Search Results

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.

An instrument is described which allows the load bearing capacity of a lubricant to be measured under pressures, shear rates and dynamic loadings typical of those found in automotive crankshaft journal bearings. Using Newtonian lubrication approximations, a theoretical analysis of the Journal Bearing Simulator has been carried out. Good agreement between theory and experiment is found demonstrating that the instrument is a fundamentally sound method for measuring lubricant load bearing capacity.

An American Society for Testing and Materials (ASTM) Task Force was formed in 1984 to: 1) establish a series of reference oils, 2) measure the minimum bearing oil film thicknesses provided by the oils in fired engines, and 3) interpret the results in terms of oil rheological properties. Minimum oil film thickness (MOFT) measurement and analysis techniques using a capacitance method were developed. At steady-state operating conditions, laboratories evaluated a matrix of eighteen monograde and multigrade oils, blended with four widely-used viscosity index improvers (VIIs). Analyses showed increasingly better correlations between MOFTs and viscosities of both single grade and multigrade oils as temperatures and shear rates used to calculate viscosities more-closely approximated in-bearing conditions.

A 288 hour cyclic durability test has been developed Co determine bearing wear in the big-end bearings of a 2.8 litre V-6 gasoline engine. Bearing wear for a 5W30 oil with a high-temperature, high-shear (HTHS) viscosity of 2.5 mPa.s is at least as low as that for SAE 30 to SAE 50 monograde oils. Minimum oil film thickness, (MOFT)0, under steady-state operating conditions has also been determined in the big-end bearing of the same engine. There is a critical value of (MOFT)0 below which catastrophic wear occurs. Correlations between (MOFT)0 and high-temperature, high-shear-rate viscosity show that the latter parameter is deficient as a measure of lubricant load bearing capacity. It is concluded that measurement of (MOFT)0 in an operating engine is the optimum test for the routine assessment of lubricant performance in a journal bearing.