METHOD FOR OPERATING A MOTOR VEHICLE COMPRISING AN ALL-WHEEL DRIVE THAT CAN BE ENABLED AND DISABLED BY DETERMINING AN ANGULAR ACCELERATION OF COMPONENTS, WHICH ARE UNCOUPLED WHEN THE ALL-WHEEL DRIVE IS DISABLED

Abstract

A method for operating a motor vehicle including an all-wheel drive that can be enabled and disabled, and a drive train including two clutches actuated by a control unit for enabling and disabling the all-wheel drive, and components rotating between the two clutches, which components are driven when the all-wheel drive is enabled and are uncoupled from the remaining drive train when the all-wheel drive is disabled. In order to allow early detection of defects and, in particular, bearing defects of the rotating components, and to determine the applied drag torque even without knowing the oil temperature, in one embodiment, when the all-wheel drive is disabled, the rotational speed (n) of at least one of the uncoupled components is measured in a time interval, and an angular acceleration of the uncoupled components is determined therefrom.

Claims

1-9. (canceled)

10. A method for operating a vehicle comprising: an all-wheel drive that can be enabled and disabled, and a drive train with two clutches actuated by a control unit for enabling and disabling the all-wheel drive, and a plurality of components rotating between the two clutches, the plurality of components are driven when the all-wheel drive is enabled and are uncoupled from the remaining drive train when the all-wheel drive is disabled, wherein when the all-wheel drive is disabled, the rotational speed (n) of at least one of the plurality of uncoupled components is measured in a time interval and an angular acceleration (a) of the uncoupled components is determined.

11. The method according to claim 10, wherein a defect is indicated when the determined angular acceleration (a) fails to meet or exceeds a predetermined critical threshold value.

12. The method according to claim 10, wherein a drag torque (DT) is calculated on the basis of the determined angular acceleration (a) and a moment of inertia or mass moment of inertia (J) of the plurality of uncoupled components.

13. The method according to claim 12, wherein the drag torque (DT) is calculated for a plurality of different rotational speeds (n) measured in the time interval.

14. The method according to claim 12, wherein the drag torque (DT) is calculated according to the relationship
DT=J×α+DecTconst−AccTconst where J is the moment of inertia or mass moment of inertia and a is the angular acceleration of the uncoupled components, where DecTconst is a constant decelerating torque, and AccTconst is a constant accelerating torque.

15. The method according to claim 12, wherein the torque for acceleration is pilot-controlled as a function of the calculated drag torque (DT) upon the next active acceleration of the uncoupled components.

16. The method according to claim 15, wherein the pilot control of the torque for accelerating the uncoupled components is only carried out as a function of the calculated drag torque (DT) when the period of time between the measurements of the rotational speed (n) of at least one of the uncoupled components and the engagement of the all-wheel drive is less than a predetermined period of time.

17. The method according to claim 12, wherein the calculated drag torque (DT) is compared to a stored reference drag torque (DTref) and, as a result, a conclusion is drawn about a wear-induced or wear-compensated component of the drag torque (DT).

18. The method according to claim 17, wherein a current oil temperature is determined in an axle drive from the wear-compensated component of the drag torque (DT).

Description

[0018] The invention is explained in more detail in the following by means of an exemplary embodiment shown in the drawing.

[0019] FIG. 1 shows a schematic representation of a drive train of a motor vehicle with an all-wheel drive that can be disabled, in which the rear axle can be enabled and disabled;

[0020] FIG. 2 shows the chronological progression of the rotational speed of a connecting shaft of the drive train after the all-wheel clutch and a separating clutch in the drive train have been disengaged;

[0021] FIG. 3 shows a schematic representation of a drive train of another motor vehicle with an all-wheel drive that can be disabled, in which the front axle can be enabled and disabled;

[0022] The motor vehicle 1 schematically shown in FIG. 1 has a drive train 2 with an internal combustion engine 3 and a gearbox or manual transmission 4 downstream of the internal combustion engine 3. An output of the gearbox 4 is connected to a permanently driven primary axle 5 of the motor vehicle 1, which is the front axle. A further output of the gearbox 4 can be connected to a secondary axle 7 that can be enabled, which is the rear axle, via an all-wheel clutch 6. When the all-wheel clutch 6 is disengaged, the torque of the internal combustion engine 3 is completely applied to the primary axle 5. When the all-wheel clutch 6 is engaged, the torque of the internal combustion engine 3 is distributed both to the primary axle 5 and to the secondary axle 7.

[0023] The secondary axle 7 comprises an axle drive 8 with a limited slip differential 9, which is connected to the output side of the all-wheel clutch 6 through a connecting shaft 10 in the form of a universal shaft, two lateral auxiliary cardan shafts 11, which are connected to the wheels 12 of the secondary axle 7 and the limited slip differential 9, as well as a separating clutch 13. The all-wheel clutch 6 and the separating clutch 13 are disengaged and engaged by a control unit 14 of the motor vehicle 1, in order to separate the connecting shaft 10 and the axle drive 8, in addition to the limited slip differential 9, from the gearbox 4 and from the auxiliary cardan shafts 11 in order to disable the all-wheel drive, and/or to connect to the gearbox 4 and the auxiliary cardan shafts 11 upon engagement of the all-wheel drive.

[0024] The all-wheel clutch 6 is a friction or disc clutch with two disc packages 15, 16 immersed in an oil bath, of which disc package 15 is connected in a torsionally resistant manner to an output shaft 17 of the gearbox 4 and disc package 16 is connected to the connecting shaft 10 in a torsionally resistant manner. The separating clutch 13 is designed as a dog clutch and comprises a shift element 18, which is connected to the control unit 14 via a control line 19. A rotational speed sensor 20 is attached at the axle drive 8, which measures the rotational speed of a differential cage 22 of the limited slip differential 9 and transmits this to the control unit 14 via a signal line 21.

[0025] The enabling and disabling of the all-wheel drive is carried out by the control unit 14 as a function of the respective driving situation. The disabling of the all-wheel drive is used to achieve consumption savings when the all-wheel drive is not required by minimizing the drag torque of the axle drive 8 of the secondary axle 7.

[0026] When the all-wheel drive is disabled, the engaged all-wheel clutch 6 and the engaged separating clutch 13 are disengaged at time t1, as indicated in FIG. 2 by the two curves A and B. As soon as the all-wheel clutch 6 is completely ventilated and the separating clutch 13 is completely disengaged, as shown in FIG. 2 at time t2, the connecting shaft 10 with the disc packages 16 of the all-wheel clutch 6 as well as the axle drive 8 with the limited slip differential 9 and/or the rotating components thereof are uncoupled from the gearbox 4 on one side and from the auxiliary cardan shafts 11 of the secondary axle 7 on the other side and rotate freely. As a result of the drag torque, which acts upon the uncoupled components 8, 9, 10, 16, the rotational speed n of the connecting shaft 10 subsequently gradually decreases to zero, as shown in FIG. 2 by curve C.

[0027] The rotational speed n of the differential cage 22 of the limited slip differential 9, which is measured by the rotational speed sensor 20 in short time intervals Δt and is transmitted to the control unit 14 by the signal line 21, also decreases as the rotational speed n of the connecting shaft 10 decreases.

[0028] The negative angular acceleration a or deceleration of the differential cage 22 and thus also the connecting shaft 10 as well as the remaining uncoupled components, such as, for example, of the disc package 16 and of a drive unit 23 in the axle drive 8, is calculated in the control unit 14 from the transmitted rotational speeds n and the time intervals Δt , according to the following relationship:

[00001]$\alpha =\frac{d.Math..Math.\omega }{\mathrm{dt}}=\underset{‘}{\overset{’}{\omega }}$

[0029] where ω is the angular velocity of the uncoupled components 10, 16, 22, 23, and is the first derivative of the angular velocity ω over time t or the gradient of angular velocity ω.

[0030] If the calculated angular acceleration a after a short debounce time, which depends on the speed and on the longitudinal acceleration of the motor vehicle 1 at time point t1, exceeds a predefined value determined in test bench tests, which indicates that the uncoupled components 10, 16, 22, 23 are being decelerated more quickly than expected, this may point to a defect, whereby there is a high probability of a bearing defect in one of the pivot bearings in the connecting shaft 10.

[0031] In addition, the calculated angular acceleration a after the debounce time is associated with a drag torque DT of the secondary axle as follows:

[00002]$\alpha =\frac{\mathrm{DT}+\mathrm{DecTconst}+\mathrm{AccTconst}}{J}$

[0032] where DecTconst is a constant decelerating torque component, AccTconst is a constant accelerating torque component, and J is the moment of inertia or mass moment of inertia of the uncoupled components 10, 16, 22, 23.

[0033] DecTconst and AccTconst are determined in test bench tests and stored in the control unit 14. The moment of inertia or mass moment of inertia J of the uncoupled components 10, 16, 22, 23 is known and is likewise stored in the control unit 14.

[0034] Thus, the drag torque DT of the secondary axle 7 can be determined in the control unit 14 according to the following relationship:

DT=(J×α)+AccTconst−DecTconst

[0035] The drag torque DT determined by the control unit 14 can be used in order to optimize the pilot control of the drag torques applied by the internal combustion engine 3 upon the next enabling of the all-wheel drive. If a temperature sensor is not installed in the axle drive 8 and there is no model of the dependency of the drag torque DT on the oil temperature in the axle drive 8 stored in the control unit 14, the time between the measurement of the rotational speeds n being included in the calculation of the drag torque DT and the enabling of the all-wheel drive must not be too long, of course, in order to exclude interim temperature changes.

[0036] When the influence of the oil temperature in the axle drive 8 on the drag torque DT is known by preceding test bench tests and the temperature dependency of the drag torque DT is stored in the control unit 14, statements can furthermore be made regarding a portion of the drag torque DT induced by wear or compensated by wear. For this purpose, the measurement of the rotational speeds n as well as the determination of the angular acceleration α and the calculation of the drag torque DT take place at the start of a driving cycle, as long as the temperature of the uncoupled components 10, 16, 22, 23 and of the lubricating oil in the axle drive 8 still correspond to the ambient temperature and thus the oil temperature is known. The calculated drag torque DT is then compared to a reference drag torque DTref determined through test bench tests for the same temperature and stored in the control unit 14. If the calculated drag torque DT exceeds the stored drag torque DTref by a certain amount, this amount corresponds to the wear-induced component of the drag torque DT.

[0037] If the wear-induced component of the drag torque does not change with the temperature and the dependency of the non-wear-induced component of the drag torque DT on the oil temperature in the axle drive 8 is stored in the control unit 14, the wear-induced component portion of the drag torque DT can, in turn, be used to make statements regarding the current temperature in the axle drive 8.

[0038] The motor vehicle 1 schematically shown in FIG. 3 differs from the motor vehicle in FIG. 1 in that the permanently driven primary axle 5 is the rear axle and the secondary axle that can be enabled via the all-wheel clutch 6 is the front axle 7 of the motor vehicle 1. The limited slip differential 9 there is formed as the axle differential and is separated from the axle drive 8. Corresponding parts are characterized by the same reference numbers, as shown in FIG. 1. As previously described, the rotational speed of one of the uncoupled components 10, 16, 22, 23 is measured in a time interval here as well, and an angular acceleration a of the uncoupled components 10, 16, 22, 23 is determined as a result. The remaining previously described method steps are also the same.