This paper introduces the development, types and characteristics of electric wheel drive technology, and the advantages of electric wheel drive technology, probes into the key technical problems in current electric wheel drive technology and the development trend of electric wheel electric vehicles, and puts forward some corresponding development suggestions. Keywords: electric wheel; Electric car; Driving Technology (I) Introduction With the increase in the number of cars and the increasing shortage of energy, people's awareness of environmental protection has gradually increased. Cars bring convenience, speed and comfort to human beings, but also cause increasingly serious environmental pollution and increasing energy shortage, and the leading position of fuel engines in modern automobile power systems is gradually shaken. At present, electric vehicles are becoming more and more popular as the only means of transportation that can achieve zero emissions. As an important development direction of electric vehicles, electric wheel technology has attracted more and more attention from automobile developers because of its unique technical advantages. As an independent driving component, the electric wheel is a unique driving unit, which integrates the motor transmission mechanism, brake and hub. Electric vehicles with electric wheel technology generally have the advantages of flexible control, compact structure, environmental protection and high transmission efficiency. (II) The development of electric wheel drive technology The earliest electric wheel structure was invented by American Robert in the early 1950s, and its structure is shown in figure 1. The hub device comprises a motor, a reduction mechanism and a brake. The output torque of the motor is transmitted to the input shaft of the reduction mechanism. After deceleration, the increased torque is transmitted to the rim, which finally drives the wheel to rotate. This structure was first used in large-scale mining dump trucks, and was introduced by General Electric Company of the United States in 1968. In 1970s, China also began to develop large-scale mining electric wheel dump trucks. Since 1977, Hunan Xiangtan electric machinery factory successfully developed the first prototype of electric wheel dump truck, a series of electric wheel dump trucks have been produced one after another. At present, the performance of electric wheel dump trucks in China is improving day by day, and some models have reached the international leading level. In the early 1990s, Tsinghua University Light Electric Vehicle Research Group first applied the idea of electric wheels to the development of electric bicycles, and developed the electric wheels with half-axle birdcage structure, thus becoming the first unit in the world to apply the transmission structure of electric wheels to electric bicycles. This kind of electric hub adopts brush motor, reduction gear and clutch. Half-axle birdcage structure is to expand the middle axle, that is, the middle section of bicycle axle, into a "birdcage", and divide the axle into two sections, that is, the left and right half-axle structure, in which a disc motor is placed. The characteristic of this "birdcage" is that the motor is well protected, and there is no external force acting on the motor except the working torque. Its structure is shown in Figure 2. The internal structure of the whole hub is exquisite and compact, with a total weight of 35kg and a volume of φ190 mm×10 mm. Electric wheel electric vehicles are considered to have incomparable advantages over centralized motor-driven electric vehicles and traditional electric vehicles, and are an ideal choice for high-end fuel cell vehicles in the future. Many automobile companies and research institutions in the world are conducting research on electric wheel electric vehicles. Since 199 1 Japanese applied for a patent in the United States, Japan has been in a leading position in the research of electric wheels for electric vehicles. (III) Structural types and characteristics of electric wheels According to the driving types of electric wheels, electric wheels can be divided into deceleration driving type and direct driving type. High-speed motor with inner rotor is often used for electric wheels driven by deceleration, which generally has high speed and low torque. In order to meet the actual speed requirements of wheels, it is usually necessary to match the corresponding deceleration mechanism. The deceleration mechanism is generally installed between the motor and the wheel to decelerate and increase the torque, so as to ensure that the electric vehicle can obtain enough torque at low speed. The electric wheel driven by deceleration has the advantages of high specific power, light weight, high efficiency, low noise and low cost. However, due to the high rotating speed of the motor, it is necessary to reduce the rotating speed with a reduction mechanism to obtain a larger torque, so the mass of the whole electric wheel as a unsprung weight is still heavier than that of the traditional diesel locomotive. Most of the reduction mechanisms are planetary gear reduction devices with compact structure and large reduction ratio, and some of them use external meshing cylindrical gear reduction devices, but the axial size is too large and the radial mass distribution is uneven. In order to reduce the unsprung weight of electric wheels, direct-drive electric wheels appeared. This kind of electric wheel cancels the deceleration mechanism in the deceleration drive electric wheel, which greatly reduces the unsprung weight and simplifies the structure of the whole electric wheel. This kind of electric wheel mostly adopts external rotor motor, and the external rotor is directly installed on the rim of the wheel to drive the wheel to rotate. However, electric vehicles generally need large torque when starting, that is to say, the motor installed in the direct-drive electric wheel must have good torque characteristics and can provide large torque at low speed. In addition, it must have a wide range of torque and speed adjustment. The outer rotor motor for the direct-drive electric wheel has the advantages of simple structure, small axial size, capability of controlling torque in a wide speed range, quick response and high efficiency because there is no reduction mechanism. In order to get more torque, it is necessary to increase the size and mass of the motor, but the cost is high, the efficiency is very low and the noise is very high when accelerating. (IV) Advantages of electric wheel drive After the electric vehicle adopts electric drive technology, the power transmission between the energy source and the driving motor adopts flexible cable, which gets rid of the design constraints of traditional mechanical transmission and brings many advantages to the whole vehicle: (1) With electric wheel technology, the power of a single motor can be distributed to multiple motors under the same power demand. Therefore, the requirements for electrical and mechanical transmission parts can be reduced, which is convenient for design and production. On the large mining truck, the electric wheel structure is used to realize the large torque that is difficult to be transmitted by mechanical transmission. (2) The clutch, transmission, transmission shaft, differential and other parts are eliminated, which simplifies the transmission system and is beneficial to the realization of the automobile lightweight goal; Because the processing cost of precision mechanical parts is reduced, the production cost of the whole vehicle is expected to be reduced; The wheels are directly driven by motors, or even integrated into a whole, which is convenient for electromechanical integration. (3) Because the mechanical transmission part is cancelled, the transmission efficiency of the electric vehicle is improved compared with the electric vehicle with mechanical transmission system. (4) Improve the passing performance of automobiles. This mainly comes from two aspects. On the one hand, the simplified transmission system can improve the ground clearance of vehicles. On the other hand, the measures of all-wheel drive and independent control of driving wheels can make maximum use of the ground adhesion ability. (5) Flexible cable link is adopted between the electric wheel and the power supply, which occupies less space and the layout design of the whole vehicle is very flexible. For electric buses, it is convenient to realize low-floor luggage and more flexible passenger position design, and there is also room for batteries. The design freedom of vehicle mass distribution is great, and the axial load distribution is more reasonable. (V) Key technologies of electric wheels Electric wheels have broad application prospects in electric vehicles due to their own structural characteristics. However, at present, the key technology of electric wheel has not been completely broken, mainly including the following four key technologies: (1) Developing a motor with wide speed regulation range, large torque variation range and compact structure. (2) Cooling, sealing and anti-vibration technology of rescue motor. (3) Develop a reducer with high efficiency, compact structure and light weight. (4) Electronic differential with high reliability and good performance. (VI) Development trend of electric wheels The promotion of electric wheels in automobiles is mainly restricted by two factors. On the one hand, we should solve the key technology of electric wheel; On the other hand, after the key technologies are solved, the cost of electric wheels should be greatly reduced, and users can accept the increased cost after using electric wheels. There are still many problems to be solved in adopting electric wheel technology in automobiles, which will not be popularized soon. The comfort and speed of cars are relatively high, and the increase of unsprung weight caused by electric wheels will lead to the decrease of ride comfort, which needs to be further solved. The automobile has a wide speed range, and the constant speed ratio reducer is adopted, which requires high torque of electric wheels and has certain technical difficulties. The diameter of automobile wheels is small, which not only makes it difficult to install electric wheels, but also has many problems to be solved in sealing, cooling and vibration resistance of electric wheels. Electric wheel technology is used more and more in buses. The wheel speed of the bus is low. After using the fixed speed ratio reducer, the performance of electric wheels can meet the requirements of vehicle driving performance; After the electric wheel structure is adopted for passenger cars, especially low-floor passenger cars, the function that the original central transmission structure can only be completed by the main reducer and the auxiliary reducer can be easily realized, which not only simplifies the transmission system, but also helps to solve the influence of unsprung weight on ride comfort caused by electric wheels. The cost increase brought by electric wheels accounts for a small proportion in the bus cost, which is acceptable to users. (7) Conclusion As a new technology, the electric wheel technology has a compact structure, which can improve the driving performance and driving performance of the vehicle and is beneficial to the layout of the whole vehicle. It has broad application prospects in light vehicles such as electric bicycles, electric vehicles or heavy mining vehicles. Although some key problems in electric wheel technology have not been completely solved, compared with traditional cars, electric vehicles using electric wheel technology do have many incomparable advantages. Therefore, electric vehicles with electric wheel technology are the development direction of electric vehicles in the future. References: 1. Peng Qian. General situation and trend of development of large electric wheel dump trucks [J]. Mining Machinery, 2000 (2): 12- 13. 2. Song Youchuan, Jin. Types and characteristics of electric wheels [J]. Urban Public Transport, 2004 (4): 16- 18. 3. Chen Yong, Zhang Jianrong and Zhang Daming. Application and development of electric wheel technology in electric vehicles [J]. Mechanical Design and Manufacturing, 2006 (10):169-171.