Wet Clutch Basic Function - GF27.51-P-1004A
Model All with transmission 725
Overview
This document contains information on:
- General
- Function requirements
- Function
- Wet clutch, general
- Function sequence
General
The wet clutch is a starting device that reduces the frictionally engaged connection between the engine and the automatic transmission to a minimum on a stationary vehicle running at idle speed. When starting off it causes an increase in engine torque and enables continuously variable speed and torque adjustment.
Function requirements
- Ignition ON (circuit 15)
- Engine in operation
Function
Power transmission in the wet clutch ensues through the non-positive connection of the internal and external plates, which are pressed together by the clutch piston. The pressure in the clutch piston is regulated by the wet clutch's control solenoid valve.
The radial and the axial low dimension of the clutch in comparison to the torque converter, produces very low masses and inertia torques at simultaneously higher torque capacity. The low inertia torque enables an extremely dynamic engine start-up with short response times. The disadvantage of low heat capacity resulting from the low masses and reduced oil volume is compensated for by a cooling concept integrated into the clutch.
The use of an electronically controlled clutch as a starting device enables different starting-off strategies to be implemented. The start-off can be comfortable or performance-orientated, depending on the driver's preference.
Wet clutch, general
The fully integrated transmission control electric controller unit control unit regulates the pressure from the wet clutch solenoid, which then actuates the wet clutch regulating valve.
A lubricating oil volume flows through the housing of the wet clutch. In the process a lubrication oil pressure adjusts which has an opening action on the clutch piston. The lubrication oil pressure is fed back hydraulically to the regulating valve and is taken into account when adjusting the piston pressure.
The flow volume of the lubricating oil and a special geometry of the clutch linings ensure optimum cooling of the wet clutch. A specified valve pressure is calculated from the specified clutch torque by means of an inverted distance model of the wet clutch. The wet clutch solenoid adjusts the valve response pressure, whereby the output quantity is a specified valve current. A subordinate flow regulator sets the required valve flow at the wet clutch solenoid.
Function sequence
The wet clutch function sequence is divided up into the following subfunctions:
- Switch-on process
- Crawling
- Starting off
- Slip control
- Clutch change
- Stop vehicle
The shift strategy of condition transitions depends on engine load and vehicle speed or vehicle acceleration.
Engagement process:
The engagement process controls the filling of the clutch piston and the application of the clutch.
Creeping: In this condition, the specified clutch torque is calculated so that the vehicle starts to roll automatically when the driver deactivates the service brake.
Moving off:
The wet clutch enables a strategy-capable startup to be executed. A suitable start-off strategy is selected to match the driver request involved (accelerator pedal position and operating gradient).
Slip control:
If the startup process is complete, then slip control of the wet clutch is activated. The specified clutch torque is calculated so that a microslip is adjusted at the wet clutch. This helps to dampen drivetrain vibrations.
Clutch change:
During a gear change in the main transmission, a so-called clutch change, the specified clutch torque on the wet clutch is calculated so that the required clutch torque and the required slip are set for all the shift types involved.
Stop vehicle:
The specified clutch torque is calculated to match the type of deceleration involved to ensure that the clutch is opened at precisely the correct moment.
Wet clutch temperature model:
A temperature model of the wet clutch and the measured state variables are used to calculate the temperature on the wet clutch's steel lamellas. If the calculated temperature exceeds a limit value, then protective functions are activated to match the driving condition that then minimize the friction power on the wet clutch.
Substitute values are calculated in the event of mechanical and hydraulic sensor faults. If a hydraulics fault is detected, the wet clutch solenoid is flushed out in a controlled manner.