## 2 Classification of HEVs
### HEV Powertrain Configuration
#### 2.2.3 Power-Split Powertrain
The output-split powertrain may perform poorly at low vehicle speeds compared to the input-split configuration [78]. [PERSON] et al. (2019) [79] introduced a single planetary gear-based dual-mode hybrid powertrain, which incorporates both input and output split. To assess its efficacy, they benchmarked this novel configuration against two established power-split designs, the THS-P610 and Volt-II, following comprehensive optimization. This hybrid powertrain enabled the vehicle to operate in either pure electric, series hybrid, or parallel hybrid modes when the power-split operation was not engaged. (3) Multi-mode power distribution hybrid system configurations are combinations of more than two power distribution schemes [80; 81], allowing for complex power distribution methods [80]. The compound split is a multi-mode power distribution hybrid powertrain  
Figure 4: Configuration of Power-split HEV [44]. configuration subtype with no configuration between the MG and the output shaft or engine [44]. The Lexus HS 250 h, Lexus RX 400 h, Toyota Camry and Honda, Lexus GS 450 h, and Lexus LS 600 h utilize the compound-split configuration [68]. [PERSON] et al. (2020) [82] proposed a systematic approach for designing, analyzing, evaluating, and screening the 2-PG multi-mode power-split hybrid powertrain system configuration. Additionally, they took into account all possible 2-PG configurations, power component connection locations, actuator counts, and combinations of various operating modes. [PERSON] et al. (2019) [83] investigated the optimization of a composite power distribution configuration in the PHEV bus to reduce fuel consumption and battery degradation. They explored potential composite power distribution configurations with two planetary gear sets. Multi-mode optimization addressed driveline issues, with composite power-split configurations performing better in terms of fuel economy, acceleration, and motor size. In recent years, the development of planetary gear hybrid powertrain (PGHT) systems has been mainly aimed at the fuel economy of HEVs. [PERSON] et al. (2021) [84] introduced the Torsional Vibration Considered for PHEVs, with the objective of achieving desirable energy economy and ride comfort. Contributing work to the PGHT includes [PERSON] et al. (2021) [85] and [PERSON] et al. (2022) [86]. Power-split HEVs offer better performance compared to series and shunt HEVs, but they also lead to complex structures, increased integration and control difficulties, and increased costs. The differences in the inherent characteristics of engine and motor operating efficiency, frequency response characteristics, torque characteristics, etc., result in EMSs that play a decisive role in the overall vehicle performance improvement of HEVs. However, different types of powertrain configurations of HEVs determine the selection of different algorithms and optimization potentials [87], but all of them aim to improve energy utilization efficiency, reduce energy consumption and environmental pollution, and enhance vehicle performance and driving experience. Through rational energy management, hybrid vehicles are able to choose the optimal energy source and distribution method in different driving scenarios to achieve more efficient and environmentally friendly energy utilization.