METHOD FOR OPERATING A DRIVE TRAIN OF A MOTOR VEHICLE

Abstract

A method of operating a drive-train (1) of a motor vehicle, which has a drive aggregate (2), a transmission (3) with a hydrodynamic torque converter (4), an engine brake device (6) and a drive output (5). The engine brake device (6) which is activated when the drive-train (1) is operating in an overdrive mode, during a coasting process, before carrying out an overdrive downshift, when the turbine rotational speed (n_tu) of the torque converter (4) is lower than the engine idling rotational speed (n_mot_LL) of the drive aggregate (2). The engine brake device (6) is deactivated at a point in time chosen such that, when the overdrive downshift is carried out, a load change in the drive-train (1) is avoided.

Claims

1-9. (canceled)

10. A method for operating a drive-train (1) of a motor vehicle, which comprises a drive aggregate (2), a transmission (3) with a hydrodynamic torque converter (4), an engine brake device (6) and a drive output (5), the method comprising: activating the engine brake device (6), when the drive-train (1) is operating in an overdrive mode during a coasting process, before carrying out an overdrive downshift, when a turbine rotational speed (n_tu) of the torque converter (4) is lower than an engine idling rotational speed (n_mot_LL) of the drive aggregate (2), and selecting deactivation of the engine brake device (6) at a point in time such that when the overdrive downshift is carried out, a load change in the drive-train (1) is avoided.

11. The method according to claim 10, further comprising determining, during the coasting process, either an engine rotational speed (n_mot) of the drive aggregate (2) or a turbine rotational speed (n_tu) of the torque converter (4), and turning off the engine brake device (6) at a point in time when the rotational speed (n_mot) of the drive aggregate (2) or the turbine rotational speed (n_tu) of the torque converter (4) reaches a rotational speed value which is above the engine idling rotational speed (n_mot_LL) by a certain offset.

12. The method according to claim 11, further comprising taking into account, a braking gradient of the drive-train (1), a current vehicle mass or a switch-off delay time of the engine brake device (6) for either determining the offset or determining the point in time for deactivating the engine brake device (6).

13. The method according to claim 10, further comprising deactivating the engine brake device (6) to carry out the overdrive downshift with the converter bridging clutch of the torque converter (4) engaged.

14. The method according to claim 10, further comprising ignoring a switch-off command to deactivate the engine brake device (6) when an emergency braking operation is recognized, and maintaining the engine brake device (6) actuated beyond the determined switch-off time to assist the emergency braking.

15. A control unit for operating a drive-train (1) of a motor vehicle, the control unit comprising: at least a receiving interface designed to receive signals from signal emitters; a data processing unit designed to process the signals received; and a sending interface designed to emit control signals to control elements, the control unit deactivating an engine brake device (6), which is activated when the drive-train (1) is operating in an overdrive mode during a coasting process before an overdrive downshift is carried out when a turbine rotational speed (n_tu) of the torque converter (4) is lower than an engine idling rotational speed (n_mot_LL) of the drive aggregate (2), at a point in time chosen such that when the overdrive downshift is carried out, a load change in the drive-train (1) is avoided.

16. The control unit according to claim 15, wherein the control unit comprises control means for carrying out method in which the engine brake device (6), which is activated when the drive-train (1) is operating in an overdrive mode during a coasting process before an overdrive downshift is carried out when a turbine rotational speed (n_tu) of the torque converter (4) is lower than an engine idling rotational speed (n_mot_LL) of the drive aggregate (2), is deactivated at a point in time chosen such that when the overdrive downshift is carried out, a load change in the drive-train (1) is avoided.

17. The control unit according to claim 15, wherein the control unit is designed as a transmission control unit (8).

18. A computer program product with program code means, stored on a computer-readable data carrier, for carrying out a method, wherein an engine brake device (6), which is activated when a drive-train (1) is operating in an overdrive mode during a coasting process before an overdrive downshift is carried out when a turbine rotational speed (n_tu) of a torque converter (4) is lower than an engine idling rotational speed (n_mot_LL) of a drive aggregate (2), the engine brake device (6) is deactivated at a point in time chosen such that when the overdrive downshift is carried out, a load change in the drive-train (1) is avoided, and the computer program product is run on a control unit having at least a receiving interface designed to receive signals from signal emitters, a data processing unit designed to process the signals received, and a sending interface designed to emit control signals to control elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Preferred further developments emerge from the subordinate claims and the following description. An example embodiment of the invention, to which it is not limited, is described in greater detail with reference to the drawings, which show:

[0027] FIG. 1: An example of a drive-train with an automatic transmission and a hydrodynamic torque converter, represented schematically,

[0028] FIG. 2: An example of a rotational speed variation during a coasting process, with conventional actuation of an engine brake device,

[0029] FIG. 3: An example of a rotational speed variation during a coasting process, with actuation of an engine brake device in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] FIG. 1 shows a schematic representation of a drive-train 1 of a motor vehicle, wherein the drive-train 1 shown comprises a drive aggregate 2, a transmission 3, an engine brake device 6 and a drive output 5. The transmission 3 is an automatic transmission connected in the power flow between the drive aggregate 2 and the drive output 5. The drive output is in the form of a drive axle, in this case comprising at least an axle transmission, two driveshafts and two drive wheels. An output shaft of the automatic transmission 3 is in driving connection with the drive wheels of the drive axle by way of the axle transmission and the two driveshafts.

[0031] On the input side, a hydrodynamic torque converter 4 provided with a bridging clutch is connected upstream from the automatic transmission 3. The hydrodynamic torque converter 4 comprises a pump wheel and a turbine wheel, the pump wheel being connected, rotationally fixed to a driveshaft of the drive aggregate 2 which can be connected as necessary to an input shaft of the automatic transmission 3 by way of the bridging clutch and a vibration damper. The turbine wheel of the torque converter 4 is connected to the input shaft of the automatic transmission 3.

[0032] The automatic transmission 3 shown in FIG. 1 is of planetary configuration and has four gearsets and five shifting elements, the shifting elements being in the form of frictional brakes and frictional clutches. A total of eight forward gears and one reverse gear can be engaged, and in each of these gears three shifting elements are closed and two shifting elements are open. To carry out a gearshift and thus a shifting process, at least one of the previously open shifting elements is closed or engaged and at least one of the previously closed shifting elements is opened or disengaged. The transmission scheme shown in FIG. 1 is presented purely as an example. In addition to frictional shifting elements, interlocking shifting elements can also be present.

[0033] Associated with the drive aggregate 2 there is an engine control unit 7 and associated with the automatic transmission 3 there is a transmission control unit 8. The operation of the drive aggregate 2 is controlled and/or regulated with the help of the engine control unit 7, for which purpose the engine control unit 7 exchanges data 9 with the drive aggregate 2. The operation of the automatic transmission 3 is controlled and/or regulated by the transmission control unit 8, for which purpose the transmission control unit 8 exchanges data 10 with the automatic transmission 3. Furthermore, the engine control unit 7 and the transmission control unit 8 exchange data 11 with one another.

[0034] According to FIG. 1 sensors (not shown here) provide data 12 to the transmission control unit 8, on the basis of which the transmission control unit 8 controls and/or regulates the operation of the automatic transmission 3, for example data about a position or a degree of actuation of a brake pedal. Other sensors (not shown here) supply further data 13 to the engine control unit 7, on the basis of which the engine control unit 7 controls and/or regulates the operation of the drive aggregate 2, for example data about a position or a degree of actuation of an accelerator or gas pedal.

[0035] During traction operation of the drive-train 1, drive torque from the drive aggregate 2 is passed via the hydrodynamic torque converter 4 and the automatic transmission 3 to the drive output 5. In contrast, during overdrive operation of the drive-train 1, starting from the drive output 5, torque in the drive-train 1 is passed via the automatic transmission 3 and the hydrodynamic torque converter 4 in the direction toward the drive aggregate 2. The drive aggregate 2 then delivers engine braking torque which, when the drive aggregate 2 is in the form of an internal combustion engine, is determined essentially by the load change work. The engine brake device 6 is provided in order to increase the engine braking action of the drive aggregate 2 still further during overdrive operation. The operation of the engine brake device 6 can be controlled and/or regulated both by the engine control unit 7 and also by the transmission control unit 8, for which purpose the engine control unit 7 and/or the transmission control unit 8 exchange data 9, 14 with the engine brake device 6.

[0036] FIG. 2 shows a variation of a turbine rotational speed n_tu and a variation of an engine rotational speed n_mot of the drive aggregate 2 during a conventional coasting process of the motor vehicle. During the coasting process the drive aggregate is operated in overdrive mode. At the beginning of the rotational speed variation the engine brake device 6 is activated, whereby the engine braking action of the drive aggregate 2 is increased. The converter bridging clutch of the hydrodynamic torque converter is closed, which can be recognized in that the variation of the turbine rotational speed n_tu and the variation of the engine rotational speed n_mot overlap. During the coasting process of the motor vehicle, with the converter bridging clutch closed and the engine brake device 6 activated, coasting downshifts are carried out at times t1 and t2. At low rotational speeds the engine braking torque is smaller than at higher rotational speeds. Since after a coasting downshift has been carried out the engine rotational speed n_mot of the drive aggregate 2 is higher again, there is consequently also a higher engine braking torque again after the coasting downshift. When the converter bridging clutch is closed and the engine brake device 6 is activated, it is advantageous while an overdrive downshift is being carried out to deactivate the engine brake device 6. This simplifies the synchronization process when shifting into the new gear, since the braking torque produced by the engine brake device 6 plays no part in the synchronization process.

[0037] When an engine idling rotational speed n_mot_LL is reached at time t3, the converter bridging clutch of the torque converter is opened and the engine rotational speed n_mot of the drive aggregate 2 is held at least approximately at the engine idling rotational speed n_mot_LL by an idling control or idling regulation, as shown in FIG. 2 by the dot-dash line n_mot. Since during the coasting process of the motor vehicle its travel speed decreases, the turbine rotational speed n_tu decreases and assumes a course represented by the broken line n_tu,

[0038] At time t4 another overdrive downshift is carried out. At that time t4 the turbine rotational speed n_tu is already lower than the engine idling rotational speed n_mot_LL of the drive aggregate 2. Due to the overdrive downshift, the turbine rotational speed n_tu increases again and at time-point t5 crosses over the engine idling rotational speed n_mot_LL of the drive aggregate 2. When the speed of the vehicle decreases after the overdrive downshift during the coasting process, the turbine rotational speed n_tu also falls again in the newly engaged gear to below the engine idling rotational speed n_mot_LL of the drive aggregate 2. Thus, after the overdrive downshifts have been carried out the turbine rotational speed n_tu crosses twice over the engine idling rotational speed n_mot_LL of the drive aggregate 2, so that in the drive-train 1 two undesired load changes take place, which have a negative influence on the driving comfort during the coasting process.

[0039] To be able to avoid this two-time crossover of the turbine rotational speed n_tu and the engine idling rotational speed n_mot_LL of the drive aggregate 2 and the concomitant undesired load changes in the drive-train 1, in the method according to the invention a switch-off time is determined for the engine brake device 6 of the motor vehicle which was actuated during the coasting process. FIG. 3 shows a variation of a turbine rotational speed n_tu and a variation of an engine rotational speed n_mot of the drive aggregate 2 during a coasting process of the motor vehicle, with actuation of an engine brake device 6 according to the invention. First, as in FIG. 2, the drive aggregate 2 is operated in an overdrive mode, the engine brake device 6 is active and the converter bridging clutch of the hydrodynamic torque converter is closed. During the coasting process of the motor vehicle, with the converter bridging clutch closed and the engine brake device 6 activated overdrive downshifts are carried out at times t7 and t8.

[0040] According to the present invention a switch-off time t9 for the active engine brake device 6 is now determined. It is provided that the engine brake device 6 is deactivated already when an engine rotational speed n_mot of the drive aggregate 2 or a turbine rotational speed n_tu of the torque converter 4 reaches a rotational speed value which is above the engine idling rotational speed n_mot_LL by a certain offset. The offset can be specified variably, for example as a function of a braking gradient of the drive-train 1, a vehicle mass and other boundary conditions such as a switch-off lag time of the engine brake device 6. The braking gradient of the drive-train 1 can for example be determined from the variation of the rotational speed n_mot of the drive aggregate 2, from the variation of the turbine rotational speed n_tu or from the variation of a transmission drive output rotational speed.

[0041] The offset can be determined by the transmission control unit 8 which, when the engine rotational speed n_mot of the drive aggregate 2 or the turbine rotational speed n_tu of the torque converter 4 reaches the rotational speed value, emits a control signal for the deactivation of the engine brake device 6 to the engine brake device 6 directly or to the engine control unit 7, which then emits the signal for deactivating the engine brake device 6 to the engine brake device 6.

[0042] According to FIG. 3 this rotational speed value is reached at time t9, whereupon the engine brake device 6 is deactivated. Thereafter, a rotational speed variation takes place which is flatter compared with FIG. 2 due to the less pronounced engine braking effect. Due to this less pronounced engine braking action the engine idling rotational speed n_mot_LL is reached at a later time t10, at which the converter bridging clutch of the torque converter 4 opens and the engine rotational speed n_mot of the drive aggregate 2 is held by an idling control or idling regulation at least approximately at the engine idling rotational speed n_mot_LL, as shown in FIG. 3 by the dot-dash line n_mot. Since during the coasting process of the motor vehicle the travel speed decreases, the turbine rotational speed n_tu too is reduced and assumes a variation shown by the broken line n_tu.

[0043] The shifting points for the overdrive downshifts carried out during overdrive operation are determined as a function of an existing vehicle deceleration or a magnitude equivalent thereto. Thus, if the vehicle decelerates sharply an overdrive downshift is triggered at an earlier time, i.e. at a higher shift rotational speed, than if the deceleration of the vehicle is less severe. If an engine brake device 6 which is active during the overdrive operation of the drive-train 1 is deactivated, this results in a more gentle deceleration of the vehicle, due to which the shifting point for a subsequent overdrive downshift is displaced and the overdrive downshift is triggered at a later time, i.e. at a lower shift rotational speed.

[0044] After the opening of the converter bridging clutch, at time t11 an overdrive downshift is again carried out. Since after the opening of the converter bridging clutch, as shown in FIG. 3 the turbine rotational speed n_tu has a lower rotational speed gradient than the turbine rotational speed n_tu in FIG. 2, in the method according to the invention the overdrive downshift when the turbine rotational speed n_tu of the torque converter 4 is lower than the engine idling rotational speed n_mot_LL can be carried out at a lower shift rotational speed and therefore correspondingly later. In this way a crossover of the turbine rotational speed n_tu, which increases again after the overdrive downshift, with the engine idling rotational speed n_mot_LL, can be avoided and consequently the driving comfort during a coasting process can be improved.

INDEXES

[0045] 1 Drive-train [0046] 2 Drive aggregate [0047] 3 Transmission [0048] 4 Torque converter [0049] 5 Drive output [0050] 6 Engine brake device [0051] 7 Engine control unit [0052] 8 Transmission control unit data [0053] 9 Data [0054] 10 Data [0055] 11 Data [0056] 12 Data [0057] 13 Data [0058] 14 Data