Cfd Simulation of Drive Unit Cooling Helps to Improve Reliability

T he cooling performance of oil is critical to the functionality and durability of vehicle drive units, such as transmissions and differentials. Traditionally, automotive R&D teams evaluate cooling performance by building prototypes, installing them in a vehicle, and conducting tests in wind tunnels. The use of free-surface multiphase flow modeling with high-performance computing (HPC) has made it possible to accurately predict oil cooling performance via readily available computing resources. Automotive leader Toyota uses this approach to evaluate more design alternatives in the early stages of the product development process. A typical drive unit consists of a case containing rotating internal parts, such as gears and shafts supported by bearings that transmit power. These are surrounded by oil and air. The oil serves various functions, including lubrication, power transmission and cooling. The major heat sources within the drive unit include meshing of the gears, sliding friction between bearings and shafts, and stirring oil as a result of gear movement. The heat generated is conveyed to and through the oil to the internal surface of the case; from there, it goes to the case’s external surface and surrounding air. Oil flow patterns within the transmission are critical to efficient lubrication, power transmission and cooling performance, and to avoid negative effects, such as churning loss, in which friction between the oil and gears revolving at high rpm can consume several horsepower. Simulating the cooling capacity of a drive unit requires predicting internal oil flow patterns involving free surfaces, external air flows, and the complex threedimensional flows of heat from the oil to the air. A key difficulty is that external air flows can be resolved with sufficient accuracy only by modeling the entire vehicle, DRIVE UNIT