Search Results

A range of multigrade oils designed for both gasoline and diesel engine operation have been characterised in terms of their load bearing capacity (LBC) and hydrodynamic friction (torque) under transiently loaded conditions using a journal bearing simulator. Base oil behaviour was also investigated. The results suggest that while LBC and torque are viscosity dominated, it is possible to decouple transiently loaded journal bearing LBC and hydrodynamic torque by modification of the bulk lubricant rheology. It would now appear possible that both LBC and frictional benefits may be obtained by from multigrade oils by judicious choice of lubricant base oil and additive systems.

In this paper we examine the question of whether the viscoelastic properties of multigrade oils can have a measurable effect on the performance of dynamically loaded journal bearings. Our investigation takes the form of a computational study in which the full set of coupled equations (kinematic and constitutive) governing the flow of the lubricant is solved using a moving spectral element method. The lubricant is modelled using the White-Metzner constitutive relationship.

The performance of journal bearings is dependent on many factors such as design, materials, load cycle and lubricant. The behaviour of lubricants in automotive bearings is of critical importance as the size of the bearing is restricted due to engine compartment space constraints and the loadings are increased with rising power output. The traditional approach to the modelling of journal bearing lubrication has been via the “lubrication approximation” introduced by Reynolds. If the lubrication approximation is not invoked, then there is no option but to solve the full set of coupled equations (kinematic and constitutive) governing the flow of the lubricant and taking proper account of the moving parts of the geometry. Until recently, this task has proved too formidable, but with current computing power, combined with efficient and accurate numerical methods, the calculation may be attempted.

Continuing interest in energy conservation and friction reduction, driven primarily by environmental concerns, provides opportunities to develop energy saving lubricants. The greatest potential energy savings come from reductions in hydrodynamic friction as typified by main and connecting rod journal bearings in automotive engines. The main approach to minimise friction losses in these bearings is to reduce the lubricant viscosity. However, this approach will inevitably reduce oil film thickness and impose even higher stresses on the lubricant. The problem is compounded by the use of multigrade oils, which contain relatively high molecular weight polymeric components, and exhibit both temporary and permanent shear thinning. Thus these lubricants exhibit non-Newtonian flow behaviour under the extreme conditions imposed by engine bearings.

Most automotive multigrade oils contain high molecular weight polymeric viscosity index improvers (VII's) and are, to a greater or lesser extent, viscoelastic fluids. For many years the effect of multigrade oil viscoelasticity on journal bearing lubricant load bearing capacity (LBC), (and implicitly minimum oil film thickness) has been a vexed question. The work described in this paper provides experimental evidence that a significant enhancement of journal bearing LBC over that generated by isoviscous Newtonian (single grade) oils can be achieved by the use of multigrade oils under simulated realistic in-senice engine conditions. This effect only occurs under operation at high eccentricity ratios. At lower eccentricity ratios no enhancement in LBC could be found. Examination of the effect of lubricant piezoviscosity on LBC failed to account for the load bearing enhancement phenomenon.

Many engineering components in engines and machines are of a counter conformal geometry (e.g. valve trains, rolling element bearings and gears) and impose not only high shear rates and temperatures but also extremely high pressures on the lubricant. The effect of pressure is to elastically deform the sliding/rolling contact geometry. Of crucial importance to the engineering and lubricant designer is the magnitude of the lubricant film thickness generated under these severe conditions. Steady state isothermal elastohydrodynamic (EHD) lubrication is now relatively well understood by the use of engineering correlations, first propounded by Dowson et. al., for both line and point contacts, which are used as design tools. However, with ever increasing demands to improve the efficiency of machines, these correlations do not satisfy all the design needs, especially under reversal conditions, where wear is a major problem.

Most automotive multigrade oils contain high molecular weight polymeric viscosity index improvers (VIIs) and are to a greater or lesser extent, viscoelastic liquids. A way of characterising lubricant viscoelasticity (as typified by the first normal stress difference) using slit die rheometry is described in this paper. This technique has been applied to measure the viscoelasticity of several conventional multigrade oils at temperatures and shear rates close to that experienced by the lubricant in an automotive journal bearing. Additionally, the potential of slit die rheometry as a technique to measure the viscosity of engine oils at shear rates up to 2 X 106s-1 is assessed.

The widespread use of overhead camshaft (OHC) rocker-follower valve-train configurations in current automotive engines allows a more compact cylinder head design and improved valve operation. Unfortunately, this valve train configuration can be difficult to lubricate, as evidenced by a number of wear problems occurring in service. As a consequence, there have been a proliferation of industry standard wear tests. Little work has been published on the rheological behaviour of the lubricant in these severe non-conformal contacts. A motored cylinder head utilising a cam and rocker-follower-valve train configuration has been instrumented in order to measure the oil film thickness (OFT) in an exhaust valve contact by means of an electrical capacitance technique. The experimental apparatus and data acquisition system are described, together with the subsequent data processing.

A motored cylinder head utilising a cam and inverted bucket follower configuration has been instrumented to measure the oil film thickness in the elastohydrodynamic (EHD) contact separating cam and follower. Measurements were carried out using a capacitive divider circuit originally designed for use with deep groove ball bearings. A method is presented to convert the electrical measurements of contact capacitance into actual oil film thickness. These oil film thickness measurements are compared with a theoretical EHD analysis for this cam and follower configuration at the test temperature of 100°C. Oil film thickness measurements are reported for seven single grade oils of viscosity range 5 to 23 mPa.s at 100°C. These oils are fully formulated with the same additive package but with base stocks varied to obtain the required viscosities. The tests were run at constant camshaft speed and temperature.