In-Wheel Motor Drive Systems for Electric Vehicles: Comparison
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Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Kritika Deepak.

There has been significant attention given to the electrification of transportation due to concerns about fossil fuel depletion and environmental pollution. Conventional drive systems typically include a clutch, reduction gear, and mechanical differential, which results in power loss, noise, vibration, and additional maintenance. However, in-wheel motor drive technology eliminates the need for these components, providing benefits such as higher system efficiency, improved wheel control, and increased passenger comfort.

  • in-wheel motors
  • axial flux motors
  • outer-rotor PMSM motors

1. Introduction

Over the past few decades, there has been widespread attention given to the issues of energy crises and environmental pollution. The global community has recognized the urgency of conserving energy and reducing emissions, as indicated by the research in [1][2]. Electrification has emerged as an alternative solution for various industrial applications, particularly in the realm of transportation, where electric vehicles have shown promise. Thanks to advances in high-performance electric motors and power electronics, electric and hybrid electric vehicles (EVs/HEVs) are being developed and studied as a substitute for conventional internal combustion engines (ICE) automobiles [3]. The Tesla Model 3, Renault Zoe, Volkswagen ID3, and Volkswagen ID4 are currently the most popular EVs in the european market [4][5][6][7]. However, the conventional central-motor drive system in EV/HEV involves linking the propulsion motor to the wheels via a chain of mechanical transmission components [8][9]. Unfortunately, this mechanical transmission setup leads to increased weight and higher maintenance costs [10]. Alternatively, an in-wheel motor (IWM) could be used to directly power the wheels without the use of mechanical transmission. Placing the motor inside the wheel rim allows for the speed and torque output of the motor to be directly transmitted to the wheel. Consequently, IWMs operate at lower speeds but with higher torque when compared to central-motor drives. This method provides numerous benefits, including increased space for passengers and batteries, as well as greater control flexibility through independent wheel control [11].
Over a hundred years ago, in 1900, the first-ever IWM driven automobile was created by Lohner–Porsche Electromobile [12]. This vehicle was propelled by two IWMs, which were powered by a forty-four cell 80-volt lead battery [13]. Later, Porsche also introduced the first functional hybrid car, using a combustion engine to drive the generator and supply power to the wheel hub motor. This car failed over the years as it weighed almost two tonnes [12]. Electric cars and gasoline-powered cars were in significant competition for a period of time. Nonetheless, the introduction of Henry Ford’s Model T in 1908 led to a shift in favor of gasoline-powered vehicles, ultimately resulting in greater affordability at a larger scale. In the following years, electric vehicles were outperformed by ICE vehicles.
The contemporary fascination with electric vehicles can be traced back to the release of the Toyota Prius in 1997, which was the world’s first mass-produced hybrid electric vehicle. This event sparked interest in modern electric vehicles in the automobile market. Moreover, the US Department of Energy states that electric vehicles are highly energy efficient as they convert more than 77% of electrical energy into power that drives the wheels. In stark contrast, gasoline-powered vehicles convert only 11% to 37% [3] of the energy stored in gasoline into kinetic power. Additionally, the rising global awareness and concern for environmental impact of combustion vehicles fueled by oil is drawing consumers to go electric.

2. Motor Topologies for IWMs

2.1. Radial Flux Motors

2.1.1. Topologies of Radial Flux Motors in IWMs

Permanent Magnet Synchronous Motors

The outer-rotor PMSM (OR-PMSMs) with neodymium PMs are a conventional choice of motor designers both in research and industrial sector for IWM-drive applications due to their high torque density. Fractional slot-concentrated winding is the preferableoption to accommodate an in-wheel installation, and opting for a high pole count results in a slimmer iron yoke design, ultimately reducing both weight and size [14][15]. The advantages of concentrated windings, such as enhanced fault tolerance and streamlined manufacturing, are well-documented in the literature [16][17]. Additionally, incorporating a multi-phase design can further improve fault tolerance. Certain OR-PMSM models include sub-motors to increase fault tolerance, and various techniques have been implemented to mitigate parasitic effects, such as eddy currents and NVH [18][19].

Switched Reluctance Motor

The outer-rotor switched reluctance machine (OR-SRM) is a potential non-PM option for IWM-DEVs [20]. While it offers low cost, compact size, and a robust structure, due to the high magnetic saturation present in both the stator and rotor steel, the flux characteristics of the machine are non-linear [21]. As a result, the machine’s efficiency is reduced and its torque performance is inferior to PMSM [22]. In [23] proposed a motor that combines the advantage of multiple teeth per stator pole and more rotor poles than stator teeth to achieve the purpose of enhancing the specific torque. The structure of two teeth can improve the output torque. Increasing the number of rotor poles results in a larger slot area for winding and allows a larger excitation current. However, the OR-SRM produces high pulsating torque, vibration, and noise, making it less desirable for IWM-drive applications. Researchers have employed multiphysics modeling and improved control methods to address these NVH issues [24][25].

Synchronous Reluctance Motor

Outer-rotor synchronous reluctance motors (OR-SynRM) are an under-explored class of motors for use as IWMs. Their torque ripple is smaller and they are quieter than OR-SRM. They also have higher efficiency than OR-SRM [26]. Figure 7 illustrates the topology of a OR-SynRM. Unlike OR-SRM, OR-SynRM can utilize the same stator as the PMSM motor. A sizing and design procedure of OR-SynRM is presented in [27] to optimize the flux barrier shape and rotor and stator size. The study took into account the structural analysis to ensure the rotor with flux barriers can withstand the stress at higher speeds. A PM-assisted OR-SynRM is presented in [28] for IWM applications. A novel dual-rotor SynRM [29] is designed and compared with a single rotor counterpart. The dual rotor motor showed higher power density and lower torque ripple when compared to a single rotor. The above mentioned research developed OR-SynRM with maximum power 5 kW, hence more research investigation is required to develop high power OR-SynRM for IWM-DEVs.

Induction Motor

While PM-based motors are currently the preferred choice for traction applications, induction motors (IMs) have historically been favored for industrial applications due to their robustness and low-cost and as the price of rare-earth materials has risen sharply, there have been efforts to develop IMs for use in electric traction applications [22][30]. Induction motors with distributed windings are the most prevalent, but their winding configuration is not favored due to the larger overall size of the machine and reduced magnetic core utilization, which is a huge limitation for IWM application [31]. In [32], a stator design with auxiliary slots and variable turns per coil is utilized to mitigate the spatial harmonics of the MMF. However, this approach entails a complex manufacturing process. In [33][34], the toroidal winding is proposed as a solution for reducing end-winding length in OR-IM. This enables the use of a startup strategy that entails a brief period of high current, resulting in lower power consumption. However, toroidal windings have some drawbacks: the winding process is complex and labor-intensive, which can lead to increased manufacturing costs and longer lead times and high leakage inductance, which can result in increased power loss and reduced motor performance [35].

2.1.2. Industrial State of the Art in Radial Flux Motors for IWMs

The industrial sector is dominated by outer-rotor PMSM motors for IWM applications. This is due to the high torque density owing to the rare earth PMs used in them. The trend shows that intensive research is required for torque dense motors without using RE magnets to compete in the industry. Elaphe has developed four IWM in their portfolio: S400, M700, M1100, and L1500. S400 is designed for light weight EV and HEVs and can operate at 48 V and 100 V DC link voltage providing 400 N m peak torque and maximum speed of 750 RPM and 1565 RPM at 48 V and 100 V, respectively. Elaphe’s M700 is suitable for medium weight passenger cars with peak torque 700 Nm and maximum speed 1500 RPM. M1100 provides peak torque 1100 Nm and maximum speed 1160 RPM. The most powerful in the range is L1500, with a peak torque of 1500 Nm and peak power of 110 kW . Lordstown Motor uses Elaphe’s L1500 in their all-electric pickup truck, making it the first commercially available EV driven with IWM [36][37]. All the motors are liquid cooled and utilizes NdFeB magnets for high torque density. The motor topology is outer-rotor PMSM [38].

2.2. Axial Flux Motors

Radial flux machines have been more popular since the beginning of electric motors’ origin, and overshadowed the axial flux machines. This is partly because axial flux machines are more complicated to manufacture and operate. The intense magnetic force along the axis between the stator and rotor can cause the rotor discs to bend, and the production process can be difficult due to factors such as slotted stator lamination, high expenses, and challenges in assembling the various parts [39]. However, recent trends have shown that the features of axial motors are extremely favourable for IWM-DEVs, such as compact axial length, high torque density and flexible structure [40]. Despite extensive research on the topologies of axial flux machines, there are still gaps in their practical use and certain aspects of their analysis and design that need further exploration. To mitigate uncertainties related to rare-earth materials, there is ongoing research on developing non-rare-earth AFMs for IWM-DEVs. An axial flux switched-reluctance motor (AF-SRM) has beendeveloped for use in IWM-DEVs. This motor has a higher torque density than its radial flux counterpart. Additionally, the inherent problem of high pulsating torque can be addressed by using a double-rotor structure. AFMs and RFMs require different materials due to the difference in direction of the flux. Non-oriented grain electric steel and amorphous soft material are commonly used in AFMs [41]. The axial flux motors has the following advantages:
  • Compact structure, especially short axial sizes which is ideal for IMW technology;
  • Small stator core volume, therefore reduced stator core loss;
  • Low weight;
  • High torque density;
Disadvantages of axial flux motors:
  • Manufacturing and assembly problems due to complicated structure;
  • High power density;
  • Higher cost;
  • Flexible and modular structure; motors can be stacked axially to increase the torque and power;
  • Prone to non-uniform airgap due to strong axial magnetic forces between stator and rotor.
    Small end-winding hence lower copper losses.

2.2.1. Topologies of Axial Flux Motors

Figure 1 shows the classification of axial flux-motor topologies based on the rotor and stator axial placement. According to the number of stator and rotor used and their relative position in the motor structure, the axial flux motors are categorized as single-stator single-rotor (SSSR), single-stator double-rotor (SSDR), double-stator single-rotor (DSSR), and multi-stator multi-rotor (MSMR). Like radial flux motors, the rotor of axial flux motors can either be permanent magnet (PM-based) or induction motors.
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Figure 1. Axial Flux Motors classification.

Single-Stator Single-Rotor (SSSR)

The single-stator single-rotor (SSSR) configuration is the most basic structure of AFMs. Due to their compact size and high torque capacity, SSSR-type AFMs are widely utilized in the servo drive, gearless elevators, and transportation industries [42]. However, the unbalanced axial force between the stator and rotor can cause structural deformation, generate vibration noise, and shorten the motor’s lifespan [43]. To achieve maximum rotational torque while minimizing axial force, several techniques have been proposed, including adjusting the portions of stator winding currents, utilizing complex bearing arrangements, using a thicker rotor disc, and implementing a current shifting angle [44].

Single-Stator Double-Rotor (SSDR)

AFM motors of the single-stator double-rotor (SSDR) type, which feature a slotted or slotless stator positioned between two rotors, exhibit excellent symmetry that effectively cancels out unbalanced axial forces. As a result, the motor’s vibration and lifespan can be improved over its lifecycle. Slotless machines are particularly advantageous because their end windings are shorter, resulting in lower copper loss and better heat dissipation. In addition, leakage and mutual inductance are reduced in slotless configurations, resulting in the elimination of slot effects such as flux ripple, cogging torque, high-frequency rotor loss, and stator teeth. This configuration is highly suitable for IWMs [45]. A SSDR type axial flux induction motor is designed in [46] to eliminate the magnetic axial force between the rotor and stator. The rotor design was optimized to reduce the torque ripple by introducing the skewing and rotational displacement of rotors, which also increases the power factor of the motor as well.

Double-Stator Single-Rotor (DSSR)

In double-stator single-rotor (DSSR) AFM motors, the rotor is sandwiched between two stators. The permanent magnet (PM) can be positioned on the surface or inside the rotor. Inserting the PM inside the rotor provides better protection against shock and corrosion compared to the surface-mounted PM structure [47]. The power density of the interior permanent magnet (IPM) structure is lower than that of the surface-mounted structure due to the need for a thicker rotor disc. Additionally, the leakage flux of PM ends and armature reactions are higher in the interior design due to the PMs being surrounded by ferromagnetic material. A DSSR can have either a slotted stator (SS type) or a slotless stator (NS type) configuration, with the rotor situated between the two stators. A DSSR-type axial flux switched-reluctance motor was used by [48] for IWM applications. The axial flux switched-reluctance motor has the advantage of magnetic force equalization in terms of stator balance compared SSSR, the existence of two stators results in two air gaps and a reduced peak inductance, leading to a lower power density.

Multi-Stator Multi-Rotor (MSMR)

MSMRs, also referred to as multi-stage AFPM machines, can be built using either DSSR or SSDR configurations. The MSMR-type AFPM motors consist of N stators and (N+1) rotors, enabling them to achieve higher torque and power density without an increase in motor diameter. These motors are well-suited for high-torque applications, such as ship propulsion. The stator windings of AFPM and RFPM machines can be connected in either parallel or series. In a multi-stage configuration, the torque and power density are improved without increasing the diameter of the machine. Compared to RFPM machines, multi-stage AFPM machines are easier to assemble due to their planar air gap [49].

2.2.2. Industrial State of the Art in Axial Flux Motors

YASA has created motors with a yokeless and segmented armature design. YASA 750R motor can produce a peak power density of >5 kW/kg, peak torque density >75 Nm/L with peak efficiency of >95% at its highest point [50]. The YASA design eliminates the torque ripple caused by stator slotting and reduces the mass of the motor. However, due to a longer airgap length, the winding inductance is lower, resulting in a reduced flux-weakening capability [51]. Avid Technologies, now acquired by Turntide technologies [52], produces highly efficient AFM for DC link operating voltage up to 800 V. Magnax provides an axial flux motor with high peak power density of 12.5 [kW/kg]. Emrax provides axial flux motors with 100–1000 Nm for both automotive and aerospace applications. Figure 2 shows a power trend comparison of commercially available IWMs for EVs. It can be noticed that some central-motor drives, such as the Porsche Taycan Turbo S, can provide an extremely high over-boost power of 560 kW but at the cost of high mass ( 76 kg) [53]. A general trend of high power with lower mass can be seen in axial flux IWMs. Motors such as Magnax AXF275, Turntide, and EMRAX motors [54][55][56] have also claimed to have peak efficiency over 95% with high power densities. Because of their shorter magnetic path, AFMs can produced a higher torque than RFMs [57]. IWMS have stricter requirements in terms of space as they need to fit in the wheel hub, hence their outer diameter and axial length is limited, affecting their torque/power capabilities directly. Research focused on the optimization of the motor geometrical parameters and cooling system to increase the torque capability and minimize the losses needs more attention.
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Figure 2. Power vs. mass trend of commercially available radial flux IWM, axial flux IWM, and central-motor drives for EVs.

3. Integrated Power Electronics in IWMs

As a result of their significant potential advantages, machine manufacturers are actively involved in developing and producing IMDs. Substituting inefficient motors, improving power density, reducing losses, and reduced prices when compared to separate motor and drive solutions are among the most important advantages of IMDs. According to [58], over the last decade, technological breakthroughs have resulted in the development of strong electronic components capable of withstanding the extreme environments required by certain techniques of integration. The integrated solution has minimized electromagnetic interference (EMI) and winding voltage surge induced by the surge impedence imbalance between the cables and windings by eliminating individual casings and extensive cable runs [59][60]. The volume of the whole drive can be reduced by 10–20% [61] and system costs by 20–40% [62]. Integration of an IWM with an inverter can reduce long connecting wires and additional housing, resulting in increased power/torque density [63]. According to [64] the latest advancements of wide bandgap semiconductors has improved the power density and ability to withstand higher temperature for elongated time of power electronics modules. Therefore, the integrated motor drive system is extremely appealing for EVs with very demanding electric drive volume, mass, and efficiency criteria. An integrated and modular drive topology is proposed using SSDR yokeless and segmented (YASA) AFPM in [65]. GaN switches are used to reduce the size of the converter. To prevent converter overheating, the stator and power converter components are thermally isolated. Protean has implemented an integrated motor drive technique for its in-wheel system. High fault tolerance is achieved in the Protean IWM system by utilizing four sub-motors and four sub-inverters. Only two DC connections are required for the power supply because the inverter is integrated with the motor. This design reduces the efficiency losses associated with the use of connecting cables between the motor and inverter. Nonetheless, there are concerns to be addressed during the development of integrated motor drives. The drive circuits must be capable of surviving harsh on-road conditions, such as high/low temperatures, dirt, water, and vibration, since they are sealed inside the wheel.

4. Cooling of IWMs

4.1. Air Cooling

Air cooling, which is the most fundamental method of cooling motors and is usually used to cool motors with a low heat density, can be accomplished through both natural air cooling on the outer surface and forced air cooling within the motor. A 25 kW air-cooled IMW is proposed in [66] with grooves on the outer surface to increase the cooling surface area exposed to wind. It is concluded that the densely arranged grooves in the same direction as the air flow direction provide the best cooling performance. The study was conducted at a continuous rated operation of 1250 RPM at 40 A and 76.5 Nm, and peak performance is not mentioned in the study. [67] designed and verified an air-cooled YASA AFPM motor. To enhance heat dissipation and power density, the novel cooling system utilized aluminum heat-spreading components on each armature section. The heat-spreading components offered a low-resistance heat channel to the surrounding air.

4.2. Oil Cooling

The oil-cooling method uses a pump to circulate oil as a cooling medium both inside and outside the motor, effectively reducing its temperature. Compared to air, oil has a higher convection coefficient, resulting in superior thermal performance. Additionally, oil can be sprayed directly on the windings for better heat dissipation. To investigate the effect of oil spray cooling on thermal dissipation for higher power output, an oil-cooling system is used for a 35 k W IWM. The cooling path for oil is provided by implementing a hollow shaft cooling channel suitable for IMW [68]. The shape of the oil cooling channel in the hollow shaft is optimized in [69]. The coil temperature in the optimized motor was reported to be reduced by maximum of 13.5 ∘C. [70] used a multi-objective genetic-algorithm optimization to optimize the cooling channel for oil. In high-torque motors, the current density often exceeds 20 A/mm2 which results in high copper losses. In general, copper losses account for the highest percentage of losses in the motor. The source of copper is in the winding, hence a direct cooling for the winding can improve the motor efficiency. In [71], an in-slot oil cooling for a PMSM is designed and verified for continuous operation with 25 A/mm2 and 35 A /mm2 for 30-s peak operation. Reference [72] reported that with the direct oil cooling for the end winding in AFPM, the oil extracted about 2.8 times more heat compared to the water jacket cooling and effectively double the torque and power density of AFPM as compared to indirect water cooling. However, this method is challenging to implement in outer rotor topology. In [73], a novel winding embedded liquid cooling for slotless motors is proposed. The results showed that proposed method can increase the continuous current density by 35% as compared to in-slot water cooling.

4.3. Water Cooling

The most prominent cooling method for commercial motors, particularly in electric traction applications, is liquid jacket cooling. Heat is expelled by a liquid coolant predominantly by convective heat transfer in the jacket. In most cases, the coolant is water. Glycol is often added to water to decrease the freezing point of the mixture. It is important to use the lowest concentration of glycol necessary to meet the freeze protection requirements because as the concentration of glycol in the solution rises, the effectiveness of the heat transfer fluid drops. The performance of a coolant jacket has been investigated in numerous studies with regard to the channel dimensions, cross-section shape, and flow rate. A comparison of spiral and axial type cooling water jackets has been investigated in [74]. The authors recommend the use of an axial water jacket due to its higher convective heat transfer coefficient compared to a spiral type water jacket. It should be noted, however, that axial water jackets have a more severe loss in pressure than spiral water jackets for the same amount of heat transfer area [75]. In [76] investigated the effect of flow-rate of the cooling water to optimize the electromagnetic performance of the motor. Based on this analysis, the parameters of the water channel structure are optimized with the suggested chaotic-mapping ant colony algorithm using the metropolis criteria to improve the heat dissipating efficiency of the cooling system. After optimization, an average increase of 23.57% in the convection heat transfer coefficient (CHTC), a reduction in the maximum stator temperature from 95.47∘C to 82.73 ∘C, and a 14.26% reduction in the peak temperature of the PM are observed. These improvements comprehensively reduce the risk demagnetization in the PM.

5. Control Methods Used for IWMs and IWM Driven EVs

5.1. Control of the IWMs

5.1.1. Field Oriented Control (FOC)

FOC is a popular vector-control technique that is commonly used for electric motor drives, particularly for PMSMs [77]. Direct and indirect field-oriented control (DFOC/IFOC) are two approaches to implement FOC for electric motor drives. The main difference between these two methods lies in how the motor current and voltage signals are controlled. In direct FOC, the stator current components in the d-q reference frame are directly controlled by the voltage components in the same reference frame. This is achieved by using a PI controller to regulate the error between the desired and actual stator current and voltage components. Direct FOC is often used in high-performance applications where fast and accurate torque control is required [78]. In contrast, indirect FOC separates the control of the motor’s magnetic field and torque by using a separate controller for each component. The magnetic field is controlled indirectly by regulating the stator current in the d-axis, while the torque is controlled by regulating the stator current in the q-axis [79]. Indirect FOC is simpler to implement than direct FOC and is commonly used in low-to-medium performance applications. Direct FOC provides faster and more accurate torque control, but requires more complex algorithms and hardware, while indirect FOC is simpler to implement but may not be as precise in some high-performance applications. FOC is renowned for its excellent transient response. Figure 3 shows the operating principle of FOC for speed control in PMSM. The principles for FOC are the same whether the PMSM has an inner rotor or outer rotor. FOC for AFPM is implemented using space vector modulation (SVM) technique in [80][81] for both no-load and with load torque conditions.
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Figure 3. Field oriented control block diagram for speed control in PMSM.

5.1.2. Direct Torque Control (DTC)

DTC is simpler in structure than FOC and works by directly controlling the voltage and frequency applied to the motor based on the desired torque and speed. All calculations are performed in the stator reference frame since precise rotor position information is not required in these systems, except during start-up of the PMSMs. The computational capabilities needed for the controllers are relatively low. These types of systems have superior dynamic features, react promptly to load variations, and are less influenced by alterations in motor properties and interferences. Nevertheless, their steady-state operation has high ripple levels of the stator current, flux linkage, and torque, especially at low speeds, which greatly restricts their usage for high-precision drives [82]. Ref. [83] compares the efficiency and performance of IM used in EVs using two different control methods: IFOC and DTC. The study shows that compared to the IFOC, the DTC approach offers benefits in terms of speed tracking and energy consumption. In [84] the implementation of DTC for controlling AFPM led to a reduction in torque ripples compared to the FOC method.

5.1.3. Model Predictive Control (MPC)

The use of model predictive control (MPC) has become increasingly popular due to advancements in microprocessors, enabling faster and more powerful computing. MPC offers advantages over conventional feedback control schemes, particularly when rapid dynamic response is necessary. It is a versatile control method that can be applied to various systems and is capable of accommodating constraints and non-linearities. MPC is also applicable in multi-variable cases and is easy to implement. Essentially, the MPC method involves using a system model to predict the future behavior of controlled variables, which is then utilized by the controller to determine the optimal actuation based on an optimization criterion. Predictive control offers the advantage of faster transient responses without the need for a cascaded structure that is usually employed in linear control schemes. However, one major hurdle in using MPC is the necessity for an accurate model, drive parameters, and increased sampling periods due to the large number of computations required.

5.2. Control of Vehicle with IWMs: Torque Vectoring and Torque Distribution

IWM-DEVs offer several advantages, such as an independently controllable four-wheel torque, high energy-utilization rate, and fast motor response speed. The precise control of applied driving and braking torque also enables the use of advanced control methods, such as energy-efficiency control allocation (EECA) algorithms and electric stability control (ESC) based on the coordinated control of driving motors to improve the system performance [85]. However, as an over-actuated system, the control of IWM-DEVs is critical for achieving optimal performance, as the system has more actuators than degrees of freedom. The IWM-DEV’s nonlinear dynamics and strong system coupling add to the complexity of the control problem [86]. Disadvantages of independent motor control [86]:
  • Non-linearity in the vehicle dynamics due to the coupling of longitudinal, lateral, and yaw dynamics, as well as the non-linear tire longitudinal and lateral characteristics;
  • The issue of over-actuation occurs due to the fact that there are four control inputs (torques on four wheels) which exceed the number of states requiring control. As a result, it is necessary to allocate torque based on specified objectives.
To obtain lateral stability control, intensive research has been carried out and two main control methods have been focused on:
  • Direct yaw moment control (DYC): DYC uses independent drive motors that are equipped on IWM-DEVs as actuators. By distributing torque in a specific manner, DYC generates a yaw moment that enables the control of vehicle lateral stability [87][88][89];
  • Active front steering (AFS) control: AFS offers an electronically controlled superimposition of an angle to the steering wheel angle, allowing for a continuous and driving-situation dependent adaptation of the steering characteristics. This additional degree of freedom enables the optimization of steering comfort, effort, and dynamics [90][91][92].
Moreover, there has been research into integrating both DYC and AFC methods for vehicle lateral stability. The authors in [93] developed control methods for this purpose. In [94] proposed an integrated MPC-based approach that uses both AFS and DYC to track the path and improve handling stability. In [95] authors suggested a hierarchical controller for four-wheel independent drive autonomous cars that incorporates an adaptive sliding mode high-level algorithm and a pseudo-inverse control allocation strategy, and assesses immeasurable disturbances through a fuzzy control system. An integrated control architecture is proposed in [96] that combines the MPC and sliding mode control (SMC) to achieve integrated control of AFC and DYC, using a driving state prediction algorithm for online risk assessment. There is a growing demand for research of the torque distribution control of the entire vehicle, as all wheels can be controlled independently [97]. A review of the design of torque distribution schemes was conducted in [98], using various algorithms such as genetic algorithm, BP neural network, particle swarm algorithm, and fuzzy control algorithm to distribute torque in IWM. The main conclusion drawn from the review was that the future key study areas include the combination of pertinent theoretical research findings and real car test verification under challenging and varied working situations. Additionally, safety and the thorough investigation of energy efficiency must be strengthened in the torque distribution scheme developed in the future. Extensive research is being conducted to improve the limited driving range of in-wheel motor-driven electric vehicles. Advanced high-energy dense batteries are being developed, while the torque vector control on each wheel is being implemented to reduce motor energy consumption [99][100][101][102]. A dual-MPC-based hierarchical control framework has been proposed in [103] to achieve optimum energy consumption and stability control for IWM-DEV. The upper layer of the framework allocates the torque vector to the front and rear wheel axles to allocate a high-efficiency work zone for IWM. The lower layer deploys a DYC input to provide continuous vehicle handling and cornering stability. Simulation results showed that the proposed controller can reduce energy consumption by 11.29% and 10.81% compared to traditional torque allocation combined linear quadratic regulator controller at a high-speed double-lane-change maneuver and U-turning maneuver, respectively, while ensuring vehicle lateral stability.
 

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