main content:
1. The basic structure of pure electric vehicles
A pure electric vehicle refers to a new energy vehicle that uses a power battery as an energy storage power source, and provides electrical energy to the drive motor through the power battery to make it run, thereby driving the electric vehicle forward. Its basic structure is shown in Figure 1.
Figure 1 Basic structure of a typical pure electric vehicle
Compared with gasoline vehicles, pure electric vehicles have the following advantages:
①Zero emission, zero pollution, low noise;
②Simple structure, easy to use and maintain;
③The energy conversion efficiency is high, and the braking energy can be recovered, thereby improving the utilization efficiency of energy;
④ It can use the "valley electricity" of the power grid for charging at night, and play a role in stabilizing the peak-to-valley difference of the power grid.
The key technologies of pure electric vehicle research and development mainly include power battery, drive motor, motor control, body and chassis design and energy management technology. From the perspective of electrical composition, pure electric vehicles can be divided into mechanical subsystems, power electronic subsystems and information subsystems.
1) Mechanical subsystem
The mechanical subsystem consists of chassis and body, drive unit, transmission and battery box. The current technical trend of pure electric vehicle mechanical subsystems is to centrally arrange all power and transmission systems on the chassis. Chassis and body separation design has become the mainstream trend of design and manufacturing. With the gradual popularization of modern control technology, there can be no mechanical links and hydraulic pipeline connections between the body and the chassis. This design concept is of great help to the overall layout of the vehicle, and can easily realize the complete integration of pure electric vehicles. Modular Design and Manufacturing of Vehicles
2) Power electronic subsystem
The power electronic subsystem consists of power battery, motor and its controller. The energy of the power battery is output to the motor through a power conversion device or an inverter device to make the vehicle move forward or backward. When supplementary power is required, the power battery can be charged through the grid-connected charging device. When the vehicle is braking, the drive motor works in a power generation state, and the energy is transferred from the motor to an energy storage device, such as a battery or a super capacitor. Under normal circumstances, the drive motor will change its working state under the control of the motor controller to meet the requirements of the working conditions. In order to reduce the volume and weight of the drive system, devices such as motors and inverters are designed and manufactured as a whole using mechatronics technology.
3) Information subsystem
The information subsystem handles the actions of the driver and monitors the operation of the vehicle and the status of the power supply, motors, controllers, and charging units. The information subsystem of the electric vehicle undertakes the task of coordinating the internal work of the vehicle. The vehicle controller of the pure electric vehicle collects data from pedals, batteries, motors and various sensors related to speed, temperature, etc., and integrates and calculates these data to determine the current working state of each part of the vehicle. The control command is used to complete the control of the entire vehicle, and the drive system, energy system, auxiliary system, etc. have their own control devices to ensure the execution of the corresponding commands.
2. The power system of pure electric vehicle
When the accelerator pedal demand information enters the vehicle controller, the vehicle controller converts the driver's driving intention into a torque request for the motor and sends it to the motor controller. The motor controller controls the power output of the inverter. Adjust the torque or speed output of the motor, and the output torque of the motor drives the vehicle through the transmission system to meet the driver's demand for driving power. When the vehicle is braking, the vehicle controller obtains the driver's braking demand by collecting the signal of the brake pedal, and then converts part or all of the braking demand into a power generation request for the motor according to the vehicle braking distribution control strategy. The motor controller will make the motor run in the power generation state according to the power generation request issued by the vehicle controller, and convert part or all of the kinetic energy during vehicle braking into electric energy and store it in the power battery, so as to realize the auxiliary vehicle braking and energy recovery. Purpose. Usually, the braking of electric vehicles is completed by the electric braking of the motor and the traditional mechanical braking. The motor braking mainly starts the auxiliary function, so the energy recovered is only a part of the total braking energy. Part of the kinetic energy that can be recovered will be converted into heat energy and dissipated by the traditional braking system. The energy recovery function can improve the efficiency of energy use and prolong the driving mileage of the whole vehicle.
Due to the diversity of electric drive characteristics of pure electric vehicles, pure electric vehicles can have various power system architectures. The following are the power forms of several common pure electric vehicles.
Figure 2 Common forms of EV power
In Figure 2(a), an electric motor, a single-speed transmission and a differential constitute the powertrain of a pure electric vehicle. The power system structure utilizes the characteristics of the constant torque of the motor in the low speed stage and the constant power in a wide range of speed changes, and uses a single speed ratio reducer to replace the multi-speed reducer. The power system uses a single speed ratio. After the reducer is installed, the requirements for the clutch are correspondingly reduced, so that the clutch can be canceled; the advantage of this transmission system is that the volume and mass of the mechanical transmission can be reduced, and the control of the drive system can be simplified; but the disadvantage of this system is that it cannot be used to change the work. In order to meet the requirements of vehicle acceleration/climbing and high-speed working conditions at the same time, it is usually necessary to select a larger power motor.
The power transmission system shown in Figure 2(b) is similar to the traditional fuel vehicle, except that the internal combustion engine is replaced by an electric motor, and together with the clutch, transmission and differential, it constitutes the power drive system of the vehicle. The electric motor outputs the driving force, and the clutch can realize the disconnection or connection of the driving force and the driving wheel. The transmission provides different transmission ratios to make the speed power (torque) curve match the load requirements. The differential is mainly used to realize the ability to turn The wheels on both sides of the vehicle are driven at different rotational speeds.
In Figure 2(c), the electric motor, the speed reducer with fixed speed ratio and the differential are further integrated, and can even be combined into one component, and the driving half shaft of the wheel is directly connected to the combined body, so that the drive system is further simplified and miniaturization. This solution is also the most common form of drive in pure electric vehicles at present.
In Figure 2(d), the mechanical differential is canceled, and the two electric motors drive the wheels on their respective sides through the reducer with a fixed speed ratio. The speed of rotation is high, so as to realize the normal turning of the vehicle.
Figure 2(e) shows the dual-motor fixed-gear direct drive mode. The drive motor and the fixed-speed planetary gear reducer are installed in the rim of the wheel. This drive system can also be called a wheel-side drive system. The drive system can be further simplified by using wheel-side transmission. The main function of the planetary gear reducer in the drive system is to reduce the speed of the motor and increase the drive torque.
In Figure 2(f), the drive system completely abandons the mechanical connection between the motor and the driving wheels, and the motor drives the wheels directly. The speed control of the motor is equivalent to the wheel speed control, that is, the vehicle speed control. Such a drive system structure puts forward special requirements for the motor, such as requiring the motor to have high torque characteristics when the vehicle accelerates or decelerates, so a low-speed outer rotor type motor is generally selected.
Figure 3 shows another drive structure of a pure electric vehicle—a dual-motor four-wheel drive system.
Figure 3 Structure of dual-motor four-wheel drive system
For pure electric vehicles with dual electric four-wheel drive, the front and rear wheels are driven by electric motors through differentials. The system can use different electric motors to drive the vehicle under different working conditions, or according to a certain The torque split ratio uses a combination of two electric motors to drive the vehicle together, maximizing the efficiency of the drive system.
The structure of a pure electric vehicle with a single power source is shown in Figure 4. Pure electric vehicles have only one energy source composed of power batteries and have a simple structure. Since they do not use internal combustion engines, they do not need any fuel injection devices, complex engine control units and various peripheral devices connected to the engine and transmission. The number of parts is reduced and the reliability of the complete vehicle is improved. Pure electric vehicles have excellent environmental performance and can achieve zero emissions during driving. In addition, pure electric vehicles are very quiet during driving, which can largely avoid and reduce the noise pollution of vehicles to the surrounding environment.
Figure 4 Structure of a pure electric vehicle with a single power source