Regenerative And Hydraulic Brake Mode, Function - GF42.22-P-1001LFH
Model 222.004/057/104/157/163/173
The front axle brake pressure sensor registers the hydraulic inlet pressure (braking by the driver using brake pedal) in the front axle brake circuit. The front pressure sensor measures the hydraulic pressure in the right front axle brake circuit. The rear pressure sensor measures the hydraulic pressure in the rear axle brake circuit.
Function requirements for regenerative and hydraulic brake mode, general
- Hybrid drive system "READY"
- Drive train operational
- Selector lever position "D"
- Vehicle speed > 8 km/h
Regenerative and hydraulic brake mode, general
The driver's brake command is registered by the pedal angle sensor and the front axle brake pressure sensor integrated in the traction system hydraulic unit, and is sent to the Electronic Stability Program control unit (N30/4).
Via chassis FlexRay (Flex E), the Electronic Stability Program continuously sends the value of the currently available regenerative braking torque, the requested regenerative braking torque and the implemented regenerative braking torque to the CDI control unit (N3/9) (for diesel engine) or ME-SFI control unit (N3/10) (for gasoline engine), and the power electronics control unit (N129/1).
The regenerative braking torque is only applied via the rear axle. The extent of the regenerative 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 charge level 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 brake command is larger than the purely regenerative braking torque that can be generated and if the pedal travel and built-up pressure in the front axle brake circuit thus increases (primary brake circuit), then hydraulic braking torque is additionally applied to the front axle.
The subsequent function sequence is subdivided into the following points:
- Regenerative brake mode function sequence
- Regenerative and hydraulic brake mode, function sequence
- Hydraulic brake mode function sequence
Regenerative brake mode function sequence
The CDI control unit (for diesel engine) or the ME-SFI control unit (for gasoline engine) forecasts the current status of the maximum available regenerative braking torque, which can be generated by the electrical machine at the rear axle. This status is sent via the powertrain CAN (CAN C1), the powertrain control unit (N127) and the suspension FlexRay to the Electronic Stability Program control unit.
On the regenerative braking system, during a fault-free operation, the hydraulic connection between the master brake cylinder and the rear axle brake circuit is blocked through the rear axle switchover valve.
After an integrated short free travel, operating the brake pedal causes the primary piston located in the master brake cylinder to be pushed.
Due to this, pressure is constantly built up in the front axle brake circuit and the brake pads are thus applied to the brake disk without generating noticeable hydraulic brake power. With the increasing pressure in the front axle brake circuit, the back pressure that arises in the blocked pressure chamber for the rear axle brake circuit (secondary circuit) leads to a corresponding brake pedal feeling. The driver's brake command is detected by the pedal angle sensor via the pedal travel and a brake pressure sensor integrated in the traction system hydraulic unit, and read in and processed by the Electronic Stability Program control unit.
The Electronic Stability Program control unit divides the overall braking torque moment requested by the driver according to 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 requested total braking torque above the rear axle can be fully regenerated, then the split into an additional hydraulic portion is no longer required. In this case the deceleration is generated solely by the alternator output. As soon as the speed drops below 10 km/h during a braking procedure, the system transitions from recuperative braking to hydraulic braking. The 3-phase AC voltage generated during regenerative braking by the electric machine is converted by the power electronics control unit into high-voltage DC voltage and supplied to the high-voltage battery. Recuperative braking is not possible at speeds lower than 3 km/h.
The braking torque actually applied is sent by the power electronics control unit via the hybrid CAN (CAN L) and chassis FlexRay back to the Electronic Stability Program control unit.
In the case of an ABS (anti-lock braking system), ESP® (Electronic Stability Program) or BAS (Brake Assist) control action, recuperative braking via the rear axle is terminated for this braking procedure and the braking torque is implemented exclusively via the hydraulic service brakes of the front and rear axles.
If the high-voltage battery is fully charged, recuperative braking is not possible. In this case, braking is performed solely by the hydraulic brake until the high-voltage battery is partly discharged again through energy takeoff and can again absorb electrical energy.
Regenerative and hydraulic brake mode, function sequence If the driver's requirement of braking torque increases and if it causes pedal travel and built-up pressure in the front axle brake circuit (primary brake circuit), hydraulic braking torque is additionally applied to the front axle.
Furthermore, if driving instability increases, the recuperative braking torque via the rear axle is decreased and, if necessary, hydraulic braking torque is generated via the rear axle brake circuit. For this, the high-pressure and return flow pump in the traction system hydraulic unit is actuated, which generates the required hydraulic pressure in the rear axle brake circuit.
As a matter of principle, the brake pedal feeling on a regenerative braking system is perceived differently from that on a conventional brake system.
Recuperative braking is deactivated for the ignition cycle if the requested recuperative braking torque cannot be provided correctly or if there is a fault in the hybrid system.
Hydraulic brake mode function sequence
If no regenerative braking torque can be generated due to overload of the electric machine or the power electronics or due to the charge level of the high-voltage battery being too high, a purely hydraulic braking torque is generated. At the front axle, this takes place in the conventional manner, i.e. via a pressure buildup in the master brake cylinder. At the rear axle, hydraulic pressure is generated during fault-free operation via the high-pressure and return flow pump in the traction system hydraulic unit.
| Overview of system components for regenerative braking system | GF42.22-P-9999LFH |