Oil flow management for a typical oil cooled electric machine for hybrid vehicle applications

Abstract

Increasing emission regulations and desire for energy efficiency are major drivers for increasing the need for hybrid vehicles. An integral part of hybrid vehicle architecture is a high power density machine, where thermal management is key design criterion. These electric machines herein referred to as machines, can be either air or oil cooled. In oil cooled machines, achieving optimum oil volume flow rates in critical regions of the machine at all shaft speeds is a necessity for efficient thermal management. However in such machines with oil supplied to the rotor, achieving optimal oil volume flow rates in critical regions at different rotor speeds is a major design challenge. As the rotor speed increases, the oil volume flow rate predictions from different rotor shaft bores becomes increasingly difficult due to significant centrifugal effects at these bores. Moreover the test and measurement set-ups near to high centrifugal regions are very difficult to either place or measure. Hence there is a very strong need of a prediction methodology which estimates oil volume flow rates distribution behavior between the stator and rotor circuits at all shaft speeds. The benefits of this methodology are to provide insights into oil flow distribution dynamics at high shaft speeds, cost savings from tests avoidance and rapid evaluation of design concepts. These benefits become more significant when the physics of oil flow distribution behavior is highly non-linear and basic design principles of 1Dimensional (D), 2D approaches are not sufficient at high shaft speeds. This paper discusses the Computational Fluid Dynamics (CFD) analysis methodology, which is a 3D approach, to predict oil volume flow rates distribution between the rotor and stator cooling circuits and associated pressure drop at different rotor speeds. Furthermore, section wise break-up of pressure drop is presented highlighting the key regions responsible for significant portion of this pressure drop. This paper further proposes the performance parameters critical for an efficient cooling circuit design and their evaluation methodology at all shaft speeds. This evaluation methodology is proposed by comparing CFD based relationships of critical input, output parameters at various shaft speeds. These proposed performance parameters and their evaluation methodology are then used for comparing 12 design cases and is very effective to select the best design alternative among them. Moreover, the sensitivity of critical input parameters like the diameter of rotor shaft bores and stator housing bores; the number of distribution holes and their diameters for oil volume flow rates are studied.