Basic overview and important components of electric vehicles
I. Overview
The electric vehicles described in this section refer to electric vehicles driven by batteries or fuel cells and driven on urban streets or intercity highways, excluding trolley buses and electric forklifts used in stations, docks or factory areas And ordinary battery car.
The world's first electric car was hand-made in 1873 by British R. Peterson. Since then, electric vehicles have developed rapidly. From the end of the 19th century to the beginning of the 20th century, electric vehicles have almost become the mainstream of automobiles. Taking the United States as an example, during the period of car ownership, steam engines accounted for 40%, electric vehicles accounted for 38%, and internal combustion engine vehicles accounted for only 22%. In 1915, the annual output of American electric vehicles reached 5,000. However, with the development and popularization of gasoline-powered vehicles, electric vehicles have gradually declined. The reason for this is the long charging time and short driving range of electric vehicles, which limits its application. However, at the end of the 20th century when environmental pollution was becoming more serious and oil resources were facing a crisis, countries around the world had a new wave of research, development and use of electric vehicles. In order to encourage the production and use of electric vehicles, many countries implement a number of preferential policies: or allocate a large sum of money to subsidize or subsidize manufacturers of electric vehicles and customers who purchase electric vehicles; or exempt electric vehicle users from license taxes and road maintenance fees And charge only half of the electricity charge at night. In 1990, the California State Assembly of the United States passed a law to enforce the promotion of electric vehicles. This regulation requires that in 1998, there must be 2% of zero-emission polluting cars in total car sales; by 2000, zero-emission polluting cars should account for 3% of total car sales; in 2001 it reached 5%; and Increased to 10% in 2003. At present, only electric vehicles can achieve zero emission pollution.
In recent years, electric vehicles have made great progress. With the development of science and technology, especially high-tech, electric vehicle technology has also made great progress.
The advantages of electric vehicles are:
1) No exhaust gas is emitted during driving, and it does not pollute the environment, so electric vehicles can be called "zero emission pollution vehicles".
2) High energy efficiency. Comparison of the total energy efficiency of electric vehicles and gasoline-powered vehicles according to 10 driving modes.
3) The vibration and noise are small, and the inside and outside of the car are very quiet.
4) Simple structure and easy maintenance.
Electric vehicles currently have the following disadvantages:
1) The mileage that can be driven on a single charge is short. Electric vehicles equipped with lead-acid batteries of the same quality as gasoline have a driving range of only 1/70 of that of gasoline-powered vehicles.
2) High cost. The high cost of batteries and motor controllers is the main reason for the high cost of electric vehicles. In addition, the battery life is short and the depreciation cost is high.
3) The charging time is long, generally need 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 an electronic controller, a power converter, an electric motor, a mechanical transmission device, and wheels. Its function is to efficiently convert the electrical energy stored in the battery into the kinetic energy of the wheels. The kinetic energy is converted into electrical energy and charged into the battery. The latter function is called regenerative braking.
The power supply system includes a power supply, an energy management system and a charger. Its functions are mainly to provide driving power to the motor, monitor the use of the power supply and control the charger to charge the battery.
Auxiliary systems include auxiliary power sources, power steering systems, navigation systems, air conditioners, lighting and defrosting devices, wipers and radios, etc., with the help of these auxiliary devices to improve the maneuverability of the car and the comfort of the passengers.
Typical electric car composition. In the figure, the double line indicates the mechanical connection, the thick line indicates the electrical connection, the thin line indicates the control signal connection, and the arrow on the line indicates the transmission direction of the electric power or the control signal.
There are various layouts of various systems on electric vehicles. This is because on electric vehicles, energy is transmitted through flexible wires instead of rigid couplings and rotating shafts. Therefore, the layout of various systems or components of electric vehicles is Great flexibility. For example, an electric car with an electric motor in front and a front wheel drive. The charger charges the battery placed at the rear of the car through the charging interface at the front of the car. When the car is running, the battery supplies power to the motor via the controller. The signal from the accelerator pedal is input to the controller and the torque or speed output by the motor is adjusted by the controller. The torque output by the motor drives the wheels through the automotive transmission system.
3. 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. The central drive system of the electric motor is composed of electric motor, fixed speed ratio reducer and differential. In this drive system, since there is no clutch and transmission, the volume and mass of the mechanical transmission can be reduced.
Another type of layout of the central drive system of the electric motor is similar to the layout of the fuel car with front-wheel drive and lateral front engine. It integrates the electric motor, fixed speed ratio reducer and differential, and the two half shafts connect the two. This is the most common application for small electric vehicles.
The electric motor and the planetary gear reducer with fixed speed ratio are installed inside the wheel without the drive shaft and differential, thus simplifying the transmission system. However, the electric wheel drive method requires two or four motors, and the control circuit is also relatively complicated. This drive method is widely used in heavy-duty electric vehicles. ?
Before the 1990s, the drive motors of electric vehicles mainly used DC motors. It has the advantages of large driving force when starting acceleration, simple speed control and mature technology. However, the armature current of the DC motor is introduced by the brush and the commutator, and sparks are generated during the commutation. The commutator is easy to ablate, the brush is easy to wear, need to be replaced frequently, and the maintenance workload is large. The friction loss in the contact part not only reduces the efficiency of the motor, but also limits the working speed of the motor.
At present, a commutator true-flow brushless motor has been released. It consists of a motor body, a rotor angle sensor, and an electronic switch control circuit. The electronic switch control circuit plays the role of commutator in ordinary DC motor. The DC brushless motor has many advantages such as simple structure, reliable operation and convenient maintenance of the AC motor, 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, BMWEI electric vehicles produced by BMW and IZA electric vehicles 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. This is because when induction motors use variable frequency speed regulation, they can eliminate the mechanical transmission, realize stepless speed change, and greatly improve the transmission efficiency. In addition, the induction motor is easy to achieve forward and reverse rotation, and the recovery of regenerative braking energy is also simpler. When a squirrel-cage rotor is used, the induction motor also has the advantages of simple structure, ruggedness, low price, reliable operation, high efficiency, and maintenance-free.
Another type of AC motor used in electric vehicles is an AC synchronous motor. When the excitation winding of the synchronous motor is replaced by a permanent magnet material, the permanent magnet synchronous motor can eliminate the brushes and slip rings, and there is no copper loss of the excitation winding, which is more efficient and smaller than the induction motor. . ?
Switched reluctance motor is recognized as a very promising electric motor drive motor. Its stator and rotor are made of laminated ordinary silicon steel sheets. There are neither windings nor permanent magnets on the rotor, and only concentrated windings are wound on the stator. Switched reluctance motors have advantages that ordinary DC motors and AC motors cannot match: â‘ Simple structure, ruggedness, low cost, can work at extremely high speeds, can adapt to high temperature and strong vibration working environment; â‘¡Starting torque Large, low-speed performance; â‘¢ Wide speed range, flexible control, easy to achieve various special requirements of torque-speed characteristics; â‘£ High efficiency in a wide range of speed and power.
Power converters for electric vehicles are used for DC-DC conversion and DC-AC conversion at different frequencies. DC-DC converter, also known as DC chopper, is used in DC motor drive system. The two-quadrant DC chopper can convert the DC voltage of the battery into a variable DC voltage and reversely convert the regenerative braking energy. The DC-AC converter is usually called an inverter and is used in an AC motor drive system. It converts the DC power of the battery into AC power with adjustable frequency and voltage. Electric vehicles generally only use voltage input inverters because of their simple structure and bidirectional energy conversion.
Fourth, the power system?
Power supply is a factor restricting the development of electric vehicles. As a power supply for electric vehicles, it 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 have the characteristics of cycle life, high efficiency, low cost and maintenance-free equivalent to the service life of the car.
The power source currently used in electric vehicles is mainly batteries, followed by fuel cells. The storage battery is an energy storage device, which realizes energy storage through external charging; the fuel cell is an energy generation device, which generates electrical energy through a chemical reaction. The battery technology is mature and the price is reasonable, while the fuel cell is considered to be the most promising source of power for electric vehicles.
The main performance indicators of the battery are: ①specific energy-the amount of energy that can be stored per unit battery mass (W · h / kg), which is an index to evaluate the quality and driving range of the electric vehicle; ② energy density-unit battery volume The stored power (W · h / L), which affects the size of the battery; ③ Specific power-the power that can be output per unit battery mass (W / kg), is to evaluate the acceleration, climbing ability and maximum speed of electric vehicles Indicators; ④ Power density-the power that can be output per unit battery volume (W / L); ⑤ Cycle life-the battery charge and discharge are called one cycle at a time, and the cycle life indicates the number of cycles that can be completed before replacing the battery. The short cycle life will increase the maintenance cost of electric vehicles. ?
1. Battery
Lead-acid batteries are widely used in electric vehicles. The main reasons are mature technology, cheap price, good reliability, and high rated voltage (2.0V). In addition, the large output current and good high and low temperature performance are suitable for electric vehicles. However, lead-acid batteries have the disadvantages of low specific energy, long charging time and short service life.
Nickel battery (Ni-Cb) battery has high specific power, high specific energy, fast charging, long service life, strong resistance to current impact, wide operating temperature range (-40 ℃ ～ 85 ℃), and large discharge current range Small changes in internal voltage, etc., make it an attractive power source for electric vehicles. However, the production cost is high (about 2 to 4 times that of lead-acid batteries), the single-cell rated voltage is only 1.2V, and the heavy metal cadmium is carcinogenic, which limits its wide application in electric vehicles.
Nickel-metal hydride (Ni-MH) batteries have many of the same characteristics as Ni-Cd batteries, but because there is no cadmium, there is no problem of heavy metal pollution, and they are called "green batteries." The cost of mass production is about four times that of lead-acid batteries. The rated voltage of the Ni-MH battery cell is 1.2V, the negative electrode is a hydrogen storage alloy after hydrogen absorption treatment, the positive electrode is nickel hydroxide, and the electrolyte is a KOH solution.
Sodium-sulfur (Na-S) batteries have high specific power and specific energy, but their high operating temperature, coupled with sodium's activation and corrosiveness, must be robust and safe in structural design. Na-S batteries use molten sodium as the negative electrode, molten sulfur as the positive electrode, and ceramic β-Al2O3 as the electrolyte, and as an ion conduction medium and a separator of the molten electrode to avoid self-discharge of the battery.
Lithium ion (Li-Ion) batteries have developed rapidly since they were introduced in the early 1990s. Although lithium-ion batteries are still in the development stage, they are used in electric vehicles such as Nissan FEV, Nissan Prairic Joy and Altra. It has the advantages of high rated voltage of the monomer, high specific energy and energy density, and long service life. The disadvantage is that the self-discharge rate is high.
2. Fuel cell
A fuel cell is a device where fuel and oxidant directly convert their chemical energy into electrical energy through an electrode reaction. The fuel cell does not need to be charged, as long as the fuel and oxidant are continuously supplied from the outside, it can continuously and stably generate electricity. The fuel for electric vehicle fuel cells is hydrogen and methanol, and the oxidant is air. The fuel cell has the advantages of high specific energy, long service life, less maintenance workload, and continuous high-power supply. In addition, fuel cell electric vehicles can achieve the same driving range as fuel vehicles. (Pictured below left)
According to different electrolytes, fuel cells can be divided into five categories: alkaline fuel cells, phosphoric acid fuel cells, proton exchange membrane fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. Suitable for electric vehicles are alkaline fuel cells and proton exchange membrane fuel cells. In a fuel cell, fuel acts as a working substance for the negative electrode, and an oxidation reaction occurs at the negative electrode; oxygen (air) acts as a working substance for the positive electrode, and a reduction reaction occurs at the positive electrode. In alkaline fuel cells, hydrogen and oxygen (air) are adsorbed on electrodes made of activated carbon, and the two electrodes are placed in KOH electrolyte. If an external circuit is connected, current will flow through the load.
Using nickel as a catalyst for the positive electrode and lithium nickel oxide as the catalyst for the negative electrode can accelerate the reaction process of the battery. Proton exchange membrane fuel cells use a solid membrane as the electrolyte, the membrane is sandwiched between the positive and negative electrodes, and platinum is used as the catalyst for the electrode reaction.
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