James A. Spearot

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.

A comparison of the total capacitance and total resistance techniques for measuring the thickness of journal bearing oil films in an operating engine

Bearing oil film thickness (BOFT) values were determined for the front main bearing of a four-cylinder engine at different engine speeds, loads, and oil temperatures using two different measurement techniques. The total resistance technique assumes the oil film can be modelled as a simple ohmic resistor. The total capacitance technique assumes the oil film can be described as a simple capacitor. A comparison of results determined using both methods for a set of single-grade oils demonstrated that, although the methods agree quantitatively for certain combinations of engine test conditions and oils, the level of precision with the total capacitance technique is greater than with the total resistance technique

Relating High-Temperature, High-Shear-Rate Viscosity to Engine Operation

The argument over which values of oil viscosity best correlate with measures of engine operation has gone on for many years. Over a decade ago (1977), the membership of the Engine Oil Subcommittee of the Society of Automotive Engineers (SAE) Fuels and Lubricants Committee (now the Fuels and Lubricants Division) agreed that there were enough data to justify asking the appropriate ASTM subcommittee (D02.07) to develop methods for determining values of high-temperature, high-shear-rate (HTHS) oil viscosity and to relate them to engine operation. In 1984, an ASTM task force finished reviewing the available literature and composed a report that summarized the effects of viscosity on bearing oil film thickness, engine wear, engine friction, and fuel economy (ASTM DS-62). This paper reviews the findings of that report and updates them in light of recent studies. The case for incorporating HTHS viscosity specifications into the Engine Oil Viscosity Classification, SAE J300, is presently stronger than ever. Suggestions are offered on how this incorporation might be accomplished and on what future directions ASTM research on HTHS viscosity should take.

Engine Oil Additive Effects on the Deterioration of a Stoichiometric Emissions Control (C-4) System

Phosphorus, originating from Che commonly used engine oil additive zinc dialkyldithio-phosphate (ZDP), contributed to the deteriora tion in the performance of a stoichiometric emissions control system. Data obtained from a series of 200-hour engine-dynamometer tests suggest two separate means for phosphorus-induced system deterioration: reduced CO, N0x, and HC conversion efficiencies due to catalyst poisoning; increased oxygen sensor rich-to-lean response time and altered oxygen sensor output voltage due to sensor contamination. The presence of an alkaline metal detergent in the ZDP-contalning engine oil resulted in less deterioration of converter HC efficiency, but had no effect on the other system performance parameters.

Measuring the effect of oil viscosity on oil film thickness in engine journal bearings

The minimum oil-film thickness in the front main bearing of a 3.8 L, V-6 engine was measured at 3,000 r/min, and 140 N.m using an electrical resistance technique. For a series of seven Newtonian, single-grade oils, film thickness correlated with oil viscosity measured either in a kinematic or in a high-shear-rate viscometer. For a series of fifteen polymer-containing, non-Newtonian, multigrade oils, however, no single measure of viscosity adequately correlated with film thickness for all of the oils. By eliminating four multigrade oils from the combined single and multigrade data sets, it was possible to correlate film thickness to the viscosity (of the remaining multigrade and Newtonian oils) measured at 150 deg C and 500,000/s, conditions which are believed to be representative of temperatures and shear rates in bearing oil films. Possible explanations for the lack of correlation with the entire set of twenty-two oils are discussed.