Drive device for an all-wheel-drive motor vehicle

Abstract

A drive device for an all-wheel drive, two-track motor vehicle, in the drive train of which a first motor vehicle axle and, via a center clutch, a second motor vehicle axle are driven permanently by a drive assembly in driving operation. In the closed state of the center clutch, the second vehicle axle is engaged with the drive train, and, in the open state of the clutch, the second vehicle axle is decoupled from the drive train. In a driving situation with engaged all-wheel drive as well as with axle friction coefficients of varying size, a greater wheel torque can be taken up at the vehicle axle with a large axle friction coefficient than at the vehicle axle with a small axle friction coefficient, and a control instrument is provided, which, for engine torque limitation, limits the drive assembly to a maximum allowed engine torque.

Claims

1. A drive device for an all-wheel drive, two-track motor vehicle comprising: a drive train, wherein in a driving operation a drive assembly of the drivetrain permanently drives a first motor vehicle axle of the drivetrain and, via a center clutch in a closed state, drives a second motor vehicle axle of the drivetrain, wherein, in the closed state of the center clutch, the second motor vehicle axle is engaged with the drive assembly and, in the open state of the clutch, the second vehicle axle is decoupled from the drive assembly, wherein, in a driving situation with engaged all-wheel drive wherein one of the first and second axles is a vehicle axle with a large axle friction coefficient and the other of the first and second axles is a vehicle axle with a small axle friction coefficient, a greater wheel torque can be taken up at the vehicle axle with the large axle friction coefficient than at the vehicle axle with the small axle friction coefficient, and wherein a control instrument is provided, which, for engine torque limitation, limits the drive assembly to a maximum allowed engine torque, wherein, for prevention of an axle overload at the vehicle axle with the large axle friction coefficient, the control instrument has an analysis unit, with which the maximum allowed engine torque can be determined and adjusted, and the maximum allowed engine torque can be determined from an addition of an overload threshold value that is predetermined and deposited in the analysis unit, wherein the overload threshold value defines a threshold above which there is a danger of component damage at the vehicle axle with the large axle friction coefficient, and at least one maximum allowed potential moment of inertia fraction at the vehicle axle with the small axle friction coefficient, said moment of inertia fraction offsetting the wheel torque at the vehicle axle with the large axle friction coefficient.

2. The drive device according to claim 1, wherein the determination of the maximum allowed engine torque takes place continuously during a starting operation of the driving operation, so that the maximum allowed engine torque can be adjusted continuously.

3. The drive device according to claim 1, wherein the analysis unit has a program module, in which the maximum allowed potential moment of inertia fraction at the vehicle axle with the small axle friction coefficient, which offsets the wheel torque at the vehicle axle with the large axle friction coefficient, can be determined.

4. The drive device according to claim 3, wherein the program module has a first determination unit, in which a maximum wheel torque that can be taken up at the vehicle axle with the large axle friction coefficient can be determined, a subtraction unit, in which a maximum allowed potential moment of inertia at the vehicle axle with the large axle friction coefficient for which no axle overload exists can be determined, wherein the determination of the maximum allowed potential moment of inertia at the vehicle axle with the large axle friction coefficient takes place by subtraction of the maximum wheel torque that can be taken up at the vehicle axle with the large axle friction coefficient from the overload threshold value, and a second determination unit, in which, on the basis of the maximum allowed potential moment of inertia at the vehicle axle with the large axle friction coefficient, the potential moment of inertia fraction at the vehicle axle with the small axle friction coefficient can be determined.

5. The drive device according to claim 4, wherein, in the first determination unit of the program module, the maximum wheel torque that can be taken up at the vehicle axle with the large axle friction coefficient can be determined on the basis of a predefined maximum load state as well as an axle friction coefficient that is set to 1 via characteristic curves, and as a function of a current longitudinal acceleration of the vehicle.

6. The drive device according to claim 4, wherein, in the closed state of the center clutch, a maximum allowed potential moment of inertia at the vehicle axle with the small axle friction coefficient correlates linearly with the maximum allowed potential moment of inertia at the vehicle axle with the large axle friction coefficient.

7. The drive device according to claim 1, wherein the analysis unit has a program module, in which a minimum wheel torque that can be taken up at the vehicle axle with small axle friction coefficient can be estimated, and, for the determination of the maximum allowed engine torque, the minimum wheel torque is added to the overload threshold value.

8. The drive device according to claim 1, wherein the drive device has an electronic differential lock (EDS), wherein a vehicle wheel of the vehicle axle with the small axle friction coefficient that is turning during startup can be braked in a targeted manner by a braking torque that is generated by the braking system, as a result of which, via an axle differential, an additional torque is imposed on another vehicle wheel of the vehicle axle with the small axle friction coefficient, the magnitude of which equals that of the braking torque, and the analysis unit has an EDS program module, in which a EDS total torque that offsets the wheel torque at the vehicle axle with the large axle friction coefficient is additionally added to the overload threshold value.

9. The drive device according to claim 8, wherein, between the EDS program module and a summing element, a first compensating module is connected, and the compensating module further conveys the greater value obtained from a minimum wheel torque at the vehicle axle with the small axle friction coefficient and the EDS total torque to the summing element and retains the smaller value.

10. The drive device according to claim 9, wherein a second compensating module is connected downstream of the first compensating module, and, in the second compensating module, the greater value determined in the first compensating module is compared to a current center clutch torque, and the second compensating module further conveys the smaller value to the summing element.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention and its advantageous embodiments and enhancements as well as the advantages thereof will be explained in detail below on the basis of drawings.

(2) Shown are:

(3) FIG. 1 in a schematic illustration, the drive train of a two-track motor vehicle;

(4) FIG. 2 in a simplified flowchart, the program modules of an analysis unit of an electronic control instrument, with which an engine torque limitation can be implemented; and

(5) FIG. 3 another flowchart, in which the determination of the rear-axle moment of inertia fraction is illustrated.

DETAILED DESCRIPTION OF THE FIGURES

(6) In FIG. 1, a drive train in a two-track motor vehicle is shown. FIG. 1 and FIG. 2 have been prepared with the aim of providing a simple understanding of the invention. For this reason, the figures are merely highly simplified illustrations, which do not present a construction of the drive train or of the software architecture present in the electronic control instrument that is true to reality.

(7) In accordance with FIG. 1, the drive train of the motor vehicle has a drive assembly, which is constructed from an internal combustion engine 1 and a downstream manual transmission 3. The transmission output shaft 4 thereof is in drive connection via a toothed gear step 5 with a front-axle differential 7 of a front axle VA, by means of which the front wheels 11 are driven via the universal joint shafts 9.

(8) The transmission output shaft 4 is in fixed connection with a clutch housing of a center clutch 13, which can be coupled to a Cardan shaft 15. The rear wheels 21 of a vehicle rear axle HA are driven by the Cardan shaft 15 via a rear-axle differential 17 as well as via universal joint shafts 19. In addition, in the right universal joint shaft 19 of the rear axle HA, a dog clutch is installed.

(9) In FIG. 1, the dog clutch 23, the center clutch 13, and an engine control instrument 25 of the internal combustion engine 1 are in signal connection with a control instrument 27, which, on the basis of a large number of driving operation parameters, controls an engagement or disengagement of the all-wheel drive of the motor vehicle. For an engaged all-wheel drive, the flow of torque indicated in FIG. 1 results, after which an engine torque M.sub.mot is conveyed from the internal combustion engine 1 to the manual transmission 3 and is applied there, as a total wheel torque, at the transmission output shaft 4. The total wheel torque M.sub.ges is divided into a wheel torque M.sub.VA which is conveyed to the front axle VA, and into a second wheel torque M.sub.HA, conveyed to the rear axle HA conveyed via the closed plate clutch 13. For an engaged all-wheel drive, the control instrument 27 actuates a clutch actuator 28. Said clutch actuator generates a contact force F.sub.B, with which, when there is excess contact pressure, a slip-free or slippage-free transmission of the wheel torque M.sub.HA to the rear axle HA is ensured.

(10) The control instrument 27 controls an engine torque limitation, for which the drive assembly 1 can be limited to a maximum allowed engine torque M.sub.mot,max. In order to attain a high acceleration of the vehicle during a starting operation, the determination of as well as the adjustment of the maximum allowed engine torque M.sub.mot,max to the current driving situation take place continuously during the starting operation in order that a front-axle overload is prevented. Such a front-axle overload results, in particular, in a driving situation for which the front axle VA is driven on asphalt (that is, high front-axle friction coefficient) and the rear axle HA of the all-wheel drive is driven on an icy surface (that is, low rear-axle friction coefficient).

(11) As mentioned above, the determination and adjustment of the maximum allowed engine torque M.sub.mot,max takes place in an analysis unit 29 of the control instrument 27 during a starting operation, for example. In this way, during startup, there results a substantially faster response behavior, because the maximum allowed engine torque M.sub.mot,max can be set substantially higher than is the case for conventional calculation methods known from the prior art, without an axle overload resulting at the front axle VA.

(12) As can be seen from FIG. 2, a front-axle overload threshold value M.sub.VA, serves as starting value in the determination of the maximum allowed engine torque M.sub.mot,max. The front-axle overload threshold value M.sub.VA, is predetermined by design and is deposited in the analysis unit 29. When the front-axle overload threshold value M.sub.VA, has been attained, there exists the danger of component damage at the front axle VA.

(13) The starting value (that is, the front-axle overload threshold value M.sub.VA,) is increased in driving operation depending on the situation and, indeed, is done so by adding together torque fractions, which, in the above driving situation, relieve the front axle VA, namely, by a maximum allowed potential rear-axle moment of inertia M.sub.T,HA as well as a moment of inertia of the engine M.sub.T,mot, and either a rear-axle minimum axle torque M.sub.HA,taken up or an EDS total torque M.sub.EDS.

(14) The torque fractions given above are determined in the program modules 35, 37, 39, 41 of the analysis unit 29. Said program modules are in signal connection with a summing element 45, in which the front-axle overload threshold value M.sub.VA, is added to the torque fractions that relieve the front axle VA. The sum thereof affords the maximum allowed engine torque M.sub.mot,max.

(15) As explained below, the maximum allowed potential rear-axle moment of inertia M.sub.T,HA and the rear-axle minimum axle torque M.sub.HA,taken up are not calculated on the basis of a sensor-recorded angular acceleration d and on the basis of a mass inertia of the respective components, but are estimated in the program modules 35 and 39. In this way, the maximum allowed engine torque M.sub.mot,max can be increased initially even without having to wait for a sensor recording of the angular velocity .

(16) The determination of the maximum allowed potential rear-axle moment of inertia M.sub.T,HA that relieves the front axle VA takes place in a first program module 35 of the analysis unit 29, which is shown in more detail in FIG. 3. Accordingly, the program module 35 has a first determination unit 31, in which, initially, a maximum front-axle torque M.sub.VA,taken up that can be taken up at the front axle VA is determined. In the first determination unit 31, the maximum axle torque M.sub.VA,taken up that can be taken up at the front axle VA is determined on the basis of a predefined maximum load state as well as a front-axle friction coefficient .sub.VA that is set to 1 (for example, via characteristic curves or tables). Entering into the predefined maximum load state are the center of gravity of the vehicle as well as a predetermined maximum weight of the vehicle. Therefore, in the determination, an extreme case is assumed, in which a maximum weight is loaded on the front axle VA and the front-axle friction coefficient .sub.VA is 1.

(17) In FIG. 2, the first determination unit 31 is in signal connection with a longitudinal acceleration sensor 28, so that the maximum torque that can be taken up at the front axle M.sub.VA,taken up results as a function of the current longitudinal acceleration ax of the vehicle. When the motor vehicle is not moving, the longitudinal acceleration corresponds to a slope of the roadway. For an accelerating vehicle, the longitudinal acceleration corresponds to a total value obtained from the slope of the roadway and an acceleration of the vehicle.

(18) The maximum torque that can be taken up at the front axle M.sub.VA,taken up, which is determined in the first determination unit 31, is conveyed in FIG. 3 to a subtraction unit 33. In the subtraction unit 33, a maximum allowed potential front-axle moment of inertia M.sub.T,VA is determined and, indeed, is done so by subtraction of the maximum torque that can be taken up at the front axle M.sub.VA,taken up from the front-axle overload threshold value M.sub.VA,. The maximum allowed potential rear-axle moment of inertia M.sub.T,VA defines a threshold value that can be attained by an actual front-axle moment of inertia, without exceeding the front-axle overload threshold value M.sub.VA,.

(19) In FIG. 2, the program module 35 has, in addition, a second determination unit 34, in which, on the basis of the maximum allowed potential front-axle moment of inertia M.sub.T,VA, the maximum allowed potential rear-axle moment of inertia M.sub.T,HA can be determined.

(20) Used in the determination of the maximum allowed rear-axle moment of inertia M.sub.T,HA in the second determination unit 34 is the fact that, in the engaged state of the center clutch, the moment of inertia M.sub.T,HA at the rear axle HA correlates linearly with the moment of inertia M.sub.T,VA at the front axle VA and, indeed, does so depending on the inertia fractions of the components of the vehicle axles VA, HA and the components lying between them.

(21) Against this background, in the second determination unit 31, the mass inertias of the components of the front axle VA are set in a mass inertia ratio to the mass inertias of the components of the rear axle HA. Taking this into consideration, the maximum allowed potential moment of inertia fraction M.sub.T,HA at the rear axle HA is calculated in a simple rule of three calculation.

(22) In accordance with FIG. 2, in the second program module 37 of the analysis unit 29, the moment of inertia of the engine M.sub.T,mot (=J.sub.mot.Math.d.sub.mot), which relieves the front axle VA, is calculated, where J.sub.mot is the mass inertia and .sub.mot is the sensor-recorded angular velocity of the engine components.

(23) In the third program module 39 of the analysis unit 29, a minimum axle torque M.sub.HA,taken up that can be taken up at the rear axle HA on an icy surface is estimated, and, indeed, is done so for an estimated rear-axle friction coefficient .sub.VA of between, for example, 0.01 and 0.1.

(24) In the fourth program module 41, an additional torque M.sub.additional and a braking torque M.sub.B are summed together to give a total torque M.sub.EDS. The additional torque M.sub.additional and the braking torque M.sub.B are obtained for an EDS (EDS=electronic differential lock) engagement at the rear axle HA, which is described below.

(25) For such an EDS engagement, the drive device can have an electronic differential lock, with which a vehicle wheel 11 of the rear axle HA that turns during startup can be braked in a targeted manner by a braking torque M.sub.B that is generated by the brake system. In this way, via the rear-axle differential 17, an additional torque M.sub.additional is imposed on the other vehicle wheel 11 of the rear axle HA, the amount of which is equal to that of the braking torque M.sub.B. As already mentioned above, in the program module 41, the additional torque M.sub.additional and the braking torque M.sub.B are summed together to give a total torque M.sub.EDS. The total torque M.sub.EDS isin addition to the above moments of inertia as well as in addition to the minimum rear-axle torqueadded to the maximum axle torque M.sub.VA,taken up that can be taken by the front axle in order to obtain thereby the maximum allowed engine torque M.sub.mot,max.

(26) As can be seen further from FIG. 2, a first compensating module 45 is connected in the signal pathway between the two parallel-connected program modules 39, 41 and the summing element 45. The first compensating module 42 further conveys the greater torque value obtained from the minimum rear-axle torque M.sub.HA,taken up and the EDS total torque M.sub.EDS to the summing element 45 and retains the smaller value.

(27) Connected downstream of the first compensating module 42 is a second compensating module 44. In the second compensating module 44, the greater value conveyed from the first compensating module 42 is compared to a current center clutch torque M.sub.clutch. In this case, the second compensating module 44 further conveys the smaller value obtained from the comparison. This means that the torque taken up at the rear axle HA is limited to the magnitude of the set center clutch torque (provided the latter is less than the potentially possible rear-axle torques).