## 2 Classification of HEVs
### HEV Powertrain Configuration
#### 2.2.2 Parallel Hybrid Powertrain  
In parallel HEVs, the ICE and MG (Motor/Generator) are both directly connected to the output shaft to provide drive. The ICE connection is termed the mechanical path, while the MG connection is known as the electrical path [65]. Since the two power sources operate in different modes, parallel HEVs allow the electric motor and ICE to provide power to drive the vehicle in parallel, so the ICE and the electric motor can be driven separately or jointly, as illustrated in Figure 3. Under certain operational conditions, the vehicle can be operated by either the motor or the ICE alone when the battery's state of charge is high. Some of the torque from the ICE will be redirected to turn the MG, which will be utilized as a generator to recharge the battery pack, in order to prolong the battery life when the SOC of the battery is at a lower level. In the state of charge, the drive power required for the exercise of the vehicle can only be provided by the ICE alone [66]. The MG functions as a generator during low power demand and as a motor during high power demand, keeping the ICE operating in the high-efficiency range and thereby reducing fuel consumption. Furthermore, parallel hybrids require transmissions to align high engine speeds with low vehicle speeds [44].  
Figure 3: Configuration of parallel HEV [44].  
In parallel HEVs, the ICE serves as the primary drivetrain source, while the electric motor functions as a secondary source to increase traction power through control strategies that can successfully reduce fuel consumption [67]. In contrast to the series HEV, the parallel hybrid vehicle arrangement uses a larger ICE with a comparatively smaller and less powerful electric motor [45]. However, both the mechanical and electrical energy of a parallel HEV can be used to directly propel the vehicle, resulting in increased driveline efficiency compared to a series configuration under most operating conditions [61]. Although this approach increases performance and efficiency, it has drawbacks. The parallel design is more complicated than the series configuration, and the transmission and driveline are more costly [68]. [PERSON] et al. (2019) [69] evaluated the potential for reducing CO\({}_{2}\) emissions, drivability, and total cost of ownership of different parallel HEV architectures in comparison to conventional layouts. Although parallel HEVs have higher procurement costs, they can be effectively recovered over the life of the vehicle. [PERSON] et al. (2017) [70] studied and simulated a parallel hybrid vehicle using Autonomie software and concluded that the fuel economy increased by 68%, emissions decreased by 40%, and engine efficiency increased by 12% during the real driving cycle. There are many applications of parallel hybrid technology in the market, such as Volkswagen Passat Hybrid [71], Hyundai Sonata Hybrid, BYD Qin, Honda's Insight, Civic, Accord, and the Chevrolet Malibu Hybrid in parallel configuration [68]. Parallel HEVs are unsuitable for typical city driving with frequent stop-and-go traffic, as the constant starting and stopping significantly drains the battery. The charging mechanism is not available when the vehicle is not moving, and the ICE is unable to drive the generator to charge the batteries when the SOC is at a low battery level, which forces the engine to produce power in the area of low efficiency.