Retarder mode function

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Retarder mode function

The retarder function integrated into the turbo clutch (5) can be used when the dry clutch (2) is closed in all gear ranges. To this end, the turbine wheel brake (8) is activated and the turbine wheel (7) firmly braked against the turbo-retarder 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 the hollow shaft (4), acts as the rotor, the turbine wheel (7) as the stator. The retarder braking torque is controlled by changing the amount of oil in the working chamber of the fluid coupling (5). When starting off, the torque is controlled. The difference here is that the working chamber of the turbo-retarder clutch (5) is only charged with a regulated partial quantity of oil and it is not charge to the maximum level.

The heat produced during retarder mode is dissipated into the vehicle coolant system via the heat exchanger (15). 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 oil temperature of the turbo-retarder clutch (5).

When the maximum coolant or oil temperature is reached, the available retarder braking torque is automatically reduced down to a complete shutdown of the retarder function. This is indicated to the driver by a visual information message in the instrument cluster control unit (ICUC) (A1) or the instrument cluster screen (ICS) control unit (A153).

Retarder function request options

The retarder function is requested in the following cases:

 The service brake is actuated and the electronic brake system (EBS) control unit (A10b) requires the retarder function of the fluid coupling (5) to support the braking power.

 The cruise control is activated and the retarder function of the fluid coupling (5) is necessary for adjusting the vehicle speed e.g. for downhill driving.

 The Proximity Control Assist (code S1I) is activated and a request for the retarder function of the fluid coupling (5) is required for distance control.

 The continuous brake switch (S23 s4) on the right multifunction lever is actuated.

Preconditions

 Anti-lock braking system (ABS), anti-slip control (ASR), ESP® are not in control mode.

 The coolant temperature and oil temperature of the fluid coupling (5) have not exceeded the maximum limit value.

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Actuation process

During braking, the requested braking torque is forwarded by the drive control control unit to the fluid coupling control unit via the drive CAN. The fluid coupling control unit forwards an electrical signal to the proportional valve.

The proportional valve is connected to the auxiliary consumer circuit (a) of the vehicle's compressed-air system. As soon as the proportional valve receives an electric input signal from the fluid coupling control unit, this signal is converted into a pneumatic actuating pressure. This affects the oil in the oil reservoir (18) via the air duct (C).

At the same time, the turbine brake solenoid valve is activated and the turbine brake (8) is engaged. This then stalls the turbine wheel (7) against the housing of the fluid coupling (5).

The oil reservoir pressure sensor records the pneumatic actuating pressure in the oil reservoir (18) and converts it to an electric signal. A deviation from the permissible pressure tolerance is detected by the fluid coupling control unit and displayed as a fault message on the instrument cluster control unit or the instrument cluster screen control unit.

The opened oil feed shutoff valve (17) and the working chamber oil feed (A) enable the oil to flow through intake ports in the impeller (6) into the working chamber between the turbine wheel (7) and the impeller (6).

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 the stator, the oil is diverted and directed back to the impeller (6) at the inside diameter of the working chamber. 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 heated oil flows through the working chamber oil return (B) and the outlet restrictor (14) into the heat exchanger (15), where it dissipates the absorbed thermal energy into the coolant circuit (D). Depending on the operating condition, the cooled-down oil is then routed back over the heat exchanger (15) and the outlet check valve (16) to the working chamber, or it flows back into the reservoir (18).

When the braking procedure is completed, the proportional valve is deactivated. The actuating pressure in the oil reservoir (18) is reduced through the air duct (C) and the proportional valve. It is vented through a separate air duct in the housing gap between the turbo-retarder clutch (5) and the transmission.

At the same time, the quick drain solenoid valve is activated and pressure is applied to the oil feed shutoff valve (17). This causes the oil feed shutoff valve (17) to close whereupon the oil flow to the working chamber is then interrupted. The "residual oil" in the working chamber is returned through the heat exchanger (15) and the outlet check valve (16) into the oil reservoir (18). Once the working chamber has been drained, the quick drain solenoid valve is deactivated and the oil feed shutoff valve (17) is opened via a return spring.

The outlet check valve (16) is closed again to prevent the oil in the heat exchanger (15) draining into the oil reservoir (18).

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