## 2 Classification of HEVs
### HEV Powertrain Configuration
#### 2.2.3 Power-Split Powertrain  
Power-split HEV vehicles consist of two MGs and a driveshaft together by one or more planetary gear sets (PGS) coupled to an ICE [72; 73], as illustrated in Figure 4. The PG set, also known as the power-split device (PSD), is the central component of the power-split hybrid system [44]. In power-split hybrid vehicles, the PSD divides the power generated by the ICE and transmits it through both mechanical and electrical transfer paths to the output shaft. Functionally similar to a continuously variable transmission, the PSD synchronizes the speed of the ICE with that of the vehicle. This synchronization is crucial because it allows the ICE speed and torque to be entirely decoupled from the speed and torque of the system's output shaft. Consequently, this decoupling enables the optimization of the engine's operating point, leading to improved fuel efficiency. The electric motor can also recover kinetic energy and turn off the engine for purely electric driving under certain operating conditions. This design allows the ICE to operate at high efficiency regardless of vehicle speed. Therefore, PSD is also known as electronic CVT in power-split hybrid vehicles [74]. Power can be transferred from the driveshaft via an electrical or mechanical path thanks to the PSD [75]. In the electrical path, the PSD works similarly to series HEVs. The power split hybrid vehicle integrates the benefits of both series and parallel hybrid vehicles, effectively addressing the size constraints of series hybrids and the frequent stopping and starting issues associated with parallel hybrids. First, part of the ICE power is converted into electricity through a generator. This electricity can then be utilized to drive the electric motor or to charge the battery. In the mechanical path, the PSD functions similarly to PHEVs, allowing the ICE to generate power flow directly to the drive shaft to propel the vehicle. The ICE and the battery can supply power to the vehicle independently or in conjunction, and the battery can be charged by the engine simultaneously. Essentially, this configuration extends the all-electric range of a hybrid vehicle, providing a versatile and efficient propulsion system [66]. Power-split HEVs are based on planetary displacements as energy distribution components, which couple the electric motor with the engine for optimal energy matching. The goal of the control logic is to continuously operate the engine in the high-efficiency range. [PERSON] et al. (2019) [76] have previously observed that the efficacy of a power split hybrid powertrain system is contingent upon its specific design, which is influenced by several factors. These include the count of PGS as well as the variety, quantity, and installation position of the transmission actuators, also known as clutches or brakes. Depending on the number of PGS, power split configurations are differentiated into two distinct modalities: a single-mode configuration and a multi-mode configuration. In the single mode, there exists only one PGS, while the multi-mode configuration incorporates two or more PGS [68]. The single-mode power-split hybrid system configuration encompasses two distinct subtypes of power distribution arrangements: namely, input-split schemes and output-split schemes. (1) In the input-split configuration, the engine and generator are respectively coupled to two distinct central shafts of the planetary gear sets, while the electric motor is linked to the output central shaft [77]. The earliest power-splitting systems were developed by Thompson Ramo Wooldridge Inc. in the 1960s. Today, the Toyota Prius is the best-selling hybrid system globally. Ford Motor Company employs a similar input split concept in its Fusion Hybrid and C-Max models [44]. (2) In the output-split configuration, the engine and generator are firmly attached to the input central shaft of the planetary gear set, whereas the electric motor is linked to a separate central shaft within the planetary gear set [77].