Retarder mode function

W25.50-1029-76

Power flow diagram in retarder mode

1 Flywheel

2 Dry clutch

3 Vibration damper

4 Hollow shaft

5 Turbo clutch

6 Impeller

7 Turbine wheel

8 Turbine wheel brake

9 Brake roller freewheel

10 Transmission input shaft

Retarder mode function

In all shifting stages, the retarder function integrated into the turbo clutch (5) can be used when the dry clutch (2) is closed. To this end, the turbine wheel brake (8) is activated and the turbine wheel (7) firmly braked against the turbo clutch housing (5). At the same time, oil is routed into the working chamber between the impeller (6) and turbine wheel (7). The impeller (6) driven by the transmission input shaft (10) via the dry clutch (2) and hollow shaft then becomes a rotor and the turbine wheel (7) a stator. The retarder brake torque is regulated - as for regulation of torque when driving off - by changing the oil quantity in the turbo clutch's working chamber.

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The heat generated during retarder operation is dissipated through the heat exchanger into the vehicle's cooling system. The retarded braking torque that can be requested is limited by the gear engaged, the transmission load limit, the temperature of the coolant and the temperature of the turbo clutch's oil.

When the maximum coolant or oil temperature is reached, the available retarder brake torque is automatically reduced down to a complete shutdown of the retarder function.

This is indicated by the permanent brake indicator lamp (P2h8) flashing on and off.

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Retarder function request options

The retarder function is requested:

 If the service brake is operated and the brake-control control unit (BS) (A11) requests the use of the turbo clutch's retarder function (TK) to assist the braking power.

 If the cruise control has been activated and an actuation of the turbo clutch's retarder function (TK) is necessary to adapt the vehicle speed (for example when driving down a mountain)

 by manual operation of the engine control/permanent brake operating lever (S2)

 If the autonomous intelligent cruise control (ART) has been activated and a request for actuation of the turbo clutch's retarder function (TK) is necessary to regulate the distance

Requirements

 Antilock brake system (ABS) and acceleration slip regulation (ASR) are not in closed-loop operation.

 Voltage supply of all control unit affected and networking intact.

W25.50-1025-79

6 Impeller

7 Turbine wheel

8 Turbine wheel brake

9 Brake roller freewheel

11 Oil separator

12 Shaped breather

13 Check valve

14 Quick drain valve

15 Heat exchanger

16 Outlet restrictor

17 Outlet check valve

18 Inlet check valve

19 Air restrictor

20 Oil supply chamber

A Working chamber oil feed

B Working chamber oil return

C Vent duct

D Coolant circuit

E Oil circuit (turbine wheel brake)

F Compressed air supply

G Unpressurized line

a Ancillary consumer circuit

A3 Vehicle engine control (FMR) control unit

A11 Electronic brake system (EBS) control unit

A125 Turbo clutch (TK) control unit

B187 Oil reservoir pressure sensor

B188 Ventilation pressure sensor

B189 Turbine wheel rpm sensor

B190 Oil temperature sensor

B191 Coolant temperature sensor

CAN 1 Vehicle CAN

Y151 Proportioning valve

Y152 Fast emptying solenoid valve

Y153 Turbine wheel brake solenoid valve

P2 Instrument (INS)

P2h8 Permanent brake indicator lamp

S2 Engine control/permanent brake operating lever

Actuation process

When braking, the braking torque required by the drive control (FR) control unit (A3) is forwarded to the turbo clutch control unit (TK) (A125) over the vehicle CAN (CAN 1). The turbo clutch control unit (TK) (A125) forwards an electrical signal to the proportional valve (Y151).

The proportional valve (Y151) is connected to the auxiliary consumer circuit (a) of the vehicle's compressed-air system.

As soon as the proportional valve (Y151) receives an electrical input signal from the turbo clutch control unit (TK) (A125), the signal is converted into a pneumatic actuating pressure, which then acts on the oil in the oil reservoir chamber (20) through the vent duct (C).

At the same time, the turbine wheel brake solenoid valve (Y153) is activated and the turbine wheel brake (8) engaged. This then brakes the turbine wheel (7) hard against the turbo clutch housing.

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During the brake application the quick drain solenoid valve (Y152) is activated. The applied air pressure closes the quick drain valve (14), which is located in the outlet duct downstream of the working chamber.

The oil reservoir chamber pressure sensor (B187) records the pneumatic actuating pressure in the oil pressure reservoir, and converts it to an electrical signal. Any deviation from the permissible pressure tolerance is detected by the turbo clutch control unit (TK) (A125) and shown on the instrument cluster (INS) (P2).

The oil is pumped via the oil feed working chamber (A) and the intake restrictor (19) through intake ports in the impeller (6) to the working chamber between the turbine wheel (7) and impeller (6).

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The hollow shaft (4) driven and rotating impeller (6) accepts the oil into its rotary motion and accelerates it.

In the turbine wheel (7), which acts as a stator, the oil is deflected and fed again to the impeller (6) at the working chamber's inside diameter.

An outlet pressure is built up, which generates a braking torque on the driving impeller (6).

The air in the working chamber is displaced by the oil and routed over the profile ventilation (12) and oil separator (11) to the atmosphere.

The profile ventilation (12) is then closed in the maximum filling range.

From an outlet pressure of approx. 0.1 bar, the outlet check valve (17) opens and the heated oil flows through the working chamber oil return (B) into the heat exchanger (15), where it dissipates the absorbed thermal energy to the coolant circuit.

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The cooled oil then flows from the heat exchanger (15) over the inlet check valve (18) into the oil reservoir chamber (20). Depending on the operating condition, the oil is then fed back to the working chamber or it remains in the oil reservoir chamber (20).

Once the brake application is over, the proportional valve (Y151) is deactivated. The actuating pressure in the oil reservoir chamber (20) is reduced through the vent duct (C) and the proportional valve (Y151). It is vented through a separate vent duct in the turbo clutch housing.

The quick drain solenoid valve (Y152) is deactivated and the exhaust air is routed over the profile ventilation (12) into the working chamber.

At the same time, the quick drain valve (14) is opened and the pending outlet pressure is then reduced. The oil flows over a bypass duct into the secondary housing and straight back into the oil reservoir chamber (20), without passing through the heat exchanger (15).

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The profile ventilation (12) is now opened again, and the working chamber is connected through the oil separator (11) with the atmosphere.

The inlet check valve (18) and outlet check valve (17) are now closed again, to prevent any oil draining into the heat exchanger (15) in the oil reservoir chamber (20).

The turbine wheel brake solenoid valve (Y153) is deactivated and the reservoir pressure escapes into the atmosphere through the turbine wheel brake (8) piston.