1. Oil Film Thickness in Engine
1.2 Does the Automotive Industry Need a Standard Engine Test
1.2.6 Does the Automobile Industry Need a Bearing Film Thick
ness Test?
The answer to the central question o f this paper depends entirely on whether or not the film thickness in a journal bearing o f an operating engine can be adequately described by one or more laboratory oil Theological properties measurements. Clearly, if relatively straightforward laboratory measurements are available, there is no justification fo r a more complex, harder to control, more time-consuming and expensive engine test. SAE J300 was originally
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developed as a table o f oil viscosities from which engine designers could choose an oil suitable fo r use in their engines w ith o u t having to evaluate a large number o f oils in engine tests. The concept of such a table o f oil viscometric properties is still valid and desirable.
But are the proper rheological measurements which affect bearing oil film thickness known, and can they be easily measured in the laboratory? Although our research has progressed greatly during the last decade, and we now have the capabilities to measure film thickness in bearings more easily than ever be
fore, it must be said that the exact relationship between oil rheological properties and bearing film thickness fo r multigrade oils is yet to be determined. The data in several publications have demonstrated that the viscosity o f engine oil measured at a temperature and shear rate close to those o f an operating journal bearing is an important factor in bearing performance. No reasonable inter
pretation o f the existing data would conclude that kinematic viscosity at 100°C relates better to bearing film thickness than does a viscosity measured at bearing temperatures and shear rates.
The problem which occurs, however, is that each engine bearing and operating condition is characterized by a different shear rate and temperature. Where 150°C and 10s s—1 might be well suited to describe the operation o f some bearings at certain operating conditions, it is less suited fo r others. Complicating the issue even further is the fact that the characteristic shear rate o f an operating journal bearing is itself a function o f oil viscosity. As o il viscosity increases, film thickness increases and shear rate, in general, decreases. Thus, the selection o f any particular set o f shear rate and temperature conditions at which to specify the viscosity grade o f an engine oil must be viewed as a compromise based on the characteristics o f many engines and many operating conditions.
This concept is not w itho ut precedent, however. The pum pability specifications in SAE J300 (the Borderline Pumping Temperature, BPT) are based on an average o f the pum pability characteristics o f a set o f w idely differing engines.
The advancements which have been made over the past decade in measuring the viscosity o f oils at high temperatures and shear rates have also been significant. Although the maximum shear rates at which oil viscosities have been reported measured in the laboratory range from 2 to 5 x 106 s~* (13 ,14 ), less than the shear rates which can occur in con-rod bearings (6 ), these measure
ments are significant in that a decade ago viscosity determinations at 10* s-1 were considered the lim its o f laboratory capabilities. Regardless o f the value o f shear rate which is identified as being representative o f bearing operation, if the shear rate is produced in an operating engine, it can be reproduced in a laboratory viscometer. Viscosity values may be determined on a relative rather than an absolute basis, but they w ill be determined none-the-less.
The other rheological property which may influence journal bearing operation is oil elasticity. Although the relative influence o f this multigrade oil property is still to be verified, the measurements needed to define its effect in engines
are available, and the laboratory measurements needed to quantify its magnitude are beginning to be developed. The limited data which have been collected in recent engine bearing studies suggest that the effect of elasticity is additive to that o f any viscosity effect. If this is true, it means that a vis
cosity classification system based only on measurements of high-temperature, high-shear viscosity would define a level o f minimum performance fo r m ulti
grade oils. This would provide protection fo r the automotive industry from shear-thinning, non-elastic multigrade oils. If it can be unequivocally demon
strated that elasticity provides an extra measure of protection fo r certain multigrade formulations, then recognition o f this fact could also be incorpor
ated into a system such as SAE J300 at a later date.
With regard to the problems which might be experienced in developing an engine BOFT test, it is w orth noting that the development o f any "standard" industry engine test is a time-consuming, arduous task. In this instance, potential prob
lems would arise even in the selection of a test engine. Because the operat
ing characteristics o f passenger car and heavy-duty engines are radically d iffe r
ent, it is doubtful that either segment of the industry would accept a BOFT engine test which uses the other's engine. Journal bearings in heavy-duty engines are designed differently and for different objectives as are those in passenger car applications. Even if an engine could be selected to which both segments of the industry agree, the selection of lim its on film thickness to define different viscosity grades o f oil in such a test would present a significant hurdle. BOFT engine tests are excellent tools fo r developing an understanding o f how journal bearings operate in an engine. As a technique for defining different grades of engine oil, however, they should be considered as a last resort after it is demon
strated that laboratory rheological properties measurements can not do the job adequately.
Given the current data regarding the effects of oil rheological properties on bearing oil film thickness in engines, it is the opinion of General Motors Cor
poration, that including oil viscosities measured at a representative temperature and shear rate in the Viscosity Classification System, SAE J300 is sufficient for providing a measure o f oil high-temperature, high-shear viscometric performance.
The engine test procedures which have been developed for research projects designed to study oil rheological effects on bearing performance are very good, and the data collected from them w ill help in the selection of a temperature and shear rate which is representative o f industry bearing characteristics. However, at this time, there is no compelling reason fo r complicating the oil qualification process w ith an engine test designed to measure only journal bearing film th ick
ness.
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