The first electric car in the world was made by hand in 1873 by an Englishman, R. Day Peterson. Since then, electric cars have developed rapidly, and since the late 19th century to the early 20th century, they have become almost the mainstream of automobiles. In the United States, for example, steam-engine cars accounted for 40% of the total number of cars, electric cars for 38%, and internal combustion-engine cars for only 22%. in 1915, the annual production of electric cars in the United States reached 5,000 units. However, with the development and popularity of gasoline-powered cars, electric cars have gradually declined. The reason is that electric cars have long charging time and short driving range, which limits their application. However, at the end of the 20th century, when environmental pollution was becoming more and more serious and oil resources were facing a crisis, there was a boom in the research, development and use of electric vehicles in the world. In order to encourage the production and use of electric vehicles, many countries have implemented several preferential policies: either allocating a large amount of money to the manufacturers of electric vehicles and customers who purchase electric vehicles to give financial support or subsidies; or exempting electric vehicle users from license taxes, road fees and charging only half of the electricity at night, etc. In 1990, the U.S. State Assembly of California passed a law to compel the promotion of electric vehicles. This regulation requires that: in 1998, 2% of total vehicle sales must be zero-emission polluting vehicles; by 2000, zero-emission polluting vehicles should account for 3% of total vehicle sales; in 2001, 5%; and in 2003, 10%. At present, only electric vehicles can achieve zero emission pollution.
The advantages of electric cars are.
The world's first electric car was made by hand in 1873 by an Englishman, R. Day Peterson. Since then, electric cars have developed rapidly, and since the end of the 19th century to the beginning of the 20th century, electric cars have become almost the mainstream of automobiles. In the United States, for example, steam-engine cars accounted for 40% of the total number of cars, electric cars for 38%, and internal combustion-engine cars for only 22%. in 1915, the annual production of electric cars in the United States reached 5,000 units. However, with the development and popularity of gasoline-powered cars, electric cars have gradually declined. The reason is that electric cars have long charging time and short driving range, which limits their application. However, at the end of the 20th century, when environmental pollution was becoming more and more serious and oil resources were facing a crisis, there was a boom in the research, development and use of electric vehicles in the world. In order to encourage the production and use of electric vehicles, many countries have implemented several preferential policies: either allocating a large amount of money to the manufacturers of electric vehicles and customers who purchase electric vehicles to give financial support or subsidies; or exempting electric vehicle users from license taxes, road fees and charging only half of the electricity at night, etc. In 1990, the U.S. State Assembly of California passed a law to compel the promotion of electric vehicles. This regulation requires that: in 1998, 2% of total vehicle sales must be zero-emission polluting vehicles; by 2000, zero-emission polluting vehicles should account for 3% of total vehicle sales; in 2001, 5%; and in 2003, 10%. At present, only electric vehicles can achieve zero emission pollution.
The advantages of electric vehicles are.
1) No exhaust gas is emitted during driving, so it does not pollute the environment, so electric cars can be called "zero-emission pollution cars".
2) High effective utilization rate of energy. A comparison of the total energy utilization efficiency of electric vehicles and gasoline engine vehicles by 10 driving modes.
3) Low vibration and noise, very quiet inside and outside the car.
4) Simple structure, easy to maintain and use.
The following disadvantages of electric vehicles still exist.
1) The mileage that can be driven by one charge is short. The range of an electric car with a lead-acid battery of the same quality as gasoline is only 1/70 of that of a gasoline-powered car.
2) High cost. The main reason for the high cost of electric vehicles is the high price of batteries and motor controllers. In addition, the battery life is short and the depreciation cost is high.
3) Long charging time, usually 6-10h.
Second, the composition of electric vehicles
Electric vehicles are composed of three parts: electric drive system, power supply system and auxiliary system.
The electric drive system includes electronic controller, power converter, electric motor, mechanical transmission device and wheels, whose function is to efficiently convert the electric energy stored in the battery into kinetic energy of the wheels, and to be able to convert the kinetic energy of the wheels into electric energy to charge the battery when the car decelerates and brakes. The latter function is called regenerative braking.
The power system includes the power supply, energy management system and charger, whose functions are mainly to provide drive electricity to the motor, monitor the power usage and control the charger to charge the battery.
The auxiliary system includes the auxiliary power source, power steering, navigation system, air conditioner, lighting and defroster, wiper and radio, etc., which are used to improve the car's handling and occupant comfort.
Typical electric vehicle components. The double lines in the diagram indicate mechanical connections, the thick lines indicate electrical connections, the thin lines indicate control signal connections, and the arrows on the lines indicate the direction of electrical power or control signal transmission.
There are various arrangements of systems in an electric vehicle because energy is transmitted through flexible wires rather than through rigid couplings and shafts, so there is a great deal of flexibility in the arrangement of systems or components in an electric vehicle. An example is an electric car with a front-mounted electric motor and front-wheel drive. The charger charges the battery at the rear of the vehicle via a charging port at the front of the vehicle. When the car is running, the battery supplies power to the electric motor via the controller. The signal from the accelerator pedal is fed to the controller and the output torque or speed of the motor is regulated by the controller. The torque output from the motor drives the wheels through the vehicle drivetrain.
Electric drive system
The electric drive method of electric vehicles can be basically divided into two types: electric motor central drive and electric wheel drive. Electric motor central drive system consisting of electric motor, fixed speed ratio reducer and differential, etc. In this kind of drive system, there is no clutch and transmission, so the volume and quality of mechanical transmission device can be reduced.
Another arrangement form of electric motor central drive system, which is similar to the arrangement form of front-wheel drive, transverse front engine fuel car, integrates electric motor, fixed speed ratio reducer and differential, and two half shafts are connected to two driving wheels, and this arrangement form is most commonly used in small electric vehicles.
The electric motor and fixed-ratio planetary gear reducer are mounted inside the wheels, and there is no drive shaft or differential, thus simplifying the drive train. However, the electric wheel drive method requires two or four electric motors, and its control circuit is also more complex. This drive method is more widely used in heavy-duty electric vehicles.
The drive motor of electric vehicles mainly used DC motor before 1990s. It has the advantages of large driving force when starting and accelerating, simple speed control and mature technology. However, the armature current of DC motor is introduced by brushes and commutator, which generates electric spark when commutation, the commutator is easy to ablate, the brushes are easy to wear and need to be replaced frequently, and the maintenance workload is large. There is friction loss in the contact part, which not only reduces the efficiency of the motor, but also limits the working speed of the motor.
At present, the brushless motor without commutator has been introduced, which consists of motor body, rotor angle sensor and electronic switch control circuit. Among them, the electronic switch control circuit plays the role of commutator in ordinary DC motor. DC brushless motor has many advantages of AC motor such as simple structure, reliable operation and easy maintenance, but also has the characteristics of high operating efficiency, no excitation loss, low operating cost and good speed regulation performance. Therefore, its application in electric vehicles is increasing day by day. For example, the BMW EI electric car produced by BMW and the IZA electric car developed by Tokyo Electric Power Company both use permanent magnet DC brushless motors as electric wheels.
AC induction motors are widely used in electric vehicles because they can eliminate mechanical transmission and realize stepless speed change when adopting variable frequency speed regulation, which makes the transmission efficiency greatly improved. In addition, induction motors can easily achieve forward and reverse rotation, and the recovery of regenerative braking energy is simpler. When the squirrel cage rotor is used, the induction motor also has the advantages of simple structure, durability, low price, reliable operation, high efficiency and maintenance-free.
Another type of AC motor used in electric vehicles is the AC synchronous motor. When the excitation winding of the synchronous motor is replaced by permanent magnet material, the brush and slip ring can be eliminated and there is no copper loss of the excitation winding, so it is more efficient and smaller than the induction motor.
The switched reluctance motor is recognized as an extremely promising electric vehicle drive motor. Its stator and rotor are both made of ordinary silicon steel laminated, and there is neither winding nor permanent magnet on the rotor, only concentrated winding on the stator. The switched reluctance motor has the advantages that ordinary DC motor and AC motor can not be compared: ① simple structure, durable, low cost, can work at very high speed, can adapt to high temperature and strong vibration working environment; ② large starting torque, good low speed performance; ③ wide range of speed regulation, flexible control, easy to achieve a variety of special requirements of the torque-speed characteristics; ④ in a wide range of speed and power are high efficiency in a wide range of speed and power.
The power converter for electric vehicles is used as DC-DC converter and DC-AC converter with different frequencies. DC-DC converter, also called DC chopper, is used in DC motor drive system. The two-quadrant DC chopper converts the DC voltage from the battery to a variable DC voltage and can reverse regenerative braking energy. DC-AC converters, often called inverters, are used in AC motor drive systems to convert DC power from the battery to AC power with adjustable frequency and voltage. Electric vehicles generally use only voltage input inverters because of their simple structure and the ability to convert energy in both directions.
Fourth, the power supply system
Power supply is a factor that restricts the development of electric vehicles. As an electric vehicle power supply should have high specific energy and high specific power performance to meet the requirements of the car's power and driving range. In addition, it should also have a cycle life equivalent to the life of the car, high efficiency, low cost and maintenance-free features.
The main power source currently used in electric vehicles is the battery, followed by the fuel cell. The battery is an energy storage device, which achieves energy storage through external charging; the fuel cell is an energy generation device, which generates electricity through chemical reaction. The battery technology is mature and reasonably priced, while the fuel cell is considered to be the most promising power source for electric vehicles.
The main performance indicators of the battery are: ① specific energy - the power that can be stored per unit of battery quality (W-h/kg), which is an indicator to evaluate the quality and range of electric vehicles; ② energy density - the power that can be stored per unit of battery volume (W-h/L), which affects the size of the battery; ③ specific power - the power that can be output per unit of battery quality (W-h/L); ③ specific power - the power that can be output per unit of battery quality (W-h/L). The power output of battery quality (W/kg), is an indicator to evaluate the acceleration, climbing ability and maximum speed of electric vehicles; ④ power density - the power output of unit battery volume (W/L); ⑤ cycle life - battery charging and discharging once is called a cycle, cycle life indicates the number of cycles that can be completed before replacing the battery. Short cycle life will increase the maintenance cost of electric vehicles.
1. Battery
Lead-acid batteries are widely used in electric vehicles, mainly because of the mature technology, cheap price, good reliability and high single rated voltage (2.0V). In addition, the high output current and good high and low temperature performance are suitable for electric vehicles. However, lead-acid batteries have disadvantages such as low specific energy, long charging time and short service life.
Nickel-isolated (Ni-Cb) battery has high specific power, high specific energy, fast charging, long service life, strong resistance to current shocks, wide operating temperature range (-40℃～85℃), and small voltage change in a wide range of discharge current, which makes it an attractive power source for electric vehicles. However, the high production cost (about 2 to 4 times of lead-acid batteries), the single rated voltage of only 1.2V, and the carcinogenicity of heavy metal cadmium limit its wide application in electric vehicles.
Nickel-metal hydride (Ni-MH) batteries have many of the same characteristics as Ni-Cd batteries, but since there is no cadmium, there is no heavy metal pollution, so they are called "green batteries". The cost of mass production is about four times that of lead-acid batteries. Ni-MH batteries have a rated voltage of 1.2V, with a hydrogen-absorbing alloy as the negative electrode, nickel hydroxide as the positive electrode, and a KOH solution as the electrolyte.
Sodium-sulfur (Na-S) battery has high specific power and specific energy, but its high operating temperature, coupled with the activation and corrosiveness of sodium, therefore, the structural design must ensure the robustness and safety. the Na-S battery uses molten sodium as the negative electrode, molten sulfur as the positive electrode, ceramic β-Al2O3 as the electrolyte, and serves as an ion conduction medium and isolator for the molten electrode to avoid self-discharge of the battery.
Lithium-ion (Li-Ion) batteries have developed rapidly since their introduction in the early 1990s. Although still in the development stage, Li-Ion batteries are used in electric vehicles such as the Nissan FEV, Nissan Prairic Joy and Altra. It has the advantages of high single unit voltage rating, high specific energy and energy density and long service life, and the disadvantage of high self-discharge rate.
2. Fuel cell
Fuel cell is a device that converts the chemical energy of fuel and oxidizer directly into electricity through electrode reaction. Fuel cell does not need to be charged, and as long as the fuel and oxidizer are continuously supplied from outside, it can continuously and stably generate electricity. The fuel for fuel cells for electric vehicles is hydrogen and methanol, and the oxidizer is air. Fuel cells have the advantages of high specific energy, long service life, low maintenance and continuous high-power power supply. In addition, fuel cell electric vehicles can achieve the same driving range as fuel cars. (Figure below left)
Depending on the electrolyte, fuel cells can be divided into five categories: alkaline fuel cells, phosphoric acid fuel cells, proton exchange membrane fuel cells, dissolved carbonate fuel cells and solid oxide fuel cells. Alkaline fuel cells and proton exchange membrane fuel cells are suitable for electric vehicles. In the fuel cell, fuel is used as the working substance of the negative electrode, and the oxidation reaction occurs at the negative electrode; oxygen (air) is used as the working substance of the positive electrode, and the reduction reaction occurs at the positive electrode. In an alkaline fuel cell, hydrogen and oxygen (air) are adsorbed on electrodes made of activated carbon, and the two electrodes are placed in a KOH electrolyte, and a current flows through the load if an external circuit is connected.
Using nickel as the catalyst for the positive electrode and lithium-nickel oxide as the catalyst for the negative electrode can accelerate the reaction process of the cell. Proton exchange membrane fuel cells use a solid diaphragm as the electrolyte, which is sandwiched between the positive and negative electrodes, and platinum as the catalyst for the electrode reaction.
