SAE 5W-30 Pumpability Studies in Modern 4- and 8-Cylinder Engines: Gelation Index and MRV Effects

Abstract

Four SAE 5W-30 formulations with a range of MRV and Gelation Index properties were tested in motored 4- and 8- cylinder engines at ambient temperatures between -35°C and -38°C (below anticipated minimum start temperatures (MSTs)). A slow-cooling profile was used to enhance gelation effects in the test engines, which were motored at normal fast idle speeds. Oil pressurization after the pump was relatively rapid in all cases and did not show a large dependence on oil type or temperature. However, pressurization times at the main gallery showed a correlation to interpolated D 4684 MRV viscosities of the test oils. No correlation was observed between pumpability characteristics and D 5133 gelation index. While the two 2.0L I-4 engines gave comparable pressurization characteristics, the two 4.6L V-8s were quite different from each other. Pumpability differences between the V-8 engines were due to the presence of a plate-type oil cooler in one engine, which reduced oil pressure by 200 KPa and lead to significantly longer pressurization times. At the lowest test temperatures, the 2.0L designs showed ‘pseudo air-binding’ behaviour with all the test oils, in which gallery pressure dropped near zero after an initial pressure spike; pressure before the filter, however, continued to be registered. Low temperature rheological analysis of some of the used test oils was conducted to understand changes occurring after the relatively brief engine operation. In some cases oils with higher gelation indices showed significant decreases after engine operation, while MRV values were relatively unaffected.