Wet clutch, function - GF27.51-P-1002AHO

TRANSMISSION 724.2 in MODEL 212 

Function requirements for wet clutch, general 

• Circuit 87 (connected circuit 15) ON

Wet clutch, general 

The pressure from the wet clutch control solenoid valve (Y3/8y9) is boosted by 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 lubrication oil volume flow 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 pressure regulator adjusts the valve pressure, the regulator output quantity being a specified valve current. A low-mounted flow regulator sets the required valve flow at the wet clutch control solenoid valve.

Inverted distance model

Based on an inverted distance model, the relationship between the input and the output parameter is shown; however, this is done in inverted form, i.e. the regulating variable is received as the output parameter.

Wet clutch function sequence 

The function sequence is subdivided into the following steps:

• "Engagement process", function sequence 
• "Creeping", function sequence 
• "Start-off", function sequence 
• "Slip control", function sequence 
• "Clutch change", function sequence 
• Function sequence for "Drive change" 
• "Stopping", function sequence 

The shift strategy for the state transitions depends on the following factors:

• Engine load
• Torque of the electric machine (A79/1)
• Vehicle speed
• Vehicle acceleration
• Charge level of high voltage battery (A100g1)
• Component protection functions

The transitions between the individual conditions is controlled by a function coordinator.

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, then the wet clutch control solenoid valve is flushed out in a controlled manner.

"Engagement process", function sequence 

The engagement process controls the filling of the clutch piston and the application of the clutch.

"Creeping", function sequence 

In this condition, the specified creeping torque is calculated so that the vehicle starts to roll automatically when the driver deactivates the brake.

The specified creeping torque target for the wet clutch depends on the following factors:

• Road gradient
• Slope
• Steering angle
• braking torque
• Charge torque of the electric machine
• Torque reduction when the vehicle is at a standstill

The specified creeping torque target for the wet clutch is set by the CDI control unit (N3/9) or the ME-SFI [ME] control unit (N3/10). The specified creeping torque target is designed to simulate the functional properties of a torque converter with the wet clutch.

Specified creeping torque on inclines 

On inclines, a maximum specified creeping torque of the wet clutch is set corresponding to the start-off pattern of a torque converter. To prevent the vehicle from rolling back, the maximum specified creeping torque can be exceeded for a short period of time (dependent on the component protection of the power electronics control unit (N129/1)).

Specified creeping torque on slopes 

On slopes, the specified creeping torque of the wet clutch (when approaching maximum creeping speed) is adapted to the drag torque value of the combustion engine.

Specified creeping torque dependent on the steering angle 

When the vehicle is at a standstill and the front wheels are turned, the pilot control torque of the wet clutch must be set dependent on the steering angle such that the additional rolling resistance compared to forward creeping is compensated.

Specified creeping torque dependent on the braking torque 

With increasing braking torque, the specified creeping torque of the wet clutch is reduced to the level of the combustion engine's drag torque.

Specified creeping torque dependent on the charge torque 

While operating the wet clutch, the charge torque of the electric machine must be considered when configuring the pilot control torque for the wet clutch, i.e. with a high charge torque of the electric machine (depending on factors such as, e.g. a low state of charge (SOC) value of the high-voltage battery), the pilot control torque must be increased correspondingly.

Specified creeping torque when the vehicle is at a standstill 

If, due to problems, no rpm signals are read in via the regenerative braking system control unit (N30/6) over a certain period of time even though torques are acting upon the drivetrain, then the transmission input torque is reduced.

"Start-off", function sequence 

The wet clutch enables a strategy-capable startup to be executed. A suitable startup strategy is selected depending on engine load and vehicle speed or vehicle acceleration.

"Slip control", function sequence 

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 in turn suppresses driveline vibrations.

"Clutch change", function sequence 

During a clutch change the specified clutch torque at the wet clutch is calculated so that with upshifts no slip expansion occurs but with downshifts a slip expansion at the wet clutch occurs.

Function sequence for "Drive change" 

The wet clutch enables a change from driving mode with combustion engine to purely electric driving mode without changing the vehicle speed. The wet clutch opens after the torque of the combustion engine was transferred to the electric machine. There must never be a change in driver request at the transmission input shaft during any time of the synchronization phase.

"Stopping", function sequence 

The specified clutch torque is calculated depending on the type of deceleration.