Thermal Properties and Heat Transfer Performance of Driveline Lubricants

The removal of heat from automatic transmissions has always been a vital function of the lubricant though little attention has been given to this in form of specifications and requirements. In current transmission architectures, the majority of heat is generated by the torque converter and though within the shifting elements temperatures at the clutch interface can exceed temperatures of 300�C, these thermal transients are short lived (typically less than one second). Heat transfer is achieved by efficient hardware design optimized around average lubricant thermal properties. With the incorporation of electric motors in the transmissions of hybrid and electric vehicles, consideration has been put on the ability of the lubricant to remove heat and whether lubricants with specific thermal properties may be advantageous. Though overall these electrified transmissions are expected to operate cooler, during select operation the temperatures of the motor coils could reach temperatures in excess of 170�C for tens of seconds. The ability to remove heat from the electric motor directly impacts the motor durability, efficiency, and driving range. In this paper, we will consider the case for using the lubricant as part of the transmission design to improve heat transfer. Lubricant thermal properties that affect heat transfer will be discussed in terms of calculated �Figures of Merit� and contrasted to actual cooling curve data. We then compare these findings to those from a predictive model of electric motor temperatures and show that they provide similar insights. In real lubricants, many of the thermal properties are interdependent making it difficult to understand if their individual contributions to heat transfer are really significant. We therefore conducted a theoretical study using a DOE approach, wherein the properties are independent of each other, to gain further insight into the practical significance of each thermal property on the predictive electric motor temperature. We report these findings.