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Cold start studies were conducted on 10 different engines (including duplicates of one type), and six different multigrade oils ranging from SAE 0W-30 to 25 W-30. As expected, the results showed that for all the engines, the minimum starting temperature decreased with decreasing winter viscosity grade. The 4.0L I-6 engine had the highest minimum start temperatures (MSTs) for all the oils, while the lowest MSTs were distributed over seven of the other eight engines tested. The difference in the starting characteristics of the 4.0L I-6 engine were attributed to the fact that this engine design was relatively old (early 1980s) compared to the rest of the engines in this program (1990s). Another interesting observation was that, excluding the 4.0L I-6 engine, there was no effect on the MSTs due to engine displacement, number of cylinders or configuration. This is contrary to older studies which showed that as number of cylinders increased, minimum starting temperatures decreased. Improved starting characteristics of these modern engines are attributed to improvements in engine technology. An excellent correlation was observed between minimum start temperature and cold cranking simulator (CCS) viscosity for the modern engines. In addition, the CCS viscosity for these engines at the minimum start temperatures was 2 to 4 times higher than the present SAE J300 limits. In contrast, the MSTs for the older engine design (4.0L I-6) appear to follow the present CCS temperature and viscosity limits. The oil viscosity at the MST for the 4.0L I-6 engine was relatively constant (3,000 to 5,000 mPa-s) throughout the whole temperature range, while with the newer engines, limiting viscosity decreased with decreasing temperature (25,000 mPa-s at -19°C to 7,000 mPa-s at -37°C). This requirement for lower oil viscosities at lower temperatures is attributed to a need to compensate for increased internal engine friction with decreasing temperature. Comparison of these results with those from the 1970s shows that the older engine design (4.0L I-6) follows a similar pattern (comparable CCS viscosities and no temperature effect) to the old 4- and 6-cylinder engines, while the newer engines parallel the results for older V-8s.

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.

The Low Temperature Engine Performance Task Force Phase II pumpability test program focused on the evaluation of six engine oils which exhibited significant yield stress or gelation index in slow-cool bench tests. The test oils were designed specifically to have abnormal flow properties and are considered to be non-commercial formulations. The light duty engines that primarily were used included the 2.2L I-4, 3.8L V-6, 4.0L I-6 and 4.6L V-8 engine, with some additional work in the 2.3L I-4, 1.9L I-4 and 3.0L V-6. Four laboratories contributed work, and each used different engines, and often, different cooling profiles, in attempts to better establish the importance of yield stress and gelation index to air-binding failures in modern engines. For the majority of Phase II oils tested, air-binding failures were not detected. No pumping failures were detected with the phase II test oil LTEP 23, which had a gelation index of 16. Using the 4.6L V8 engine, an air-binding failure was detected only with LTEP 27, an oil that had caused air-binding failures in other work. The 2.2L I-4 engine did not fail by air-binding, but the 3.8L V-6 and 4.0L I-6 engines, which generally exhibit flow-limited failures, were “tricked” into producing air-binding failures by use of a reduced volume of test oil LTEP 28. This oil exhibited a gelation index of 40; it also exhibited a substantial yield stress (70–105 Pa) and a relatively low pumping viscosity when the engine pumping tests were run.

In response to a request by SAE, the Low Temperature Engine Performance (LTEP) task force was established within Section 7C in 1992 to determine the starting and pumping characteristics of modern engines using multigrade and single-grade oils, determine if there existed a “safety margin” between limiting cranking and pumping viscosities in modern engines, and assess the benefits or limitations of current lab tests for identifying oils which could result in pumpability failures in engines. This paper discusses the background surrounding the activity and the organization of the task force.