# Development of Transmission Systems for Parallel Hybrid Electric Vehicles
###### Abstract
evaluated the max fuel economy potential parallel hybrid electric vehicle by considering power electronics, drivetrain loss, and system loss [10]. The series-parallel hybrid power architecture combines both series and parallel systems to enhance the overall efficiency and the performance of the system. The series-parallel hybrid power architectures require two electric machines to work as the traction motor and generator. The series-parallel hybrid architecture has superior efficiency and performance, nevertheless the system is extremely complex and costly. The hybridization factor and function of the EM can be classified into micro hybrid, mild hybrid, and strong hybrid factors and functions. The hybridization factor is a ratio that presents the percentage of EM power in a vehicle propulsion system. According to [PERSON] [11], the hybridization factor has a considerable effect on the performance and total efficiency of the hybrid vehicles. On the basis of the hybridization factor, the design can be classified into five types [12]. According to [PERSON] [13], the distinct propulsion architecture for the parallel hybrid powertrain can be classified into pre-transmission and post-transmission. In the pre-transmission architecture, the mechanical power flow is first integrated and then adjusted to traction wheels (TWs) through transmission. In the post-transmission architecture, the mechanical power flow is first adjusted through transmission and is then integrated with TWs. [PERSON] [14] studies the differences between the performance of torque-coupled type hybrid vehicles with pre-transmission and post-transmission architectures. The result shows that the average engine efficiency slightly improved for the pre-transmission architecture; however, the overall efficiency and acceleration performance are superior for the post-transmission architecture. [PERSON] and [PERSON] presented the configuration synthesis of series-parallel hybrid transmission with eight-bar mechanisms and demonstrated the feasibility [15]. [PERSON] et al. evaluated the electrical losses and the average ICE efficiency in different operating conditions in the continuously variable transmission architecture and discontinuously variable transmission architecture [16]. [PERSON] [17] investigated the location and number of clutches for the hybrid powertrain of Toyota Prius and Chevrolet Volt, which are hybrid powertrains with a single planetary gear. The result shows that applying one clutch in the Toyota Priustransmission can improve fuel consumption by 16% under the federal urban driving cycle (FUDS). Applying two clutches from Chevrolet Volt transmission can evidently improve fuel consumption, irrespective of the city or highway driving cycle. Although various principles have been proposed, how the intriguing \(k\) ratio [18] influences the performance and efficiency of hybrid EVs has not been investigated so far. Therefore, we performed numerical simulations to examine the effect of design matching between the PIM and transmission system for single-motor parallel hybrid EVs. In this study, the Simulink/Simscape environment is used to construct a model of the parallel hybrid EV powertrain system, which PIM characteristics, locations of the gearbox at the driveline, and design of the gearbox affect. This paper focuses on the influence of \(k_{1}\) and \(k_{2}\), which are the design parameters for the PIM. The matching design of the torque-coupled-type PIM (TC-PIM) and the gearbox improves the efficiency and performance of parallel hybrid vehicles. The matching design principle for TC-PIM parameters and the location of the gearbox are based on the speed range of two power units. The simulation results show that the transmission system design correlates vehicle performance and it uses the combined power of two power units. The design matching principle that is proposed in this study can identify the effective design range of TC-PIM parameters and the gear ratio of the gearbox.