### Literature Review
The gold ratio cut-off technique was utilized to optimize the membership function of the FLC for HEV. Several fuzzy theories, such as fuzzy classification, fuzzy diagnosis, and their applications, are represented in [22]. Several applications related to fuzzy control are carried out at Siemens in Germany. Examples of these classifications, such as optimization theory, fuzzy data analysis, and fuzzy expert systems, etc., are also explained. Different optimization techniques for fuzzy controllers such as generic algorithms (GA) and Rosenbrok's algorithms are also presented. Moreover, the multilevel qualitative optimization of fuzzy controllers is explained in detail [23]. [PERSON] et al. [24] used FLC for the rule-based algorithm. A flatness control was used for the fuel cells or ultra-capacitor. A rule-based algorithm was utilized to distribute the power requirements between the sources and load in HEVs, ignoring converter losses. This EMS is flexible and does not require any of the vehicle's trajectory in advance, but its cost is high. In [25], the battery's current variation was minimized and the power flow from the SC was managed using Karush-Kuhn-Tucker (KKT) conditions. A detailed investigation was carried out on several control algorithms. Among them, rule-based control algorithms and optimization-based control algorithms are the most significant for EMS of HEVs. By utilizing the SC and battery as ESSs, the energy demand of the motor has been supplied with a fuzzy logic controlled EMS [26]. In [27], a comparison of different techniques has been carried out. Among them, for braking control mechanisms, various EMSs are significant. A combination of battery and SC with the use of a bidirectional converter related to fuel-cell HEVs are studied. However, it resulted in a complex and expensive system. It was also justified by the author that the SC is the best source of capturing braking energy. This is due to its high power density. Some researchers [28] used two ESS and Pontryagin's minimum principle (PMP) in their research to fulfill the requisite energy demand for HEVs. The main purpose of using these strategies is to conserve the available electric power. As a result, the lifetime of the battery increases, so that the battery will provide power for a longer duration. In [29], a through-the-road (Ttr) HEV scheme is presented by using FLC-based EMS. The purpose of FLC is to efficiently control the power flow mechanism; hence, the fuel economy can be improved. Comparison with other published articles has also been carried out. EMS in [30] plays a key role in improving the performance and energy efficiency of HEVs. So, most of the researchers have carried out research in this field to provide better and more efficient EMSs. A comprehensive comparison of such EMS techniques is discussed both qualitatively and quantitatively based on the bibliometric technique. These techniques validate the use of fuzzy logic-based EMSs for HEVs. It also shows that a fuzzy logic-based EMS gives a robust result and is best suited for the commercial manufacturing of HEVs. [PERSON] et al. [31] also worked on increasing the stability and efficiency of a hybrid energy storage system (HESS). As a result, the problem faced in the short life span and poor range of ESS in EVs can be minimized. For this purpose, the unscented Kalman filtering (UKF) algorithm was used to evaluate the SOC and state of power (SOP). The proposed control strategy uses two FLCs--one for SOC estimation and the other for SOP estimation. The state of power (SOP) gives information about the battery peak power. This peak power can easily be found by using the SOC of a battery. On the basis of SOC and SOP, fuzzy logic rules are built. These rules provide the required continuous power to the motor and minimize the rapid discharge of the battery. [PERSON] et al. [32] used the battery's SOC and torque required (Tr) as inputs, whereas engine torque (Te) was taken as the output of the FLC. This gives validation to using an FLC as an EMS.