With the drastic inclination towards reduction of atmospheric issues, hybrid electric vehicles are becoming the major alternative for internal combustion engine vehicles. Compared to internal combustion engine vehicles, hybrid electric vehicles are remarkable in terms of efficiency, durability and acceleration capability. However, the major drawback of hybrid electric vehicle is energy storage capability. An electric vehicle requires the energy sources with high specific power (W/kg) and high specific energy (Wh/kg) to reduce the charging time. Generally, fuel cells, batteries, ultracapacitors, flywheels and regenerative braking systems are used in hybrid electric vehicles as energy sources and energy storage devices. All these energy storage devices are connected to the different DC-DC converter topologies to increase the input source voltage. From the recent past, most of the hybrid electric vehicles are using multi-input converters to connect more than one energy source in order to improve the efficiency and reliability of the vehicle. This survey presents an assessment of present and future trend of energy storage devices and different multi-input DC-DC converter topologies that are being used in hybrid electric vehicles. In addition, different electric vehicle architectures are also discussed.

Introduction

From the past few decades the road transport vehicles have been mostly using internal combustion engine (ICE) because of its high reliability. Due to rising prices of diesel and petrol, environmental pollution and depletion of fossil fuels most of the vehicle manufacturers are looking for the alternative fuels such as natural gas, hydrogen and biofuels for the propulsion of vehicle [1,2]. But all these alternative fuels are not feasible for long-term transportation plan. The Electrical vehicle (EV) is the best solution to reduce the global warming emission gases in transportation sector [3]. EV technology is becoming more trendy in transportation sector, because they consume very less amount of crude oils and environmentally friendly. Transportation sector is the major contributor in global warming emission gases. According to union of concerned scientists, in U.S nearly 30% of global warming emission gases are due to transportation vehicles [4]. The greenhouse gas emissions of different sectors in U.S are shown in Fig. 1. By using EV the operating cost of the vehicle decreases along with carbon gases emission. The average efficiency of internal combustion engine vehicle (ICEV) is 25%, which means that only 25% of total fuel is using for the vehicle and remaining 75% fuel is wasted through heat and friction whereas EV has an average efficiency of 80% [5]. The electric vehicles include battery electric vehicles, hybrid electric vehicles, plug-in hybrid vehicles and fuel cell electric vehicles. The main challenge in the electric vehicle is the cost of energy source. The cost of the energy source is almost one-third of the total vehicle cost [6]. The automobile companies like BMW, Volkswagen, Ford, Toyota, Mitsubishi, Honda, etc. are concentrating mostly on hybrid and plug-in hybrid vehicles. In Refs. [7e9], the authors reviewed the differentmulti-input converter topologies and energy storage devices used in hybrid vehicle applications. But the developments in the converter designs made significant impact on the hybrid vehicles in recent years. In this work we made an attempt on the recent converter topologies and its control strategies applicable for the hybrid electric vehicles are presented. This work will useful for the upcoming researchers to understand the operation of the multi-input converters during vehicle designing. This paper is organized in seven parts. After this introduction section, different types of electric vehicles and their powertrain architectures are discussed in section Types of eletric vehicles. The state-of-the-art of different energy sources used in EVs are presented in section Electric vehicles energy sources. The different multi-input DC-DC converter topologies used in EVs and their control strategies are discussed in sections Multi-input DC-DC converter topologies and Control techniques of DC-DC converter. The future scope and conclusion of the work are given in sections Future scope and Conclusion.