Text/Qin Ming
In the Chinese auto market, the most popular car brands are Japanese cars and German cars, German cars in the past two years due to many failures, its reputation has declined, and Japanese cars with stable and reliable performance, deeply trusted by Chinese consumers, especially Honda and Toyota, two major Japanese car companies. China's Honda models have always been based on CVT, such as model grid, Lingpai, Civic, CR-V and other models, all match Honda's CVT transmission.
Although the current transmission widely used in automobiles is the AT automatic transmission, using a combination of hydraulic torque converter and planetary gear system, the transmission ratio of this combination is not continuous, and the automatic transmission can only achieve stepless speed change within a range of several segments. Secondly, in order to increase the number of transmission gears and expand the range of changes in the transmission ratio, more components must be added to control the power transmission path, too many parts and structural complexity, and it is easier to increase the probability of machine failure. To this end, many car manufacturers have begun to study new automatic transmission technology, CVT is one of the most promising.
CVT transmission construction and working principle
The CVT stepless gearbox is mainly composed of active cone wheels, driven taper wheels and V-shaped metal belts, while the active wheels and driven wheels are combined, consisting of movable taper wheels and fixed taper wheels, and the V-shaped steel belt is composed of multiple wedge steel sheets installed on flexible martensitic aging steel rings. The main working principle is that the power is input from the active wheel, through the V-shaped steel belt, and transmitted to the driven wheel output.
Whether it is a driven wheel or an active wheel is composed of two parts of the spinal wheel that can be slid relatively, the V-shaped steel belt is located in the groove between the intervertebral wheels, when the two sides of the spinal wheels are tight, the grooves are narrower, the V-shaped steel belt transmission trajectory comes to the outer edge of the spinal wheels, and as the two intervertebral wheels are relatively separated and farther away, the grooves are getting wider and wider, and the steel belt movement trajectory is gradually approaching the center of the spinal wheels, that is, the place where the working diameter is the smallest.
When the car has just started and the speed is low, it needs a large torque, and the working diameter of the active wheel is smaller than the working diameter of the driven wheel, and the driven wheel can obtain a large transmission ratio, so that the car can obtain enough torque to overcome the driving resistance. With the increase of vehicle speed, the working diameter of the active wheel gradually increases, the working diameter of the driven wheel becomes smaller and smaller, and the speed ratio decreases accordingly. Since the working diameter of the V-shaped steel belt on both sides can be continuously changed, the transmission ratio of this transmission is continuously changed, which is the so-called stepless variable speed.
The amount of movement of the moving cone wheel is achieved by adjusting the hydraulic pressure attached to the main and driven cone cylinder according to the requirements of the car speed change. Controlled by hydraulic control, the relative slip opening and closing between the two parts of the spindle depends on the combined effect of the throttle opening and the transmission output shaft speed. Since the adjustment of the hydraulic pressure and the V new steel belt transmission ratio are continuously changed, stepless variable speed transmission can be realized.
CVT development history, how to break through the limit step by step?
It's hard to imagine that Mercedes-Benz, which now features 9ATs and dual clutches, will be the originator of CVTs. In 1886, The German Mercedes-Benz installed the V-type rubber belt CVT on the gasoline engine cars produced by the company.
As early as 1958, Volvo Company carried the rubber belt CVT developed by the Dutch Daf (DAF) company for the first time on the Volvo 343, but because of the rubber material, the life and reliability of the entire CVT have yet to be examined.
Later, it was not until 1987 that the Dutch VDT company used metal belt CVT to break through the original technical obstacles and achieve rapid development, and the first generation of CVT was officially born. Using hydraulic mechanical control, CVT transmits torque to. 25Nm, metal steel belt width is 24mm.
Continuous improvement, the second generation of continuously variable transmission using hydraulic torque converter, electronic control, CVT transmission torque increased to 250 Nm, metal steel belt width can be 30 mm.
Upgrade of CVT torque converter. It can be made possible to bring greatly improved transmission efficiency and transmission smoothness when starting, and in order to improve its transmission efficiency, when the speed reaches a certain value. The lock-up clutch locks the torque converter and locks the clutch joint to reduce losses.
CVT is divided into chain drive and steel belt drive, what is the difference between the two structures?
CVT has two ways of power transmission, one is steel belt, the other is the chain, in general, steel belt is usually suitable for small torque engines, and chains are usually used for high torque engines, from the actual use of performance, chain is more resistant, durable, longer service life. The world's performance-oriented CVT gearboxes have adopted chain solutions, such as the early Audi Multitronic, the current Subaru Linearartronic, Nissan's 8-speed CVT and so on.
CVT steel belt is supported by two embedded steel belts as the foundation, the metal belt is stuck with hundreds of steel sheets, using this push plate structure of the steel belt, by the thrust between the metal sheets to complete the transmission of torque. The hardness of the material between the push plates is used to form a thrust, the push plate is deformed under the action of external forces, and when the power is transmitted, the active taper wheel tightly compresses the entire side of the steel belt to form a static friction force, and the active taper wheel pushes the wheel through the steel belt, such as Toyota and Honda are driven by steel belts.
The CVT chain is structurally different from the steel belt, the chain does not directly contact the taper wheel, but by pressing the middle rocker pin, to achieve torque transmission. This structure adopts the "pull" method for power transmission, and the active cone wheel presses the rocker pin of the chain to form a static friction force and drive the chain to transmit torque. Things like Audi's Multitronic CVT and Subaru Linearartronic CVT are chain drives.
Advantages and disadvantages of steel belt tradition and chain drive
At present, the latest stepless transmission transmission torque has already exceeded to 450N·m, subaru's Forester, Auhoo, Lishi and other models of the Linearronic CVT can withstand a maximum torque of 350N·m, the maximum torque capacity of 400N·m, and Nissan's Jatcto also uses chain transmission CVT torque capacity of 380N·m.
Honda's new generation of CVT stepless transmission application of "G-Design Shift" control technology, the use of higher strength steel belt transmission, although the torque is not as good as the chain transmission, but more lightweight, compact also reduces friction, improve transmission efficiency, while improving fuel economy, to achieve a more sensitive response speed, mainly mounted on the Accord, Odyssey, Alexand and other models, the maximum withstand torque how to have 250N ·m.
As a Toyota Holding company specializing in the production of transmissions, Aisin is not only well-established in AT, but has also been providing CVT for Toyota's small-displacement models, such as the RAV4 and Corolla, which do not have much torque.
It can be seen that the maximum torque capacity of the steel belt transmission is larger than the chain transmission, because the CVT cone wheel is inclined, and the taper wheel squeeze will form an outward extrusion thrust, especially the large torque output will produce a slip situation, and once the cone wheel is depressurized, the entire CVT steel belt will be instantly scattered. Relatively speaking, the chain adopts tensile transmission, when the cone wheel is pressed, it will produce an inward pulling force, which will form a tighter and tighter trend, and the probability of slippage is relatively small, as long as the chain material can withstand, the transmission torque can be increased.
Also because of the prevention of steel belt slippage, the need for the vertebral wheel and steel belt steel sheet to squeeze, the generation of enough static friction, in order to ensure the transmission of the steel belt, the greater the static friction that can be generated, the transmission resistance will certainly increase, so the transmission efficiency will be reduced. The chain only produces local contact between the rocker pin and the cone wheel, the contact surface is much smaller, the friction and resistance are much smaller, so the transmission efficiency will be relatively higher.
When hundreds of steel plates of the chain type are working, they produce a certain amount of jitter, which will produce noise, so many chain CVTs are equipped with a buffer to reduce jitter and noise. CVT chain chain is thicker, so the torque is also greater, and the hardness of the material is far more than that of the steel belt, which is undoubtedly longer from the perspective of wear and service life.
But the chain is more complex structure and more steel sheets, and the rocker pin and contact area is smaller, requiring better materials and higher parts process, then the cost will certainly be higher, so it seems that the steel belt transmission has no advantage over the chain transmission at all, but in order to obtain profits, the lower cost price for these car manufacturers can also make more profits, so many models in the market still use steel belt transmission, but there is no need to resist the steel belt transmission CVT, after all, CVT itself has good properties, Compared with other AT, DSG and other gearboxes are the smoothest and most fuel-efficient.
In the next issue, we will explain the characteristics of the unique CVTs of Toyota, Honda and Nissan, and why is Honda's CVT more well received in the Chinese market?