Method for actuating a clutch at low temperatures

This application is a national stage completion of PCT/EP02/09712 filed Aug. 30, 2002 which claims priority from German Application Serial No. 101 48 203.5 filed Sep. 28, 2001.

The instant invention relates to a method for actuating a clutch for a hydraulically actuatable clutch at low temperatures.

In a hydraulically actuated clutch system such as often used in motor vehicles, there exists the problem that the clutch cannot be opened quickly enough especially at very low temperatures. It thus can happen that in a full brake application the engine becomes stalled. The reason for this is the high viscosity of the oil at low temperatures.

DE 196 39 377 has disclosed a control for an automatic clutch in which the transmissible torque, at gears or running steps having a high reduction ratio between engine rotational speed and rotational speed of the traction wheels and/or at low vehicle speed, exceeds the respective engine torque by a smaller amount than at gears or running steps with less reduction ratio and/or at higher vehicle velocity. The kinetic clutch torque follow up serves to improve the comfort and to minimize the shifting time.

In addition, U.S. Pat. No. 6,024,673 describes a brake-dependent clutch opening in which control instrument opens the clutch with a specific deceleration during a braking operation.

The problem on which this invention is based is to outline a method for clutch actuation at low temperatures which ensures a satisfactory operation of the clutch.

It is accordingly proposed, especially at low temperatures, to implement a situation-dependent pre-opening of the clutch when a braking is probably coming. The probability of braking exists, for example, when the driver leaves the accelerator pedal. In such a situation and according to the invention, the clutch is pre-opened so as only to transmit somewhat more than the actual engine torque. When a braking subsequently takes place, part of the clutch path has already been covered so that, despite the low adjusting velocity resulting from the viscosity, the clutch promptly becomes open whereby the engine is protected from stalling.

A clutch is advantageously pre-opened only at low temperatures at which, due to the high viscosity of the hydraulic fluid, the clutch actuator velocity is insufficient. This means that below a certain temperature limit the inventive follow-up of the clutch torque is activated; above said temperature the clutch is closed with over-adaptation.

To prevent a too frequent clutch actuation, it is possible according to the invention to omit said pre-opening of the clutch in situations where a braking is improbably or uncritical, as is the case, for example, at high rotational speeds or gears.