Comparative study on fuel saving potential of series-parallel hybrid transmission and series hybrid transmission

The series hybrid transmission (SHT) and series-parallel hybrid transmission (SPHT) have become an important development direction in the field of hybrid electric vehicles (HEVs) in China. The main difference between a SHT and SPHT is that the engine can directly drive the vehicle through the addition of one clutch in the SPHT. The SPHT is thus more complex in structure, but its fuel economy advantages, as compared with the SHT, have remained controversial. Therefore, this study provides a thorough comparison between the SHT and SPHT for different types of vehicles and driving cycles. A global optimal control strategy based on dynamic programming (DP) is developed to evaluate the economic performance of the SHT and SPHT. Particularly, a mode-switching penalty matrix is designed in the control strategy based on the energy consumed during the mode-switching process. To exclude the influence of the configuration parameters and power sources, a large number of simulations are conducted for different parameter combinations. The results show that the SPHT has notable advantages in fuel saving compared with the SHT, especially in high-speed driving cycles for type C vehicles, while there is a negligible difference in the fuel consumption between the two for type A vehicles under urban driving cycles. Under the Worldwide Harmonized Light Vehicles Test Cycle (WLTC), the SPHT has a fuel saving of approximately 7.2% more than the SHT for type B vehicles. This study provides a new theoretical basis on technical path selection for the SHT and SPHT.