Method for operating a drive train

A method of operating a drive-train of a motor vehicle in which the drive-train comprises a hybrid drive with an internal combustion engine and an electric machine, a clutch that is connected between the internal combustion engine and the electric machine of the hybrid drive, a transmission connected between the electric machine of the hybrid drive and a drive output of the drive-train, and either a primary retarder connected between the electric machine of the hybrid drive and the transmission or a secondary retarder connected between the transmission and the drive output for warming up hydraulic oil when the clutch, connected between the internal combustion engine and the electric machine of the hybrid drive, is disengaged and a gear is engaged in the transmission, the method comprising the steps of:

• producing a braking torque in the drive-train by the primary retarder or the secondary retarder;
• and
• operating the electric machine of the hybrid drive in a torque-controlled manner such that the electric machine delivers a torque equal to a sum of a target torque, determined in accordance with a driver's wish, and the braking torque produced by the primary retarder or the secondary retarder.

The method according to claim 7, further comprising the step of determining the braking torque produced by the primary retarder or the secondary retarder as a function of one of:

• a hydraulic oil temperature and an external temperature, and
• an amount of electrical energy available in an electrical energy accumulator.

The method according to claim 7, further comprising the step of determining the braking torque produced by the primary retarder or the secondary retarder in such a manner that a sum of the braking torque produced and the target torque, determined as a function of the driver's wish, does not exceed a permissible maximum torque of the electric machine.

A method for operating a drive-train of a motor vehicle in which the drive-train comprises a hybrid drive with an internal combustion engine and an electric machine, a clutch connected between the internal combustion engine and the electric machine of the hybrid drive, a transmission connected between the electric machine of the hybrid drive and a drive output of the drive-train, and a primary retarder connected between the electric machine of the hybrid drive and the transmission for warming up hydraulic oil when the clutch, connected between the internal combustion engine and the electric machine, is disengaged and a neutral position is engaged in the transmission, the method comprising the steps of:

• producing a braking torque with the primary retarder in the drive-train;
• and
• operating the electric machine of the hybrid drive in a rotational speed controlled manner such that the electric machine works in opposition to the braking torque produced by the primary retarder.