ABSTRACT Aeration properties of lubricants is an increasing concern as the design of powertrain components, specifically transmissions, continue to become more compact leading to smaller sumps and higher pressure requirements. Although good design practices are the most important factors in mitigating the aeration level of the fluid, the fluid properties themselves are also a contributing factor. This paper investigates the aeration properties of specific base oils commonly used to formulate modern transmission fluids using the GM Aeration Bench Test found in GMN10060. The test matrix includes thirteen different fluids representing a cross-section of base oil types, manufacturers, and viscosity grades. Per the procedure found in GMN10060, the bench test measures the aeration time, deaeration time, and percent maximum aeration of the fluid at three temperatures, 60°C, 90°C, and 120°C. In the end, the results are compared with four commercially available transmission fluids. INTRODUCTION Original equipment manufacturers (OEMs) have started to decrease the mass and size of their engines and transmissions, in order to improve fuel economy. Reducing the physical size of the powertrain can give fuel economy benefits; however, one undesirable side effect from a performance point of view is that the lubricating fluid volumes are often decreased drastically. Both engines and automatic transmissions typically use pumps to pressurize and transfer the lubricant inside the device. As the fluid volumes are decreased and/or volumetric pump rates are increased, the residence time of the fluid in the sump is decreased. Residence time is important to aeration because sufficient time is required for the lubricant to release any air that has been entrained. If the residence time is decreased, the fluid then has less time to release theentrained air. If the residence time is decreased excessively, the pump will start picking up fluid with air entrained in it, causing pumping inefficiency, wear, fluid chemical degradation, and unstable pressures in hydraulic circuits. If the fluid becomes grossly entrained with air, it may develop enough stable foam such that the fluid can exit through the vents of the device, leading to lubricant volume reductions and possible concerns with operational safety. [ 1] In order to meet the new powertrain design requirements, the aeration properties of lubricants need to improve. The factors measured on fluids for this study are aeration time, deaeration time, and maximum percent aeration. The GM test procedure utilizes a density meter to measure the percent aeration of a fluid. The meter reads a baseline density to produce a baseline aeration of the fluid at the point where the percent aeration reaches a maximum. For the purpose of this test, the aeration time is the time to reach maximum aeration of the fluid, and the deaeration time is the time for the fluid to return to the base state from the maximum aeration. The ideal fluid would have a long aeration time, a short deaeration time, and a low percent maximum through the life of the fluid. In order to develop an improved fluid, the components and characteristics of the fluids that have the greatest affect on aeration need to be determined. Typically over 80% of a finished fluid formulation is base oil, so it seemed to be a logical place to start understanding aeration characteristics. The following study evaluates thirteen commonly used base stocks for formulating automotive lubricants with respect to the effect of base oil group, viscosity, and base oil blending on aeration properties ( Table 1). The main criteria evaluated is the maximum percent aeration in the GM Aeration Test. The base oil properties were compared against four modern transmission fluids; three are Group III based products, and one is a Group IV based product. Effects of Base Stocks on Lubricant Aeration2011-01-1210 Published 04/12/2011 Joan Petit, Cheryl O'Brien and Roy Fewkes General Mot