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configurations [33]. The series hybrid configuration is mostly used in heavy vehicles, military vehicles, and buses [34]. An advantage of this topology is that the ICE can be turned off when the vehicle is driving in a zero-emission zone. Moreover, the ICE and the electric machine are not mechanically coupled; thus, they can be mounted in different positions on the vehicle layout drive system [35]. 2.2. Parallel hybrid electric vehicles (PHEV) In a PHEV, mechanical and electrical powers are both connected to the driveline, as shown in Figure 3. In the case of parallel architectures, good performance during acceleration is possible because of the combined power from both engines [35]. Different control strategies are used in a preferred approach. If the power required by the transmission is higher than the output power of the ICE, the electric motor is turned on so that both engines can supply power to the transmission. If the power required by the transmission is less than the output power of the ICE, the remaining power is used to charge the battery packs [36]. Moreover, mechanical and Figure 2. Schematic of series hybrid electric vehicles (SHEV). Figure 3. Schematic of parallel hybrid electric vehicles (PHEV). Trends and Hybridization Factor for Heavy-Duty Working Vehicles http://dx.doi.org/10.5772/intechopen.68296 7