**2. Drivetrain architecture**

In general, the HEVs are a combination of conventional and full-electric vehicles using both ICE and electric motor/generator for propulsion. Various topology of HEVs (e.g. series, parallel, series–parallel) exist depending on how the comprising power sources and components are connected through the vehicle structure. The parallel architecture for a passenger HEV is considered for the drivetrain topology of this study. The parallel drivetrain utilizes more than one direct power source in its architecture to provide energy for the propulsion system. The ICE and electric motor (EM) in such a topology can be coupled/decoupled to the wheels when required which brings more degree of freedoms (DoFs) of operating the vehicle in different modes. Hence, the traction force can be provided by means of both ICE and EM or either of them independently leading to lower number of energy conversions and consequently lower losses in such a topology compared to the series HEVs [8]. In a parallel drivetrain the wheels can receive the generated power from the EM plus the one received from ICE. Since the EM can operate as an electric generator in such a topology, the battery pack can be charged during regenerative braking or when the ICE output power is greater than the required power at the wheels. **Figure 2** illustrates a schematic of the considered parallel HEV architecture.
