How Continuously Variable Transmission works
There is no specific gear position, and the operation is similar to that of the automatic transmission. However, the change of the speed ratio is different from the shifting process of the automatic transmission, but it is continuous, so the power transmission is continuous and smooth.
There are V-type rubber belt type, metal belt type, multi-disc type, steel ball type, roller turntable type and other structures, most of which use metal belts and variable radius rollers to transmit power. Through the change of the radius of the active roller and the passive roller, the gear ratio is changed. In theory, this type of transmission is very efficient, but it must be built in the case of the power transmitted by the load. Since the friction between the steel strip and the roller is used to transmit power, the operation of the steel strip and the roller is very demanding. In order to effectively transmit power, no slippage is allowed between the steel strip and the roller, and the original heat energy has been generated. If it is slipped again, it may cause the internal parts to burn or be seriously worn out. In order to increase the static friction, the most direct way is to increase the pressure between the steel strip and the roller. However, the friction is increased, and the power transmission loss is also increased, which inevitably increases fuel consumption. And the strength of the steel strip is also a major focus. Therefore, CVT gearboxes have the advantages of comfort, high efficiency and energy saving. The disadvantage is that the current CVT gearbox cannot withstand large torques. Otherwise, it is necessary to compensate with higher fuel consumption.
The CVT (Continuously Variable Transmission) differs from the stepped type in that its gear ratio is not a discontinuous point, but a series of continuous values, such as from 3.455 to 0.85. The CVT structure is simpler and smaller than the conventional transmission. It has neither the gear pair of the manual transmission nor the complex planetary gear set of the automatic transmission. It mainly relies on the main, driven wheel and metal belt to achieve stepless change of the speed ratio.
The principle is that several sets of gears of the same size as the ordinary gearbox are combined under the control to form different speed ratios, and the pedals like bicycles rotate the wheels at different speeds by the large and small wheels and the chain. Because the different thrusts produce different thrusts for each set of gears, the output speed of the gearbox also changes, thereby achieving a slow rotation without dividing the grade.
The CVT uses a transmission belt and a ratchet wheel with variable groove width for power transmission. That is, when the ratchet changes the groove width, the contact radius of the drive belt and the driven belt on the driven wheel is changed accordingly, and the transmission belt generally uses a rubber belt, a metal belt and a metal chain. CVT is truly stepless. Its advantages are light weight, small size and few parts. Compared with AT, it has high operating efficiency and low fuel consumption. However, the shortcomings of the CVT are also obvious. The transmission belt is easily damaged and cannot withstand large loads. It can only be used for low-power and low-torque vehicles with a displacement of about 1 liter, so the automatic transmission occupancy is less than 4%. . In recent years, after extensive research by major automobile companies, the situation has improved. CVT will be the development direction of automatic transmissions.