Method for Engaging a Torque Converter Lock-up Clutch in a Power Transmission of a Mobile Machine

The present invention relates to a method for engaging the converter lock-up clutch in a power-shift transmission of a working machine according to the preamble of claim 1.
To reduce the power loss in a hydrodynamic torque converter, it is provided, according to the prior art, to use a converter lock-up clutch in power-shift transmissions, by way of which the turbine of the converter can be connected to the impeller. When the converter lock-up clutch is engaged, a loss-free drive connection thereby results between the drive engine and the transmission of the vehicle.

In applications known so far involving working machines, such as dump trucks, graders, and crane vehicles, the converter lock-up clutch is engaged and disengaged depending on the rotational speed of the turbine, with consideration for the converter characteristic curve. In that particular case, a disengaged converter lock-up clutch is actuated for engagement when the rotational speed of the turbine exceeds a threshold value at which the same level of turbine torque is present when the converter lock-up clutch is engaged and when the converter lock-up clutch is disengaged. According to the prior art, an engaged converter lock-up clutch is actuated for disengagement when the rotational speed of the turbine falls below a threshold value at which the same level of turbine torque is present when the converter lock-up clutch is engaged and when the converter lock-up clutch is disengaged. These switching points are determined on the basis of the difference in the rotational speed between the turbine and the impeller.

Due to this mode of operation, the vehicle is accelerated in the lower gears when the converter lock-up clutch engages, in the partial load range, due to an increase in engine torque caused by the reduction of engine speed that

• occurs during engagement, and this vehicle acceleration is typically perceived by the driver as unpleasant shift shock.

Document DE 37 12 498 C2 makes known a method for the integral control of an engine and an automatic transmission, between which a torque converter is disposed, the torque converter comprising a lock-up clutch which can be engaged in at least one shift stage of the automatic transmission, wherein the engine torque is regulated, during a shifting procedure, in order to suppress shift shocks.

In the known method, regulation of engine torque is initiated during upshifting, depending on the state of the lock-up clutch that is determined: if the lock-up clutch is in the disengaged state, regulation of engine torque is initiated when the currently detected engine speed falls below the previously detected engine speed, and, if the lock-up clutch is in the engaged state, regulation of engine torque is initiated when the currently detected engine speed falls below a value obtained by multiplying the rotational speed of the output shaft by the transmission ratio of the previously selected shift stage of the automatic transmission.

The problem addressed by the present invention is that of providing a method for engaging the converter lock-up clutch, in a power-shift transmission of a working machine, the implementation of which improves the quality of the engagement of the converter lock-up clutch, in particular in the partial load range. Furthermore, the acceleration and thus the jerking sensation, that occurs when the converter lock-up clutch is engaged, should be greatly reduced or prevented.

This problem is solved by the features of claim 1. Further embodiments and advantages, according to the invention, will become apparent from the dependent claims.
According thereto, a method for engaging the converter lock-up clutch is provided, wherein the engagement of the converter lock-up clutch is influenced ' by engine control, during the engagement process, by reducing the increase in engine torque, i.e., the gradient of the engine torque.

For this purpose, the available engine torque or turbine torque is determined, prior to an engagement of the converter lock-up clutch, and, depending on the engine torque or turbine torque that was determined, the gradient of the engine torque is controlled by way of the transmission control unit during engagement and, optionally, for a defined period of time following the engagement such that acceleration of the vehicle, and thus a jerking sensation during engagement, is reduced or prevented.

In that case, engine torque is ascertained by the engine control unit and transmitted, via the CAN (Controller Area Network), to the transmission control unit; the turbine torque is determined by reference to the converter characteristic graph stored in the transmission control unit.

Preferably the gradient of the engine torque is controlled during the engagement, depending on the engine torque or turbine torque that was available prior to the engagement of the converter lock-up clutch, by reference to a first characteristic curve stored in the transmission control unit.

According to a development of the invention, the gradient of the engine torque can also be controlled, for a defined period of time after the converter lock-up clutch is engaged, and this preferably occurs by reference to a second characteristic curve stored in the transmission control unit. It can be provided that torque is controlled, in a ramped manner, such that it matches the torque corresponding to the position of the accelerator pedal.

Furthermore, it can be provided that the second characteristic curve matches the first characteristic curve, and, after the engagement, the first characteristic curve is used to control the gradient of the engine torque.

The concept, according to the invention, makes it possible to reduce or prevent the increase in engine torque during engagement of the converter lock- ' up clutch, in particular during partial load engagement, thereby reducing the jerking sensation that occurs during engagement.

Claims

1. A method for engaging the converter lock-up clutch in a power-shift transmission of a working machine, characterized in that the available engine torque or turbine torque is determined prior to an engagement of the converter lock-up clutch, and, depending on the engine torque or turbine torque that was determined, the gradient of the engine torque is controlled by way of the transmission control unit during engagement and, optionally, for a defined period of time following the engagement such that acceleration of the vehicle, and thus a jerking sensation during engagement, is reduced or prevented.

2. The method for engaging the converter lock-up clutch in a power-shift transmission of a working machine according to claim 1, characterized in that the engine torque is ascertained by the engine control unit and is transmitted via the CAN to the transmission control unit, wherein the turbine torque is determined by reference to the converter characteristic graph stored in the transmission control unit.

3. The method for engaging the converter lock-up clutch in a power-shift transmission of a working machine according to claim 1 or 2, characterized in that the gradient of the engine torque is controlled, during the engagement, depending on the engine torque or turbine torque that was available prior to the engagement of the converter lock-up clutch by reference to a first characteristic curve stored in the transmission control unit.

4. The method for engaging the converter lock-up clutch in a power-shift transmission of a working machine according to claim 1, 2 or 3, characterized in that, for the case in which the gradient of the engine torque is controlled by means of the transmission control unit for a defined period of time after engagement, this occurs by reference to a second characteristic curve stored in the transmission control unit.

5. The method for engaging the converter lock-up clutch in a power-shift transmission of a working machine according to claim 4, characterized in that engine torque is controlled in a ramped manner such that it matches the torque corresponding to the position of the accelerator pedal.