Regenerative And Hydraulic Brake Mode, Function - GF42.22-P-1001RFH
Model 205.012/013/047/053/054/147/212/213/247/253,
Model 253.353/354/953/954/99
Hydraulic diagram (except model 253.353/953)
Hydraulic diagram (model 253.353/953)
Function requirements, general
- Hybrid drive system "READY"
- Drive train operational
- Selector lever position "D"
- Vehicle speed above 8 km/h
Regenerative and hydraulic brake mode, general
The pedal angle sensor (B37/1) and the front-axle brake pressure sensor as well as - except on model 253.353/953 - the front pressure sensor in the traction system hydraulic unit record the driver's braking request.
The Electronic Stability Program control unit (N30/4) reads in the signals from the pedal angle sensor, front-axle brake pressure sensor and the front pressure sensor directly.
The Electronic Stability Program control unit continuously communicates over the suspension FlexRay (Flex E), powertrain control unit (N127) and the engine CAN (CAN C) with the CDI control unit (N3/9) (Diesel engine) or the ME-SFI [ME] control unit (N3/10) (Gasoline engine) as well as on model 253.353/953 over the suspension FlexRay with the power electronics control unit (N129/1) about the value for the currently available, requested and the implemented regenerative braking torque.
The regenerative braking torque is implemented through the drivetrain. The extent of the braking torque that can be used or applied depends greatly on the limits of driving stability and the performance of the electric machine (A79/1), the power electronics and the state of charge and - on model 253.353/953 - the temperature of the high-voltage battery (A100g1).
The braking torque at the front axle continues to be applied hydraulically. When the driver's brake command is recorded, purely regenerative braking torque is applied only through the rear axle depending on the required brake power.
If the driver's braking request is greater than the braking torque that can be created purely through regenerative braking, additional hydraulic braking torque is built up at both axles.
The subsequent function sequence is subdivided into the following sections:
- Regenerative braking mode
- Regenerative and hydraulic brake mode
- Hydraulic braking mode
Regenerative braking mode
The CDI control unit or the ME-SFI [ME] control unit calculates the maximum available regenerative braking torque that can be generated by the electric machine, and sends appropriate information over the drive train CAN (CAN C1), the powertrain control unit and the suspension FlexRay to the Electronic Stability Program control unit.
In the regenerative braking mode, the connection between the master brake cylinder and a reservoir chamber of the front axle brake circuit or - on model 253.353/953 - of both brake circuits is released by switching a valve. Following a brief free travel, operation of the brake pedal causes the primary piston to be displaced in the master brake cylinder. The brake fluid is then moved into the pressure reservoir. At the same time, enough pressure is built up in both brake circuits for the brake pads to be applied to the brake disks without immediately creating a noticeable braking power.
The Electronic Stability Program control unit divides the overall braking torque moment requested by the driver to match the driving condition into a regenerative part (to be applied by the drive train) and a hydraulic part (to be applied over the service brake).
If the overall braking torque can be generated through solely regenerative means, a splitting into an additional hydraulic part is not required. As soon as a speed of roughly 10 km/h is dropped below during a brake application, a switch is made to hydraulic braking. The 3-phase A/C voltage generated by the electric machine is converted by the power electronics control unit into high voltage direct voltage and fed to the high-voltage battery. Regenerative braking is not possible at speeds lower than 3 km/h.
Information on the currently implemented braking torque is sent by the power electronics control unit over the hybrid CAN (CAN L), powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.
In the event of an ABS, ESP® or BAS system control intervention, regenerative braking at the rear axle is interrupted for this brake application and the braking torque is applied solely by the hydraulic service brake.
Once the high-voltage battery is fully charged, regenerative braking is stopped.
Regenerative and hydraulic brake mode
If the braking torque requirement continues to increase, a hydraulic braking torque is implemented at the front and rear axle.
As driving instability increases, the regenerative braking torque is canceled and, if necessary, hydraulic braking torque is also generated. For this, the high-pressure and return flow pump in the traction system hydraulic unit are actuated, which generates the required pressure in the brake circuits.
As a matter of principle, the brake pedal feeling on a regenerative braking system is perceived differently from that on a conventional brake system.
Regenerative braking is switched off for the driving cycle if certain faults occur in the brake control system, if the requested regenerative braking torque is not correctly provided or if there is a fault in the hybrid drive system.
Hydraulic braking mode
If the electrical machine or the power electronics are overloaded or the high-voltage battery is fully charged, no regenerative braking torque is generated. All braking force is generated hydraulically. This takes place at the front and rear axle in the conventional manner; i.e. via a pressure buildup in the master brake cylinder by the driver.
| Overview of system components for regenerative braking system | GF42.22-P-9999RFH |