Actually infinite

CVT: How a continuously variable transmission works

(Bild: Nissan)

Five, six, now eight or nine or even ten: the number of gears in automatic transmissions has continued to increase in recent years. The so-called CVT transmission has always had an infinite number of steps in principle - and yet it is called a "continuously variable transmission".

In the classic automatic transmission, power is transmitted via toothed or planetary gears. If these are cleverly interconnected, you get different transmission ratios. This is important in order to keep the combustion engine in the optimum speed range at all times: If it has to rev too fast, wear and consumption increase; if it revs too slowly, it develops no power and the car jerks at low revs. In the worst case, the engine dies completely.

The greater the spread between the individual ratios, the more frequently and permanently the engine can be kept in the optimum working range. Ideally, it is then capable of both rapid acceleration in a low gear and reasonably economical driving at high speeds.

In the CVT ("Continuously Variable Transmission"), the engine power is not transmitted in the direction of the axle via differently sized gear wheels, but via two pairs of conical pulleys over which a thrust link belt or a type of chain runs. The two cones with their tips pointing towards each other can change their distance from each other continuously; if they come closer together, the chain has no choice but to slip outwards, increasing its radius. If the cones move further apart, the chain "slips" downwards.

(Bild: Nissan)

Using the analogy of a cogwheel, the first case is a large diameter and the second a small diameter. Between these two poles, there are theoretically an infinite number of possible positions for the cones and therefore the chain. The pusher link belt itself consists of hundreds of individual steel elements that are connected with bands of high-alloy steel. In the transmission, it transfers the engine power to the drive axle, always in the best possible ratio depending on the speed and power requirement.

CVT cars score points, at least in theory, with low fuel consumption and high driving comfort - the gear ratio change is smooth and stepless, there are no shift jerks. One of the disadvantages is the unpleasant sound of the drive, the so-called rubber band effect. Driving speed and engine noise don't seem to go well together, as the gearbox immediately compensates for the increase in speed and the engine does not change its speed.

This is another reason why the technology has never really caught on in Europe, whereas it is a great success in Japan, for example. Nevertheless, European manufacturers such as Audi ("Multitronic"), Mercedes ("Autotronic") and DAF ("Variomatic") also have or have had it. Ford also offered them under the name "Durashift". Some manufacturers program their CVT with shifting steps to ensure a familiar driving experience and thus improve acceptance.

(Bild: Bosch)

CVT also for electric cars
Continuously variable transmissions are no longer only used in combination with the combustion engine. Bosch, for example, recently presented a continuously variable automatic transmission for electric cars, which is primarily intended for use in vehicles with high weight or high speeds. The supplier promises lower energy consumption, improved efficiency and performance and a pleasant, smooth driving experience.