## 2 Classification of HEVs
### HEV Classification by Hybridization Rate
In addition to producing no emissions at the time of use, FHEVs in pure electric mode are perfect for removing localized pollutants in urban areas while preserving the ability to travel great distances inside the city when utilizing an ICE [49]. However, this benefit comes at the expense of complexity and expense [50]. Contrary to conventional vehicles, including MHEVs, which harness the engine's power to operate auxiliary components like water pumps, fuel pumps, and air conditioning (A/C) compressors, and utilize combustion's waste heat for cabin heating,FHEVs necessitate the operation of these systems entirely on electric power [51; 52]. Full hybrids are best suited to the stop-and-start driving patterns found in cities [53]. Compared to micro and mild hybrids, FHEVs have considerably smaller engines and demand more advanced energy management systems. When compared to non-HEVs, FHEVs typically have fuel efficiency gains of 40-45%. The Chevrolet Tahoe Hybrid, Toyota Prius and Camry Hybrid, Ford C-Max, Honda CR-Z, and Kia Optima Hybrid are a few examples of FHEVs now on the road [45]. PHEV: Plug-in HEVs (PHEVs), which offer lower emissions and better fuel efficiency than traditional fossil fuel-powered cars, have become a popular substitute. Unlike conventional HEVs (HEVs), plug-in hybrids can be recharged directly from the grid, resulting in additional all-electric range and further reducing engine fuel consumption [54]. PHEVs can run on electricity for long periods of time thanks to high-capacity electrical components. PHEV, as a multiple power source system, uses ICE and lithium-ion battery packs, supercapacitors, etc., to provide power [55]. In typical applications, the ICE serves as the primary power source, while energy storage mechanisms, including batteries and supercapacitors, function as secondary or supplementary sources, either independently or in conjunction, to facilitate the propulsion of the vehicle [56]. In order to maximize fuel efficiency and prolong battery life, PHEVs require appropriate energy allocation between the two sources [57]. In the current market, two variations of PHEV exist: extended-range electric vehicles and hybrid PHEVs. The extended-range PHEVs typically adopt a series arrangement, where the gasoline engine primarily generates electricity for the electric motor to propel the vehicle until the batteries are depleted, at which point the engine activates. Conversely, in hybrid PHEVs, the engine is predominantly employed to directly provide power to the vehicle. In terms of the torque source for the drive wheels, the operational modes of PHEVs are categorized as centralized drive and distributed drive [58]. Presently, the majority of PHEVs favor centralized drive systems due to their advanced level of technological maturity [59]. The Chevy Tahoe Hybrid, Toyota Prius and Camry Hybrid, Ford C-Max, Honda CR-Z, and Kia Optima Hybrid are a few examples of fully electric vehicles currently on the road [45]. PHEVs that are selling the best in 2018 include the Mitsubishi Outlander, BYD Tang, and BYD Qin [60]. The comparison of the above four types of HEV is shown in Table 1[44]. HEVs combine the benefits of a gasoline engine and an electric motor to provide better fuel economy or more power than conventional vehicles. Most hybrids use several advanced technologies [61]: