## 2 A Comprehensive Overview of Developments in Vehicle Technology
### Hybrid Electric Vehicles  
HEVs utilize a combination of two different energy sources, ICE and electric motor (EM), to achieve maximum efficiency while operating the vehicle. There are three main categories of HEVs: mild HEVs, full HEVs, and plug-in HEVs. Mild HEVs offer similar benefits to micro-HEVs, but have a higher EM power output ranging from 7 to 12 kW/ 150 V. Both the EM and ICE work together to drive the car, increasing fuel efficiency by up to 30% [18]. The downside is that the EM and ICE cannot function independently since they share the same shaft [19]. Honda Civic, Honda Accord, Saturn Vue 2009, and GMC Sierra 1500 truck are a few examples of commercial mild HEVs.  
Most automakers have shifted towards the production of full HEVs to meet the demand for greater fuel efficiency. This kind of HEV is equipped with a dual power path for both the ICE and electric motor, allowing for independent or combined operation. Fuel economy can be enhanced by 40% without any loss in performance by utilizing this technology. Full HEVs typically make use of large energy storage systems (ESS), like UC or batteries, and the electric motor operates at approximately 50 kW/330 V. The full HEV system is categorized into four distinct types: series HEV, parallel HEV, series-parallel HEV, and complex HEV.  
Vehicles classified as series full HEVs rely on the electric motor as the primary source of propulsion, like battery electric vehicles (BEVs). These vehicles feature a high-powered internal combustion engine generator to recharge the battery. This design offers the benefit of reducing the size of the energy storage system, due to the generator power and fuel capacity. Series full HEVs can effectively use the generated energy during regenerative braking to recharge the ESS. However, this design choice results in lower vehicle efficiency, approximately 25.7%, which is the lowest among full HEV types. Despite this, the series full HEV is ideal for city driving patterns with frequent stops and starts [2].  
Parallel full HEVs utilize separate propulsion systems for the ICE and the EM, which are joined through a mechanical coupler. This enhances the vehicle's fuel efficiency by up to 43.4%, but the downside is a lower battery capacity. Despite this, the parallel full HEV is still favored over the series full HEV due to its higher efficiency and the reduced size of its electric motor and EMSs [2].  
The full HEV vehicles that have a series-parallel configuration utilize both a mechanical and an electrical coupler to combine the ICE and the EM. This configuration offers the best of both series and parallel full HEV systems, but the setup can be quite intricate and expensive.  
The complex full HEV configuration shares similarities with the series-parallel full HEV, but with the added advantage of power converters linked to the EM, making it a more efficient and controlled system. However, it is more complicated and expensive. All full HEV structures permit vehicle manufacturers to utilize existing technologies like motors, engines, and ESS, which helps to keep the cost of vehicles low [20; 21; 22]. Examples of commercially available full hybrids, like the Nissan Tino, Lexus LS 600 h, and Toyota Prius,utilize the series-parallel full HEV configuration, while others, such as the Ford Escape, Honda Civic hybrid, and Honda Insight, utilize the parallel full HEV configuration.  
Plug-in HEVs are another form of HEV that enable to charge the energy storage system from an external source. Besides the ICS, recharging the ESS is made possible by linking it to the electrical grid. This feature allows for the use of a larger ESS, thereby extending the vehicle's range. Figure 4 displays a schematic diagram of a series-parallel HEV [23].