A motor vehicle automatic gearbox, with infinitely-variable ratio, may be operated in a constant speed mode, or in an acceleration mode in which the motor revolution speed is increased or reduced in steps.

The variator consists of two pairs of cone pulleys, the primary set of pulleys and the secondary set of pulleys as well as of an enlacement means, which for instance can be formed as a plate link chain and which serves as a force transmissionelement. The primary set of pulleys is driven by the engine, the engine torque is transferred via the chain to the secondary set of pulleys and is guided into the axle drive. One cone pulley each is moveable on a shaft, whereby the run diameter of thechain and thus the ratio is infinitely-variable. The changes of the ratio are made jerk-free and without any interruption of the tractive force.

It is, therefore, the object of the invention to create a gearbox, which conveys an improved driving experience.

With an automatic gearbox of the type mentioned above this object is achieved according to the invention in that the revolution speed in the acceleration mode can be increased or reduced in steps.

Different from the conventional gearbox, in which all changes of the revolution speed in the acceleration mode take place in infinitely-variable manner, the revolution speeds according to the invention are predetermined so as to create a similarimpression as with a gearbox having shifting steps. If the driver triggers a change of the revolution speed by more or less strongly activating the acceleration pedal in the acceleration mode, then the transition to the new revolution speed is effectedby a jump or leap of the revolution speed, which is effected within a very short time interval. In this way the infinitely-variable automatic gearbox according to the invention reacts like a manually shifted gearbox or an automatic gearbox withregulating or shifting steps.

An infinitely-variable increase of the revolution speed in the acceleration mode corresponds to shifting down in a conventional gearbox.

In accordance with a first form of embodiment of the automatic gearbox according to the invention it is provided that during an acceleration phase the rise of the revolution speed is independent from the ratio at least in sections. In this modeof operation there is no constant ratio as is the case for example with a conventional transmission, but there are phases, in which the revolution speed is increased e.g. in infinitely-variable manner, what corresponds to a changing ratio. Aside, alsophases can be provided, in which the revolution speed can be increased or reduced in steps. Thereby, it is particularly preferred that during the starting phase of the acceleration mode the revolution speed is brought to a higher revolution speed valuein steps, subsequently, the speed can be continuously further increased in infinitely-variable manner. This variant can be called hybrid mode with infinitely-variable increase of the revolution speed.

In accordance with a second alternative embodiment of the invention, however, it may also be provided that the revolution speed can be increased during an acceleration mode with an approximate constant ratio. With this variant the impression ofan automatic gearbox with fixed gears is created and during acceleration successively several rises of the revolution speed with a respective constant ratio are passed through, in order to achieve the desired final speed. This variant can be calledhybrid mode with fixed gears.

With the hybrid mode with fixed gears it can be provided that after an acceleration phase subject to the position of the acceleration pedal a further stepped rise of the revolution speed or a stepped reduction of the revolution speed can beadjusted. In case by the position of the acceleration pedal it is signalized that the driver desires a stronger speed increase, a second volatile increase of the revolution speed can be effected, which corresponds to shifting down, in order to achievethe desired final speed in a shorter time. In the other case, when the driver activates the acceleration pedal less strongly, a volatile reduction of the revolution speed may be effected, which corresponds to shifting up. Subsequently, the desiredfinal speed can be achieved by a further, however reduced, increase of the revolution speed.

With the automatic gearbox according to the invention it is particularly convenient, if the regulating steps for the increase or reduction of the revolution speed are predetermined as a characteristic line or field, if necessary, subject tofurther values such as the position of the acceleration pedal or the speed. In this manner a regulating step is associated with each driving situation.

The automatic gearbox according to the invention can be designed and constructed such that in the acceleration mode up to achieving the maximum speed, five to ten, in particular seven regulating or shifting steps are provided. By the multiplevolatile or jump-like or step-like reductions of the revolution speed, a particularly sporty and dynamic driving impression is created.

It is not necessary that with the increase and reduction of the revolution speed the same regulating steps are passed through, but also separate regulating steps can be determined.

A particularly high ease of use can be achieved with the automatic gearbox according to the invention, if the stepped change of the revolution speed can be activated subject to the selected driving program. For instance it may be provided thatthe stepped change of the revolution speed according to the invention can be activated merely in the driving program S, which means a sporty driving characteristics. In the driving program D, however, the stepped change of the revolution speed cannot beactivated and the automatic gearbox behaves like a conventional CVT-gearbox. The appropriate driving program, in which the stepped change of the revolution speed can be activated, can conveniently and expediently be selected by activating a transmissionselector lever for the individual driving programs.

It may, however, also be provided that the stepped change of the revolution speed in the acceleration mode can be activated automatically subject to the driving characteristics. For instance, sporty driving characteristics can be detected withthe aid of the longitudinal or lateral acceleration and of further parameters. Therewith it is possible to effect the stepped change of the revolution speed according to the invention dependent upon the situation, e.g. if a sporty drivingcharacteristics has been recognized.

It may also be provided that in a conventional automatic gearbox, the stepped change of the revolution speed can be upgraded by an update of the software. The software for the gearbox control is stored in a flash memory, which can bereprogrammed if needed. In this way it is possible to update older automatic gearboxes to the updated state of the software.