How does a CVT transmission work?

Depending on the manufacturer they are called Ecotronic, Multitronic, Autotronic, Variomatic or Multidrive. Of course, this refers to continuously variable automatic transmissions, which are now increasingly common in modern passenger cars. CVT transmissions are often referred to as CVT transmissions, short for Continuously Variable Transmission, because the English term has also become established in German. In contrast to other transmissions, the transmission between the input shaft and the output shaft is not effected by different gear pairs, but by a push link belt, a V-belt, a plate chain or a slat chain. However, before we go into detail about the technology and its advantages and disadvantages, a brief digression is appropriate as to why a transmission is necessary at all.

Variable Automatic CVT transmission

Task of a gear unit

Every combustion engine is determined by its minimum and maximum speed. Within this range, only a limited torque is available. In order to circumvent this mechanical restriction, torque and speed are converted by the gearbox. At high speeds, the wheels ultimately turn much faster than the crankshaft. On the other hand, high torque is required above all when accelerating. The speed range between the engine’s maximum torque and its maximum power is known as the elastic range and in normal passenger cars it is between approx. 2,000 and 4,500 rpm. Conversion by the transmission thus allows the engine to operate in the elastic range, while the transmission ratio produces the desired drive speeds and torques to suit the driving situation. The transmission can also reverse the direction of rotation and thus enable reversing.

Structure of a CVT transmission

Whereas in manual and stepped automatic transmissions, gear pairs or planetary gear sets provide the power transmission and transmission ratio – large gears with many teeth mean high torque and low speed – the inner workings of a CVT transmission are clearly different. On the drive shaft, which is connected to the engine either by a torque converter or a clutch, the primary bevel gear pairing is found instead. The opposite circular cones are closer together on the shaft and taper with increasing distance. The belt, the push link belt or the chain is in the starting position between both conical pulleys at the very bottom. However, oil pressure can press one conical pulley against the other stationary conical pulley, which presses the drive belt outwards on the walls of the two conical pulleys. The radius of rotation and thus the transmission ratio changes. This principle applies analogously in the opposite direction to the output shaft. The two pairs of conical pulleys are also called variators.

The axial displacement of the two halves of the disc is effected electro-hydraulically, whereby the movable part of the secondary variator is of course diagonally opposite the movable conical disc on the input shaft. If this were not the case, the chain or belt could not run straight. The transmission control regulates the oil pressure of both variators, taking into account the selector lever position, driving program, accelerator pedal position and, of course, the driven speed. The increase in the radius of rotation on one shaft must always correspond to the decrease in rotation on the other shaft, otherwise the chain or belt will break.

Advantages of a CVT transmission in detail

A stepless conversion of torque and speed offers many advantages, especially with regard to driving comfort and efficiency. After all, the engine can always operate in the optimum range. Using a driving program such as “Eco” or “Sport”, the driver determines the maximum engine speed and the transmission does the rest. The CVT transmission is therefore theoretically both sportier and more economical than any manual transmission. After all, the optimum transmission ratio is available continuously and without interruption of tractive power. The lack of interruption in traction not only allows dynamic driving pleasure, but also avoids jerking or bouncing of the car when other automatic transmissions have to clutch. In addition, the corresponding driving program can also limit the engine’s noise development, as it can no longer tour above the specified value. Finally, a CVT transmission is relatively simple in its basic design and can be implemented at low cost and also saves space. It is therefore also ideally suited for small cars.

Disadvantages of a CVT transmission in detail

A look at the disadvantages of CVT transmissions reveals the weak points of the design. In passenger cars, the entire power transmission is often effected via a push link belt. It consists of the shear links and steel bands that hold the shear links together and make them flexible. These steel bands or steel band packages can tear if the torque becomes too high. They are also constantly bent due to the circulation in the gearbox, which can lead to material fatigue and also ends in the breaking of the shear link belt. On the other hand, plate link chains such as those used in the Multitronic from Audi can transmit torques of up to 400 Nm, but have to contend with whistling noises caused by the vibration of the chain. This problem can be suppressed by varying the length of the chain links. On the one hand, the axially movable conical pulleys must be able to be moved smoothly, but on the other hand they must be connected to the shaft in a rotationally fixed manner. This represents a design challenge, as Nissan drivers, among others, have discovered in recent years. The roller bearings press against the guide of the shaft, which wears out and eventually allows the taper disk to rotate. As a result, it can no longer be moved axially, and in the worst case, the thrust link belt will break.

Regular gearbox oil changes, as prescribed by e.g. Mercedes-Benz, should also be taken seriously, as the gearbox oil is responsible for pressure build-up and lubrication. Usually it is filtered and brought to the right temperature by a heater cooler. Whoever disregards the prescribed change intervals risks jerking and finally tearing of the thrust link belt. The engine revs up, but propulsion is not noticeable. Finally, the lack of thrust interruption and speed change can result in an unfamiliar driving sensation, which is sometimes perceived as unpleasant.

Current developments in CVT transmissions

The main focus of current development is on the reliability of the units and their maintenance intervals. After all, a gearbox oil change is not exactly favourable. At the same time, the disadvantages are being addressed by various developments. Toyota, for example, offers CVT transmissions that combine gear pairs and continuously variable transmission ratios. This means that power is transmitted positively when starting up, before the thrust link belt takes over. The driving feel of a normal gear shift can also be simulated by the electronics following predefined transmission ratios. Although this means that the advantages of the continuously variable transmission are to some extent lost, the human being is a creature of habit.

Hints for driving with CVT transmission

In addition to adhering to the prescribed transmission oil changes, there are a number of things you should consider when driving yourself. Under no circumstances should the selector lever be set to P(arken) or even R(reverse gear) when driving. Damage to the gearbox and possibly the rest of the drive would be the result. In most cases this is not even possible due to electronic monitoring. The so-called revving of the engine when the brakes are applied should also be avoided. Revving is even more dangerous in the position N(eutral), in order to change to forward gear D(rive). The transmission is not designed for such loads. Hold the car on the hill with the brake and not with the forward torque, in order not to wear out a possible clutch unnecessarily. There is generally also no reason to change to neutral at traffic lights. The transmission is capable of compensating for the continuous engine speed with a wide variety of systems – e.g. either as IVT (infinitely variable transmission) with additional planetary gear sets or via the torque converter. If you really want to save fuel at traffic lights, you’d better switch off the engine completely.

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