are fatigue, sliding surface wear, erosion due to cavitation phenomena and all these

effects are directly or indirectly related to the shape of the oil pressure field in shaft-bearing

interface. An accurate estimation of oil pressure distribution characteristics and measurement

of oil film thickness in the shaft-bearing interface would allow better prediction of the bearing

performance and reliability.

The goal of this work is to develop a method for calculating the oil film thickness (OFT),

oil film pressure (OFP) in the shaft-bearing interface and its validation by experimental results.

The detailed analysis of the lubrication condition around the position of minimum oil film

thickness conditions is based on finite difference method of integration of the Reynolds

equation coupled to the elastic deformations determined by a detailed finite element model.

Four non contact eddy current gap sensors mounted on the main bearing of a single cylinder

engine are used to measure the oil film thickness and the journal orbit for variable speed and

variable loading conditions.

Approach

To arrive at the OFT, the instantaneous eccentricity was used as a starting point along

with the operating and physical parameters. OFP distribution corresponding to these

conditions was calculated by a numerical solution of the Reynolds equation. The loading

information thus obtained was applied to a FEM model of the bearing shell in Hypermesh

environment coupled to Nastran solver, to determine its elastic deformation. This was fed back

to the instantaneous eccentricity and an iterative technique leading to the OFT under

elastohydrodynamic conditions was developed by matching the Load Carrying Capacity at

every step. The benefit of this model is that is somewhat accurately predicts OFT and the

corresponding OFP for the real case of an elastic bearing shell.

The journal orbit was plotted using Mobility method, and compared with the

experimentally measured orbit. A correlation was established between the two, leading to a

validation of the elastohydrodynamic lubrication model developed.

Flowchart Representing Analysis Methodology

Flowchart Representing Operation of Matlab Code

Experimental Bearing Model

Results

Experimental Bearing Model – OFT Iteration 2 (Load = 3.11 kN)

Pressure Variation for the Elastic Case Bearing Load = 17 kN (Pressure in MPa)

Pressure Distribution along length of bearing Pressure distribution along width of bearing

Pressure Force, Inertia Force and Total Force 1250 rpm and 3 Nm

Polar Plots of Bearing Loading 1250 rpm and 3 Nm

Results – Oil Film Thickness vs CAD 1250 rpm and 3 Nm

Results – Oil Film Thickness vs CAD 1250 rpm and 3 Nm

Results - Journal Orbit 1250 rpm and 3 Nm

Indicated Elastic Deformation = 9 μm

Results - Displacement Indicated by Finite Element Model 1250 rpm and 3 Nm

Results – Comparison of Journal Orbit determined by Mobility Method with

Experimental Result 1250 rpm and 3 Nm

Mobility Method Experimental Result