A critical review of radial field in-wheel motors: technical progress and future trends

In-wheel motors (IWMs) are considered ideal drivetrains for electric vehicles (EVs), but their applications remain preliminary. In particular, the torque density of IWMs cannot meet the performance requirements of all vehicle types. This review reports the evolutionary progress of IWMs toward torque density improvement and discusses four critical technologies together for the first time: deceleration mode, electromagnetic topology, heat dissipation, and in-wheel structure. The direct drive, outer rotor, and water cooling IWMs are well-suited to most passenger vehicles. Furthermore, the adaptability of IWMs to vehicle types is analyzed. Medium and large passenger and sport utility vehicles have limited installation space for the reducer and largely depend on IWMs’ torque. When the torque weight density of an IWM with structural components improves, IWMs will be adopted widely. Further evolution of IWMs will involve employing novel materials, refined design optimization, and seamless structural integration. Novel materials will enhance the torque output capability and transcend existing limitations. The intelligent design optimization balances torque and efficiency, achieving the required energy conversion quality. The degree of structural integration determines the weight and reliability of the entire IWM and its auxiliary parts.