GB2303185A

GB2303185A - Method of controlling a CVT to reduce wheel slip

Info

Publication number: GB2303185A
Application number: GB9617111A
Authority: GB
Inventors: Peter Baeuerle; Gerhard Keuper; Joachim Luh; Karl-Heinz Senger; Peter Weiberle
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-03-31
Filing date: 1994-03-30
Publication date: 1997-02-12
Anticipated expiration: 2014-03-30
Also published as: GB9617111D0; GB2303184A; GB9617051D0; GB2303185B; GB2303183A; GB9617050D0; GB2303184B; GB2303183B

Abstract

The invention relates to a method for operating a motor vehicle having an electronically controlled, continuously variable transmission (CVT), wherein, by means of a manually operated actuating device 20, a continuous adjustment of the transmission ratio occurs. The adjustment preferably occurs at a constant rate of change and may be carried out with or without engine control such that the engine speed remains within permissible limits.

Description

DESCRIPTION METHOD FOR OPERATING A MOTOR VEHICLE HAVING A CONTINUOUSLY

VARIABLE TRANSMISSION

2303185 The invention relates to a method for operating a motor vehicle, in particular a passenger motor vehicle, having an electronically controlled, continuously variable transmission (CVT = continuously variable transmission).

The present application is a divisional out of GB2276683 (Application No. 9406276.7) which relates to a method of operating a motor vehicle having an electronically controlled CVT in which the engine braking effect is exploited to maintain or substantially maintain a measured vehicle velocity.

Reference is also made to two related divisional applications. relates to a method of operating a motor vehicle having an electronically controlled CVT wherein the transmission ratio is adjusted to support braking of the motor vehicle. RW7C)5n. 1 relates to a method of operating such a motor vehicle wherein a manually operated device is used to continuously adjust the transmission ratio.

Transmissions having continuous transmission ratio adjustment using hydraulic controls are available on the market. These transmissions, which -2are installed in motor vehicles in particular in passenger motor vehicles, have the disadvantage that the cost of controlling the transmission ratio adjustment is relatively large. Therefore, generally only the variable characteristic curve is produced.

Development is currently being carried out on continuously variable transmissions which are electronically controlled and wherein different performance characteristics (for example for an economy mode, normal mode or sport mode) are available, improving the drivability and the attractiveness of the motor vehicle.

In accordance with the invention, there is provided a method for operating a motor vehicle having an electronically controlled, continuously variable transmission (hereinafter referred to as "the CVT11), wherein any slip at the driven wheels of the motor vehicle is detected and, in dependence thereupon, the transmission ratio is continuously adjusted at the transmission in such a manner that the slip is reduced or completely eliminated (hereinafter referred to as ',the slip control operation,').

Preferably, in addition it is ascertained whether a braking operating is taking place and, in the braking operation, continuous adjustment of the transmission occurs, changing the transmission ratio so as to support the braking of the motor vehicle (hereinafter referred to as "the braking support operation").

Preferably, in addition, by means of a manually operated control device a continuous adjustment of the transmission ratio of the gears is performed, the adjustment being carried out within the limits of the largest and the smallest transmission ratio for the duration of the manual operation (hereinafter referred to as the "inching operation,').

The method of operation in accordance with the invention has the advantage that additional functions in the driving operation can be achieved, which enhance the usability of the motor vehicle and the safety and the enjoyment of driving.

It is possible to detect any slip at the driven wheels of the motor vehicle and, in dependence thereupon, to continuously adjust a further, in particular a smaller, transmission ratio at the transmission in such a way that the slip is reduced or removed completely. This is a slip- control operation. Safety is improved.

It is further possible, in addition to ascertain whether a braking operation is taking place and to continuously adjust the transmission during braking, changing the transmission ratio, in particular increasing it, to support the braking of the motor vehicle. This is a a braking-support operation. By virtue of the corresponding continuous adjustment of the transmission ratio of the transmission, an engine braking effect is produced in addition to the braking effect of the braking system of the motor vehicle.

Finally, it is possible, in addition to carry out continuous adjustment of the transmission ratio of the transmission by means of a manually operated actuating device, wherein the adjustment is carried out within the limits of the largest and the smallest transmission ratio for the duration of the manual actuation (inching operation). This adjustment of the transmission ratio is particularly carried out during normal driving. It is not stepped, as in the prior art, so that a switching process is carried out from one gear to the next, but rather it occurs continuously, and the driver carries out the adjustment by actuating the actuating device (for example a key or keys). It is possible to provide a selector key in a special passage of the switching console, where it forms the actuating device.

It is advantageous if, during the brakingsupport operation, independently from the actual driving condition, a constant engine braking moment is produced by virtue of the transmission adjustment. It -5is, however, alternatively also possible, during the braking-support operation, in dependence upon the value of longitudinal deceleration produced by the braking effect of the brakes, to produce an appropriate engine braking moment by virtue of a corresponding adjustment of the transmission ratio. In this case, when the longitudinal deceleration is high, a correspondingly high engine braking moment i available and when the longitudinal deceleration is smaller, only a correspondingly smaller engine braking moment is available. It is, in particular, possible for the fraction of total braking moment which is due to the engine braking effect to be kept constant by corresponding adjustment of the continuously variable transmission.

It is possible, in the inching operation, to carry out the adjustment of the transmission ratio with a constant rate of change. Alternatively, however, the rate of change of the transmission ratio may be dependent upon selected operating parameters.

The ability to perform the slip-control operation and/or the brakingsupport operation and/or the inching operation without interfering with the engine control is especially preferred. Alternatively, it is, however, also possible that in the case of the known types of operations the continuous adjustment of -6the transmission ratio is accompanied by interference in the engine control.

The transmission ratio of the gears is adjusted automatically, i.e. without the interference of the driver, during the slip-control operation and/or braking support operation.

The ability to carry out a controlled adjustment of the transmission ratio during the slip-control operation and/or braking support operation and/or inching operation is especially preferred. The control of the continuous adjustment is thus not provided with the aid of a characteristic curve or similar but rather by virtue of a control operation.

In order not to cause any damage to the engine, the transmission ratio is only adjusted during the slip-control operation and/or braking support operation and/or inching operation in such a way that the engine rotational speed remains within permissible limits. The control or regulating operation is acted upon accordingly.

Specific methods of operation of a continuously variable transmission will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram of a transmission of a motor vehicle, Fig. 2 is a flow diagram relating to a downhill engine braking operation not forming part of the present invention, Fig. 3 is a flow diagram relating to the slipadjustment operation according to the present invention, Fig. 4 is a flow diagram relating to the brakingsupport operation, Fig. 5 is a flow diagram relating to the inching operation, Fig. 6 is a second flow diagram relating to the downhill engine braking operation not forming part of the present invention, Fig. 7 shows the controlled system during the downhill engine braking operation, Fig. 8 is a first block diagram relating to the downhill engine braking operation, Fig. 9 is a second block diagram relating to the downhill engine braking operation and Fig. 10 is a third block diagram relating to the downhill engine braking operation.

The various above-mentioned types of operation are referred to again hereinunder.

During the downhill engine braking operation, the actual velocity of the motor vehicle at the moment downhill-driving is detected is stored. Downhill- -8driving is detected when the following conditions are fulfilled: the angle of the throttle flap is at a minimum or equal to zero; the longitudinal acceleration is positive. The velocity ascertained is stored and the gear transmission ratio is automatically controlled, using the possibility of carrying out continuous adjustment, in such a manner that the motor vehicle is held at the stored velocity (initial velocity) using the engine braking effect without any interference in the engine control. This naturally takes place within the scope of the permissible engine rotational speed. This special function is abandoned as soon as the driver accelerates again, i.e. actuates the throttle flap. If the brake pedal is actuated by the driver during downhill driving, then the velocity after releasing the brake pedal is stored as a new desired value and used for subsequently controlling the velocity while continuing the downhill driving.

Fig. 1 clearly illustrates in block diagram form, the arrangement for the downhill engine braking operation. The continuously variable transmission 1 is connected to a computer 3 by way of an electrical connection 2. Various information is fed as input variables to the computer 3 by way of electrical wiring 4. A sensor 5 supplies the motor vehicle velocity, a sensor 6 the throttle flap angle of the engine of the motor vehicle and a sensor 7 feeds the computer 3 with information regarding the position of the brake pedal of the motor vehicle. The data originating from the sensors 5 - 7 is processed by the computer 3, which passes control data, for the purpose of controlling the transmission, via its output, by way of the electrical connection 2, to the continuously variable transmission 1.

Fig. 2 is a flow diagram illustrating the mode of operation of the computer shown in Fig. 1. The throttle flap angle and the longitudinal acceleration are detected in a first step 8 of the computer 3, in order to be able to establish whether the motor vehicle is being driven downhill. The throttle flap angle is supplied by the sensor 6; the longitudinal acceleration can be ascertained by appropriately processing the signal of sensor 5, which detects the velocity. It is possible to determine the longitudinal acceleration by differentiating the velocity with respect to time. In the subsequent step 9 of the computer 3, the computer detects the actual velocity (sensor 5) prevailing at the commencement of downhill driving. The velocity is stored in the next procedural step 10. Finally, the computer 3 determines in step 11 the adjusting data for the -10continuously variable transmission, in such a manner that the velocity of the motor vehicle during downhill driving corresponds to the velocity stored in step 10. This data, which is fed to the continuously variable transmission 11, then results in the appropriate adjustment so that the motor vehicle maintains the desired velocity during the downhill driving. If, during downhill driving the motor vehicle driver actuates the motor vehicle brakes and this is detected by the sensor 7 and passed to the computer 3, then in step 12 of the computer 3 the motor vehicle velocity prevailing after the release of the motor vehicle brakes, which were actuated during the downhill engine braking operation, is fed as a new value to the computer, i.e. this new velocity is detected in step 9 and stored in step 10 and subsequently processed further in step 11.

A method for adjusting or controlling the motor vehicle velocity during the downhill driving mode of operation (step 11) is described with reference to Figs. 7 to 10 below.

In addition to resetting the motor vehicle velocity after a braking process induced by the driver actuating the motor vehicle brake during overrun, it is also possible to reset the motor vehicle velocity detected at the commencement of hill driving (desired velocity) during certain other operating conditions. Such operating conditions prevail if the difference between the desired velocity and the instantaneous velocity exceeds a predetermined threshold. This is explained in detail hereinunder with reference to Fig.

6.

The motor vehicle velocity detected at the commencement of downhill driving is stored during step 60 (corresponding to step 10 of Fig. 2) as a desired motor vehicle velocity Vsoll. The instantaneous motor vehicle velocity Vist, detected by sensor 64, is then compared in step 61 (corresponding to step 11 of Fig. 2) with the desired motor vehicle velocity Vsoll and the transmission ratio of the continuously variable transmission is changed in such a manner that the instantaneous motor vehicle velocity Vist is adjusted to the desired motor vehicle velocity Vsoll, During this control process, the validity of the desired motor vehicle velocity Vsoll is continuously checked. To this end, during step 62 the difference [Vsoll VistF1 between the instantaneously valid desired motor vehicle velocity Vsoll and the instantaneous preferably low-pass filtered, motor vehicle velocit VistF is formed and compared with a predetermined threshold dhyst. If this difference exceeds the threshold, then the desired motor vehicle velocity -12Vsoll is reset in step 63 to the value VistF + dhyst] and supplied to the control process 61 (or 11).

If the difference does not exceed the threshold dhyst, then a further adjustment to the old value of the desired motor vehicle velocity VS011 occurs in step 61 (or 11).

Step 63 preferably uses the low-pass filtered signal of the instantaneous motor vehicle velocity, in order that short-term fluctuations in the instantaneous motor vehicle velocity, for example as a result of uneven ground, do not lead to the desired motor vehicle velocity being reset.

The method illustrated in Fig. 6 guarantees that the desired velocity is adjusted to the motor vehicle velocity when decelerating on a slight incline. Consequently, in the case of a subsequent rising incline, the motor vehicle is immediately propelled, without accelerating unexpectedly.

The slip at the driven wheels of the motor vehicle is detected by suitable means (for example sensors) during the slip-control operation. In dependence upon the value detected, the transmission ratio of the gearing is continuously electronically controlled, in such a manner that slip is reduced or prevented. A characterising feature resides in the fact that instead of adjusting an optional, in -13particular smaller, transmission ratio, a control process is carried out, while using the ability to carry out continuous adjustment, so that preferably any incipient slip is eliminated without interfering with the engine control of the motor vehicle. Alternatively, it is,however, also possible to use the aforementioned function in conjunction with an engine control.

It is evident from Fig.1 that the computer 3 is moreover connected by way of electrical wiring 13 to a sensor 14 which detects any incipient slip which occurs at the driven wheels of the motor vehicle. Fig. 3 is a flow diagram illustrating the associated mode of operation of the computer 3. During step 15, the slip detected by the sensor 14 is passed as an input variable to the computer 3. The slip detected is processed by the computer 3 producing in step 16 an output variable which is passed to the gear control of the automatic transmission, as a result of which the variable transmission 1 is adjusted in such a manner that the slip is eliminated. The aforementioned embodiments represent, by way of example, the most preferred embodiment of the slip-adjustment operation.

During the braking-support operation, whilst using the ability to continuously adjust the transmission, adjustment is carried out in order always -14to produce, especially dependent upon the actual driving condition, a constant engine braking moment, while a braking process is being carried out. The braking of the motor vehicle is thus supported by the engine braking process.

The transmission shown in Fig. 1 comprises, as previously mentioned above, the sensor 7, which serves to detect a braking process. During the brakingsupport operation, the occurrence of a braking process is detected by the computer 3, according to the process described with reference to Fig. 4, in step 17. The computer processes the data resulting therefrom and produces, in step 18, a corresponding output variable, which is fed to the gear control of the continuously variable transmission. Consequently, the continuously variable transmission is adjusted in such a manner that the engine braking moment is constant. This is the most preferred embodiment, by way of example, of the braking-support operation.

Alternatively to the aforementioned constant engine braking moment, it is also possible, by virtue of the corresponding continuous adjustment of the transmission, to adjust the engine braking moment to suit the value of the longitudinal deceleration of the motor vehicle. When the longitudinal deceleration is high, a correspondingly high engine braking moment is -isset; when, on the other hand, the longitudinal deceleration is small, a correspondingly small engine braking moment is set. It is advantageous to be able to assume that the portion of the entire braking moment due to the engine braking effect is constant.

The motor vehicle has an automatic transmission function by reason of its electronically controlled, continuously variable transmission, i.e. the continuously variable transmission ratio is adjusted automatically. Moreover, it is possible to simulate a manually-shifted transmission. Consequently, the driver has the possibility of selecting a desired transmission ratio within the scope of the fundamentally freely definable gear stage then definitively predetermined for the driver. In accordance with the invention, it is moreover possible additionally, in the inching operation, to use the ability to carry out a continuously variable adjustment with the aid of an inching actuating unit (key or the like) not to select any discreet gear stages, but rather to continuously set a desired larger or smaller transmission ratio. While the actuating unit is actuated, the transmission ratio changes, preferably at a constant rate of change. This change can naturally only occur within the limits of the largest and smallest possible transmission ratio.

Fig. 1 shows that the computer 3 receives the data of a further sensor 20 by way of an electrical line 19. The sensor 20 detects the position of an actuating device which is actuated by the motor vehicle driver, preferably in the inching operation. The data originating from the sensor 20 is passed, in step 21, according to the method described with reference to Fig. 5, to the computer 3, which forms therefrom in step 22 an output variable which is fed to the gear control of the continuously variable transmission 1. Depending upon whether the motor vehicle driver performs the inching operation in the direction of a higher or of a lower transmission ratio, the continuously variable transmission is adjusted accordingly. This is the especially preferred embodiment by way of example of the inching operation.

Referring to Figs. 7 to 10, a method of adjustment of the motor vehicle velocity in the downhill drive mode is now described.

Fig. 7 is a block diagram of the control system. The controller R shown in detail in the following Figs. 8, 9 and 10 supplies, via an output, as a correcting variable, the desired value for the engine rotational speed NEso11 The subordinate transmission -17ratio control 71 adjusts the gear transmission ratio of the continuously variable transmission accordingly. Thus, when adjusting the engine rotational speed NE, a deceleration occurs which can be represented as T1 or T2. The engine rotational speed NE is, in the case of a closed coupling, equal to the gearbox input rotational speed. The engine rotational speed NE is present at the output side of the subordinate transmission ratio control 71. The propulsive moment MSchub produced by the engine is ascertained in block 72 from the engine rotational speed NE by means 9f an engine propulsive moment characteristic curve. The propulsive moment MSchub is multiplied by the multiplication process 74 by the gear transmission ratio i. The gearbox output moment Mab is present at the output side of the multiplication process 74. The gear transmission ratio i is i = NE: Nab The gearbox output rotational speed Nab reproduces the rotational speed of the output gearbox. In the amplifying step 76.1, the gearbox output moment Nab is amplified by a factor K to obtain the propulsive force FSchub effective on the motor vehicle.

The resistance to movement of the motor vehicle is ascertained in block 75 in dependence upon the -18instantaneous motor vehicle velocity VFhzg and the incline GE being instantaneously negotiated. Essential influencing variables which are used to ascertain the resistance to movement in block 75 are, for example, the air drag on the motor vehicle and effects of the tyres. The unit 75 then outputs the variable FW which represents the total force acting on the motor vehicle excluding the propulsive force. This total force FW is added in block 77 to the propulsive force FSchub acting on the motor vehicle. This produces the value of the total force FG acting on the motor vehicle. A division 78 by the motor vehicle mass MFhzg provides the motor vehicle acceleration aFhzg, which is recalculated by an integration 79 to form the instantaneous motor vehicle velocity VFhzg The gear output rotational speed Nab can be ascertained in block 76.2 from the instantaneous motor vehicle velocity VFhzg by multiplying by a value which is a constant for the motor vehicle.

Fig. 8 is a first block diagram of the method of carrying out the downhill driving operation. As already mentioned, the desired motor vehicle velocity Vsoll and the instantaneous motor vehicle velocity Vist are supplied to the controller 81. As illustrated with reference to Figs. 9 and 10, a -19desired value NE'Soll of the engine rotational speed is output by the controller 81. This desired value of the engine rotational speed is supplied to the transmission ratio controller 82, which ascertains a primary pressure P P for the continuously variable transmission 83 from the difference between the desired value for the engine rotational speed and the instantaneous actual value NEist of the engine rotational speed. The transmission ratio i is adjusted at the transmission 83 and, by means of this primary pressure and by a multiplication 84 with the instantaneous motor vehicle velocity Vist and a multiplication by a parameter K, which is fixed for the motor vehicle, (step 85), the actual engine rotational speed NEist is ascertained. The basic point of the method is therefore that the engine rotational speed or the gearbox input rotational speed is compared as a controlled variable with a corresponding command variable and, in dependence upon the result of this comparison, the transmission ratio of the gear is changed in the sense of an approximation of the controlled variable to the command variable.

The mode of operation of the controller 81 is now explained in detail with reference to Fig. 9. For this purpose, the values of the desired velocity VS011 and of the instantaneous actual velocity Vist of the motor vehicle are supplied to the PD controller 91. The PD controller described in detail with reference to Fig. 10 below, calculates from the desired and actual velocity the necessary propulsive force Fsoll This necessary propulsive force Fsoll is limited by means of the unit 92 to negative values, since the control process should only have a braking effect. By virtue of this limitation 92, the propulsive force ascertained by the PD controller 91 is set to zero, which represents a forward thrust. This is, for example, the case if the predetermined desired velocity is greater than the actual velocity. In this case, the motor vehicle is prevented by virtue of the unit 92 from actively accelerating.

The propulsive force F'soll desired by the engine is then present at the output side of the limitation 92. This propulsive force, in addition to the gearbox output rotational speed Nab, is supplied to the linearization block 93. The linearization block behaves as follows.

The linearization block determines, from the preceding propulsive force signal, the required engine rotational speed NES011. This linearization is based on the following considerations. In detail, it is possible for the drive torque to use:

The gear transmission ratio:

i = NE/Nab = Mab/ME (1) 0 wherein NE is the gearbox input rotational speed, which in the case of a closed coupling corresponds to the engine rotational speed, Nmot, and Nab represents the gearbox output rotational speed already mentioned. ME is the gearbox input moment and Mab is the gearbox output moment.

MSchub f(NE) KS(Nmot-Nmoto) = KSWE-Nmoto) (2), wherein the values KS and Nmoto are values which are specific to the motor vehicle or the engine. The parameter Nmoto is the engine rotational speed at which the engine does not produce any or produces only a slight braking moment.

It follows from (1) and (2) Mab = (NE/Nab)KS(NE-Nmoto) The recalculation of the gearbox drive moment Mab on the motor vehicle provides:

-22 F Schub = KSFMab (4) = KSFKS(NE/Nab)(NE-Nmoto)(5) After combining the two constants to form K Schub:

K Schub = KSFKS it follows:

(6) FSchub = KSchub(NE/Nab)(NE-Nmoto)(7) The calculation rule for the linearization block 93 is thus:

2 NESoll 0.5Nmoto + [0.25Nmoto +Fsoll(Nab/KSchub)3 1/2 The above mentioned calculation rule thus represents the transmitting behaviour of the linearization block 93. A desired value NEsoll for the engine rotational speed is thus ascertained by means of the linearization block 93 from the desired propulsive force F'soll and the output rotational speed Nab. Therefore the linearization block 93 determines from the preceding propulsive force signal the required engine rotational speed NES011, which is limited in the unit 94 to minimum and maximum values NEmin and NEmax. The limitation 94 prevents desired value NE'soll is then output by the By reason of the for the P1) controller -23action of the rotational speed the engine from racing. A for the engine rotational speed rotational speed limitation 94. linearization it is now possible to be proportioned at a linear I-T, or I-T2-level, The structure of the discontinuous PD controller 91 is illustrated in Fig. 10.

It has proved to be expedient to smooth the signal Vist of the instantaneous motor vehicle velocity with a low-pass filter 101. The smoothed motor vehicle velocity signal VistF is then available at the output side of the low-pass filter 101. The desired signal, lying at the output side of the PD controller 91, for the propulsive force Fsoll (adjusting signal) is composed of a feedback of the controlled deviation (proportional portion) and a superimposition of the differentiated velocity signal al'istF (D-portion).

In order to form a control portion P, the signal VsOll, representing the desired motor vehicle velocity, is supplied to the addition unit 103 with a positive sign. This signal is superimposed on the actual motor vehicle velocity ViStF which has been smoothed by the low-pass filter 101. The signal VistF 1 representing the actual velocity which has been smoothed by the low-pass filter is supplied to the addition unit 103 with a negative sign. The controlled deviation between the desired and actual value of the motor vehicle velocity thus lies at the addition unit 109 as the P portion.

The D control portion is found in such a manner that the signal Vist representing the instantaneous motor vehicle velocity is processed by thelow-pass filter 101 to form the smoothed signal V istF" A value of the actual motor vehicle velocity is stored in the memory unit 102 and, with negative signs, superimposed in the addition unit 104 on an instantaneous actual velocity value following the detection time T.

Division by the detection time in the unit 105 produces a low-pass filtered signal aistF representing the actual motor vehicle acceleration. The units 102, 104 and 105 therefore function as a differentiator.

The low-pass filtered actual motor vehicle acceleration aistF is fed to a further low-pass filter 106 and filtered to form the signal alistF The low pass filtered actual motor vehicle acceleration is multiplied in unit 107 by a time constant. The dynamics of the control process can be controlled in this way. A signal a''istF representing the actual motor vehicle acceleration of the motor vehicle is now -25available as a D controller portion at the output side of unit 107.

By means of the addition unit 109, the above described P controller portion is additively superimposed on the D controller portion and can be further processed by means of the amplifying step 108 to form the signal representing the desired propulsive force Fsoll, The method by which the signal Fsoll, representing the desired propulsive force, is processed as an output signal of the PD controller 91 has been described with reference to Fig. 9.

The method ensures a constant velocity during downhill driving. In contrast to a stepped automatic transmission or known CVT-automatic transmissions, the driver is provided with more comfortable downhill driving, since he need not brake continuously. wear on the brake linings is also reduced.

The linearization block renders possible the stable operation of the controller in the case of all velocities and all loads. Smooth controlled behaviour is achieved by virtue of the special construction of the PD controller which filters and regenerates the acceleration.

Claims

1. A method for operating a motor vehicle having an electronically controlled, continuously variable transmission (hereinafter referred to as "the CVT"), wherein any slip at the driven wheels of the motor vehicle is detected and, in dependence thereupon, the transmission ratio is continuously adjusted at the transmission in such a manner that the slip is reduced or completely eliminated (hereinafter referred to as ',the slip control operation,').

2. A method as claimed in claim 1, wherein additionally is it ascertained whether a braking operation is taking place and, in the braking operation, continuous adjustment of the transmission occurs, changing the transmission ratio so as to support the braking of the motor vehicle (hereinafter referred to as ',the braking support operation").

3. A method as claimed in claim 1 or claim 2, wherein by means of a manually operated control device a continuous adjustment of the transmission ratio of the gears is performed, the adjustment being carried out within the limits of the largest and the smallest transmission ratio for the duration of the manual operation (hereinafter referred to as the "inching operation").

A method as claimed in any preceding claim, wherein said motor vehicle is a passenger motor vehicle.

5. A method as claimed in claim 2, wherein the transmission ratio is increased during the brakingsupport operation.

6. A method as claimed in any preceding claim wherein the transmission ratio is reduced in the slip control operation.

7. A method as claimed in claim 2 or claim 5, wherein in the braking support operation a constant engine braking moment is produced by adjusting the transmission.

8. A method as claimed in claim 2, claim 5 or claim 7, wherein in the braking support operation, an appropriate engine braking moment is produced by correspondingly adjusting the support in dependence upon the value of the longitudinal deceleration produced by the braking effect of the brakes.

9. A method as claimed in claim 8 wherein the engine braking moment is adjusted in such a manner that its fraction of the total braking moment remains constant or that its fraction of the total braking moment varies according to a predetermined characteristic.

10. A method as claimed in claim 8 wherein the -28total braking moment is adjusted with the aid of a characteristic curve.

11. A method as claimed in claim 3, wherein in the inching operation, the adjustment of the transmission ratio occurs with a constant rate of change.

12. A method as claimed in any preceding claim, wherein the slip adjustment operation and/or the braking support operation and/or the inching operation is/are carried out without interference in the engine control.

13. A method as claimed in any of claims 1 to 11, wherein the slip adjustment operation and/or the braking support operation and/or the inching operation occur(s) with an interference in the engine control.

14. A method as claimed in any preceding claim, wherein a controlled adjustment of the transmission ratio occurs in the slip control operation.

15. A method as claimed in claim 3 or claim 11, wherein a controlled adjustment of the transmission ratio occurs in the inching operation.

16. A method as claimed in any preceding claim, wherein in the slip control operation and/or the braking support operation and/or the inching operation, the adjustment of the transmission is carried out only in a manner such that the engine rotational speed remains within its permissible limits.

17. A method of operating a motor vehicle having an electronically controlled, continuously variable transmission substantially as herein described with reference to Fig. 3.