Abstract Toyota Motor Corporation developed a new hybrid system, Multi Stage Hybrid System, for the Lexus flagship coupe LC500h with the aim of achieving an excellent balance between fuel economy and acceleration performance. The Multi Stage Hybrid Transmission used in this new hybrid system includes a shift device located immediately after the power split device and motor. Compared with previous hybrid systems, the new hybrid system improves fuel economy by reducing electrical loss in the optimal gears, which are selected depending on the driving state. The system also improves acceleration performance by increasing the driving force at low and medium vehicle speeds in lower gears. In addition, the range of the power split device that cooperates with the shift device was widened to enable both an electrically-controlled continuously variable transmission mode and a ten-speed transmission mode, which creates a direct shift feeling to improve driving pleasure. This system contains many independent variables given by the interactions of the engine, generator, motor, and shift device. Thus, conventional control methods were not appropriate for devising the shift control of the system. In addition, shift performance may also be affected by the particular constraints of a hybrid system, such as battery limitations. This paper describes the shift control system for the Multi Stage Hybrid Transmission that was developed to resolve these issues. 1. Introduction When Toyota developed the first hybrid system for rear-wheel drive (RWD) Lexus GS450h in 2006, a two-stage motor speed reduction device was introduced to expand the control range and efficiency of the motor [ 1]. This system was also adopted for all-wheel drive (AWD) Lexus LS600h in 2007. To improve both fuel economy and acceleration performance, the Multi Stage Hybrid Transmission was developed to enable more high efficiency operating points than previous hybrid systems. The Multi Stage Hybrid Transmission includes a new shift device with both reduction and overdrive functions. This shift device is located behind the power split device and motor as shown in Fig. 1. Figure 1. Transmission Cross-section. Figure 2. Multiple High Efficiency Operating Points.Development of Shift Control System for Multi Stage Hybrid Transmission2017-01-1150 Published 03/28/2017 Kenta Kumazaki, Tooru Matsubara, Nobufusa Kobayashi, Shunya Kato, Kazuyuki Shiiba, Ikuo Ando, Hiromichi Kimura, and Hiroatsu Endo Toyota Motor Corporation CITATION: Kumazaki, K., Matsubara, T., Kobayashi, N., Kato, S. et al., "Development of Shift Control System for Multi Stage Hybrid Transmission," SAE Technical Paper 2017-01-1150, 2017, doi:10.4271/2017-01-1150. Copyright © 2017 SAE InternationalThe theoretical efficiency is shown in Fig. 2. Theoretical efficiency here is efficiency of the transmission calculated with a certain power conversion loss between mechanical and electrical power in motor and generator. Optimally selected gears improve the overall fuel economy and heat management performance. Fourth gear, as an overdrive gear, improves fuel economy at highway speeds, and first gear improves acceleration performance and heat management performance by gear reduction. Combined fuel economy is improved compared with a previous RWD hybrid vehicle with a V6 engine, and acceleration performance (time from 0 to 60 mph) is improved compared with a previous AWD hybrid vehicle with a V8 engine. 2. System Configuration This section explains the basic architecture of the transmission by describing its general specifications and operating states. 2-1 Specifications The general specifications are shown in Table 1, and a schematic diagram of the transmission is shown in Fig. 3. Table 1. General Specifications of New Hybrid System. Figure 3. Schematic Diagram of Transmission. The driving force of the new hybrid system and the previous hybrid systems are shown in Fig. 4. In 1st gear, the Multi Sta