It is well known that in the architecture of new energy pure electric vehicles, the vehicle controller VCU, motor controller MCU and battery management system BMS are the most important core technologies, which have great influence on the power, economy, reliability and safety of the vehicle. Important influence, there are still certain technical constraints in the three core power systems of motor, electronic control and battery, which are reported in overwhelming articles. The only thing not mentioned is the mechanical automatic transmission system, as if it does not exist, there is only a gearbox, and it can’t make a fuss.
At the annual meeting of the Gear Technology Branch of the Chinese Society of Automotive Engineers, the topic of automatic transmission for electric vehicles aroused great enthusiasm among the participants. In theory, pure electric vehicles do not need a transmission, only a reducer with a fixed ratio. Today, more and more people realize that electric vehicles need automatic transmissions. why is that? The reason why domestic electric vehicle manufacturers make electric vehicles without using transmissions is mainly because people initially misunderstood that electric vehicles do not need transmissions. Then, it is not cost-effective; the industrialization of domestic automobile automatic transmission is still at a low level, and there is no suitable automatic transmission to choose from. Therefore, the “Technical Conditions for Pure Electric Passenger Vehicles” does not stipulate the use of automatic transmissions, nor does it stipulate the limits of energy consumption. The fixed ratio reducer has only one gear, so that the motor is often in a low-efficiency area, which not only wastes precious battery energy, but also increases the requirements for the traction motor and reduces the driving range of the vehicle. If equipped with an automatic transmission, the speed of the motor can change the working speed of the motor, greatly improving the efficiency, saving electric energy, increasing the driving range, and increasing the climbing ability in low-speed gears.
Professor Xu Xiangyang, deputy dean of the School of Transportation Science and Engineering, Beihang University, said in an interview with reporters: “The multi-speed automatic transmission for electric vehicles has broad market prospects.” The electric motor of pure electric passenger vehicles has a large low-speed torque. At this time, the motor The efficiency of the electric vehicle is extremely low, so the electric vehicle consumes a lot of electricity when starting, accelerating and climbing steep slopes at low speed. This requires the use of gearboxes to reduce motor heat, reduce energy consumption, increase cruising range, and improve vehicle dynamics. If there is no need to improve the power performance, the power of the motor can be reduced to further save energy, improve the cruising range, and simplify the cooling system of the motor to reduce costs. However, when an electric vehicle starts at a low speed or climbs a steep slope, the driver will not feel that the power is insufficient and the energy consumption is extremely high, so the pure electric vehicle needs an automatic transmission.
Sina blogger Wang Huaping 99 said that everyone knows that extending the driving range is the key to the popularization of electric vehicles. If an electric vehicle is equipped with a transmission, the driving range can be extended by at least 30% with the same battery capacity. This point of view was confirmed by the author when communicating with several electric vehicle manufacturers. BYD’s Qin is equipped with a dual-clutch automatic transmission independently developed by BYD, which significantly improves the driving efficiency. It stands to reason that it is good to install a transmission in electric vehicles, but there is no manufacturer to install it? The point is not having the right transmission.
If you only consider the acceleration performance of electric vehicles, one motor is enough. If you have a lower gear and better tires, you can achieve much higher acceleration at the start. Therefore, it is generally believed that if an electric car has a 3-speed gearbox, the performance will also be significantly improved. It is said that Tesla has also considered such a gearbox. However, adding a gearbox not only increases the cost, but also brings additional efficiency loss. Even a good dual-clutch gearbox can only achieve more than 90% transmission efficiency, and it also increases the weight, which will not only reduce the power , will also increase fuel consumption. So it seems unnecessary to add a gearbox for extreme performance that most people don’t care about. The structure of the car is an engine connected in series with a transmission. Can an electric car follow this idea? So far, no successful case has been seen. Putting it in from the existing automobile transmission is too large, heavy and expensive, and the gain outweighs the loss. If there is no suitable one, only a reducer with a fixed speed ratio can be used against it.
As for the use of multi-speed shifting for acceleration performance, this idea is not so easy to realize, because the shifting time of the gearbox will affect the acceleration performance, and the power will be sharply reduced during the shifting process, resulting in a large shift shock, which is harmful to the whole vehicle. The smoothness and comfort of the device will have a negative impact. Looking at the status quo of domestic cars, it is known that it is more difficult to create a qualified gearbox than an internal combustion engine. It is the general trend to simplify the mechanical structure of electric vehicles. If the gearbox has been cut off, there must be sufficient arguments to add it back.
Can we do it according to the current technical ideas of mobile phones? The hardware of mobile phones is developing in the direction of multi-core high and low frequency. At the same time, various combinations are perfectly called to mobilize various frequencies of each core to control power consumption, and it is not just one high-performance core that goes all the way.
On electric vehicles, we should not separate the motor and the reducer, but should combine the motor, the reducer and the motor controller together, one more set, or several sets, which are much more powerful and performant. . Isn’t the weight and price much more expensive?
Analyze, for example, BYD E6, the motor power is 90KW. If it is divided into two 50KW motors and combined into one drive, the total weight of the motor is similar. The two motors are combined on a reducer, and the weight will only increase slightly. Besides, although the motor controller has more motors, the current controlled is much less.
In this concept, a concept was invented, making a fuss on the planetary reducer, connecting an A motor to the sun gear, and moving the outer ring gear to connect another B motor. In terms of structure, the two motors can be obtained separately. The speed ratio, and then use the motor controller to call the two motors, there is a premise that the motor has a braking function when it is not rotating. In the theory of planetary gears, two motors are installed on the same reducer, and they have different speed ratios. The motor A is selected with large speed ratio, large torque and slow speed. The speed of the B motor is faster than the small speed. You can choose the motor at will. The speed of the two motors is different and not related to each other. The speed of the two motors is superimposed at the same time, and the torque is the average value of the output torque of the two motors.
In this principle, it can be extended to more than three motors, and the number can be set as needed, and if one motor is reversed (AC induction motor is not applicable), the output speed is superimposed, and for some slow speeds, it has to be increased. The combination of torque is very suitable, especially for SUV electric vehicles and sports cars.
The application of multi-speed automatic transmission, first analyze the two motors, BYD E6, the motor power is 90KW, if it is divided into two 50 KW motors and combined into one drive, the A motor can run 60 K m / H, and the B motor can run 90 K m / H, the two motors can run 150 K m / H at the same time. ①If the load is heavy, use the A motor to accelerate, and when it reaches 40 K m / H, add the B motor to increase the speed. This structure has a characteristic that the on, off, stop and rotation speed of the two motors will not be involved or restricted. When the A motor has a certain speed but is not enough, the B motor can be added to the speed increase at any time. ②B motor can be used to medium speed when no load. Only a single motor can be used for medium and low speeds to meet the needs, and only two motors are used at the same time for high-speed and heavy-duty loads, which reduces energy consumption and increases cruising range.
In the design of the whole vehicle, the setting of the voltage is an important part. The power of the driving motor of the electric vehicle is very large, and the voltage is above 300 volts. The cost is high, because the higher the withstand voltage of electronic components, the higher the cost. Therefore, if the speed requirement is not high, choose a low-voltage one. A low-speed car uses a low-voltage one. Can a low-speed car run at high speed? The answer is yes, even if it is a low-speed car, as long as several motors are used together, the superimposed speed will be higher. In the future, there will be no distinction between high and low speed vehicles, only high and low voltage vehicles and configurations.
In the same way, the hub can also be equipped with two motors, and the performance is the same as above, but more attention is paid to the design. In terms of electronic control, as long as the single-choice and shared mode are used, the size of the motor is designed according to the needs, and it is suitable for micro-cars, commercial vehicles, electric bicycles, electric motorcycles, etc., especially for electric trucks. There is a big difference between heavy load and light load. There are gears automatic transmission.
Using more than three motors is also very simple to manufacture, and the power distribution should be appropriate. However, the controller may be more complicated. When one control is selected, it is used separately. The common mode can be AB, AC, BC, ABC four items, a total of seven items, which can be understood as seven speeds, and the speed ratio of each item is different. The most important thing in use is the controller. The controller is simple and troublesome to drive. It also needs to cooperate with the vehicle controller VCU and the battery management system BMS controller to coordinate with each other and intelligently control, making it easy for the driver to control.
In terms of energy recovery, in the past, if the motor speed of a single motor was too high, the permanent magnet synchronous motor had a voltage output of 900 volts at 2300 rpm. If the speed was too high, the controller would be seriously damaged. This structure also has a unique aspect. The energy can be distributed to two motors, and their rotation speed will not be too high. At high speed, the two motors generate electricity at the same time, at medium speed, B motor generates electricity, and at low speed, A motor generates electricity, so as to recover as much as possible. Braking energy, the structure is very simple, the energy recovery rate can be greatly improved, as far as possible in the high-efficiency area, while the spare is in the low-efficiency area, how to obtain the highest energy feedback efficiency under such system constraints, while ensuring braking Safety and flexibility of process transition are the design points of energy feedback control strategy. It depends on the advanced intelligent controller to use it well.
In terms of heat dissipation, the heat dissipation effect of multiple motors is significantly greater than that of a single motor. One motor is large in size, but the volume of multiple motors is dispersed, the surface area is large, and the heat dissipation is fast. In particular, lowering the temperature and saving energy is better.
If it is in use, in case of a motor failure, the non-faulty motor can still drive the car to the destination. In fact, there are still benefits that have not been discovered. That’s the beauty of this technology.
From this point of view, the vehicle controller VCU, motor controller MCU and battery management system BMS should also be improved accordingly, so it is not a dream for an electric vehicle to overtake on a curve!
Post time: Mar-24-2022