The present invention relates to a method for controlling a drive line provided for torque transmission between an engine and vehicle wheels of a vehicle and having two power-shift clutches arranged in parallel with one another in the power transmission path. 
German Published Patent Application No.  196 52 244 describes methods for controlling a vehicle drive line having just one power-shift clutch.  In these methods, the point of engagement of the power-shift clutch is determined while the vehicle is stationary. 
The term point of engagement or kiss point is intended to mean the position of the pressure plate in which one friction face of the clutch disc makes contact with the friction face of the flywheel and the other one makes contact with the friction face of the pressure plate.  That is, when the power-shift clutch is engaged, torque transmission starts from the point of engagement. 
U.S. Pat.  No.  5,711,409 describes methods for controlling two single-disc power-shift clutches, which are arranged in the power transmission path ahead of a so-called twin-clutch transmission of a vehicle, parallel to one another, which can be actuated in a special way and in which the two friction discs are arranged axially between two hydraulically actuable pressure plates and can be supported on an abutment plate arranged axially between them, the special feature being that the two pressure plates can be supported directly against one another via a transmission member that can be compressed resiliently in the axial direction and is arranged radially outside the friction discs, thereby making it possible to reduce or influence the respective engagement force resulting at one friction disc by the preloading force or spring force of the transmission member and by the pressure of the pressure plate associated with the other friction disc.  In these methods, either a conventional process of overlapping control of the power-shift clutches or a torque-free gear-change phase are provided or a third variant, in which one power-shift clutch is engaged as soon as the other is torque-free, is provided during a gear change. 
It is an object of the present invention to provided a method in which the point of engagement or kiss point of the power-shift clutches may be determined while driving.  This is advantageous compared with the conventional methods because the point of engagement is, inter alia, dependent on temperature and therefore shifts in the same gear during a relatively long journey without it being possible to adapt it.  A knowledge of the respective point of engagement is indispensable, for example, for automatic control of a so-called twin-clutch transmission. 
The method according to the present invention does not require any additional outlay on hardware but uses conventional actuators and electronic control units which are present in a conventional manner. 
The method according to the present invention may be applied to power-shift clutches that open actively under the action of an actuator and to power-shift clutches that close actively under the action of an actuator. 
The method according to the present invention may furthermore also be applied to power-shift clutches in which a spring arrangement, e.g., a diaphragm spring, produces a contact pressure, with the result that only 50%, for example, of the rated torque of the engine may be transmitted.  To transmit a higher torque, an actuator must increase the contact pressure.  To disengage a power-shift clutch of this kind, the actuator must also be able to apply a force to the diaphragm spring.  A different contact pressure to be provided by the spring arrangement may also be chosen.  The spring arrangement may be able to transmit, for example, between 10% and 90% of the rated torque of the engine when the actuator is not applying any contact force. 
In the method according to the present invention, it is also possible to use two power-shift clutches, of which one is designed for active opening under the action of an actuator and the other is designed for active closing under the action of an actuator. 
The method according to the present invention may also be applied to single- or multi-disc dry or wet clutches.