Method for operating a motor vehicle with on-demand all-wheel drive

Abstract

A vehicle includes a powerplant, a front axle having first and second wheels and a differential operably coupled to the powerplant. A power-takeoff unit (PTU) is connected to the differential. A rear axle has third and fourth wheels and a gearbox connected to the PTU without a center differential. The gearbox has a first clutch configured to selectively couple the third wheel to the PTU and a second clutch configured to selectively couple the fourth wheel to the PTU. A controller is programmed to determine, during a turn, which of the third and fourth wheels is an outer rear wheel, determine whether there is a positive or negative torque on the outer rear wheel, and disengage, or keep disengaged, the one of the first and second clutches that is associated with the outer rear wheel in response to a negative torque on the outer rear wheel.

Claims

1. A method for operating a motor vehicle with on-demand all-wheel drive without a central differential, wherein the motor vehicle has a first partial clutch for disengaging a first rear wheel from a drivetrain of the motor vehicle and a second partial clutch for disengaging a second rear wheel from the drivetrain of the motor vehicle, the method comprising: determining, during a turn, which of the rear wheels is an outer rear wheel, determining whether there is a positive or negative torque on the outer rear wheel, and disengaging, or keeping disengaged, at least the partial clutch that is associated with the outer rear wheel in response to a negative torque on the outer rear wheel.

2. The method of claim 1, wherein the determination of the positive and negative torque is based on a speed of an outer front wheel and a speed of an inner front wheel.

3. The method of claim 2, wherein the determination of the positive and negative torque is further based on a mean value of the speeds of the front inner and outer wheels and a speed of the outer rear wheel.

4. The method of claim 1 further comprising: detecting operating parameters of the drivetrain that are indicative of a torque output; and engaging the one of the partial clutches associated with the outer rear wheel in response to the operating parameters indicating an increasing torque output.

5. The method of claim 4 wherein the operating parameters include slippage of front wheels and gas pedal actuation.

6. A vehicle comprising: a powerplant; front axle including first and second wheels and a differential operably coupled to the powerplant; a power-takeoff unit (PTU) connected to the differential; a rear axle including third and fourth wheels and a gearbox connected to the PTU without a center differential, the gearbox including a first clutch configured to selectively couple the third wheel to the PTU and a second clutch configured to selectively couple the fourth wheel to the PTU; and a controller programmed to: determine, during a turn, which of the third and fourth wheels is an outer rear wheel, determine whether there is a positive or negative torque on the outer rear wheel, and disengage, or keep disengaged, the one of the first and second clutches that is associated with the outer rear wheel in response to a negative torque on the outer rear wheel.

7. The vehicle of claim 6, wherein the controller is further programmed to, determine, during the turn, which of the first and second wheels is an inner front wheel and which of the first and second wheels is an outer front wheel.

8. The vehicle of claim 7, wherein the determination of the positive and negative torque is based on a speed of the outer front wheel and a speed of the inner front wheel.

9. The vehicle of claim 8, wherein the determination of the positive and negative torque is further based on a mean value of the speeds of the front inner and outer wheels and a speed of the outer rear wheel.

10. The vehicle of claim 6, wherein the controller is further programmed to engage the one of first and second clutches in response to an increasing torque output of the powerplant.

11. The vehicle of claim 6, wherein the controller is further programmed to engage the one of first and second clutches in response to a gas pedal being depressed.

12. The vehicle of claim 6, wherein the controller is further programmed to engage the one of first and second clutches in response to an increased slippage being sensed at the first or second wheels.

13. The vehicle of claim 6, wherein the powerplant is an engine.

14. The vehicle of claim 6, wherein the PTU is directly connected to the differential.

15. A control unit for operating a motor vehicle with on-demand all-wheel drive without a central differential, wherein the motor vehicle has a first partial clutch for disengaging a first rear wheel from a drivetrain of the motor vehicle and a second partial clutch for disengaging a second rear wheel from the drivetrain of the motor vehicle, the control unit comprising: a controller programmed to: determine, during a turn, which of the rear wheels is an outer rear wheel, determine whether there is a positive or negative torque on the outer rear wheel, and disengage, or keep disengaged, at least the partial clutch that is associated with the outer rear wheel in response to a negative torque on the outer rear wheel.

16. The control unit of claim 15, wherein the determination of the positive and negative torque is based on a speed of an outer front wheel and a speed of an inner front wheel.

17. The control unit of claim 16, wherein the determination of the positive and negative torque is further based on a mean valve of the speeds of the front inner and outer wheels and a speed of the outer rear wheel.

18. The control unit of claim 15, wherein the controller is further programmed to: detect operating parameters of the drivetrain that are indicative of a torque output; and engage the one of the partial clutches associated with the outer rear wheel in response to the operating parameters indicating an increasing torque output.

19. The control unit of claim 18 wherein the operating parameters include slippage of front wheels and gas pedal actuation.

20. The control unit of claim 15, wherein the controller is further programmed to engage the one of the partial clutches associated with the outer rear wheel in response to an increasing torque output of a powerplant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic representation of a motor vehicle while taking a bend.

(2) FIG. 2 shows a procedure for operating the motor vehicle shown in FIG. 1.

DETAILED DESCRIPTION

(3) Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

(4) Referring to FIG. 1, a motor vehicle 2, such as a passenger vehicle, is shown. The motor vehicle 2 has on-demand all-wheel drive in which the two front wheels 4a, 4b are permanent and the two rear wheels 6a, 6b are on-demand. The motor vehicle 2, in the present exemplary embodiment, has an engine 10, a manual transmission 12, and a clutch 14 arranged between the engine 10 and the transmission 12. For speed synchronization, a front differential 16 is provided between the front wheels 4a, 4b. A (power take-off unit) PTU 18 transmits torque to the rear wheels 6a, 6b. A reduction gearbox 8 is provided between the PTU 18 and the rear wheels 6a, 6b.

(5) In FIG. 1, the vehicle is cornering a right-hand turn, and the left front wheel 4a is the outer front wheel, the right front wheel 4b is the inner front wheel, the left rear wheel 6a is the outer rear wheel, and the right rear wheel 6b is the inner rear wheel. When taking a left-hand turn, the front wheels 4a, 4b and rear wheels 6a, 6b case change their roles as outer and inner front wheels 4a, 4b or outer and inner rear wheels 6a, 6b.

(6) A clutch assembly with a first partial clutch 8a and a second partial clutch 8b is between the two rear wheels 6a, 6b. The clutch assembly is operably coupled to the gearbox 8. The partial clutches 8a, 8b selectively couple the rear wheels 6a, 6b to the gearbox 8. Closing the clutches 8a, 8b engages all-wheel drive. When the clutches 8a, 8b are open, the rear wheels are disconnected from the engine. The first partial clutch 8a and the second partial clutch 8b are each configured as friction plate clutches in the present exemplary embodiment and are controlled by a control unit 20 depending on the driving situation, as will be explained in detail below. The control unit 20 may comprise hardware and/or software components for this purpose and for the tasks and function described below.

(7) The motor vehicle 2, in the present exemplary embodiment, does not include a central differential nor a rear-axle differential. In other words, the all-wheel drive of the motor vehicle 2 may also be perceived as a clutch-controlled all-wheel drive.

(8) Due to a lack of speed synchronization between the rear wheels 6a, 6b and the drivetrain which also drives the front wheels 4a, 4b, torsional stresses and noises can occur when turning, e.g., when parallel parking. When turning, the front wheels 4a, 4b follow a larger radius and have to move at higher speeds V.sub.FL, V.sub.FR than the rear wheels 6a, 6b. The rear wheels 6a, 6b are, however, forced to adopt the average speed of the two front wheels 4a, 4b, insofar as the two partial clutches 6a, 6b on the rear axle are engaged. In addition, the two rear wheels 6a, 6b are forced to turn at the same speed when the clutches 8a, 8b are locked. Depending on the design of the chassis of the motor vehicle 2, this leads to understeer. Moreover, there may be greater wear, particularly of the tires on the rear wheels 6a, 6b, and increased fuel consumption.

(9) In order to counteract this, a control unit 20 is designed to determine which of the two rear wheels 6a, 6b is to be treated as an outer rear wheel 6a, 6b when the motor vehicle 2 is taking a bend. In addition, the control unit 20 is designed to determine whether there is a positive or a negative torque on the outer rear wheel 6a, 6b. Furthermore, the control unit 20 is designed for disengaging, or keeping disengaged, the partial clutch 8a, 8b which is assigned to the outer rear wheel 6a, 6b of the motor vehicle 2 when a negative torque is present on the outer rear wheel 6a, 6b.

(10) In order to determine whether there is a positive or negative torque on the outer rear wheel 6a, 6b, in the present exemplary embodiment the control unit 20 evaluates a value that is representative of a speed V.sub.FL of the outer front wheel 4a in each case and a further value which is representative of a speed V.sub.FR of the inner front wheel 4b in each case. In the present exemplary embodiment, the control unit 20 uses equation 1 for this purpose:

(11) $\begin{array}{cc}\mathrm{sign}\left(\mathrm{Tr}{q}_{xx}\right)=\mathrm{sign}.Math.\frac{V_{\mathrm{FL}}+V_{\mathrm{FR}}*i_{\mathrm{PTU}}}{2*i_{\mathrm{RDU}}}-V_{xx}.Math.& \left(\mathrm{Eq}.1\right)\end{array}$

(12) In this case, V.sub.xx stands for the speed of the outer rear wheel 6a, 6b, i.sub.PTU for a transmission ratio of the PTU 18, and i.sub.RDU for a transmission ratio of the reduction gearbox 8. The function sign (Trq.sub.xx) supplies a value which is representative of whether there is a positive or negative value, i.e., torque, at the outer rear wheel 6a, 6b. In the present exemplary embodiment, a value of +1 represents a positive torque, a value of −1 represents a negative torque, and a value of 0 represents no torque.

(13) In other words, an arithmetic mean based on the speed V.sub.FL of the respective outer front wheel 4a and the speed V.sub.FR of the respective inner front wheel 4b is determined. The arithmetic mean is then compared with the speed V.sub.xx of the outer rear wheel 6a, 6b. For this purpose, in the present exemplary embodiment, the speed V.sub.xx of the outer rear wheel 6a, 6b is subtracted from the arithmetic mean.

(14) In order to minimize the influence of disruptive signals, the control unit 20 detects and evaluates values representative of a speed V.sub.FL of the outer front wheel 4a in each case and values representative of a speed V.sub.FR of the inner front wheel 4b in each case and also values representative of the speed V.sub.xx of the outer rear wheel 6a, 6b over a period of time.

(15) In the present exemplary embodiment, the following formula (equation 2) is used for this purpose:

(16) $\begin{array}{cc}\mathrm{Buffer}\left(t\right)={\int }_{t0}^t{\left(\frac{V_{\mathrm{FL}}+V_{\mathrm{FR}}*i_{\mathrm{PTU}}}{2*i_{\mathrm{RDU}}}-V_{xx}\right)}^3& \left(\mathrm{Eq}.2\right)\end{array}$

(17) In other words, there is a numerical integration over the predetermined period of time. The total value can then be compared with a threshold value, in order to determine whether the respective partial clutch 8a, 8b is to be disengaged or kept disengaged.

(18) The cubic nature of the equation 2 ensures that small negative torques cannot lead to the threshold value being reached too quickly. The period of time starts to elapse as soon as a negative torque has been detected. The elapsing of the period of time is set back to zero when either the threshold value is reached or the torque is positive again.

(19) Furthermore, in the present exemplary embodiment, the control unit 20 is designed to detect operating parameters of the drivetrain which are indicative of an increasing torque output. In addition, in the present exemplary embodiment, the control unit 20 is designed to engage the partial clutch 8a, 8b that is associated with the outer rear wheel 6a, 6b of the motor vehicle 2 when the operating parameters of the drivetrain are indicative of an increasing torque output. In this case, the operating parameters in the present exemplary embodiment involve increasing slippage at the front wheels 4a, 4b and/or gas pedal actuation which leads to increased slippage at the wheels. The slippage at the front wheels 4a, 4b may be slippage at one of the front wheels 4a, 4b in each case or at both front wheels 4a, 4b. In this way, a drive moment can be promptly provided, in particular also an imminent load change.

(20) Referring to FIG. 2, a procedure for operating the motor vehicle 2 shown in FIG. 1 is explained. At a first step S100, the control unit 20 determines which of the two rear wheels 6a, 6b is to be treated as an outer rear wheel 6a, 6b when the motor vehicle 2 is taking a bend. If, for example, a right-hand bend is detected, the control unit 20 assigns the value 1 to a three-valued variable K in the present exemplary embodiment. If, on the other hand, a left-hand bend is detected, the control unit 20 assigns the value −1 to the variable K. During straight driving, the variable K is assigned 0. The respective rear wheel 6a, 6b is then selected according to the value of the variable K.

(21) If the vehicle is taking a right-hand or left-hand bend, in a further step S200, the control unit 20 determines whether there is a positive or negative torque on the outer rear wheel 6a, 6b. If the control unit 20 has determined a positive torque, a three-valued variant D, in the present exemplary embodiment, is assigned the value 1. If, on the other hand, the control unit 20 has determined a negative torque, the variable D is assigned the value −1. If neither a positive nor a negative torque has been determined, the variable D is assigned the value 0.

(22) For this purpose, the control unit 20 evaluates values that are representative of a speed V.sub.FL of the outer front wheel 4a in each case and further values that are representative of a speed V.sub.FR of the inner front wheel 4b in each case and also the value that is representative of a speed V.sub.xx of the outer rear wheel 6a, 6b.

(23) In order to suppress signal noises, the control unit 20, in the present exemplary embodiment, detects a plurality of values that are representative of a speed V.sub.FL of the outer front wheel 4a in each case and a plurality of values that are representative of a speed V.sub.FR of the inner front wheel 4b in each case over a period of time and evaluates these.

(24) In a further step S300, the control unit 20 command the partial clutch 8a, 8b associated with the outer rear wheel 6a, 6b of the motor vehicle 2, to be disengaged or kept disengaged by suppressing an engagement signal when there is a negative torque on the outer rear wheel 6a, 6b. The control unit 20 emits a corresponding disengagement signal OS for this purpose.

(25) In a further step S400, the control unit 20 detects operating parameters BP of the drivetrain which are indicative of a torque output.

(26) In a further step S500, the control unit 10 brings about engagement of the partial clutch 8a, 8b which is assigned to the outer rear wheel 6a, 6b of the motor vehicle 2 when the operating parameters BP of the drivetrain are indicative of an increasing torque output. In this case, increasing slippage at the front wheels 4a, 4b and/or gas pedal actuation are/is evaluated. The control unit 20 emits a corresponding engagement signal SS for this purpose. In this way, undesirable noise generation can be suppressed, reduced oversteering or increased understeering prevented, and tire wear and also fuel consumption by the motor vehicle 2 reduced.

(27) While example embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.