## 2 Classification of HEVs
### HEV Classification by Hybridization Rate  
The ratio of the power generated by an electric motor in a HEV to the total power used by the vehicle's hybridization is known as the degree of hybridization [43]. Automotive cars can be categorized as Micro, Mild, Full, or Plug-in Hybrid based on the level of hybridization [44]. Micro HEVs: The hybridization level of micro HEVs is less than 5%. In micro HEVs, the engine is turned off when the driver applies the brake pedal, and the vehicle is started again by the electric motor, which takes the shape of a tiny integrated alternator/starter. The ICE powers the car after it is moving [45]. Using a micro HEV system results in an approximate 5-10% boost in fuel economy. The BMW 1 and 3 series, the Fiat 500, the SMART car, the Peugeot Citroen C3, the Ford Focus and Transit, and the Mercedes-Benz A-class are a few examples of micro HEVs that are now in use. Mild HEVs: The degree of hybridization in mild HEVs is less than 20%. The mild HEV and micro HEV are extremely similar; however, the mild HEV may provide power assistance during vehicle propulsion due to the larger integrated alternator/starter motor and batteries, which offer higher electric motor power outputs ranging from 7 to 12 kW/150 V [5]. Because mild HEVs have a larger electric motor, they may save more fuel by using regenerative braking and torque assistance. In real-world driving, mild hybrid vehicles often result in a 20-25% boost in fuel efficiency when compared to non-hybrid vehicles [45]. The power performance of MHEVs is heavily dependent on engine performance, as they do not use high-voltage batteries and their batteries and motors are relatively low-powered [46], which makes them cost-effective and allows them to be brought to market quickly compared to battery electric and hydrogen fuel cell vehicles, which improves average fuel economy across the fleet [47]. As illustrations of commercially available mild-HEVs, the Honda Civic, Honda Accord, the 2009 Saturn Vue, and the GMC Sierra 1500 truck stand out among numerous options. The distinction between micro and mild HEVs primarily lies in their ability to deliver torque to the vehicle. Specifically, micro HEVs lack the capacity to do so, as the electric motor in these vehicles is primarily utilized for enabling start-stop functionality and powering accessories, rather than directly contributing to torque output. Full HEVs: FHEV is a system that carries both an internal combustion engine and a high-voltage electric motor as dual power sources, along with the following features: (1) independent drive is possible in pure electric mode; (2) the electric motor and internal combustion engine can work together; (3) it is capable of regenerative braking energy recovery; (4) no external charging required. Full HEVs exhibit a hybridization level that surpasses 50%, indicating a significant integration of electric propulsion components. Full HEVs incorporate significantly larger batteries and more robust electric motors compared to their micro and mild hybrid counterparts. However, this enhanced level of hybridization often results in a higher cost of ownership [48]. In the design of this type of HEV, a dual power path is incorporated for the ICE and the electric motor, thereby enabling both independent and coordinated modes of operation [5]. The full hybrid system is capable of more than just using the generator to start and stop the engine. It can also absorb some energy during braking and deceleration, run the engine at a constant speed while driving, and adjust the amount of energy it produces based on how much is needed to charge the generator or drive the wheels. The electric motor can help drive the wheels while the automobile is accelerating or under high load, boosting the engine's power output and improving the overall performance of the vehicle. The electric motor can help drive the wheels while the automobile is accelerating or under high load, boosting the engine's power output and improving the overall performance of the vehicle.