CVT – continuously variable transmission with an external control, with automatically infinitely variable gear ratio, choosing the most optimal one according to the external load and engine speed, thereby making it possible to maximize the use of power.
In car transmissions there are two types of CVTs are proliferated: V-belt and the toroid.
V-belt variator consists of a number (one or two) of belt drives, which are formed by the conical pulley discs due to biasing and moving apart of which vary the pulley diameter and correspondingly, the transmission ratio. Different companies have developed their design V-belt CVT, the transmission belt is used instead of the Multitronic chain and puts Honda recruited from metal plates belt, but the principle remains the same.
Why is V-belt? The belt has a trapezoidal cross-sectional shape and the “wedged” into the pulley their side surfaces. When these surfaces are deteriorated, due to its shape, it cuts deeper into the pulley and is still in a good coupling with it.
For car breakaway conventional clutch or small torque converter is used, that is blocked after the start of movement. Pulleys Disk Management provides by electronic system of actuators, sensors and control unit.
How to change the gear ratio? The device is driving pulley cheeks when exposed to centrifugal forces gradually compressed and pushed further V-belt from a pulley center. The driven pulley while expands while it, and belt smoothly rolls closer to the center of the pulley. The larger the engine speed – the more compressed drive pulley and decompressed driven one, thereby changing the gear ratio from the crankshaft to the rear wheel.
Toroidal variator is arranged otherwise – it consists of coaxial discs and rollers transmit drive from one point to another. To change the gear ratio the rollers and their radii changing position, which rollers run in wheels. Since all force is concentrated at the contact patch, then to rotate the rollers have to use particular apparatus capable of overcoming the force pressing the roller to the disc.
CVT pros and cons
Structurally weak areas of automotive CVTs are the following: for V-belt it is belts, and for the toroidal – contact patch and drive roller, where the force of the pressure reaches 10 tons. Therefore, using special high-tech materials helps to make high-reliable CVTs, close to the conventional automatic transmission reliability. But because of the load on the belt or the contact patch CVTs cannot be used with heavy-duty engines and vehicles for the transportation of goods.
If trucks are not suitable for continuously variable transmission, the passenger car is quite acceptable, and here CVT has a great future, especially as the technology is not standing still.
If you compare the dynamic characteristics of the car was fitted with a CVT, it may be puzzling – why in the same vehicle with CVT acceleration is slower than with a manual, it should be the opposite, since the CVT better use of the engine power?
It’s all in the habit of – many motorists are very unhappy that the car with the CVT “whines the whole time on a single note.” Most drivers are accustomed to the familiar rising engine noise, and many firms are going to meet clients, especially setting up an electronic transmission control unit. In fact, during normal settings of the unit the acceleration is faster.
Note that the variable speed type is more advanced than the transmission with automatic transmissions. This is manifested in better acceleration dynamics, lower fuel consumption, a smoother ride. At the same time, CVT is simpler in design than the “automatic”.