System composition of pure electric cars

The pure electric car includes three subsystems, namely electric drive subsystem, energy subsystem and auxiliary subsystem, as shown in Figure 1.

System composition of pure electric cars
Figure 1 System composition of pure electric cars

The function of the electric drive subsystem: The circuit of the electronic controller is connected to the brake pedal and the accelerator pedal, and the circuit inputs the brake pedal and accelerator pedal signals to the electronic controller to obtain the driver’s driving intention, and then control the motor to drive the car And for braking energy recovery.
The function of the energy subsystem: to supply power to the electric drive subsystem and auxiliary subsystems to ensure a stable source of energy for all parts of the car; estimate the remaining power of the power battery, and when the power battery energy is insufficient, it can prompt to charge the power battery. Replenish the energy of the car in time.
Like traditional fuel cars, pure electric cars are also equipped with auxiliary subsystems such as power steering, air conditioning, and audio. The difference is that these systems completely use the electric energy stored in the power battery. The role of the auxiliary subsystem is to complete the power steering and interior. Air conditioning temperature adjustment and night lighting functions.

Different subsystems can be divided into different parts. As far as the electric drive subsystem is concerned, it can be divided into electrical and mechanical systems. Among them, the electrical system includes subsystems such as motors, power converters, and electronic controllers; the composition of the mechanical system mainly includes mechanical transmissions and wheels. The electrical and mechanical systems of the electric drive subsystem have a variety of combinations. The basic layout can usually be divided into four types: mechanical drive arrangement, motor-drive axle combination, motor-drive axle integrated, and wheel teaching motor distributed.

(1) Mechanical drive layout
The mechanical drive arrangement means that in a pure electric car, the motor drives the car mechanically. This arrangement is based on keeping the basic structure of the traditional automobile transmission system unchanged, and replacing the internal combustion engine of a traditional automobile with an electric motor. The overall structure of the drive system is very different from that of a traditional fuel automobile. Figure 2 shows the basic principle of this arrangement.

System composition of pure electric cars
Figure 2 Mechanical drive layout

The torque output by the motor is transmitted to the transmission through the clutch. After the transmission is used to decelerate and increase the torque, it is transmitted to the final drive through the transmission shaft, and then after the differential action of the differential, the power is transmitted to the drive wheels from the half shaft to drive the car. Driving.
The working principle of the mechanical drive arrangement is similar to that of a traditional car. The clutch is used to switch on or cut off the power transmission between the drive motor and the wheels when necessary; the transmission is a set of gear mechanisms that can provide different speed ratios. The driver can follow the driving needs. To select different gears to achieve different deceleration and torque-increasing effects, so that the car can obtain high torque at low speed and small torque at high speed; the mechanical differential in the drive axle can realize the left and right wheels of the car when turning Driving at different speeds is the same as a traditional car.
The transmission of a pure electric car using this drive form can be simplified accordingly. Generally, there are two gears. There is no need to set multiple gears like the transmission on a traditional car, and there is no need to set reverse gears, but use the drive motor. The reverse rotation of the car realizes reverse driving, so its transmission is relatively simple. This structural form retains the transmission components, transmission shafts, rear axles and half shafts of traditional automobiles, saves much design work, and is relatively easy to control, and is suitable for retrofitting on the original traditional automobiles. However, due to the long transmission chain between the motor and the driving wheels, its transmission efficiency is relatively low, which also weakens the advantages of the high efficiency of the motor, but it is conducive to the research and development personnel to concentrate on the development of the motor and its control system , So the early development of pure electric cars often adopted this arrangement.

(2) Motor-drive axle combined type
Further simplification on the basis of the structure of the mechanical drive arrangement, a motor-drive axle combined configuration can be obtained, which is also the drive system arrangement widely used in pure electric cars at present.

Compared with the mechanical drive arrangement, this configuration eliminates the need for clutches and transmissions, and adopts a fixed speed ratio final drive, which simplifies the transmission system and improves the transmission efficiency. At the same time, it also improves the quality of the car’s mechanical system. The volume is reduced, which is conducive to the layout of the whole car. In addition, the use of the reducer can also improve the distribution of the operating points of the motor when the car is running, thereby increasing the efficiency of the motor. This type of drive system layout is to add a final reducer and differential at the output shaft of the drive motor end cover. The motor, fixed speed ratio reducer, and differential are combined into a drive unit, Amplify the output torque of the drive motor through the deceleration effect of the fixed speed ratio. The transmission part of this arrangement is relatively compact, high in efficiency, and easy to install.

The drive motor of a pure electric car has a relatively wide speed range. In addition, the output characteristic curve of the motor is relatively close to the ideal driving characteristic curve required when the car is traveling, and the motor-drive axle combined drive arrangement can make full use of this advantage of the drive motor. The transmission system of this configuration uses a fixed speed ratio reducer, differential and half shaft to transmit the driving torque of the motor, which simplifies the power transmission system and can effectively expand the car. The layout space of the power battery and the riding space of the car. In addition, this configuration also has good versatility and interchangeability, easy to install and use on the traditional car chassis, and more convenient to maintain. However, this type of arrangement has higher requirements for the speed regulation of the drive motor. Compared with the mechanical drive arrangement, this configuration requires the motor to provide a larger torque in a narrower speed range. According to the driving mode of traditional cars, there can be two ways: front-drive motor (FF) or rear-drive motor (RR).

(3) Motor-drive axle integral type
Compared with the motor-drive axle combined type, the motor-drive axle integrated drive system further reduces the number of mechanical transmission components of the power transmission system, thus further improving the transmission efficiency of the entire power transmission system, and at the same time, it can save a lot of space.

The overall configuration of the motor-drive axle is no longer modified on the traditional car drive system. Its structure is very different from that of the traditional car, and it has formed a unique drive system layout for pure electric cars. This configuration facilitates the use of electronic centralized control, making it possible to gradually realize automobile networking and automatic control.

The motor-drive axle integrally integrates the motor, the fixed-speed ratio reducer and the differential into a whole, and the wheels are driven through two half shafts, which is similar to the layout of the traditional internal combustion engine car with the front-wheel drive of the transverse front-wheel of the engine. According to the different connection modes of the motor and the drive axle, the arrangement of the motor-drive axle integrated drive system has two types: coaxial integrated and double integrated.

The motor shaft of the coaxial integral drive system is a specially manufactured hollow shaft. A reduction mechanism and a differential are installed at the output shaft of one end of the motor. The half shaft is directly driven by the differential, and one half shaft passes through the hollow shaft of the motor to drive the wheel at the other end.

Because this configuration uses a mechanical differential, the car is similar to a traditional car when turning, and its control is relatively simple.

The dual integrated drive system is also called a dual-motor drive system. The wheels on the left and right sides of this configuration are directly driven by two motors through a fixed speed reducer. This configuration eliminates the mechanical differential. An electronic differential is installed between the left and right motors. The electronic differential is used to meet the steering needs of the car. The speed of each drive motor can be independently adjusted and controlled. One of the outstanding advantages of the electronic differential is that it can make pure electric cars have better flexibility, and can easily introduce ASR control, and improve the passability of the car by controlling the driving torque of the wheels or active braking of the driving wheels. And dynamics on complex road conditions. In addition, the electronic differential also has the advantages of small size and low mass. It can improve the performance of pure electric cars through precise electronic control when the car is turning. Due to the addition of the drive motor and the power converter, the initial cost is increased and the structure is more complicated. Compared with the coaxial drive system, the reliability of precise control of the two drive motors under different conditions needs to be further improved. This layout is very different from the previous ones. Only when the layout of the drive system of the pure electric car is developed to this step, it is possible to fully reflect the advantages of the pure electric car.

The layout of the motor-transaxle integral drive system on the car also has two forms: front motor front drive (FF) and rear motor rear drive (RR). The integral drive system has the advantages of compact structure, high transmission efficiency, small mass, small size, convenient installation, etc., and has good versatility and interchangeability, and has been applied in small electric cars.

(4) Decentralized hub motor
On the basis of the integrated motor-drive axle, the mechanical drive system is further simplified and the mechanical transmission parts are reduced, and the decentralized configuration of the hub motor can be obtained. This driving method is to install the driving motor in the wheel hub, and the output torque of the motor directly drives the driving wheel to rotate, thereby realizing the driving of the car.

This arrangement also cancels the half shaft in the motor-drive axle integrated drive arrangement, and its structure is simpler and more compact, and the overall car mass is smaller. Compared with traditional automobiles, the in-wheel motor distributed pure electric automobile completely releases the space occupied by the mechanical power transmission system of the traditional automobile, so that there is enough space for the power battery, luggage compartment, etc. to be arranged. At the same time, it can also independently control each drive motor, which is conducive to improving the steering flexibility and active safety of the car. It can make full use of the adhesion of the road surface and facilitate the introduction of electronic control technology. This kind of arrangement can better reflect the advantages of pure electric cars than the arrangements described above. The problem that must be solved by using the in-wheel motor distributed power system is how to ensure the directional stability of the car. At the same time, the drive motor of the power system and its deceleration device must be able to be arranged in the limited wheel space, and the volume of the drive motor is required. Smaller.

The in-wheel motor distributed pure electric car is a major research hotspot at present, but this configuration has not appeared in recent years. As early as 1900, Porsche developed a front-wheel-drive two-seater pure electric car called Lonard. The two front wheels of the car were equipped with hub motors. Later, because internal combustion engine cars are significantly better than pure electric cars in terms of driving range and power performance, internal combustion engine cars have become the mainstream, while pure electric cars have slowed down the pace of development to a large extent. In-wheel motors are distributed pure electric cars. Therefore, the automobile did not continue to be researched and developed, and it did not move toward industrialization.

At present, many automobile manufacturers, universities, research institutes, etc. in various countries have conducted a lot of research on in-wheel motor distributed pure electric cars. The Chinese University of Hong Kong has developed a four-wheel drive and four-wheel steering multi-directional sports car. It realizes in-situ steering and lateral movement by controlling the drive and steering of the four wheels. The focus is on using the motor efficiency map to optimize the torque distribution of the four-wheel drive. The control strategy to achieve the purpose of energy saving, and achieved good results.

At present, Nissan’s FEV and Ford’s Ecostar have adopted the decentralized arrangement of in-wheel motors. General Motors also said that it will adopt this arrangement in its pure electric cars and hybrid cars. The decentralized in-wheel motor is the development trend of the layout of the pure electric car drive system in the future.