# Development of Transmission Systems for Parallel Hybrid Electric Vehicles
###### Abstract  
This study investigated the matching designs between a power integration mechanism (PIM) and transmission system for single-motor parallel hybrid electric vehicles. The optimal matching design may lead to optimal efficiency and performance in parallel hybrid vehicles. The Simulink/Simscape environment is used to model the powertrain system of parallel hybrid electric vehicles, which the characteristics of the PIM, location of the gearbox at the driveline, and design of the gearbox influenced. The matching design principles for torque-coupled-type PIM (TC-PIM) parameters and the location of the gearbox are based on the speed range of the electric motor and the internal combustion engine. The parameters of the TC-PIM (i.e., \(k_{1}\) and \(k_{2}\)) are based on the \(k\) ratio theory. Numerical simulations of an extra-urban driving cycle and acceleration tests reveal that a higher \(k_{ratio}\) has greater improved power-assist ability under a pre-transmission architecture. For example, a \(k_{ratio}\) of 1.6 can improve the power-assist ability by 8.5% when compared with a \(k_{ratio}\) of 1. By using an appropriate gear ratio and \(k_{ratio}\), the top speed of a hybrid electric vehicle is enhanced by 9.3%. hybrid electric vehicle; power integration mechanism (PIM); transmission system; pre-transmission 2019 1538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 12390/app9081538 1390  
The improvement of power storage technology [3; 4] and alternative clean electric power sources [5; 6] remains challenging. Therefore, oil-electric hybrid-powered electric vehicles (EVs) may be a promising solution for environment-friendly vehicles until the major challenges of pure EVs are overcome [7]. The development of hybrid vehicles comprises four challenges: research and development of a low-carbon energy carrier, system control strategy, power unit, and transmission system. Among them, the transmission system is the primary factor in integrating mechanical power and performance for hybrid vehicles. For smooth coordination between the two power outputs, the power integration mechanism (PIM) design is the most crucial for the parallel hybrid powertrain. The propulsion systems of oil-electric hybrid vehicles comprise two power units. On the basis of the power flow of power units, the power architecture of hybrid-powered vehicles can be classified into series hybrid, parallel hybrid, and series-parallel hybrid power architectures. For the series hybrid power architecture, engine power and traction power are provided to the generator and the traction motor, respectively. This architecture must include two electric machines: one with a generator and traction motor and another with an internal combustion engine (ICE). Due to the engine powering the generator, the overall efficiency of the series hybrid power architecture is generally higher than that of the parallel hybrid power architecture. However, system performance depends on the specification of the traction motor. The system performance of the series hybrid architecture is worse than that of the parallel hybrid architecture [8]. For the parallel hybrid power architecture, engine power can be used as the traction power or generator power, and the electric machine can work as a traction motor or generator depending on the system control strategy. In this system, an engine and a motor or each can provide the required traction power alone; thus, the performance of the parallel hybrid vehicles is higher than that of series hybrid vehicles. The parallel hybrid architecture can comprise an electric machine (motor/generator) and ICE [9]. [PERSON] et al. developed a time efficient energy management strategy for the parallel plug in a hybrid electric vehicle based on the convex optimization and the simulated annealing algorithm [9]. [PERSON] et al.