Method of operating the power train of a vehicle

Abstract

A method of operating a drive-train (1) of a motor vehicle, in particular an agricultural or municipal utility vehicle, having a drive engine (2), a motor vehicle transmission (4) and at least one drive axle (6). A connection between the drive engine (2) and the at least one drive axle (6) of the motor vehicle is separate automatically by way of a function, and a service brake (11) of the motor vehicle has to be actuated for activation of the function. To ensure an appropriate starting gear selection in various operating situations, during the course of the function triggered by a braking process, a brake pressure (p.sub.Brems), applied during the process, is determined and a starting gear is automatically determined as a function of the brake pressure (p.sub.Brems).

Claims

1. A method for operating a drive-train of a motor vehicle having a drive engine, a motor vehicle transmission and at least one drive axle, the method comprising: automatically separating a connection between the drive engine and the at least one drive axle of the motor vehicle by a function, actuating a service brake of the motor vehicle to activate the function, during a course of the function, triggered by a braking process, determining a brake pressure applied for the braking process, and automatically determining a starting gear as a function of the brake pressure.

2. The method according to claim 1, further comprising, if a first brake pressure limit value is exceeded, automatically overriding a pre-set starting gear selection.

3. The method according to claim 2, further comprising specifying, in addition to the first brake pressure limit value, at least a second brake pressure limit value which is higher than the first brake pressure limit value.

4. The method according to claim 1, further comprising visually indicating the starting gear determined as a function of the brake pressure.

5. The method according to claim 1, further comprising carrying out selection of the starting gear to be engaged when the speed of the vehicle is below a limit value.

6. The method according to claim 1, wherein the starting gear is determined for the motor vehicle transmission which is in the form of an automated synchronous transmission or a partially powershiftable transmission.

7. The method according to claim 1, further comprising detecting the brake pressure of at least one brake circuit of the service brake.

8. The method according to claim 1, further comprising carrying out a selection of the starting gear with reference to a correlation between brake pressures and group gears and powershift gears stored in a form of a table.

9. A control unit for a motor vehicle transmission arranged in a drive-train of a motor vehicle, which is connected to a first device for regulating a function for separating a connection between a drive engine and at least one drive axle of the motor vehicle, and a second device for automatic selection of a starting gear of the motor vehicle transmission, the function activated by the first device brings about a separation of the connection between the drive engine and the at least one drive axle of the motor vehicle, for the activation of the function a service brake of the motor vehicle has to be actuated, and, when the function that is triggered by a braking process is carried out, a brake pressure applied for the process is determinable and, as a function of the brake pressure, a starting gear is automatically determined, and for an automatic selection of the starting gear, a brake pressure of at least one brake circuit of the service brake must exceed a first brake pressure limit value in order for the starting gear to be determined as a function of the brake pressure.

10. The control unit according to claim 9, the control unit carrying out a method for operating the drive-train comprising: automatically separating the connection between the drive engine and the at least one drive axle of the motor vehicle by the function, actuating the service brake of the motor vehicle to activate the function, during a course of the function triggered by a braking process, determining the brake pressure applied for the braking process, automatically determining the starting gear based on the brake pressure, and automatically overriding a pre-set starting gear selection, if the first brake pressure limit value is exceeded.

11. The control unit according to claim 9 in combination with a computer program product for a control unit which carries out the method of operating the drive-train wherein a routine for activating the function by corresponding control commands stored in a software module is implemented, the method comprising: automatically separating the connection between the drive engine and the at least one drive axle of the motor vehicle by the function, actuating the service brake of the motor vehicle to activate the function, during a course of the function triggered by a braking process, determining the brake pressure applied for the braking process, automatically determining the starting gear as a function of the brake pressure, and automatically overriding a pre-set starting gear selection, if the first brake pressure limit value is exceeded.

12. The control unit according to claim 11, further comprising a data carrier with the computer program product.

13. A method of operating a drive-train of a motor vehicle having a drive engine, a motor vehicle transmission, at least one drive axle, the method comprising: initiating the method by actuating a service brake of the motor vehicle, when a drive connection between the drive engine and the at least one drive axle of the motor vehicle is disengaged, determining, via a pressure sensor, a braking pressure applied for actuating the service brake; automatically determining, via a transmission control unit, a starting gear of the transmission based on the determined braking pressure; selecting, via the transmission control unit, a pre-set starting gear if the determined braking pressure is less than or equal to a first pressure limit; selecting, via the transmission control unit, another starting gear if the determined braking pressure is greater than the first pressure limit and lower than or equal to a second brake pressure limit value; selecting, via the transmission control unit, a further starting gear if the determined braking pressure is greater than the second pressure limit; and engaging, via the transmission control device, the selected one of the pre-set starting gear, the another starting gear, and the further starting gear for starting the motor vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An advantageous embodiment of the invention, which is explained below, is illustrated in the figures, 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, in accordance with a preferred embodiment of the invention; and

(4) FIG. 3: A table with an example of a correlation between brake pressures and group and powershift gears.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) 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 still more preferably a tractor. The drive-train 1 comprises a drive engine 2 in the form of an internal combustion engine which, on a drive output side, can be connected by way of an interposed starting element 3 to a downstream motor vehicle transmission 4. Whereas the starting element 3 is preferably in the form of a dry-running or even wet-running friction clutch, the motor vehicle transmission 4 is in particular a manual transmission, particularly preferably an automated synchronous transmission or a partially powershiftable transmission.

(6) As can also be seen in FIG. 1, on its drive output side the motor vehicle transmission 4 is connected to an axe transmission 5 of a drive axle 6, so that by virtue of the axle transmission 5 drive movement of the drive engine 2, geared by the motor vehicle transmission 4, is distributed to two drive wheels 7 and 8 of the drive axe 6. With each of the drive wheels 7 and 8 is associated a respective wheel brake 9 and 10 of a service brake 11 of the motor vehicle, by the respective actuation of which the drive wheel 7 or 8 concerned is braked. The drive wheels 7 and 8 can also be braked independently of one another in order to increase the maneuverability of the motor vehicle. The service brake 11 comprises at least one brake circuit in which at least one pressure sensor 19 is integrated. By means of the at least one pressure sensor 19, in each case the braking pressure p.sub.Brems applied during braking can be detected.

(7) Along with the drive engine 2, the starting element 3, the motor vehicle transmission 4, the axle transmission 5 and also the service brake 11 are in each case associated a respective control unit 12 or 13 or 14 or 15 or 16, which control units are connected to other control units and sensorsnot represented further herewith which, and with one another, they communicate by way of a data bus system 17 of the motor vehicle. The control unit 12 regulates the drive engine 2, while by the control unit 13 a function for the automated opening and closing of the starting element 3 can be regulated in order to prevent forward crawling of the motor vehicle. Besides this automated operation of the starting element 3, however, the starting element 3 can also be opened and shut manually as the vehicle's driver wishes, and for that purpose the control unit 13 is connected to a clutch pedal (not shown here).

(8) The control unit 14 is associated with the motor vehicle transmission 4 and can regulate it in the manner of an automated manual transmission, although here too it is possible to carry out shifts in a manual mode in accordance with the vehicle driver's wishes. Moreover, a preselected starting gear can be set by the driver of the vehicle, which after the motor vehicle has been braked to a standstill is engaged automatically by the control unit 14 for restarting by controlling the manual transmission. Finally, the control unit 15 is associated with the axle drive 5 and a rotational speed sensor 18, and the control unit 16 with the service brake 11. The control unit 16 receives signals from a pressure sensor 19 for evaluation, which signals indicate the brake pressure p.sub.Brems.

(9) As a special feature, by means of the control unit 14 the selection of the starting gear can be determined as a function of the brake pressure p.sub.Brems which has been applied in each case for braking the motor vehicle to rest. Depending on the brake pressure p.sub.Brems determined in each case the operating situation of the motor vehicle at the time can be deduced, in order to select a starting gear appropriate for the operating situation concerned for restarting. Thus an operating situation can be avoided, in which the drive input rotational speed of the drive engine falls because of the load to a speed which will not enable the operation of the drive engine to be maintained on restarting, which inter alia can happed when restarting on an incline. For that purpose the control unit 14 regulates the motor vehicle transmission 4 in accordance with the method shown in the flow chart of FIG. 2:

(10) At the beginning of the process, in a step S1 it is queried whether the function for the automated opening and closing of the starting element 3 is activated. If not, then the process returns to the beginning and proceeds no farther.

(11) On the other hand, if the result in step S1 is positive, the process advances to a step S2. In step S2 the presence of at least one condition is checked, which is a prerequisite for the automatic opening of the starting element 3. That includes the actuation of the service brake 11, which in any case can be determined, among other things, by monitoring the brake pressure p.sub.Brems. The actuation or release of the service brake 11 leads to an actuation of brake lights associated with a brake pedal mechanism. The actuation conditions of the two wheel brakes 9 and 10 can be checked with reference to actuation conditions of the respective brake light switch and can be adopted as a further condition which is a prerequisite for triggering the function for opening the starting element 3. In addition, in step S2 it is queried whether a rotational speed n.sub.Mot of the drive engine is lower than a rotational speed limit n.sub.Grenz. A further condition in step S2 is whether a vehicle speed v.sub.Fahrz is lower than a limit value v.sub.Grenz, which triggers the determination of the brake pressure p.sub.Brems applied. If all the conditions mentioned are satisfied in step S2, the process advances to a step S3, whereas if not, the process reverts to before step S2.

(12) In step S3 the starting element 3 is automatically opened and the connection between the drive engine 2 and the motor vehicle transmission 4 is disconnected, so that thereafter no forward crawling of the motor vehicle can take place. In addition, in step S3 the brake pressure p.sub.Brems and the brake pressure variation applied for braking the motor vehicle until it has come to rest is evaluated by the control unit 16.

(13) In step S4 it is checked whether the brake pressure p.sub.Brems has exceeded a first brake pressure limit value p.sub.Grenz1. If the result in step S4 is negative, then in a strep 35 the motor vehicle transmission 4 is acted upon by the control unit 14 so as to engage a preset starting gear. The preset starting gear corresponds to a gear selection appropriate for restarting on level ground. Thus, as the preset starting gear a higher group gear with a lower powershift gear can be provided. The preset starting gear can be specified by the driver of the vehicle. On the other hand, if the result in step 34 is positive the process advances to a step 36.

(14) In step S6 it is checked whether the brake pressure p.sub.Brems has exceeded a second brake pressure limit value p.sub.Grenz2. If the brake pressure p.sub.Brems has not exceeded the value of the second brake pressure limit p.sub.Grenz2 the process advances to a step 37. If the brake pressure p.sub.Brems lies between the first brake pressure limit value p.sub.Grenz1 and the second brake pressure limit value p.sub.Grenz2, the control unit 14 interprets this to mean that the motor vehicle has been brought to rest on an incline. On an incline the driving resistance occurring when restarting can result in the preset starting gear chosen being too high, so that due to the load the drive input rotational speed of the drive engine falls to a speed which does not enable the operation of the drive engine to be maintained when restarting. To overcome that situation, in step S7 the preset starting gear is overridden, so that at least the group gear is automatically reduced by at least one step. The powershift gear can if necessary also be reduced.

(15) If the brake pressure p.sub.Brems exceeds the second brake pressure limit value p.sub.Grenz2, the process advances to a step S8. Exceeding the second brake pressure limit value p.sub.Grenz2 is interpreted by the control unit 14 to mean that the motor vehicle is on a steep incline. The control unit 14 then controls the motor vehicle transmission 4 in such manner that a starting gear an additional step lower compared with step S7 is automatically engaged.

(16) For example, with a brake pressure p.sub.Brems below the first brake pressure limit value p.sub.Grenz1 the starting gear preset is group gear 3 and powershift gear 1, which corresponds to an operating situation of driving the motor vehicle on level ground. If the first brake pressure limit value p.sub.Grenz1 is exceeded, the preset starting gear is overridden and the starting gear now set is group gear 2 and powershift gear 1. If the brake pressure p.sub.Brems also exceeds the second brake pressure limit value p.sub.Grenz2, then as the starting gear group gear 1 and powershift gear 1 are engaged.

(17) The respective result of the evaluation in step S4 is transmitted from the control unit 16 to the control unit 14, which controls the motor vehicle transmission 4, by way of the data bus 17. In addition the change of starting gear that has taken place as a function of the brake pressure p.sub.Brems can be shown to the driver of the vehicle on a display device of the motor vehicle, in particular an operating terminal in the case of agricultural or municipal utility vehicles.

(18) In the control unit 16 various values can be stored for the first brake pressure limit value p.sub.Grenz1 and for the second brake pressure limit value p.sub.Grenz2. In that way the operation of the motor vehicle with one or more trailers attached can be allowed for. The brake pressure limit values can be selected as a function of the coupling of one or more trailers. For this, the number of coupled trailers can be specified by the driver of the vehicle or detected automatically by a control unit. Furthermore, more than two brake pressure limit values can be provided.

(19) FIG. 3 shows a table with an example correlation of particular brake pressures p.sub.Brems with particular combinations of a group gear and a powershift gear, which are selected as a function of the brake pressure p.sub.Brems determined. In the table, in the line Gear selection the first number denotes the respective group gear or synchronous gear while the second number stands for the associated powershift gear. The table can be parameterized freely, so that an adaptation of the brake pressure, group gear and powershift gear is possible depending on the transmission and/or the type of vehicle concerned. A brake pressure of 0 bar corresponds to the highest selectable starting gear that enables an unloaded vehicle to start off on level ground. With a brake pressure p.sub.Brems detected by the sensors which is in a range higher than 10 bar but smaller than or equal to 20 bar, as the starting gear the combination 3-3 for the group gear and the powershift gear is selected automatically. The other pairs of values in the table of FIG. 3 work analogously. The size of the interval for the brake pressure p.sub.Brems allocated to a particular starting gear to be selected, optionally enables a finer or coarser grading of the correlation.