Cold Starting and Pumpability Studies in Modern Engines — Results from the ASTM D02.07C Low Temperature Engine Performance Task Force Activities: Phase I Pumpability Testing

Abstract

LTEP Phase 1 pumpability testing focused on overnight cooling evaluation of LTEP 1–7 reference oils. Generally, each engine/oil combination was cooled in 16 hours to the desired test temperature before motoring for the pumpability evaluation. All tests in which limiting pumping criteria was achieved indicated failure by flow limited behavior rather than by air-binding failures. Two approaches were investigated for relating the time to attain a pressure at two specified engine locations to the lubricant's properties. One was to develop correlations directly between pressurization time and the lubricant's temperature in the sump, and then use these correlations to calculate the minimum pumping temperature and the corresponding maximum pumpable viscosity. The other was to develop correlations between pressurization time and the lubricant's viscosity. The first approach compared times to reach a given pressure after the oil pump (Pump Out) or the oil distribution passage downstream from the filter (Near Galley) for the observed lubricant sump temperature. A first-order exponential decay was found to give the best overall correlation for all the engine/oil combinations. All four test engines exhibited a dependency of Near Galley pressurization time on sump temperature, except for combinations involving LTEP 1 oils and the 4.6 L engine, where data were limited by cold room capabilities. Near Galley pressurization was also found to be a more stringent criterion than Pump Out in most cases. Using a 60 second 10 kPa limit, minimum pumping temperatures (MPTs) were calculated for each engine/oil combination, along with a certainty level based upon degree of extrapolation. Based upon these MPTs, limiting ASTM D 3829 viscosity was approximately 93 Pa∙s. The results also indicated that, as in the startability studies, the viscosity limit for the 4.0 L I6 engine was compatible with those of the December 1994 SAE J300 Viscosity Classification Specification, while the other engines were more in line with the December 1995 J300 Viscosity Classification Specification limit. In the second approach to pumpability analysis, a correlation between oil viscosity and pressurization time for each engine at the Pump Out or Near Galley location was developed. A linear model relating lubricant viscosity to pressurization time was found to be adequate. Analysis was done to compare viscosities as measured by ASTM Test Method for Predicting the Borderline Pumping Temperature of Engine Oil (D 3829), Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature (D 4684) and Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using, a Temperature-Scanning Technique (D 5133). The model equations provided tools to calculate the limiting viscosity for these engines. As with the first approach, the results indicated that the viscosity limit for the modern engine designs was more in line with the current April 97 J300 Viscosity Classification Specification limit. Additional work was conducted to model the time/oil pressure curves from the LTEP work. It was found that most of the pressurization curves could be modeled well using a logit function.