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functionalities because these machines have a large variety of functional drives [20]. The first part of this report gives an overview of the components of the electrification solution and hybrid/electric architectures, discussing the advantages related to the different solutions. The machines are then schematically described and compared, showing the hybrid architectures of the proposed solutions. Finally, the introduction of a specific hybridization factor is pro- posed as a first classification of the main hybrid work vehicles [21, 22]. 2. HEV power train configurations The SAE defines a hybrid vehicle as a system with two or more energy storage devices, which must provide propulsion power either together or independently [23]. Moreover, an HEV is defined as a road vehicle that can draw propulsion energy from the following sources of stored energy: a conventional fuel system and a rechargeable energy storage system (RESS) that can be recharged by an electric machine (which can work as a generator), an external electric energy source, or both. The expression “conventional fuel” in the SAE definition con- strains the term HEV to vehicles with a spark-ignition or a compression-ignition engine as the primary energy source. However, the United Nations definition of HEV [24] mentions consumable instead of conventional fuel. On this basis, the primary energy source in an HEV is not necessarily the engine hydrocarbon fuel, or biofuels but can also be the hydrogen fuel cell. The term electric-drive vehicle (EDV) is used in Ref. [25] to define any vehicle in which wheels are driven by an electric motor powered either by a RESS alone or by a RESS in combi- nation with an engine or a fuel cell. Some types of EDV belong to the subset of plug-in electric vehicles (PEVs) [25, 26]. Compared with conventional internal combustion engine vehicles, HEVs include more elec- trical components, such as electric machines, power electronics, electronic continuously vari- able transmissions, and advanced energy storage devices [27]. The number of possible hybrid topologies is very large, considering the combinations of electric machines, gearboxes, and clutches, among others. The two main solutions, series and parallel hybrid, can be combined to obtain more complex and optimized architectures. There is no standard solution for the optimal size ratio of the internal combustion engine and the electric system, and the best choice includes complex trade-offs between the power as well as between cost and perfor- mance [28]. The power train configuration of an HEV can be divided into three types: series, parallel, and a combination of the two [29]. 2.1. Series hybrid electric vehicles Series hybrid electric vehicles (SHEVs) involve an internal combustion engine (ICE), genera- tor, battery packs, capacitors and electric motors as shown in Figure 2 [30–32]. SHEVs have no mechanical connections between the ICE and the wheels. The ICE is turned off when the bat- tery packs feed the system in urban driving. A significant amount of energy is supplied from the regenerative braking. Therefore, the engine operates at its maximum efficiency point, leading to improved fuel efficiency and lesser carbon emission compared with other vehicle Hybrid Electric Vehicles6