Terence W. Bates

The viscoelastic properties of multigrade oils and their effect on journal-bearing characteristics

Abstract In this paper, we readdress the long-standing question as to whether the viscoelastic properties of multigrade oils can have a measurable effect on lubrication characteristics. To facilitate this, we first investigate the rheometrical properties of a number of multigrade oils with similar shear-viscosity responses. Specifically, we employ a Weissenberg Rheogoniometer and a Lodge Stressmeter to provide a measure of the normal stresses. We then investigate the behaviour of the oils in a journal bearing simulator. We find that viscoelasticity does indeed produce a measurable and beneficial effect on lubrication characteristics at the higher eccentricity ratios and that this effect correlates well with a characteristic relaxation time.

A Correlation Between Engine Oil Rheology and Oil Film Thickness in Engine Journal Bearings

Oil film thickness measurements made in the front main bearing of an operating 3.8 L, V-6 engine were compared with rheological measurements made on a series of commercial and experimental oil blends. High-temperature, high-shear-rate viscosity measurements correlated with the film thicknesses of all single-grade and many multigrade oils. However, the film thicknesses provided by some multigrade oils were larger than could be accounted for by their high-temperature, high-shear-rate viscosities alone. Although the pressure-viscosity coefficients of some of the oils were significantly different from those of the majority of oils tested, they were not oils which produced unusual film thicknesses. As a consequence, correcting oil viscosities for the estimated pressures acting within the bearing was unsuccessful in improving the correlations. The correlations were improved, however, by accounting for the elastic properties of the multigrade oils. Measurements of oil relaxation times at high temperatures and shear rates showed large differences in elastic properties among the test oils. A good correlation (R/sup 2/ = 0.73) was obtained from a multiple linear regression of film thickness as a function of both high-temperature, high-shear-rate viscosities and relaxation times.

EFFECTS OF ISOTROPIC PRESSURE ON THE HIGH TEMPERATURE HIGH SHEAR VISCOSITY OF MOTOR OILS

The viscosity of 10W/40 motor oils formulated with different viscosity index (VI) improvers has been measured at pressures up to 200 MPa (2000 bar) over a wide temperature and shear rate range. The response of viscosity to pressure was found to depend on the chemical nature of the VI improver at both low and high shear rates. As a consequence, the ranking of the 150 degree C viscosities of polymer-thickened oils can be different at high pressures to that observed in conventional atmospheric pressure viscometers. These effects of VI improver type on high pressure viscosity appear to be relevant to wear in journal bearings of fired engines; thus in experiments carried out in the ALI Bearing Distress Test, bearing weight loss of multigrade oils containing different VI improvers correlated better with high pressure, high shear viscosities than with high shear viscosities measured at atmospheric pressure.

Effect of Oil Rheology on Journal Bearing Performance Part 3—Newtonian Oils in the Connecting-Rod Bearing of an Operating Engine

The connecting-rod, big-end, bearing of a 2.8 litre, V-6, gasoline engine has been instrumented to allow measurement of oil film thickness during engine operation

Co-ordinating European Council (CEC) Contribution to the Measurement of High-Temperature, High-Shear-Rate Viscosity and Its Relationship to Engine Operation

Co-ordinating European Council (CEC) studies showed good agreement among results obtained with different types of high-shear viscometers. Curve-fitting procedures based on the Extended Cross Equation provided a means of comparing results from different viscometers and permitted the interpolation and extrapolation of viscosities to a standard temperature and shear rate, for example, 150°C and 10 6 s - 1 . These studies contributed significantly to the development of the first low-cost, commercial, rotational viscometer capable of measuring viscosity over appropriate temperature and shear rate ranges. Satisfactory precision was demonstrated, and international standard procedures were established for the Ravenfield viscometer. Engine studies showed strong linear correlations of high-temperature, high-shear (HTHS) viscosity with engine friction, engine-bearing oil flow, and fuel saving. Results of engine wear tests, however, did not appear to be viscosity dependent. Areas for future study, for example, measurement of bearing oil film thickness, are indicated, and the interaction of CEC with the European Committee of Common Market Automobile Constructors (CCMC), ASTM, and the Society of Automotive Engineers (SAE) is described.

Effect of Oil Rheology on Journal Bearing Performance: Part 2 - Oil Film Thickness in the Big-End Bearing of an Operating Engine

The instrumentation of the big end bearing of a 2.8 Litre v-6 gasoline engine to measure bearing oil film thickness during engine operation is described. The electrically insulated bearing and the journal form a capacitor with the oil film as the dielectric. Synchronous measurement of the capacitance enables the determination of the minimum oil film thickness, hm, as a function of crank angle. The minimum value of hm for a complete 720 degrees crankshaft rotation can be measured to a precision of about 5 per cent; it is little affected by chemistry of the engine oil performance package or by engine torque but decreases significantly with increasing oil temperature and increasing crankshaft speed (a).