Method and control system for operating a drive train

1. A method for operating a drive train of a motor vehicle, the drive train comprising a prime mover ( 1 ) with an internal combustion engine ( 2 ) and an electric machine ( 3 ), a separating clutch ( 6 ) connected between the internal combustion engine ( 2 ) and the electric machine ( 3 ), and a transmission ( 5 ) connected between the prime mover ( 1 ) and a driven end ( 4 ), the method comprising:

in response to at least one first defined operating condition, coupling a previously decoupled internal combustion engine ( 2 ) by actuating the separating clutch ( 6 ) to engage;

in response to at least one second defined operating condition, aborting the coupling of the internal combustion engine ( 2 ) by actuating the separating clutch ( 6 ) to disengage; and

determining an absolute value of a torque currently transmitted or currently transmittable by the separating clutch ( 6 ),

wherein the separating clutch ( 6 ) is disengaged at different rates depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ), and

wherein actuating the separating clutch ( 6 ) to disengage comprises

disengaging the separating clutch ( 6 ) at a first speed when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is less than a limiting value, and

disengaging the separating clutch ( 6 ) at a second speed when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is greater than the limiting value,

wherein the second speed is less than the first speed.

2. The method of claim 1 , wherein disengaging the separating clutch ( 6 ) at the first speed comprises disengaging the separating clutch ( 6 ) at the first speed in a step-wise manner when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is less than the limiting value.

3. The method of claim 1 , wherein disengaging the separating clutch ( 6 ) at the second speed comprises disengaging the separating clutch ( 6 ) at the second speed continuously with a torque gradient when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is greater than the limiting value.

4. The method of claim 3 , wherein disengaging the separating clutch ( 6 ) at the second speed comprises disengaging the separating clutch ( 6 ) at the second speed continuously along a ramp with a torque gradient when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is greater than the limiting value.

5. The method of claim 1 , wherein disengaging the separating clutch ( 6 ) at the second speed comprises:

determining a torque gradient, and disengaging the separating clutch ( 6 ) in a torque-controlled manner based on the determined torque gradient; and

determining a specified rotational speed for the internal combustion engine ( 2 ), and operating the internal combustion engine ( 2 ) in a speed-controlled manner based on the determined specified rotational speed.

6. The method of claim 5 , wherein determining the torque gradient comprises determining the torque gradient depending on a torque ( 66 MK0), the torque (ΔMK0) decreasable at the separating clutch ( 6 ) such that the electric machine ( 3 ) provides a driver-input torque, the torque gradient increasing with the torque (ΔMK0).

7. The method of claim 6 , further comprising determining the torque (ΔMK0) with the following

ΔMK0=(M FW −M GE )−(M EM-MIN −M EM-IST ),

wherein

M FW is a current driver-input torque,

M GE is a current transmission input torque,

M EM-MIN is a minimally possible torque of the electric machine ( 3 ), and

M EM-IST is a current torque of the electric machine ( 3 ).

8. The method of claim 5 , wherein determining the torque gradient comprises determining the torque gradient depending on a current gradient of a transmission input torque, the torque gradient increasing with the current gradient of the transmission input torque.

9. The method of claim 5 , wherein determining the specified rotational speed comprises determining the specified rotational speed depending on a current differential speed at the separating clutch ( 6 ) and depending on a current gradient of the differential speed at the separating clutch ( 6 ).

10. The method of claim 9 , wherein, when the current differential speed at the separating clutch ( 6 ) and the current gradient of the differential speed at the separating clutch ( 6 ) are each less than a respective limiting value, either determining no specified rotational speed for the internal combustion engine ( 2 ) or determining a current rotational speed of the electric machine ( 3 ) as the specified rotational speed for the internal combustion engine ( 2 ).

11. The method of claim 9 , wherein, when the current differential speed at the separating clutch ( 6 ) and/or the current gradient of the differential speed at the separating clutch ( 6 ) is greater than a corresponding limiting value, determining the current rotational speed of the electric machine ( 3 ) plus an offset as the specified rotational speed for the internal combustion engine ( 2 ).

12. A control system for operating a drive train of a motor vehicle, comprising:

an engine control unit ( 8 ) for open-loop control and/or closed-loop control of an internal combustion engine ( 2 );

a hybrid control unit ( 10 ) for open-loop control and/or closed-loop control of an electric machine ( 3 ); and

a transmission control unit ( 9 ) for open-loop control and/or closed-loop control of a transmission ( 5 ),

wherein the transmission control unit ( 9 ) or the hybrid control unit ( 10 ) actuates a separating clutch ( 6 ) to engage in order to couple a previously decoupled internal combustion engine ( 2 ) in response to at least one first defined operating condition, the separating clutch ( 6 ) connected between the internal combustion engine ( 2 ) and the electric machine ( 3 ),

wherein the transmission control unit ( 9 ) or the hybrid control unit ( 10 ) aborts the coupling of the internal combustion engine ( 2 ) by actuating the separating clutch ( 6 ) to disengage in response to at least one second defined operating condition, the transmission control unit ( 9 ) or the hybrid control unit ( 10 ) also determining an absolute value of a torque currently transmitted or currently transmittable by the separating clutch ( 6 ),

wherein the transmission control unit ( 9 ) or the hybrid control unit ( 10 ) actuates the separating clutch ( 6 ) to disengage by disengaging the separating clutch ( 6 ) at a first speed when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is less than a limiting value, and disengaging the separating clutch ( 6 ) at a second speed when the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ) is greater than the limiting value, wherein the second speed is less than the first speed, and

wherein the transmission control unit ( 9 ) or the hybrid control unit ( 10 ) disengages the separating clutch ( 6 ) at different rates depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch ( 6 ).

13. A control system, comprising an engine control unit ( 8 ) for open-loop control and/or closed-loop control of an internal combustion engine ( 2 ), a hybrid control unit ( 10 ) for open-loop control and/or closed-loop control of an electric machine ( 3 ), and a transmission control unit ( 9 ) for open-loop control and/or closed-loop control of a transmission ( 5 ), wherein the control system is configured for implementing the method of claim 1 on a control side.