CVT(Continuously Variable Transmission) technology is a continuously variable transmission technology, which uses a transmission belt and a variable working diameter of the main and driven wheels to cooperate to transmit power. Because the CVT can achieve continuous changes in the transmission ratio, so as to obtain the best match between the drive train and the engine operating conditions, improve the fuel economy and power of the vehicle, and improve the convenience of the driver and the comfort of the occupants, so it It is an ideal automobile transmission device.
Overview of the development of CVT technology
The development of CVT technology has a history of more than 100 years.GermanyBenzThe company is the originator of the use of CVT technology in automobiles. As early as 1886, the V-type rubber belt CVT was installed on the gasoline engine cars produced by the company. In 1958, Dr. H. Van Doorne of the Dutch DAF company successfully developed a double-V rubber belt CVT named Variomatic and equipped it on the Daffodil car manufactured by DAF. Its sales exceeded 1 million. However, due to the rubber belt CVT, there are a series of shortcomings: the power is limited (the torque is limited to 135Nm or less),clutchThe work is unstable, and the energy loss of hydraulic pump, transmission belt and clamping mechanism is large, so it is not generally accepted by the automotive industry.
However, research on improving the performance of the transmission belt and the limit of CVT transmission power has been ongoing. The hydraulic torque converter is integrated into the CVT system. The clamping force of the driving and driven wheels is electronically controlled. Energy-saving pumps are used in the CVT. The transmission belt is made of metal. The belt replaces the traditional rubber belt. The new technological advancement overcomes the original technical shortcomings of the CVT system, leading to the emergence of the second-generation CVT with larger transmission torque capacity and better performance.
In the 1990s, the automotive industry paid more and more attention to the research and development of CVT technology, especially in mini vehicles, CVT was considered a key technology. The rapid development of global science and technology makes new electronic technology and automatic control technology continue to be used in CVT.
In the first half of 1997, Japan's Nissan Corporation developed a CVT for use in 2.0L cars. On this basis, Nissan developed a CVT with a manual shift mode designed for mid-size cars in 1998. The new CVT uses a newly developed high-strength wide steel belt and a high hydraulic control system. Through the use of these advanced technologies to obtain greater torque capacity, Nissan has researched and developed CVT electronic control technology. The change of the transmission ratio implements full-speed electronic control. The car can always control the engine braking according to the speed when the car is going downhill. On slippery roads, the speed ratio can be increased smoothly to prevent skidding. Nissan plans to expand the application range of its CVT from 1.0L to 3.0L cars.
Japan's Mitsubishi Corporation has chosen the CVT to transmit the power of the direct injection engine smoothly and without energy loss to drive the car. The V-belt/transmission wheel mechanism can ensure the smooth and uninterrupted transmission of engine power at all speeds. CVT eliminates the phenomenon of gear-shifting in traditional automatic transmissions, thereby obtaining more satisfactory response and control. Mitsubishi plans to use a direct injection engine (1.5L or smaller) combined with a CVT.
Japan's Fuji Heavy Industries also has 15 years of experience in developing CVT. In May 1997, Fuji Heavy Industries assembled its Vistro mini-car with a fully computer-controlled E-CVT (CVT with six-speed manual shift mode). The driver can perform six-speed transmission without operating the clutch. Fuji Heavy Industries uses an electronically controlled CVT with a lock-up torque converter on the Pleo mini-car. The slip of the torque converter can be kept to a minimum through a small range lock. The planetary gear is used to switch forward gears. Reverse gear. The transmission ratio ranges from 1:10-5.5:1.
In the first half of 1999, Ford of the United States and ZF of Germany cooperated to produce CVTs for Ford's cars and light trucks. The newly established joint venture in Batavia and Ohio will produce the CFT23 CVT designed for Ford with electronic management functions from 2001. The CVT designed by ZF is a torque converter type transmission that uses steel belts produced for front-wheel drive cars with a transverse engine. ZF can also produce CVT series for front-wheel drive cars and rear-wheel drive cars with longitudinal engines. ZF said: Compared with the four-speed automatic transmission, the CVT system can increase acceleration performance by 10% and fuel economy by 10%-15%. Compared with the lock-up torque converter, the CVT system is more efficient without leaking oil. Ford is designing a traction drive CVT that matches all light trucks in the company, including rear-wheel drive and all-wheel drive trucks. Traction drives use movable slides along special synovial fluid instead of belts and wheels. The relative position of the sliding parts determines the transmission ratio, and the power is transmitted by a layer of very thin oil between the parts.
German ZF company began to provide steel belt driven VT1 CVT for Rover 216 in mid-1999. Such CVTs include helical gears or transmissions, suitable hydraulic systems, and wet clutches. The ECU integrated in the system allows the further combination of mechanical, hydraulic and electronic systems, which makes better use of the unique advantages of various systems.
German Bosch’s electronic CVT control system is based onsensorAnd the electronic/hydraulic module based on the actuator unit control. Bosch has integrated independent components, actuators,sensorIt forms a separate module with the transmission shift ECU, and the transmission manufacturer only needs to add an integrated control unit.
How CVT works
The main structure and working principle of the CVT are shown in Figure 1. The system mainly includes basic components such as a driving wheel set, a driven wheel set, a metal belt, and a hydraulic pump. The metal belt is composed of two bundles of metal rings and hundreds of metal sheets. Both the driving wheel group and the driven wheel group are composed of a movable disc and a fixed disc. The pulley on one side close to the cylinder can slide on the shaft, and the other side is fixed. The movable disc and the fixed disc are both conical surface structures, and their conical surfaces form a V-shaped groove to engage with the V-shaped metal transmission belt. The power output by the engine output shaft is first transmitted to the driving wheels of the CVT, then to the driven wheels through the V-belt, and finally to the wheels through the reducer and differential to drive the car. When working, the driving wheel and the movable disc of the driven wheel are moved axially to change the working radius of the driving wheel, the tapered surface of the driven wheel and the V-shaped transmission belt, thereby changing the transmission ratio. The axial movement of the movable disc is achieved by the driver by adjusting the pressure of the driving wheel and the driven wheel hydraulic pump cylinders through the control system as needed. Since the working radius of the driving wheel and the driven wheel can be adjusted continuously, stepless speed change is realized.
In the hydraulic system of the metal belt continuously variable transmission, the function of the driven cylinder is to control the tension of the metal belt to ensure the efficient and reliable transmission of power from the engine. The driving oil cylinder controls the position of the driving cone wheel to move in the axial direction. The metal belt in the driving wheel group moves along the V-shaped groove. Since the length of the metal belt remains unchanged, the metal belt on the driven wheel group changes in the opposite direction along the V-shaped groove. The radius of gyration of the metal belt on the driving wheel set and the driven wheel set changes to achieve a continuous change in the speed ratio.
When the car starts to start, the working radius of the driving wheel is smaller, and the transmission can obtain a larger transmission ratio, thus ensuringDrive axleCan have enough torque to ensure that the car has a higher acceleration. As the vehicle speed increases, the working radius of the driving wheel gradually increases, and the working radius of the driven wheel decreases accordingly, and the transmission ratio of the CVT decreases, allowing the car to travel at a higher speed.
Technical characteristics of CVT
1. Economical
CVT can realize continuously variable transmission in a fairly wide range, so as to obtain the best match between drive train and engine working conditions, and improve the fuel economy of the whole vehicle.GermanpublicThe company installed 4-AT and CVT on its Golf VR6 sedan to conduct ECE urban cycle and ECE suburban cycle tests, which proved that CVT can effectively save fuel (see Table 1)
Table 1 Comparison of fuel consumption of Volkswagen Golf VR6 cars with 4-AT and CVT
Test fuel consumption
4-AT
CVT
ECE urban circulation, L/100km
14.4
13.2
ECE suburban/long-distance circulation, L/100km
10.8
9.8
90km/h uniform speed, L/100km
8.3
7.0
120km/h, L/100km
10.3
9.2
2. Motivation
The reserve power of the car determines the climbing ability and acceleration ability of the car. The greater the reserve power of the car, the better the power of the car. Due to the continuously variable transmission characteristics of the CVT, the transmission ratio with the largest reserve power can be obtained, so the power performance of the CVT is significantly better than the mechanical transmission (MT) and the automatic transmission (AT). Table 2 shows the comparison of the power performance of Chrysler Voyager cars with 4-AT and CVT respectively. The cars with CVT have better power performance.
Table 2 Comparison of power performance of Krassler Voyager sedan with 4-AT and CVT
project
4-AT
CVT
Acceleration time (s)
0-30km/h
2.5
2.5
0-100km/h
13.2
12.2
3. Emission
The CVT has a wide range of speed ratios, which enables the engine to work under optimal conditions, thereby improving the combustion process and reducing exhaust gas emissions. ZF Company installed its own CVT into a vehicle for testing, and its exhaust emissions were reduced by approximately 10% compared to a 4-AT-equipped vehicle.
4. Cost
The structure of the CVT system is simple, and the number of parts is less than that of AT (about 500) (about 300). Once automakers start mass production, the cost of CVT will be smaller than AT. As the use of this system can save fuel, with mass production and system and material innovation, the production cost of CVT components (such as transmission belts or transmission chains, driving wheels, driven wheels and hydraulic pumps) will be reduced by 20%-30% .
Future development trend of CVT technology
The future development of CVT technology can be analyzed from the following four aspects.
1. CVT components
Push-type transmission belts and transmission chains will be further strengthened in torque transmission capacity and specificity. Due to the rapid increase in the number of products, along with the further automation of the product process, the cost will be greatly reduced.
CVT dedicated hydraulic pumps will be promoted. The small electronic drive pump used for automatic tripping and emergency braking, and the engine drive pump used for normal working conditions work together to further improve the efficiency of the entire transmission.
The optimized design of the pulley will not only reduce the quality of the system and reduce the cost, but also ensure the maximum transmission torque between the driving and driven wheels and the transmission belt. The electronic control differences of different components, microprocessors and test equipment have led to very high research and manufacturing costs, which will be reduced through electro-hydraulic control modular design and large-scale production, thereby making flexible functions and low costs organic combination.
As more and more CVTs enter the market, manufacturers have begun to research and develop CVT-specific transmission working fluids, which will further optimize the working characteristics of CVTs.
2. CVT transmission
A large number of different arrangements are possible, not only due to vehicle drive differences and requirements (FWD, RWD, AWD), but also due to the continuing requirement to increase the transmission ratio coverage.
Electronic technology will bring faster and more precise control of transmission ratio, speed, pressure and torque, ensure better adjustment of engine and transmission, and provide different driving modes, such as sports, comfort and cruise control. So that users can get all-round "driving fun".
3. Integrated control of engine and CVT
More precise and faster CVT control will be integrated with engine control into the entire drive train management system, which will further reduce fuel consumption and emissions. The first CVT drive train prototype with an integrated engine management unit has been tested in a driving cycle.
4. Hybrid CVT drive train
CVT will assume an important role in the design of hybrid powertrains with flywheel energy storage devices. The fuel consumption of hybrid vehicles with CVT drive trains may be reduced by 30%, and emissions may be reduced by 50%.
Application of CVT transmission
In 1987, Japan's Subaru successfully launched a car equipped with a CVT transmission on the market. Ford and Fiat in Europe also equip VDT-CVT on cars with displacements ranging from 1.1L to 1.6L. With the development of technology, the energy crisis has triggered a global awareness of energy conservation and environmental protection. Based on the experience of the first generation of CVTs, a CVT with better performance and greater torque capacity has been developed. At present, in order to improve the competitiveness of their products, the major automobile manufacturers all over the world are vigorously carrying out the research and development of CVT. At present, NISSAN, TOYOTA, FORD, GM, AUDI and other famous automobile brands sell cars equipped with CVT transmissions. The annual output of CVT cars in the world has reached nearly 500,000. It is worth noting that the car market equipped with CVT has penetrated into the North American market from Japan and Europe at the beginning. Therefore, continuously variable transmission cars are the main trend of today's car development.
Our country has a huge automobile sales market, and the automobile industry is one of our national industries. However, the automatic transmission (AT) required by my country's automobile industry is all dependent on imports, which makes domestic cars equipped with ATs, the cost increase greatly, and equipment self-developed and produced CVT transmissions, the cost increase is not large, indicating that the CVT market prospects are exciting optimism.
At present, my country is considering the issue of developing automatic transmissions for cars. Since the "Ninth Five-Year Plan" period, the development and research of car metal belt continuously variable automatic transmission has been included in the country's major scientific and technological research plan to track the development of world technology and develop vehicles suitable for my country's national conditions.
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