Principle and Defects of Continuously Variable Transmission

 　　Principle and Defects of Continuously Variable Transmission
The automatic transmission uses a planetary gear mechanism to change gears. It can automatically change gears according to the degree of the accelerator pedal and changes in vehicle speed.The driver only needs to manipulate the accelerator pedal to control the vehicle speed.   
Generally speaking, there are several types of automatic transmissions commonly used in automobiles: hydraulic automatic transmissions, hydraulic transmission automatic transmissions, electric transmission automatic transmissions, stepped mechanical automatic transmissions, and continuously variable mechanical automatic transmissions.Among them, the most common one is a hydraulic automatic transmission.The hydraulic automatic transmission is mainly composed of a hydraulically controlled gear transmission system, which mainly includes two parts, an automatic clutch and an automatic transmission.It can automatically shift gears according to the opening of the accelerator and changes in vehicle speed.  
Continuously Variable Transmission (CVT): 
The continuously variable transmission is composed of two sets of variable speed wheels and a transmission belt.  
Therefore, it has a simpler structure and a smaller volume than the traditional automatic transmission.In addition, it can freely change the transmission ratio, so as to realize the full-range stepless speed change, so that the speed of the car changes smoothly, without the "pause" feeling of traditional transmission.  
The continuously variable transmission is a kind of automatic transmission, but it can overcome the shortcomings of ordinary automatic transmissions such as "sudden shift", slow throttle response, and high fuel consumption.  
CVT transmission has the characteristics of light weight and short size, good fuel economy, smooth power output, etc., and is gradually being used by more manufacturers, but new technologies will also bring new problems.  
Will the CVT continuously variable transmission car you drive encounter the following situations when driving?For example, when you start in D block, you will be cautious every time you start, waiting for the red light every morning to make you feel flustered; when following the car, step on the brakes to move forward and make your heartbeat.Although the symptoms are not serious, they can shock you when they appear suddenly.  
CVT-type continuously variable transmission is a continuously variable automatic transmission that has only gradually emerged in cars in recent years. Because of its unique technology and short time to enter the market, it will naturally encounter some relatively new problems.Recently, it is occasionally heard that domestically-made Audi cars with Multitronic continuously variable transmission will have the following symptoms:  
1. Braking problem: When braking at a slow speed, the speed will flicker when it is about to stop but not stop, suddenly drop to a very low level, only about 200 to 300 revolutions, and then rise again (that is, 1/10 second).When idling to slow braking, you can notice that the tachometer pointer suddenly drops and recovers quickly in about 1/10 second.It feels like the vehicle suddenly picks up or sprints forward.This phenomenon is more obvious with softer braking.  
2. Acceleration problem: Audi with Multitronic continuously variable transmission feels slower when accelerating, and sometimes the speed doesn't change much even at higher throttle, and then it suddenly rushes out.  
3. Another phenomenon is that the car does not move when starting with forward gear (D), and it is useless to step on the accelerator.At this time, you must first engage the neutral gear (N) and then the forward gear (D) before you can start normally.  
The above problems do not appear all the time, but appear randomly, so they are more troublesome.When the above problems occur, keep a cool head and don't panic to prevent misoperation.After the above situation occurs, pay attention to it as soon as possible, and repair it as soon as possible. If it is within the warranty period, the manufacturer can treat it free of charge. (The Audi warranty period is 1 year or 2 kilometers, whichever comes first.)  
The solution is to replace the valve body in the transmission and further reset the control program.The price of the valve body is more expensive, about 16000 yuan, plus the transmission oil and other auxiliary materials and labor costs, which will cost a total of nearly 2 yuan.The maintenance time is about 3 hours.Symptoms can disappear after replacing the valve body, but it is better to reset the program.  
In foreign countries, individual vehicles equipped with continuously variable transmissions will also encounter some strange problems.For example, a MINI owner reported that the engine speed of the CVT MINI car he was driving would suddenly rise to 60~3200rpm when driving at a steady speed of 3400km/h. At this time, the gear is D gear instead of S gear in sports mode.  
No matter whether it is a cold car or a hot car, the above situation has been encountered.The engine speed remains at 50rpm at a constant speed of 70km/h or 2000km/h.The above situation is disturbing. After going to the repair shop to upgrade and re-identify the control software, the fault was finally resolved.Another 03 CVT Audi owner also encountered problems encountered by domestic owners, occasionally the engine speed was rough when it was about to stop.  
The structure of the CVT is completely different from that of the automatic gearbox. Its interior is mainly based on two variable-diameter transmission wheels and a transmission belt on the middle to transmit torque.Both ends of the transmission belt are wound on a conical wheel disc, and the outer diameter of the wheel disc changes steplessly, so that the transmission ratio between the two wheels changes from "large ratio small" to "small ratio large". This realizes variable speed.  
The electronic commands for shifting are all executed by the electronic hydraulic control system. If a small link in the control system goes wrong, it will affect the driving of the vehicle.It seems that new technologies will also encounter new problems, but each technology always has to go from naive to mature.
It is relative to the automatic transmission on the market, because now the automatic transmission cars also have gears, one by one up and down, but the computer is used to control the up and down.But it’s different for CVT. There is no shifting. Just like an adjustable desk lamp, it can be a little brighter or brighter. It is not a step-by-step light. The CVT is so big. The advantage is that there is no impact when changing gears.And its transmission is completed by a variable diameter metal belt (CVT steel belt) instead of hydraulic oil or gears.Audi's multitronic? uses a continuously variable transmission.

XNUMX. What is CVT?
CVT means continuously variable transmission, its English full name is Continuouslv Variable Transmission, or CVT for short.It was the Dutch who invented this kind of variable speed transmission mechanism, and the transmission with its device is also called a continuously variable transmission or a continuously variable transmission.The big difference between this type of transmission and ordinary automatic transmission is that it eliminates the need for complex and heavy gear combined variable transmission, and only uses two sets of pulleys for variable transmission.The design concept is very ingenious by changing the contact radius of the driving wheel and the driven wheel transmission belt.Because CVT can achieve continuous changes in the transmission ratio, it can get a good match between the drive train and the engine working conditions, improve the fuel economy and power of the vehicle, and improve the convenience of the driver and the ride comfort of the occupants, so it is Ideal automotive transmission.The CVT car also has its own gears, such as parking gear P, reverse gear R, neutral gear N, forward gear D, etc., but the car forwards automatically shifting gears very smoothly, without the feeling of sudden jump. 
At present, only four models of Audi, Fit, Palio (Siena, Weekend Wind) and Cowin can be seen in the domestic market that adopt CVT technology.

Second, the working principle of CVT
The main structure and working principle of 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 type 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, so as to ensure that the drive axle can 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 decreases, and the working radius of the driven wheel increases accordingly, and the transmission ratio of the CVT decreases, allowing the car to travel at a higher speed.

Third, the technical characteristics of CVT
1. Economy
The CVT can achieve continuously variable transmission in a wide range, so as to obtain a good match between the drive train and the engine working conditions, and improve the fuel economy of the vehicle.
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, it can obtain a transmission ratio with a large reserve power, so the power performance of the CVT is significantly better than the mechanical transmission (MT) and the automatic transmission (AT).
3. Emission
The CVT has a wide range of speed ratios, which enables the engine to work in good working 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 4% compared to a 10-AT-equipped vehicle.
4. Cost
The structure of the CVT system is simple, and the number of parts (about 500) is less than that of AT (about 300). Once the car manufacturer starts 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% .

Fourth, the advantages and disadvantages of the heavily used stepped automatic transmission (AT)
The heavily used hydraulic automatic transmission (AT) does not need a clutch to shift gears, with few gear changes and large changes, and a smooth connection. Therefore, it is easy to operate, which brings convenience to drivers and comfort to riders.But there are many shortcomings. First, it responds slowly to speed changes and is not sensitive to manual waves. Therefore, many car users like to drive manual waves. Second, fuel consumption is uneconomical, transmission efficiency is low, and torque range is limited. In recent years, electronic control technology has been introduced. This problem has been improved; the third is that the mechanism is complex and repairs are difficult.The high-speed circulating hydraulic oil in the torque converter will produce high temperature, so use the specified high-temperature hydraulic oil.In addition, if the car cannot be started due to lack of battery power, it cannot be started by a cart or trailer.If hauling a faulty vehicle, pay attention to keeping the driving wheel off the ground to protect the automatic gear from damage.

Five, the difference between CVT and ordinary automatic transmission
Ordinary people often confuse the two concepts of automatic transmission and continuously variable transmission (CVT).In fact, the two transmission structures and working principles are completely different. The CVT structure is simpler and smaller than the traditional transmission. It has neither the numerous gear pairs of a manual transmission nor the complicated planetary gear set of an automatic transmission.It mainly relies on two sets of variable speed wheels to achieve stepless change in speed ratio.The old CVT mostly used rubber belts as transmission elements, but the disadvantages were limited force, easy to slip, and low transmission efficiency.Therefore, it can only be used on motorcycles and mini-cars with less horsepower.Many automobile companies have been committed to solving this problem for more than ten years.Today, Audi is the first to "lead the times" in this technology. It uses a unique multi-piece chain belt drive and an excellent computer-controlled hydraulic system to transmit powerful torque. The big feature of CVT is that it is fuel-efficient. CVT abandons the traditional automatic transmission's hydraulic transmission that wastes energy and adopts new technology to improve fuel usage. The fuel usage rate of CVT will be higher than that of manual transmission and automatic transmission, and it can save 5% to 15% of fuel than traditional automatic transmission vehicles.
With the continuous advancement of technology and the continuous maturity of CVT technology, the manual transmission (MT) and the stepped automatic transmission (AT) will eventually be replaced by the CVT. The continuously variable transmission is the main trend of today's automobile development.

What is a continuously variable transmission?How does it work?
Now, among domestic and imported cars, there is a relatively new term-CVT, that is, continuously variable transmission.Transmission is a type of automatic transmission. Since the application was relatively narrow in the past, not many people know it.
The English full name of continuously variable transmission is Continuouslv Variable Transmission, CVT for short, and the transmission with its device is also called continuously variable transmission or continuously variable transmission.The big difference between this type of transmission and ordinary automatic transmission is that it eliminates the need for complex and heavy gear combined variable transmission, and only uses two sets of pulleys for variable transmission.The design concept is very mysterious by changing the contact radius of the driving wheel and the driven wheel transmission belt.
It was the Dutch who invented this variable speed transmission mechanism.The V-shaped rubber belt was used for the conveyor belt at the beginning, and the material was poor and the transmission torque was small, so it had no practical value. In 1979, Dutch DAF engineers switched to metal belts, perfecting and developing a practical CVT system.It has the advantages of fewer parts, small size, light weight, higher transmission efficiency and lower fuel consumption compared with ordinary automatic transmissions.But the shortcomings are also obvious, that is, the transmission belt is easily damaged and cannot bear a large load, but it is very marketable in small-power cars.Fuji, Fiat, and Ford of Germany have all introduced CVTs, improved them, and used them in their cars. In 1984, the Fuji car factory, which is good at applying new technologies, developed an electronic CVT ECVT with a computer device and installed it on the Justy (J1987) car in February 2 and launched it on the market.
Take Fuji car as an example, CVT is composed of 4 major subsystems, namely electromagnetic clutch system, electronic and hydraulic control system, direction conversion (forward/reverse) system, and continuously variable transmission system. The core part is the continuously variable transmission system (CVT). System).The main components of the continuously variable transmission system are two pulleys and a steel belt. The steel belt is sleeved on the two pulleys.The pulley is composed of two wheel walls in a figure eight shape. The groove in the middle of the two wheel walls forms a V shape. One of the wheel walls is controlled by a hydraulic control mechanism, which can be separated and drawn according to different engine speeds. , The V-shaped groove also widens or narrows accordingly, raising or lowering the steel belt, thereby changing the diameter of the contact between the steel belt and the pulley.The two pulleys are adjusted in the opposite direction, that is, when the groove of one of the pulleys gradually becomes wider, the groove of the other pulley will gradually become narrower. 
   
Continuously Variable Transmission System (CVT)
In this way, when the car is running at a slow speed, the groove width of the active pulley can be made larger than that of the passive pulley, and the circumferential radius of the steel belt of the active pulley is smaller than the circumferential radius of the steel belt of the passive pulley. Park, so it can transmit a larger torque.When the car turns to high speed gradually, one side wall of the driving pulley moves inward, and the groove width becomes smaller to force the steel belt to rise up to the high top, while the side wall of the passive pulley is just the opposite, moving outward to enlarge. The groove width forces the steel belt to lower, that is, the circumferential radius of the active pulley steel belt is greater than the circumferential radius of the passive pulley steel belt, and it becomes a large circle with a small circle, so the speed requirement of the car at high speed can be guaranteed.
The continuously variable transmission car also has its own gears, such as parking gear P, reverse gear R, neutral gear N, forward gear D, etc., but there is no sudden jump when the car is automatically shifted forward, and it is very stable.
Due to the development of material technology in recent years, the endurance of metal belts has been improved, so that CVT is no longer limited to the application of small-power cars, but also has its traces in high-end cars. The Audi A1999 6-liter sedan launched in the European market in 2.4 was the first to be equipped with a CVT.According to the factory, the high torque of Audi's 2.4-liter V6 engine is 230 Newton meters, while the high torque output of the CVT is 299 Newton meters, which is more than enough.At the same time, the continuously variable transmission has a wide transmission ratio, which is far more flexible than manual transmissions and ordinary automatic transmissions. The key technical structure is simple. You only need to change the circumferential radius of the pulleys at both ends of the metal belt to change speed. The parameters of the transmission can be changed at any time to adapt. The needs of different regional markets.Of course, CVT still has its application limitations. The key is still the bearing capacity of the metal belt. Therefore, it can only be applied to light vehicles at present.
Recently, CVT assembly and introduction are being carried out on some domestically-produced cars, such as Audi, Guangben's Fit, and Chery's Cowin and so on.

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