INTRODUCTION Modern automobiles aim at providing transportation at higher efficiency with reduced environmental impact. Every component in a vehicle is carefully examined for an opportunity to improve fuel economy. Among them, an automatic transmission system undergoes continuous design refinements. The main function of the transmission system is to transfer drive torque from an engine to the wheels according to driver demands and operating conditions. In a typical design, multiple planetary gear sets are connected in a complex manner to achieve a desired number of gear ratios. Wet clutches [ 1] are commonly utilized to couple or de-couple gear elements to alter torque paths for automatic ratio-changing. When a clutch is actuated for engagement, its slip torque is directly transferred to a transmission output shaft, affecting both drivability of the vehicle and comfort of the occupants [2 ]. When the clutch is open, its viscous drag contributes to the measureable loss of fuel efficiency. Accordingly, the clutch remains one of the critical components in the transmission system where opportunities continues to exist for design improvements. Figure 1. Illustration of clutch packs in an automatic transmission Figure 1 illustrates a transmission system which includes 5 clutch packs in a nested structure. The Automatic Transmission Fluid (ATF) is supplied from the oil pump, travels through the center shaft, and Two-Phase MRF Model for Wet Clutch Drag Simulation Pengchuan Wang and Nikolaos Katopodes University of Michigan Yuji Fujii Ford Motor Company ABSTRACT Wet clutch packs are widely used in today’s automatic transmission systems for gear-ratio shifting. The frictional interfaces between the clutch plates are continuously lubricated with transmission fluid for both thermal and friction management. The open clutch packs shear transmission fluid across the rotating plates, contributing to measurable ener gy losses. A typical multi-speed transmission includes as many as 5 clutch packs. Of those, two to three clutches are open at any time during a typical drive cycle, presenting an opportunity for fuel economy gain. However, reducing open clutch drag is very challenging, while meeting cooling requirements and shift quality targets. In practice, clutch design adjustment is performed through trial-and-error evaluation of hardware on a test bench. The use of analytical methodologies is limited for optimizing clutch design features due to the complexity of fluid-structure interactions under rotating conditions. This article presents a two-phase Multiple Reference Frame (MRF) CFD model for simulating wet clutch behaviors, accounting for detailed design geometry. The model employs the V olume of Fluid (VOF) method to determine the air-fluid interface inside a computational domain. Model setup and simulation parameters, including initial conditions, boundary conditions, and relaxation factors are evaluated in terms of conver gence behaviors. The model capabilities are validated against experimental data. Convergence of the CFD solver is demonstrated, capturing peak drag location as a function of rotating speed, until the phase fraction drops to a small value. The CFD model provides analytical insight into complex fluid interactions for grooved rotating plates, complementing hardware-based clutch design processes. CITATION: Wang, P., Katopodes, N., and Fujii, Y ., "Two-Phase MRF Model for Wet Clutch Drag Simulation," SAE Int. J. Engines 10(3):2017, doi:10.4271/2017-01-1127.Published 03/28/2017 Copyright © 2017 SAE International doi:10.4271/2017-01-1127 saeeng.saejournals.org 1327Downloaded from SAE International by Imperial College London, Saturday, September 08, 2018flows radially to lubricate all the internal components. ATF enters each clutch pack from its inner hub, travels through the frictional interface, and is discharged from the drain holes of the clutch housing. Under any driving conditions, two to three clutc