Method for operating a drive train of a motor vehicle

Abstract

A method of operating a drive-train of a vehicle having a drive engine, at least one drive axle and at least one further axle which can be drivingly connected by a pressure-controlled switching device. The switching device is acted upon by an opening pressure to disconnect the axle. When a service brake system of the vehicle is actuated, the axle is automatically connected by actuating the switching device, which is actuated as a function of a braking mode determined during actuation of the brake system, in such manner that when the brake system is actuated in a first braking mode, the switching device is actuated by a first control pressure to transmit a first torque, and if at least one parameter that characterizes the occurrence of a second braking mode is exceeded, the switching device is actuated by a second control pressure to transmit a second, higher torque.

Claims

1. A method of operating a drive-train of a motor vehicle comprising a drive engine, at least one drive axle and at least one further axle which is drivingly connectable by a pressure-controlled switching device, the switching device being acted upon by an opening pressure to disconnect the further axle, and, when a service brake system of the motor vehicle is actuated, the further axle being automatically connected by actuating the switching device, the method comprising: actuating the switching device as a function of a vehicle braking mode determined during the actuation of the service brake system of the motor vehicle, when the service brake system is actuated in a first braking mode, actuating the switching device by a first control pressure in order to transmit a first torque, and actuating the switching device by a second control pressure, if at least one parameter that characterizes an occurrence of a second braking mode is exceeded, in order to transmit a second torque that is higher than the first torque.

2. The method according to claim 1, further comprising monitoring a variation of the braking pressure applied by the service brake system, as the characteristic parameter, to determine the occurrence of, or a change to, the vehicle braking mode.

3. The method according to claim 1, further comprising monitoring a brake pedal value, as the characteristic parameter, to determine the occurrence of, or a change to, the vehicle braking mode.

4. The method according to claim 1, further comprising monitoring a variation of a speed of the vehicle, as the characteristic parameter, to determine the occurrence of, or a change of, the vehicle braking mode.

5. The method according to claim 1, further comprising limiting the actuation of the switching device by the second control pressure to a duration of a retardation process.

6. The method according to claim 1, further comprising specifying the first control pressure as a function of the first torque to be transmitted.

7. The method according to claim 1, further comprising monitoring a rotational speed during the actuation of the switching device by the first control pressure.

8. A control unit for a drive-train of a motor vehicle comprising: a drive machine, at least one drive axle, at least one further axle which is drivingly connectable by a pressure-controlled switching device, the switching device being actuatable by an opening pressure to disconnect the further axle, and, when a service brake system of the motor vehicle is actuated, the further axle being automatically connectable by actuating the switching device, the control unit being designed to actuate the switching device as a function of a vehicle braking mode of the motor vehicle determined during an actuation process, the control unit being designed to determine actuation of the service brake system and, in a first braking mode, to actuate the switching device by a first control pressure in order to transmit a first torque, and to detect that at least one parameter hat characterizes an occurrence of a second braking mode being exceeded, and, the control unit, in the second braking mode, actuating the switching device by a second control pressure in order to transmit a second torque that is higher than the first torque.

9. The control unit according to claim 8, wherein the control unit carries out a method of operating the drive-train of the motor vehicle including: actuating the switching device as a function of the vehicle braking mode determined during the actuation of the service brake system of the motor vehicle; when the service brake system is actuated in the first braking mode, actuating the switching device by the first control pressure in order to transmit the first torque; if the at least one parameter that characterizes the occurrence of the second braking mode is exceeded, actuating the switching device by the second control pressure in order to transmit the second torque; and monitoring as the characteristic parameter a variation of the braking pressure applied by the service brake system to determine the occurrence of, or a change to, the vehicle braking mode.

10. The control unit according to claim 8 in combination with a computer program product by which a method of operating the drive-train of the motor vehicle can be carried out with a routine for actuating the switching device is implemented by corresponding control commands stored in a software module, the method including actuating the switching device as a function of the braking mode determined during the actuation of the service brake system of the motor vehicle; when the service brake system is actuated in the first braking mode, actuating the switching device by the first control pressure in order to transmit the first torque; if the at least one parameter that characterizes the occurrence of the second braking mode is exceeded, actuating the switching device by the second control pressure in order to transmit the second torque.

11. A data carrier with the computer program product according to claim 10.

12. A working machine comprising a drive machine, at least one drive axle and at least one further axle which is drivingly connected and disconnected by a pressure-controlled switching device, the working machine comprising a control unit for a drive-train of a motor vehicle having the drive machine, the at least one drive axle and the at least one further axle which is drivingly connected by the pressure-controlled switching device, the switching device being actuatable by an opening pressure to disconnect the further axle , and, when a service brake system of the motor vehicle is actuated, the further axle being automatically connectable by actuating the switching device, the control unit being designed to actuate the switching device as a function of a vehicle braking mode of the motor vehicle determined during an actuation process, the control unit is designed to determine an actuation of the service brake system and, in a first braking mode to actuate the switching device by a first control pressure in order to transmit a first torque, and to detect that at least one parameter that characterizes an occurrence of a second braking mode being exceeded and, in the second braking mode, to actuate the switching device by a second control pressure in order to transmit a second torque that is higher than the first torque.

13. The working machine according to claim 12, wherein a first valve arrangement is provided to supply the opening pressure and the first control pressure and the second control pressure, and the first valve arrangement comprises an electro hydraulically regulated valve by which the first control pressure and second control pressure are regulated.

14. The working machine according to claim 12, wherein a second valve arrangement is provided to supply the opening pressure and the first control pressure, and the second valve arrangement is acted upon by the opening pressure and the first control pressure, and a bypass valve is provided to supply the second control pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An advantageous embodiment of the invention, which is explained below, is illustrated in the drawings, which show:

(2) FIG. 1: A schematic view of a drive-train of a motor vehicle;

(3) FIG. 2: A flow chart of a method for operating the drive-train of FIG. 1, corresponding to a preferred embodiment of the invention;

(4) FIG. 3: A schematic view of a drive-train of a motor vehicle according to a second embodiment; and

(5) FIG. 4: A variation of control pressures for actuating a switching device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) FIG. 1 shows a schematic view of a drive-train 1 of a motor vehicle, which is preferably an agricultural or municipal utility vehicle and more preferably still a tractor or agricultural motor tractor. The drive-train 1 comprises a drive machine 2 which is in the form of an internal combustion engine and can be connected on a drive output side to a downstream motor vehicle transmission 3. The motor vehicle transmission 3 is in particular a manual gearshift transmission. In addition the drive-train comprises a service brake system 4 that can be connected to the motor vehicle transmission 3.

(7) As can also be seen in FIG. 1, on its drive output side the motor vehicle transmission 3 is connected to an axle transmission (not shown) of a drive axle 6, whereby via the axle transmission a drive input movement of the drive machine 2 geared up or down by the motor vehicle transmission 3 is distributed to two drive wheels 7 of the drive axle 6. Furthermore, on its drive output side the motor vehicle transmission 3 is connected to a pressure-controlled switching device 5 which on its drive output side can be connected to an axle transmission of a further axle 9, which has wheels 9a. The pressure-controlled switching device 5 makes it possible optionally to connect the drive-train 1 only with the drive axle 6 and thus to tow the axle 9, or by drivingly connecting the further axle 9, to operate in the all-wheel mode. For this purpose the switching device 5 is in the form of a friction clutch, in particular a wet-operating disk clutch of the normally-closed type.

(8) With each drive wheel 7 of the drive axle 6 is associated a wheel brake 8 of the service brake system 4 of the motor vehicle, which when actuated selectively brakes the drive wheel 7 concerned, Thus, the drive wheels 7 can also be braked independently of one another in order to increase the maneuverability of the motor vehicle. By connecting the axle 9 by means of the switching device 5 the braking torque transmitted by the service brake system 4 to the drive axle 6 is also transmitted to the axle 9. The actuation of the service brake system or the actuation condition of the two wheel brakes 8 can be checked with reference to the actuation condition of brake light switches.

(9) In each case a control unit 10 or 11 or 12 is associated respectively with the drive machine 2, the motor vehicle transmission 3 and the service brake system 4, which units are connected to and communicate with other control unitsnot shown further herevia a data bus system of the motor vehicle.

(10) The service brake system 4 comprises a pressure sensor 13 by means of which the braking pressure produced by the service brake system 4 is monitored. The signals provided by the pressure sensor are transmitted to the control unit 12 in order to determine a braking pressure variation therefrom. The signals received and evaluated by the control unit 12 are transmitted to the control unit 11 by way of the data bus system.

(11) Associated with the switching device 5 is a rotational speed sensor 14 which serves for the monitoring of the switching device 5 in the form of a friction clutch. To avoid damaging the friction clutch by prolonged slipping, a rotational speed n.sub.Aus detected by the rotational speed sensor 14 on the output side, which is transmitted to the connectable axle 9 by the switching device 5, is sent to the control unit 11 of the motor vehicle transmission 3. An input-side rotational speed at the input of the switching device 5 can be determined by the control unit 11 with reference to the gear ratio of the transmission and the input rotational speed of the transmission. If the rotational speed difference determined by comparing the input-side rotational speed with the output-side rotational speed n.sub.Aus exceeds a threshold value, then the engagement of the axle 9 is deactivated in order to avoid damage to the switching device 5 due to slipping.

(12) The activation of the at least one further axle 9 by the switching device 5 takes place automatically when the service brake system 4 is actuated. The service brake system 4 can be actuated by an operator or by an assistance system or an autonomous system for driving the motor vehicle.

(13) The switching device 5 is actuated by the control unit 11 of the motor vehicle transmission 3. For this, the control unit 11 actuates a valve arrangement 15 which provides, respectively for the activation and deactivation of the switching device 5, a necessary first control pressure p.sub.St1 or an opening pressure p.sub.offen. To deactivate the axle 9 the switching device 5 is acted upon by the opening pressure p.sub.offen, i.e. in the case of a normally-closed clutch the opening pressure p.sub.offen acts in opposition to a spring force that keeps the switching device 5 closed in the unpressurized condition so that the axle 9 is deactivated, i.e. it is towed. The spring force required for this is applied by a pressure-independent actuator system. Actuation by the first control pressure p.sub.St1 causes the switching device 5 to close so that a defined first torque M.sub.1 that depends on the control pressure p.sub.St1 can be transmitted by the switching device 5 to the connected axle 9. The order of magnitude of the first torque M.sub.1 is chosen such that prolonged all-wheel operation by connecting the axle 9 is possible without damage. The valve arrangement 15 comprises an electro-hydraulically regulated valve such as a proportional valve, which enables adaptation at least of the first control pressure p.sub.St1.

(14) If a situation arises in which an emergency braking operation is needed, a second torque M.sub.2 must be transmitted to the drive axle 6 and the connected axle 9. The second torque M.sub.2 is higher than the first torque M.sub.1, and corresponds to the maximum torque that can be produced by the service brake system 4. This happens by acting upon the switching device 5 with a second control pressure p.sub.St2, which is also supplied via the valve arrangement 15.

(15) As a special feature, the control unit 11 can bring about an automated connection and disconnection of the axle 9 by controlling the switching device 5 as a function of a braking mode of the motor vehicle determined during the actuation of the service brake system 4, in order to brake all the wheels 7, 9a of the motor vehicle during a braking process. For this the control unit 11 controls the switching device 5 in accordance with the process shown in the flow chart of FIG. 2:

(16) At the beginning of the process, in a step S1 it is first questioned whether the service brake system 4 has been actuated. For this, the switch positions of the brake light switches is checked and evaluated. If the service brake system 4 has not been actuated, the process reverts to the beginning and monitoring of the actuation of the service brake system 4 is continued.

(17) In contrast, if the result in step S1 is positive, the process advances to a step S2 in which at least one condition is examined which characterizes the occurrence of a first or a second braking mode. Step S2 distinguished between an ordinary braking operation that corresponds to a first braking mode, and an emergency braking operation that corresponds to a second braking mode. For this the braking pressure p.sub.Brems detected by the pressure sensor 13 is compared with a limit value p.sub.Grenz which, if exceeded, indicates an emergency braking operation. In addition or alternatively, the actuation of a brake pedal associated with the service brake system 4 can be monitored. By evaluating the displacement path covered when the brake pedal is actuated the type of braking operation, i.e. the braking mode can likewise be concluded. Furthermore the speed variation v.sub.Fahrzeug at the time can also be considered in order to be able to conclude the occurrence of the first or the second braking mode. For this the speed variation v.sub.Fahrzeug can be compared with a comparison variation v.sub.Vergleich of the vehicle speed stored in the control unit, which characterizes the occurrence of an emergency braking operation. Depending on the evaluation in step S2, the process advances to a step S3 or a step 34.

(18) Step S3 is based on the detection of the first braking mode, in which the motor vehicle is slowed normally. In the first braking mode the switching device 5 is acted upon by the first control pressure p.sub.St1 so that the axle 9 is connected. The control pressure p.sub.St1 is chosen such that the first torque M.sub.1 is transmitted from the drive axle 6 to the connected axle 9, which enables prolonged operation without damage.

(19) If in step S2 it is found that the conditions for the occurrence of the second braking mode are fulfilled, the process advances to step S4. In step 34 the switching device 5 is acted upon by the second control pressure p.sub.St2 in accordance with the second braking mode. Acting upon the switching device 5 with the second control pressure p.sub.St2 causes the second torque M.sub.2 to be transmitted to the axle 9.

(20) In step S5 it is checked whether the service brake system 4 is still actuated. For this the switch positions of the brake light switches is examined. Likewise, the position of the brake pedal can be taken into account to conclude whether the service brake system 4 is still actuated.

(21) In the case when the braking operation has ended, the system advances to step S6. In contrast, if the service brake system 4 is still actuated, the system reverts to before step S2 in order to determine whether any change of a parameter that characterizes the second braking mode has occurred during the persisting braking process, which leads to a change from the first to the second braking mode. This, for example, can be the case if the operator of the motor vehicle at first brakes in a normal manner but then, because of changing external boundary conditions, an emergency braking operation becomes necessary.

(22) In step S6 the control pressure p.sub.St1 or p.sub.St2 active at the time is changed to the opening pressure p.sub.Offen so that the torque transmission by the switching device 5 to the axle 9 is cut off. After step S6 the process is ended.

(23) By virtue of the design of the method for operating the drive-train 1 of the motor vehicle according to the invention, during a braking operation it can be ensured that as a function of a braking mode detected the connection of the at least one axle 9 by the switching device 5 takes place in such manner that in a regular braking operation a first torque M.sub.1 is transmitted, which enables prolonged all-wheel operation without damage, whereas in the case of an emergency braking operation a second torque M.sub.2 is transmitted in order to produce maximum deceleration of the motor vehicle.

(24) FIG. 3 shows a schematic view of the drive-train 1 of a motor vehicle according to a second embodiment. This second embodiment differs from the embodiment shown in FIG. 1 only by a different actuation of the switching device 5, so the indexes for identical components remain the same. To actuate the switching device a valve arrangement 16 is provided, which comprises hydraulic valves which, respectively, supply one of the control pressures p.sub.offen or p.sub.St1. The actuation takes place in process steps already explained with reference to the flow chart of FIG. 2. In order to change to the second braking mode in the case of an emergency braking operation, an additional bypass valve 17 is provided which is also controlled by the control unit 11. The opening of the bypass valve depressurizes the switching device so that the switching device 5 is kept closed by the spring force and transmits the second braking torque M.sub.2 to the connected axle 9. In this embodiment as well, in accordance with step S2 it is examined whether during the braking process a change from the second braking mode to the first braking mode occurs. In such a case the bypass valve 17 is closed so that the switching device 5 is actuated by the first control pressure p.sub.St1.

(25) The representation in FIG. 4 shows an exemplary control pressure variation 18 for actuating the switching device 5, which is in the form of a normally-closed clutch. The control pressure p.sub.Offen controls the switching device 5 in such manner that if the service brake system 4 is inactive, all-wheel drive is deactivated and the axle 9 is towed. If the service brake system 4 is actuated by an operator or an assistance system and if the evaluation of the characteristic parameter shows that the first braking mode is in force, then the switching device 5 is acted upon by the first control pressure p.sub.St1. If evaluation of the characteristic parameter shows that the second braking mode is in force or that a change has taken place from the first braking mode to the second braking mode, then the switching device 5 is acted upon by the control unit 11 with the second control pressure p.sub.St2.

INDEXES

(26) 1 Drive-train 2 Drive machine 3 Motor vehicle transmission 4 Service brake system 5 Switching device 6 Drive axle 7 Wheels 8 Wheel brakes 9 Axle 9a Wheels 10 Control unit 11 Control unit 12 Control unit 13 Pressure sensor 14 Rotational speed sensor 15 Valve arrangement 16 Valve arrangement 17 Bypass valve 18 Control pressure variation M.sub.1 First torque M.sub.2 Second torque n.sub.Aus Rotational speed p.sub.Offen Opening pressure p.sub.St1 First control pressure p.sub.St2 Second control pressure p.sub.Brems Braking pressure p.sub.Grenz Limit pressure v.sub.Fahrzeug Speed variation v.sub.Vergleich Comparison speed variation