A Critical Review of Emerging Technologies for Electric and Hybrid Vehicles

Emerging topics such as environmental protection and energy utilization have pushed research and development of electric vehicles and hybrid electric vehicles (EVs/HEVs). In the last few decades, numerous technologies have been developed for EV/HEV importance. In this article, key research topics in the area of EVs/HEVs, namely electric machines, electrochemical energy sources, wireless charging infrastructure


I. INTRODUCTION
With changing consumer behavior, government regulations and sustainability requirements, research on electromobility applications, in particular electric vehicles and hybrid electric vehicles (EVs/HEVs), has drawn significant attention.It is anticipated that by 2040, annual sale of EVs/HEVs is projected to outsell gasoline and diesel vehicles, with estimated sale over 48 million per year.As one of the most promising electromobility applications, the development of EVs/HEVs has become a hot research topic.
This paper aims to provide a critical review of emerging technologies for EV/HEV applications.It aims to cover key research topics, including electric machines, electrochemical energy sources, wireless charging infrastructure and latest EV/HEV models.The purpose of this paper is to provide potential readers, including engineers, researchers and scholars, a critical review of latest technologies in area of EV/HEV applications.

II. ELECTRIC MACHINES
Due to the definite merits of high efficiency, high power and torque densities, permanent-magnet (PM) machine has served as a promising candidate for EVs/HEVs.Meanwhile, other types of machines, e.g., induction machine (IM), would field synchronous machine (WFSM) and reluctance machine, have also gained attention for automotive applications in the past 20 years.For example, IMs have been employed in Tesla EVs and wound-rotor synchronous machines in Renault Zoe EV and BMW iX3 e-Drive.

A. INDUCTION MACHINES (IMS)
PM machines have been widely applied in many applications due to its high torque density and efficiency.However, PM machines suffer from some disadvantages, e.g., unstable rareearth supply, low flux-weakening capability and irreversible demagnetization.Therefore, global efforts are underway in developing non-PM machines, among which the IM is the most successful type for EV/HEV applications.Being an AC machine, an IM exhibits two windings separated by an air-gap [1], [2], where all electromagnetic energy is transferred by inductive coupling from the primary winding to the secondary winding.The IM-based drives offer the advantages of robust structure, low cost, high reliability, and maintenance-free [3].Therefore, the squirrel-cage IMs are widely employed in EVs/HEVs for high torque/power density, high efficiency, strong physical structure, and variable speed range [4].However, most IMs have lower efficiency and less torque density than PM machines [5].To address this situation, an IM with copper rotor cage is utilized for the driving system of Tesla EVs to improve the torque density and efficiency, as shown in Fig. 1 [6], [7].Besides, the innovative rotor manufacturing called as short-circuit ring and connection with the copper rod is proposed by Siemens [8].With optimized material and suitable cooling technique, the power density and efficiency of the IM can be almost the same as with synchronous motors [9].In addition, an axial-flux IM, with double-sided axial fluxes, is also an attractive solution.This structure can significantly increase torque density and has the potential to become a compact and cost-effective machine [10], [11].Another high efficiency IM with compact stator winding form, which allows modifying overhang deviation and ruling thermal condition, is proposed in [12].Apart from developing the structures with improved torque density, a novel control method in the feedback channel based on morphological filters is proposed to suppress torque ripple by current harmonics [13].
Apart from the squirrel-cage IMs, the doubly-fed IM (DFIM) has also proved to be a competitive candidate for EVs/HEVs with the improved efficiency and extended torquespeed characteristics compared to its singly-fed counterpart.A dual-electrical-port control scheme of cascaded brushless doubly-fed IM (CBDFIM) is proposed to achieve doubled constant torque and constant power regions compared to its singly-fed counterpart with the same equivalent pole pair number [14].Another single-electrical-port control scheme for four-quadrant operation of cascaded doubly-fed IM (CD-FIM) is proposed and theoretically demonstrated.It is found that the doubly-fed synchronous operation of the CDFIM can achieve a wider constant torque region and constant power region than the singly-fed synchronous operation [15].

B. WOUND FIELD SYNCHRONOUS MACHINES (WFSMS)
For EV/HEV applications, another successful machine type is the wound rotor synchronous machine (WRSM) [16], as depicted in Fig. 2. To remove brush and ring in WRSMs, the wound field switched flux machine (WFSFM) is proposed [17], [18], as depicted in Fig. 3.The above two machines both belong to the family of WFSMs with wound field winding in rotor and stator, respectively.
The conventional WRSMs are widely applied as low-speed generators [19]- [22], while the rotor field winding excitation system has a large volume and low power density.Therefore, the brushless operations of WRSMs are studied with various topologies [23]- [27].
WFSFMs have both armature and field windings on stator, which realize brushless operation [28], [29].According to the winding arrangements, it can be divided into three types, namely, the one with both armature and field coils wound on one tooth [17], the one with half tooth wound by armature coils and the other by field coils [30], and the one with overlapped armature and field coils [29].According to iron steel topology, it can be further divided as single stator [28]- [30] and double-stator [31].It should be noted the latter one has a higher slot area and higher slot packing factor, and hence potentially higher torque/power density.

C. RELUCTANCE MACHINES
Switched reluctance machines (SRMs) have attracted significant attention due to their inherent advantages, such as simplicity, robustness, excellent fault-tolerance, and wide speed range.During the last few decades, as a PM-free brushless machine, the SRM-based drive systems have accelerated developments due to the supply limitation and price fluctuation of rare-earth materials [39].Hence, SRMs are considered as strong candidates for EVs/HEVs.
In 1998, a 300Nm SRM with outer diameter/axial length of 296mm/ 96mm was reported, where the torque density of 38Nm/L was achieved in experiments though the current density as 50A/mm 2 [40].In 2008, the SRM for mild HEVs with outer diameter/axial length of 410mm/93mm was built with the electromagnetic torque of 300Nm.It could provide the torque density of 25Nm/L and the highest efficiency of 96% [41].In 2010, the SRM with electromagnetic torque of 1800Nm, torque density of 30Nm/L was reported with outer diameter/axial length of 640mm/190mm [42].
Tokyo University of Science compared the electromagnetic performance of SRMs with 12/8 and 18/12 structures, from the point of view of torque density, efficiency, and torquespeed range.It was found that the performance of 18/12 machine is better [43].In [44], a 50-kW prototype SRM was built and comprehensively tested as shown in Fig. 4 .The maximum efficiency, output power, and torque density reached 95.4%, 54.1kW, and 38Nm/L, respectively.These values were competitive to the IPM motor employed in Toyota Prius 2003 (i.e., an HEV).
Although SRMs with comparable torque/power density to the IPM motors are reported, no single electrified passenger vehicle with an SRM exists in market [45]- [47] due to noise, vibration, torque ripple, and unique inverter, which indicates that the SRMs need further development.
Synchronous reluctance machine (SynRM) can be regarded as a potential candidate for alternative of PM machines in EVs/HEVs due to low cost and environmentally friendly.Different from SRMs, rotors of SynRMs are relatively complicated [48], which can be divided as axially laminated (AL) rotor [49], [50] and multi barriers (MB) [51]- [54] rotor, as shown in Fig. 5.
SynRM with AL rotor has higher saliency ratio that can help achieve higher torque density.However, AL rotor suffers from high manufacturing difficulty and fragile structure, which limit its application.Although the saliency ratio is reduced, MB rotor is more robust and simpler.Therefore, MB rotor receives considerable attention in recent years.In general, SynRM suffers from drawbacks as complicated rotor structure, non-linear characteristic and high torque ripple.The optimization of machine topology is studied in [50], [51], and the effect of saturation is considered in [52], [53].The acoustics issue by torque ripple is optimized in [54].In short, SynRM consists of some inherent problems and its application in EVs/HEVs is still in the developing stage.

D. STATOR-PM MACHINES
As a topology of novel brushless PM machines, stator-PM machines having magnets and armature windings on its stator (the so-called as stator-PM machines), exhibit the merit of convenient heat dissipations [55].The compact and robust rotor structure also makes stator-PM machines as potential candidates for high-speed applications.Meanwhile, the airgap flux regulations can be realized by hybrid excitations of PMs and field windings.In general, stator-PM machine features the definite advantages of robust structure, high power density and efficiency, high fault-tolerant capability.These advantages allow stator-PM motors to be a suitable candidate for EVs/HEVs.
Generally speaking, the flux-switching PM (FSPM) machine attracts more attention since it exhibits largest torque/power density within the stator-PM machine family.The most typical structure was proposed by E. Hoang in 1997, as shown in Fig. 6(a) [56], where its stator consists of 12 "U-shaped" silicon steel and 12 magnets.In order to reduce the PM usage, a new E-shaped structure is proposed [57], as shown in Fig. 6(b).It is found that the improved topology can reduce the PM usage by half while the torque remains almost the same.However, this topology suffers from poor overload capability.In order to further increase the slot area, another C-shaped structure is proposed in [58], as shown in Fig. 6(c).Similarly, the amount of PM is half as compared to that of traditional FSPM machine.To further improve the torque and PM utilization, a V-shaped structure is proposed in [59], as shown in Fig. 6(d).Compared with the traditional topology, a pair of magnets with opposite magnetizing directions are placed on each stator tooth, and the two magnets can therefore form a V-shaped structure.To further improve the utilization of PMs, a FSPM machine with a multi-tooth structure based on the above-mentioned V-shaped structure is proposed in [60].To expand the speed adjustment regions, the air-gap flux regulations can be realized by hybrid excitations of PMs and field windings [61]- [67].

E. FIELD MODULATION MACHINES
Field modulation machine (FMM) is a general designation for a class of machines with modulation effect.FMMs include stator-PM machines and PM vernier (PMV) machines [68]- [71].In general, FMMs utilize magnetic flux modulator to modulate the speed and pole-pair numbers of the initial magnetic field [72], [73].Most of the air-gap magnetic field harmonics in FMMs can contribute to torque, and hence an improved torque density can be achieved [74].According to air-gap field modulation theory [75], [76], the pole-pair of airgap magnetic field harmonics in field modulation machines can be expressed as where p indicates the pole-pair of the magnetic field due to PM or armature windings, n s expresses the pole-pair of the magnetic flux modulator, m and i are the modulation coefficients.
The PM layouts of stator-PM machines are usually fixed [77], [78].Meanwhile, the PMV machines consist of a more flexible PM layout, where PMs can be mounted on rotor, on stator teeth, stator yoke, and both stator and rotor [79]- [82], as shown in Fig.In [81], it is confirmed that the PMV machines in Fig. 7(a) can offer a higher no-load EMF and torque handling capability.S. L. Ho pointed out that the performances of PMV machines with PMs on the rotor in Fig. 7(a) are better than those with PMs on the stator teeth in Fig. 7(b) or in the stator yoke in Fig. 7(c) [82].S. Shimomura found that the PMV machine with multi-slot on the stator teeth [see Fig. 7(d)] can reduce the armature current and loss to improve efficiency [80].In addition, by changing the rotor PMs from surface mounted to spoke array, the torque and power density of the PMV machine can be further improved [83].

F. HIGH-SPEED MACHINES
EV/HEV machines should not only satisfy specific requirements in performance and efficiency, but also noise, vibration and harshness, cost [84], [85].With the rapid development of traffic electrifications, high-speed machine has attracted more and more attention.This type of machines has been applied in the automotive industry because of its characteristics of high power density, high efficiency, lightweight and low cost.The main types of high-speed machines used in vehicle drive, supercharger and vehicle compression include PM synchronous machine (PMSM), IM, SRM, and SynRM.In recent years, PMSM has outstanding advantages in power density, torque density, efficiency and speed regulation range and is widely used in EVs/ HEVs [86], [87].For example, commercially available Honda, Toyota, Tesla and BMW vehicles use PMSM as its propulsion motor.The disadvantage of PMSM comes from the fact that it adopts rare earth PM, which has a high cost and risk of thermal demagnetization.IM has no magnets in the rotor and is characterized by its robustness.The limitation of this topology may lie in the cooling system, where heat is generated on the rotor and stator sides.SRM does not rely on PMs and is very robust for harsh environments and fault-tolerant operation [88].However, SRMs suffer from high noise and low power factors [89], [90].SynRM has been studied more recently [86], [91], [92].By using the reluctance torque, the amount of PM so as the cost are reduced, but SynRM still suffers from serious problems of rotor deformation and magnetic flux leakage during high-speed operation [93], [94].
Many automobile companies have studied the high-speed machines and formed a series of products.The high-speed machine of BWM i3 pure EV is a PMSM, which consists of maximum power 125kW, peak torque 250Nm, maximum speed 11400rpm, and mass 42kg [95], as shown in Fig. 8(a).The Tesla Model 3 uses a 6-pole 54-slot PMSM with a maximum speed of 18,000rpm and a maximum power of 196kW.This PMSM consists currently the highest speed on the market [87], as shown in Fig. 8(b).Oxford University uses axial flux PM machines, which leads to a series of produce line, such as YASA-400 and YASA-750.Taking YASA-400 as an example, it consists of the maximum speed 7500rpm, and the maximum power 165kW [96], as shown in Fig. 8(c).General Motors (GM) has launched an advanced automotive technology project codenamed ELT093 [97].Its SynRMs can provide rotation speed 16650rpm, and the torque 250Nm, as in Fig. 8(d).
Z. Yang [84] presented the design and comparative evaluation for PMSM, IM, and SRM for EV/HEV applications.Simulation and analytical results show that each machine topology demonstrates its own unique characteristic.IM and SRM have lower peak power density, i.e., 50kW/48kg and 50kW/42kg respectively, comparing with 50kW/30kg for PMSM.The PMSM, IM and SRM for a supercharger application operating for a short duty only at a constant speed were compared in detail [98].A comparison between the PMSM and SPM machines is carried out for application to an EV, in terms of performance at given inverter ratings [99].

III. ELECTROCHEMICAL ENERGY SOURCES
Electrochemical energy source can be used to power not only electric motor drives, but also other auxiliary parts, such as air conditioners, lightings, audiovisual systems within the EVs/HEVs.Existing electrochemical energy sources are able to provide either high specific energy or high specific power (or also known as energy density and power density), but not at the same time.There are plenty of electrochemical energy sources available in the market, while they can be roughly categorized into three key groups, namely batteries, fuel cells and ultracapacitors.

A. BATTERIES
Battery is an electrochemical conversion device that can convert between active materials and electric energy [100].It typically contains three key parts, namely positive electrode, negative electrode and electrolyte.Depending on different applications, many various types of batteries have been developed in last few centuries.For EV/HEV applications, there are mainly three major types of candidates, namely lead-acid battery, nickel-based batteries and lithium-based batteries [101].
Lead-acid battery is widely regarded as one of the first commercially available battery in EV/HEV applications.Typical lead-acid battery employs lead dioxide as positive electrode and metallic lead as negative electrode.Unlike conventional lead-acid battery, an improved lead-acid battery, so-called as value regulated lead-acid (VRLA) battery, consists of electrolyte of two forms, namely absorbed electrolyte and gelled electrolyte.These two-formed structure allows VRLA battery to reduce evaporation, leakage and vibration, as compared to its conventional counterparts.Even conventional lead-acid and VRLA batteries consists of different topologies, both share the same chemical reactions as: The first nickel-based battery was invented more than a century ago and this type of battery has been widely employed in many applications, including EVs/HEVs.There are plenty types of nickel-based batteries, such as nickel-iron (Ni-Fe), nickel-cadmium (Ni-Cd), nickel-zinc (Ni-Zn) and nickelmetal-hydride (Ni-MH) batteries, while the most common one in EVs/HEVs is Ni-MH battery.Similar as other nickel-based batteries, Ni-MH battery employs nickel hydroxide as its positive electrode.Meanwhile, it employs metal hydride as negative electrode and potassium hydroxide solution as electrolyte.There are various types of metal hydride available, while the most common one is AB 2 alloy and AB 5 alloy.The typical chemical reaction of Ni-MH battery can be described as: Lithium battery employs various types of materials as its positive electrode while lithium metal as its negative electrode, as how it is named.However, lithium metal leads safety problems, including storage, transportation and operation.Hence, to improve the situations, intercalation material is used to replace lithium metal as its negative electrode, and hence forming the lithium-ion (Li-ion) battery.Typical Li-ion battery employ lithium intercalation compounds as both its positive and negative electrodes.During charging and recharging processes, lithium ions are inserted and removed between positive and negative electrodes.In principle, there is no net change during movement of lithium ions.Depending on various type of employed materials, chemical reaction of Li-ion battery can be described as: To evaluate performances of batteries, three major key index, namely specific energy, specific power and cycle life, are usually used.Specific energy is the energy capacity available with a battery while specific power is the energy delivered by a battery within a particular period.Cycle life is the number of maximum charging and recharging cycle of a battery can undergo without significant again effect.The comparisons between typical batteries are listed in Table 1.

B. FULL CELLS
Full cell is an electrochemical energy sources that can convert chemical energy into electrical energy [102].Full cell shares both similarity and dissimilarity as battery does.For the similarity, both two devices share similar structure and hence consume fuels and oxidants to perform chemical reactions.For the dissimilarity, the battery stores the fuel and oxidants within the system while the fuel cell stores these materials outside the system.For EV/HEV applications, there are four promising candidates, namely alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), proton exchange membrane fuel (PEMFC) and direct methanol fuel cell (DMFC).
AFC has been generally accepted as the first practical fuel cell while it was firstly proposed by Sir William Robert Grove in 1800s.Typical AFC operates with pure hydrogen, pure oxygen and circulating electrolyte.It enjoys definitely merits of high efficiency, mature technology and cost-effectiveness.It contains two porous electrodes, separated by matrix saturated alkaline solution.Before of efficient heat transfer rate, potassium hydroxide (KOH) is widely commonly employed in AFC.AFC is operated under redox reaction, i.e., reductionoxidation reaction as: (5)

TABLE 2. Comparisons of Typical Fuel Cells
PAFC is one of the most mature technologies among all fuel cells and it has been widely regarded as the first commercialized fuel cell in electric power industry.It utilizes liquid phosphoric acid as its electrolyte while platinum-coated carbon as catalyst.Electrons can move from negative electrode to positive electrode via external circuit, while positively-charged hydrogen ions is able to transfer from positive electrode to negative electrode via acidic electrolyte.Since PAFC needs to operate in a relatively higher temperature range, phosphoric acid is usually employed as electrolyte.When PAFC generates electricity, it produces water as by-product as: PEMFC employs proton conducting membrane in solid polymer form as electrolyte, while sometimes it is also called as ion exchange membrane fuel cell or solid polymer electrolyte fuel cell.PEMFC is renowned its high power density and it was the first fuel cell that employed in NASA's Gemini space project in 1960s [103].To improve stability and lifetime of membrane, operating temperature of this type of fuel cell is usually kept low.Since its operating temperature is relatively low, as compared with other fuel cells, it needs expensive platinum metal as catalyst for chemical reactions.When PEMFC generates electricity, it produces water as similar as PAFC does.
Unlike other fuel cells that employ hydrogen as major fuel, DMFC instead utilizes methanol as its direct fuel.As compared with using hydrogen, using methanol enjoys simpler storage and more compact system.By using methanol as its direct fuel, DMFC can generate electric energy based on electrochemical reactions.Meanwhile, methanol can be firstly transformed as hydrogen reformate gas mixture and stored within the fuel cell and this system is known as indirect methanol fuel cell (IMFC).IMFC behaves similarly as other fuel cell do, which all of them uses hydrogen as its major fuel.Depending on design of DMFC, it can produce carbon dioxide and water as: To evaluate performances of fuel cells, four major key index, namely common fuel, power density, temperature and efficiency, are used.Power density is the total power generated by a full cell per unit of volume, while temperature is operating temperature.The comparisons between typical full cells are listed in Table 2.

C. ULTRACAPACITORS
Capacitor is a simple device with two metallic electrodes separating by dielectric, while it can store and release electrons in different occasions [104].Classical capacitors can be grouped into two major types, namely electrostatics capacitor and electrolytic capacitor.The former one uses nonelectrolytic materials to act as dielectric component, while the latter one uses electrolytic materials to act as dielectric component.Even though the electrolytic capacitor consists of higher capacitance than the electrostatics one, both of them are still far from sufficient capacity for EV/HEV applications.While capacitors have suffered from low capacity for many years, a new technology known as ultracapacitor has been invented to become a promising solution.There are typically two major types of ultracapacitors available, namely double layer capacitor and pseudocapacitor.
The concept of double layer capacitor allows the charge to be stored on the electrode surface with separated electrode and electrolyte [105].This idea was not very popular before the technology breakthrough of nanotechnology.Upon support of the latest nanotechnology, capacitor electrodes can be developed based on highly porous materials.This new material characteristic allows the double layer capacitor to become highly capacitance.It should be noted the operating principle of double layer capacitor is generally regarded as a non-Faradic process, i.e., no chemical reaction is associated with charging/discharging process.
Pseudocapacitor is another ultracapacitor with similar idea and topology of double layer capacitor.Pseudocapacitor differentiates itself by oxidation-reduction reactions at its interface and Faradic process, i.e., chemical reaction is associated with charging/discharging process.Since Faradic process is involved within pseudocapacitor, it generally can offer higher capacitance and higher energy density than double layer capacitor.
Comparisons among four major electrochemical energy sources, namely batteries, fuel cells, capacitors and ultracapacitors, and gasoline are shown in Fig. 9.

IV. WIRELESS CHARGING INFRASTRUCTURES
Aiming to mitigate the manual charging with user interaction and eliminate the galvanic connection, the wireless power transfer (WPT) technique offers an unobtrusive and hassle-free charging method regardless of the harsh environment [106]- [108].In addition, no need to mechanically move charging cable will particularly promote automatic wireless charging technology, and then be more attractive for public transportation with opportunity wireless charging.Among several coupling mechanisms for WPT techniques such as inductive coupling, capacitive coupling, magnetodynamic coupling, and microwaves, as listed in Table 3, the magnetic-resonant coupling (MRC) based inductive power transfer (IPT) has been recognized as the most feasible and the most commonly applied for wireless EV charging application [109]- [111].

A. PARK-AND-CHARGE OF STATIONARY CHARGING
In order to facilitate the park-and-charge (PAC) process for the EVs, the IPT technology is extended to be plug-less, in which the primary coil is installed on the floor of a garage or in a parking lot and the secondary coil is installed on the vehicle, as shown in Fig. 10.The driver needs no bothering about those cumbersome and dangerous charging cables.The use of this system is very easy and the charging process takes place automatically once the driver parks the EV correctly [112]- [114].This plug-less PAC not only increases user convenience, but also offers a means of overcoming the standardization of charging plugs.Based on magnetic resonant coupling, the primary and secondary coils having the same resonant frequency can wirelessly transfer power efficiently with high power density, while dissipating relatively little energy in non-resonant objects such as vehicle bodies or drivers.
The latest research and development of WPT for PAC are active and diversified, such as compensating the misalignment between magnetic couplers, realizing bidirectional WPT between chargers and EVs, integrating power transfer and information transfer within the same channel.Some state-of-the-art works are summarized below: 1) For realistic PAC, the magnetic coupler design plays a key role for effective WPT.For instance, the compound primary pad using uneven pitch distances of the spiral winding has been proposed to offer a uniform magnetic flux density at most of the charging area, hence solving the problem of misalignment.Meanwhile, the bipolar primary pad has been developed, which can interoperate with simple secondary pads to achieve power transfer with large lateral tolerance [115].2) Since EVs can serve as mobile power plants to support and stabilize the power grid with renewables, the development of vehicle-to-grid (V2G) technology is promising [116].By incorporating the WPT into the V2G, a bidirectional power interface has been developed to facilitate the simultaneous charging and discharging of multiple EVs.Various bidirectional resonant inverters have recently been developed for wireless V2G, aiming to improve the power level, power flow control and fault-tolerant ability [117].
3) The simultaneous wireless power and information transfer (WPIT) technology is being actively developed for EV charging, which desires power transfer from the charger to the vehicle and data communication between the charger and on-board battery management system.
A WPIT system has been proposed in which the fundamental component of the triangular current waveform is employed to transfer power, and its third-order harmonic component is selected to transfer information [118].

B. COIL DESIGN OF WIRELESS EV CHARGING
The development of WPT typical coil technologies is depict in Fig. 11.As the energy carrier in WPT system, the transmitter and receiver pads play an important role in emitting and receiving the magnetic flux lines.The typical coil topologies for the transmitting coil design and the receiving coil design are shown in Fig. 12, which can be used at the EV side for both park-and-charge and move-and-charge.In the early development of WPT, the rectangular pad was proposed for many years, which consists of four fillets [119].This topology mainly improves the flux area; and the flux leakage in the edge can be reduced.However, the low efficiency and the large total flux leakage are indispensable.Thus, it is normally accepted for the transmitting coil design according to the specific requirement.In order to facilitate a low reluctant flux path and reduce the flux leakage, ferrite bars are presented to add to the back of the coil pad, as shown in Fig. 12(a).Also, an aluminum plate is created at the back of the whole coil to hold up the flux distribution.This topology takes the advantage that the structure is easy to build with the symmetric flux distribution around the center.However, the transfer distance, namely the main flux height, is limited by the pad size.
Based on the CP topology, an improved topology called DDP, as shown in Fig. 12(b), was proposed, where the currents in double coils are in opposite direction.The flux path can be created to narrower and taller due to the parallel field pattern along the ferrite bars [120].Hence, the transmission distance can be extended with high efficiency.However, this topology behaves poor interoperability characteristic when the receiver pad is centrally aligned, as well as the CP topology.To overcome this interoperability problem, the DDQP topology was design to generate both parallel and perpendicular magnetic field [121].The DDQP topology was placed one additional coil on DDP topology, which is shown in Fig. 12(c).Since the DDQP topology can generate the polarized and non-polarized field by regulating the coil current, the system flexibility is higher than other topologies.But the drawback is the increased number of coils, which leads to the increased cost.
Comparing to the DDQP topology, another high flexibility topology called BPP was introduced to generate the parallel and perpendicular filed, where the two coils are partially overlapped, as shown in Fig. 12(d).In the meantime, the system complexity and cost are reduced by eliminating one coil.According to the different operations of the current direction, the BPP topology can work in CRP mode, single coil mode, and DD mode.Hence, the BPP topology can be effectively suitable for multi-mode secondary pad design, which has a big potential for wireless EV charging [122].In addition, the comparison of commonly used coil pads with factors and features is summarized and listed in Table 4.

C. MOVE-AND-CHARGE OF DYNAMIC CHARGING
Rather than stopping or parking, the EV prefers to be wirelessly charged during moving.Namely, an array of power transmitters is embedded beneath the roadway (so-called the charging zone or lane) while a receiver is mounted at the bottom of the EV.This move-and-charge (MAC) technology has high potentiality to fundamentally solve the long-term problems of the EV.Namely, there is no need to install so many batteries in the EV, hence dramatically cutting its initial cost; and the EV can be conveniently charged at the charging zone during driving, hence automatically extending the driving range.Differing from PAC, the system configuration for MAC is more challenging.
The transmitters installed beneath the road surface can be either pad or rail design.The pad design includes many primary pads where the size of each pad is equal to or less than that of a vehicle.Based on one power inverter per section, the primary pads can be separately excited by using power switches and sensors as shown in Fig. 13(a).These pad-based transmitters inevitably involve a large amount of primary pads, power inverters or power switches and sensors, thus suffering from huge investment costs and high installation complexity.In contrast, the rail design involves only a primary rail or actually a long primary coil and a power inverter to feed multiple EVs, as shown in Fig. 13(b), which takes the definite advantage of much lower investment cost and much lower installation complexity than the pad design.For the sake of more flexible maintainability and scalability, the rail design usually adopts the arrangement of sectionalized roadway in  which it uses one power inverter per section to feed multiple EVs.
Because of the potentiality to fundamentally solve the longterm problems of EVs, research and development of MAC technology have been overwhelming.Some state-of-the-art works for MAC are summarized below: 1) The online electric vehicle (OLEV) project conducted by KAIST has successfully implemented the rail-based MAC system.It has solved various MAC problems such as high-frequency current-controlled inverters, continuous power transfers, and cost-effective improvement.
Innovative coil designs and roadway construction techniques make the system efficiency of the OLEV reach up to 83% at an output power of 60 kW with a resonant frequency of 20 kHz.Different magnetic couplers using a rail have been well developed for the move-and-charge system, such as the U-type, E-type, W-type, I-type, and S-type rails [123]- [125].2) Due to the mobility of the EV, the misalignment between the primary and secondary coils of the pad-based MAC system inevitably affects the performance of existing WPT techniques.The homogeneous WPT technique has been proposed [126], [127], which utilizes the alternate winding design to gaplessly assemble primary coils to enhance the magnetic flux density, and the vertical-and-horizontal secondary coil to improve the capability of acquiring energy, especially in the area of the coils gap.Hence, it can effectively improve the power transfer performance of this pad-based MAC system.3) In order to allow authorized EVs to perform charging and avoid unauthorized EVs stealing wireless power when they are running on the rail-based MAC system, the energy encryption technique has been proposed [128].Namely, the operating frequency is purposely adjusted to follow a predefined sequence over a predefined frequency band (so-called the security key) while the primary rail is synchronously tuned to have the resonant frequency matching with the operating frequency, the transmitted energy is thus encrypted.When the secondary coil is also synchronously tuned to have the same resonant frequency in accordance with the key, the authorized EV can receive the desired energy; otherwise, without the knowledge of the security key, the unauthorized EV cannot decrypt the encrypted energy or receive the desired energy [129].

V. MODERN ELECTRIC AND HYBRID VEHICLES
Internal combustion engine (ICE) vehicles have been involved in wide applications for over one hundred years, whereas these types of vehicles fail to meet the modern requirements, such as high fuel efficiency and low carbon emissions.Identified as one of the most viable solutions to the problems associated with ICE vehicles, EVs/HEVs have received significant attention in the past few decades.In this section, a few popular models with variation, namely Toyota Prius, Tesla Model S Plaid, and Porsche Taycan Turbo S are reviewed to highlight the development status of modern EVs/HEVs.It is just to illustrate different technologies with examples rather than a review of many EVs available in the market.

A. TOYOTA PRIUS XLE AWD-E
Toyota Prius is one of the most popular HEVs in the market, while it is famous for its excellent fuel efficiency.Fig. 14 shows the 2021 Prius XLE AWD-e that inherits the merits of the fourth-generation model of Toyota, and it employs all-wheel drive technology to improve driving performances.The fourth-generation Prius model has made collective innovations to deliver a 18% improvement in fuel efficiency, including a redesigned engine, an evolved hybrid transaxle, a compact Ni-MH battery, and a miniaturized power control unit [130].
As shown in Fig. 14(b), the most significant change in the fourth-generation Prius comes from its hybrid transaxle.Specifically, the drive motor and generator are located over multiple axles, and the planetary gear arrangement has been replaced with parallel gears.As a result, the mechanical loss is reduced by 20%, and the unit length is reduced by 47 mm.Another highlight is the high-performance engine can provide thermal efficiency of 40%, which is achieved by the strong airflow in combustion chambers, large-volume cooled exhaust gas recirculation (EGR), dual cooling system, and the newly designed water-cooling jacket.In addition, the use of low-loss components in the power control unit cuts losses by 20%, thus the unit size can be reduced by 33% accordingly.The detailed specifications of the 2021 Prius XLE AWD-e are summarized in Table 5.With the aforementioned improvements, the Prius XLE AWD-e can achieve an outstanding fuel efficiency of 49 mpg for combined working conditions.

B. TESLA MODEL S PLAID
Tesla has four series of EVs [131], namely Model X, Model Y, Model 3, and Model S. The first three EVs have drawn significant attention due to the desirable performances, while the Model S is widely regarded as the flagship of Tesla EVs.Fig. 15 shows the latest Tesla Model S Plaid, which has utilized the Tri-motor all-wheel drive (AWD) system.For existing AWD EVs, the complicated mechanical structures are adopted for power distribution from a single engine to all wheels, thus reducing the system efficiency.By contrast, for the Tri-motor AWD in Model S Plaid, one motor is placed in the front and the other two motors are arranged in the rear part of the EV.These three motors can be independently controlled to drive the front and rear wheels, and hence achieving fast acceleration and improved efficiency.The specifications of Model S Plaid are listed in Table 6.It should be noted Model S Plaid can reach peak power of 1020 hp with top speed of 322 km/h.In addition, the speed acceleration from 0 to 100 km/h can be achieved within 2.1 seconds due to the promising powertrain.The driving range per charge can reach 637 km, which is much higher than the existing EV counterparts.

C. PORSCHE TAYCAN TURBO S
Porsche Taycan is the first attempt from Porsche in the EV market [132].As the most powerful version of Porsche Taycan series, the Porsche Taycan Turbo S has been viewed as one of the best EVs in the world.Fig. 16 shows the Porsche Taycan Turbo S, which employs the two-speed transmission on the rear axle.This new powertrain is an unique technology from Porsche.There are two gears in the two-speed transmission system, i.e., one of the gears is utilized for initial speed acceleration and the other one is employed to keep the high power and efficiency at high speeds.
The specifications of Porsche Taycan Turbo S are listed in Table 6.It should be noted Porsche Taycan Turbo S can reach peak power of 751 hp with top speed of 260 km/h.In addition,  the speed acceleration from 0 to 100 km/h can be achieved within 2.8 seconds.The driving range per charge can reach 340 km with approximately 9 h charging time.

VI. CONCLUSION
Due to the increasing concerns on energy crisis and energy utilization, development of EVs/HEVs has become a one-way train.There are plenty exciting technologies invented in the last few decades while an overview of emerging technologies can benefit many stakeholders.This paper aims to serve as a starting point for engineers, researchers and scholars who are keen to develop their interests in this area.In this paper, several key topics, namely electric machines, electrochemical energy sources, wireless charging infrastructures and latest EV/HEV models, are discussed in detail.This paper targets to provide the readers a blueprint for them to begin their own journey in the field.

FIGURE 1 .
FIGURE 1.The copper rotor of IM used in Tesla Model S [7].

10 .
Typical configuration for park-and-charge IPT system.

FIGURE 13 .
FIGURE 13.Coil configuration of the move-and-charge system.(a) Multiple coil pads as the primary transmitter.(b) Single coil rail as the primary transmitter.