Modern electric vehicles can be generally divided into three categories: pure electric vehicles (PEV), hybrid electric vehicles (HEV) and fuel cell electric vehicles (FCEV). But in recent years, a plug-in hybrid vehicle, PHEV for short, has been derived from the traditional hybrid vehicle. In this paper, some problems and trends in electric vehicle technology research and development are briefly introduced and commented.
2. Pure electric vehicles
Pure electric vehicle refers to an electric vehicle driven entirely by a power battery. Although it has a long history of 134 years, it has been used only in some specific fields, and the market is very small. The main reason is that all kinds of batteries generally have serious shortcomings such as high price, short life, large volume and weight, long charging time and so on. See table 1 for the actual performance indexes and average market prices of lead-acid batteries, nickel-hydrogen batteries and lithium-ion batteries currently in use. According to the actual cycle life and market price, we can estimate the cost of electric vehicles per 1kWh of various power batteries. When calculating, it is assumed that the highest chargeable state of charge (SOC) of the battery is 0.9 and the discharge SOC is 0.2, that is, the actual available battery capacity only accounts for 70% of the total capacity; The power supply price from the power grid is 0.5 yuan/kWh, and the average charging and discharging efficiency of the battery is 0.75.
According to rough calculation, although it only takes 0.5 yuan/kWh to get electricity from the power grid, the cost of providing 1 kWh of electricity after lead-acid battery charging is about 3.05 yuan, including 2.38 yuan for battery depreciation and 0.67 yuan for power grid supply, while the cost of providing 1 kWh of electricity by nickel-hydrogen battery is 9.6 yuan and 65,440 yuan respectively.
At present, the domestic market uses diesel engines to generate electricity, and the price is roughly in 3 yuan/kWh. If a gasoline engine is used to generate electricity, the price of power supply is estimated to be 4 yuan/kWh, that is, the price of electric energy provided by lead-acid motors is roughly equivalent to that of diesel engines. Considering only the cost of obtaining energy, lead-acid batteries have a certain price advantage over gasoline engines, but because they are too bulky and take a long time to charge, they are only widely used in various rail cars, golf carts and golf carts with a speed of less than 50 km/h. Practice has proved that lead-acid batteries are highly competitive and practical in this low-end product market.
The main advantage of Ni-MH battery is its relatively long life, but the price of Ni-MH battery remains high because metallic nickel accounts for 60% of its cost. Lithium-ion battery technology has developed rapidly. In recent 10 years, its specific energy has increased from 100Wh/kg to 180Wh/kg, its specific power can reach 2000W/kg, its cycle life can reach more than 1000 times, and its working temperature range can reach -40 ~ 55℃. .
In recent years, due to a major breakthrough in the research and development of lithium-ion iron phosphate battery, the safety of the battery has been greatly improved. At present, many developed countries have taken lithium-ion batteries as the main direction of power batteries for electric vehicles. China has the advantage of lithium resources. In 2004, the output of lithium batteries accounted for 37. 1% of the global market. It is predicted that after 20 15, the performance-to-price ratio of lithium-ion battery is expected to reach the level that can compete with lead-acid battery and become the main power battery of electric vehicle in the future.
Figure 1 shows the quantity/performance and cost performance curves of all kinds of pure electric vehicles at home and abroad. Because of its low cost and low performance vehicles, such as electric bicycles, the annual output of China reached 20 million vehicles in 2006 alone. Although the impact 1 electric sports car produced by American general motors company has reached high power, only 50 cars have been produced because of its high price. The annual output of low-performance rail cars in China is about 7000 ~ 8000; The mini electric vehicle produced by Tianjin Qingyuan Electric Vehicle Company has a top speed of only 50km/h and an annual output of more than 1000, which may be the upper limit of the performance of pure electric vehicles acceptable in the market at present. The above electric vehicles are powered by lead-acid batteries. With the increasing performance-to-price ratio of high-performance lithium-ion batteries, in the next 5 ~ 10 years, there may be high-performance pure electric vehicles with a top speed ≥ 100km/h and a driving range ≥250km in the market.
3. Hybrid electric vehicles
Because the performance-price ratio of pure electric vehicle driven by power battery is much lower than that of traditional internal combustion engine vehicle for a long time, it is difficult to compete with traditional automobile. Since 1990s, major automobile companies have started to develop hybrid vehicles. Toyota Corporation of Japan took the lead in introducing Prius hybrid vehicles to the market on 1997, and achieved great success in Japan, the United States and European countries, with a cumulative production and sales volume of more than 600,000 vehicles. Subsequently, Honda in Japan, Ford in the United States, General Motors and some large European companies also introduced various types of hybrid vehicles to the market.
3. 1 Necessity of developing full hybrid electric vehicles
Hybrid electric vehicle refers to a vehicle with more than two power sources, one of which can release electric energy. According to different hybrid modes, hybrid vehicles can be divided into series type, parallel type and series-parallel type. According to the degree of mixing (the ratio of motor power to internal combustion engine power), it can be divided into three types: micro-mixing, light mixing and full mixing. Among them, the external belt drive starter/generator (BSG) is a typical structure of micro hybrid electric vehicle, and its motor power is generally only 2 ~ 3 kW, which can save fuel by 5 ~ 7% depending on the engine stop and fuel cut-off functions. Adding a motor/generator disk motor (ISG) at the rear end of the engine crankshaft is a typical structure of a mild hybrid electric vehicle. With pure electric drive function, it can be regarded as a typical full hybrid vehicle or series-parallel hybrid vehicle. Toyota Prius belongs to this kind of fully hybrid car. At present, the hybrid electric vehicles developed by several domestic automobile enterprises mostly adopt ISG light hybrid or BSG micro hybrid scheme, mainly because the technical difficulty and production cost of these two schemes are relatively low. However, according to the research, the fuel saving rate of hybrid electric vehicles is almost proportional to the mixing degree of automobile power and automobile production. Therefore, in the long run, it is an inevitable trend to develop full hybrid electric vehicles.
4. External plug-in hybrid electric vehicle
Plug-in hybrid vehicle is the latest generation of hybrid vehicle, which has been widely concerned by governments, automobile companies and research institutions in recent years. Experts at home and abroad believe that PHEV is expected to be widely used in a few years.
According to statistics, more than 80% of French urban residents drive less than 50km, in the United States, more than 60% of car drivers drive less than 50km, and more than 80% of people drive less than 90km. PHEV is especially suitable for wage earners who only drive to and from work five days a week with a mileage of 50 ~ 90km. PHEV adds pure electric driving conditions to hybrid electric vehicles, and increases the capacity of power battery, so that PHEV can travel 50 ~ 90 km under pure electric driving conditions. If this mileage is exceeded, the internal combustion engine will be started and the hybrid drive mode will be adopted. Therefore, the battery capacity of PHEV is generally 5 ~10 kW h, which is about 30 ~ 50% of the battery capacity of pure electric vehicles and 3 ~ 5 times that of ordinary hybrid vehicles. It can be said that PHEV is a transitional product between hybrid electric vehicle and pure electric vehicle. Compared with traditional internal combustion engine vehicles and common hybrid electric vehicles (HEV) (see Table 5), PHEV relies more on power batteries to drive vehicles, so its fuel economy is further improved, and the emissions of carbon dioxide and nitrogen oxides are less. Due to the increase of power battery capacity, the price of each car is at least 2000 dollars higher than that of the general HEV.
Figure 3 shows four different types of buses, and their battery capacity is compared with the car price, fuel consumption and exhaust emissions. It can be seen that with the increase of battery capacity, car prices will rise, but fuel consumption and exhaust emissions will decrease. Therefore, it can be considered that electric vehicles save fuel and reduce emissions at the cost of using and wearing batteries, and the substantial improvement of the cost performance of power batteries will be the key to the rapid popularization and use of electric vehicles.
Generally, the SOC of HEV power battery only fluctuates in a small range (for example, 2% ~ 3%), so the cycle life is very long, while the SOC of PHEV power battery inevitably fluctuates in a large range (for example, 40%), which belongs to deep charging and deep discharging, so the cycle life is much shorter, similar to that of pure electric vehicle (PEV). At present, PHEV uses advanced lithium-ion batteries. It can be seen from the table 1 that the energy consumption of lithium-ion battery is 1kWh, which is equivalent to three times the energy consumption of internal combustion per kwh. With the global oil price rising, the energy consumption of fuel-fired internal combustion engines will also increase, while the energy consumption of lithium-ion batteries will decrease with the technological progress and output expansion.
5. Fuel cell electric vehicles
As early as 1839, Englishman Grover put forward the principle of hydrogen-oxygen reaction to generate electricity. In the 1960s, liquid hydrogen and liquid oxygen fuel cells were developed, which were first used in aerospace and military applications by UTC Company of the United States. In recent 20 years, due to the oil crisis and the increasingly serious air pollution, the fuel cell technology represented by proton exchange membrane has attracted universal attention from all countries in the world. Major multinational automobile companies have invested heavily in developing various types of fuel cell electric vehicles (FCEV).
5. 1 main advantages of proton exchange membrane fuel cell (PEMFC)
(1) Its emission products are water and steam, with zero pollution;
(2) the energy conversion efficiency can reach 60-70%;
(3) No mechanical vibration, low noise and low heat radiation;
(4) 75% of the mass of the universe is hydrogen, and hydrogen is almost everywhere on the earth. Hydrogen is also the lightest chemical element with the best thermal conductivity and flammability.
(5) The calorific value of hydrogen is very high, and the calorific value of 1kg hydrogen is equivalent to that of 3.8L gasoline.
5.2 Technical and economic problems of fuel cell electric vehicles
In China, the Ministry of Science and Technology listed the research and development of fuel cell buses and fuel cell vehicles as "863" major scientific and technological projects in the Tenth Five-year Plan and the Eleventh Five-year Plan. And has made a series of major scientific and technological achievements, but in years of scientific research practice, some technical and economic problems have also been exposed:
(1) Fuel cell engines have short durability.
Generally, it only takes 1 0,000 ~10,200 hours (up to 2,200 hours abroad). When the fuel cell vehicle travels 40 ~ 50 thousand kilometers, the power will drop by 0 ~ 40%, which is very different from the traditional internal combustion engine that can travel more than 500 thousand kilometers.
(2) The manufacturing cost of fuel cell engines is still high.
Generally, it is estimated to be 30,000 yuan /kW (the cost abroad is about 3,000 US dollars /kW), which is a huge gap compared with the traditional internal combustion engine of only 200 ~ 350 yuan /kW. Because the key components such as proton exchange membrane, carbon paper, platinum metal catalyst, high-purity graphite powder, hydrogen recovery pump and pressurized air pump are all imported, there is no cost advantage compared with foreign countries;
(3) The fuel cell engine has poor adaptability to the working environment.
Domestic products can work at the temperature of 0 ~ 40℃, there is icing problem below 0℃, and overheating above 40℃ can not work normally; In addition, it is very sensitive to dust, carbon monoxide and sulfide in the air, and platinum catalyst is easy to be polluted and poisoned.
(4) The use cost of fuel cell vehicles is too high.
For example, high-purity (99.999%) high-pressure hydrogen (>: 200 buses cost about 80 ~ 100 yuan/kg. According to 1kg hydrogen can generate 10 kw h electric energy, only the fuel cost is about10 yuan /kWh, and the depreciation cost is 30 yuan /kWh according to the working life of fuel cell engine 1000 hours. Therefore, the total power cost is as high as 0kWh in 40 yuan. As can be seen from the comparison table 1, at least at present, the cost of providing 1kWh electric energy by fuel cell engines is far higher than that of various power batteries, which reflects that there is still a considerable distance for fuel cell vehicles to be used as the power source of automobiles.
5. Research hotspot of fuel cell electric vehicle.
Although there are so many problems, fuel cell is still the cleanest, quietest and infinitely renewable energy invented by human beings so far, which deserves our greater efforts to realize the industrialization of fuel cell electric vehicles.
6. Motors and electric wheels
Electric vehicle drive motor is an essential key component of all electric vehicles. At present, there are four kinds of motors widely used: DC brushless motor, permanent magnet brushless motor, AC induction motor and switched reluctance motor. Most electric vehicles developed in the United States and Germany use AC induction motors. The main advantages are low price, high efficiency and light weight, but the starting torque is small. Almost all electric vehicles developed in Japan use permanent magnet brushless motors. Its main advantage is that its efficiency can be 6% higher than that of AC induction motor, but it is more expensive. General permanent magnet materials can only resist heat below 120℃. Switched reluctance motor is novel in structure, and has the advantages of simple structure, reliability, low cost, good starting performance and no large impulse current. It has the advantages of AC induction motor frequency conversion speed regulation and DC motor speed regulation, but the disadvantage is that it is noisy, but there is still room for improvement.
However, for electric vehicles, because electric energy is provided by various batteries, it is expensive and precious, so it is more reasonable to use the most efficient permanent magnet brushless motor, which has been widely used in modern electric vehicles with power less than 100kW.
In addition, more and more foreign electric vehicles adopt advanced electric wheels (also known as hub motors), and the wheels are directly driven by motors (mostly permanent magnet brushless), so there are no complicated mechanical transmission parts such as gearbox, transmission shaft and drive axle of traditional cars, and the structure of the car is greatly simplified. However, the motor is required to have great torque at low speed, especially for military off-road vehicles, and the basic speed ∶ maximum speed of the motor is required to be1∶10. In recent years, the United States, Britain, France, Germany and other countries have applied electric wheel technology to military off-road vehicles and light tanks, and achieved great results. For example, the US Marine Corps developed a new hybrid off-road vehicle based on Hummer, named "Shadow", and adopted electric wheel technology. Compared with the traditional Hummer, under the same reconnaissance test conditions, Hummer consumes 472kg of fuel, while Shadow only consumes 200kg. On the same off-road section, Hummer ran for 32 minutes, while Shadow only took 13 minutes and 50 seconds. In addition, it also has the advantages of silence and no "thermal trace" in pure electric mode. With such excellent performance, it is reported that the US military has decided to stop producing the traditional Hummer and replace it with a new hybrid electric wheel-driven "shadow" military vehicle. This important development trend should be highly concerned.
7. concluding remarks
(1) Lead-acid battery is still the main power battery in the low-end electric vehicle market because its power supply cost is roughly equivalent to that of diesel engine. Lithium-ion phosphate battery technology has made rapid progress, and it is most likely to become a competitor of lead-acid battery and take the lead in becoming the main power battery in the high-end electric vehicle market.
(2) Because the hybrid electric vehicle only needs to be equipped with the power battery with the capacity of pure electric vehicle110, the cost performance of the whole vehicle is close to the market, and it will still be the main type of electric vehicle to realize industrialization in the near future. Considering China's national conditions, we should still vigorously promote the use of hybrid buses to further reduce manufacturing costs and reduce fuel consumption and emissions;
(3) After the cost performance of lithium-ion battery is further improved, plug-in hybrid electric vehicle (PHEV) is expected to become an ideal passenger car for office workers, which can greatly reduce fuel consumption and emissions, but it may be popularized and applied in developed countries first because of its high price;
(4) Although the fuel cell is an ideal clean energy source, its performance-price ratio is too low at present. It is a long way to go to achieve the cost-effective ratio of entering the market, and there must be a major breakthrough in basic materials and basic theories before entering the automobile market.