Driveline including a hydrodynamic retarder and method of operating a hydrodynamic retarder

1 . A method of operating a hydrodynamic retarder for a vehicle, the method comprising:

activating the hydrodynamic retarder at a point in time which is determined based on a rotational acceleration of a first driveline component;

wherein the point in time at which the hydrodynamic retarder is activated is further determined based on a comparison of a rotational speed of a second driveline component with a first rotational speed threshold, wherein the second driveline component includes a transmission input.

2 . The method of claim 1 , wherein the first driveline component includes a transmission output.

3 . The method of claim 1 , wherein the point in time at which the hydrodynamic retarder is activated is further determined based on a first rotational acceleration threshold.

4 . The method of claim 3 , wherein the point in time at which the hydrodynamic retarder is activated is determined based on a comparison of the rotational acceleration of the first driveline component with the first rotational acceleration threshold.

5 . The method of claim 1 , wherein the hydrodynamic retarder is drivingly engaged with the second driveline component.

6 . The method of claim 1 , wherein the hydrodynamic retarder is activated when or once the rotational acceleration of the first driveline component is equal to or greater than a first rotational acceleration threshold, and the rotational speed of the second driveline component is equal to or greater than the first rotational speed threshold.

7 . The method of claim 6 , wherein the second driveline component is drivingly connected to a vehicle engine and the first rotational speed threshold is smaller than a second rotational speed threshold which is equal to a rotational speed of the second driveline component corresponding to a no-load governed speed of the vehicle engine.

8 . The method of claim 7 , further comprising increasing a brake power of the hydrodynamic retarder when or once the rotational speed of the second driveline component is equal to or greater than the second rotational speed threshold, wherein increasing the brake power of the hydrodynamic retarder includes increasing a hydrostatic pressure inside the hydrodynamic retarder.

9 . The method of claim 8 , wherein the brake power of the retarder is increased based on the rotational speed of the second driveline component, linearly with the rotational speed of the second driveline component.

10 . The method of claim 1 , further comprising de-activating the hydrodynamic retarder when or once a rotational speed of the second driveline component is smaller than or falls below a rotational speed threshold, or when or once the rotational acceleration of the first driveline component is or falls below a rotational acceleration threshold.

11 . The method of claim 1 , further comprising reducing a brake power of the hydrodynamic retarder when or once a retarder temperature is within a predetermined range of a predetermined maximum temperature.

12 . The method of claim 1 , wherein:

the point in time is further determined based on a rotational speed of the second driveline component; and

the first driveline component is a transmission output and the second driveline component is the transmission input.

13 . A driveline, comprising:

a hydrodynamic retarder;

a first driveline component coupled to the hydrodynamic retarder;

a second driveline component, wherein the controller is configured to activate the hydrodynamic retarder when or once the rotational acceleration of the first driveline component is equal to or greater than a first rotational acceleration threshold and a rotational speed of the second driveline component is equal to or greater than a first rotational speed threshold; and

a controller configured to activate the hydrodynamic retarder at a point in time which is determined based on a rotational acceleration of the first driveline component.

14 . The driveline of claim 13 , wherein the first driveline component includes a transmission output.

15 . The driveline of claim 13 , wherein the second driveline component includes a transmission input.

16 . The driveline of claim 15 , wherein:

the point in time is further determined based on a rotational speed of the second driveline component;

the first driveline component is an output of a transmission and the second driveline component is the transmission input; and

the transmission is a multi-speed transmission.

17 . The driveline of claim 13 , further comprising a vehicle engine drivingly connected to the second driveline component, wherein the first rotational speed threshold is smaller than a second rotational speed threshold which is equal to a rotational speed of the second driveline component corresponding to a no-load governed speed of the vehicle engine.

18 . A driveline operating method comprising:

filling a hydrodynamic retarder that is rotationally coupled to a transmission in direct response to an input speed of the transmission exceeding a first threshold speed and an acceleration of an output of the transmission exceeding a threshold acceleration; and

transitioning from filling of the hydrodynamic retarder to braking operation of the hydrodynamic retarder in direct response to the input speed of the transmission exceeding a second threshold speed that is greater than the first threshold speed.