INTRODUCTION Motivation and Purpose Hybrid electric vehicles (HEVs) provide opportunities to economize fuel consumption and improve dynamic performance. However, in hybrid cars, more than one power unit (PU) is used, such as motor-generators (MGs) or an internal combustion engine (ICE). To connect between these PUs, an investigation with respect to hybrid transmissions is required [ 1, 2]. The hybrid transmissions available on the market may be in any combination of power-split, series or parallel configurations. The transmission in Toyota Hybrid System (THS) [3] can be categorized as a transmission with power-split configuration, and there are some sophisticated multi-configuration transmissions like GM V oltec GEN 2 that can switch between one parallel configuration and two power-split configurations [ 4]. In this study, a multi-configuration hybrid transmission based on power-split configuration was developed. The power-split architecture in THS, due to the characteristics of planetary gearset (PG), MG1 normally acted as a generator extracting mechanical power from the transmission. Thus, the power output capability is lower than the summation of maximum horsepower of ICE, MG1, and MG2. With regards to performance, the negative power of MG1 is unfavorable.In contrast, the power output capability in parallel hybrid is simply the summation of power from ICE and all MGs, therefore, a parallel configuration was included as an additional configuration in this study to utilize ICE and all MGs more sufficiently during large power output. During highway cruising, MG1 must keep operating to maintain power-split operation. In this study, another parallel configuration with relatively lower reduction ratio was included for cruising. It is similar to ICE direct drive in Honda Accord Hybrid Model Year (MY) 2014 (i-MMD), so that MG1 can be idle to avoid power conversion loss between mechanical and electrical power [ 4, 5, 6, 7, 8, 9, 10]. In power-split, the torque relationship between sun, carrier, and ring gear in PG is under constraint. To provide the ability for ICE to generate electricity efficiently while preserving a way to start the ICE smoothly in diverse scenarios, the series configuration was included as an additional configuration in this study. In power-split, PG has two degrees of freedom (DOF). When ICE is off, MG1 also loses its ability of torque output. Hence, there is only MG2 available in pure EV . To utilize all MGs in EV mode (such as Toyota Prius Prime and Chevrolet V olt MY2016), the two-motor configuration was also included. When more power is needed, MG1 should also be allowed to propel the vehicle.Conceptual Design and Evaluation of a Hybrid Transmission with Power-Split, Series, and Two Parallel Configurations Yan-Song Chen, Joshua Chang, I-Ming Chen, Ming-Yen Chen, and Tyng Liu National Taiwan University ABSTRACT A hybrid transmission may be in any combination of a power-split, series or parallel configuration. This study is aimed to develop a hybrid transmission with six possible configurations: power-split, series, two parallel configurations, and two EV configurations. The Function Power Graph (FPG) methodology was applied in this study. After creating and merging FPGs, a possible solution consisting of only one planetary gearset, one ICE, one MG1, one MG2, two rotating clutches, and one brake clutch was synthesized to satisfy all configuration requirements. This transmission was based on power-split configuration which can switch to other configurations. The parallel configuration I extracted more power from MG1 and ICE during lower speed driving in order to utilize the two sufficiently , and respond to the increased desire for horsepower in the market. Additionally , parallel configuration II was set up so that ICE can directly propel the vehicle during freeway cruising. Compared with power -split configuration, MG1 does not need to keep operating in parallel configuration II, which gives an add