Regenerative And Hydraulic Brake Mode, Function - GF42.22-P-1001GRH
MODEL 166.063
as of model year 2016
Function requirements for regenerative and hydraulic brake mode, general
- Hybrid 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 detected by the pedal angle sensor (B37/1) and the front axle brake pressure sensors and the front pressure sensor integrated in the traction system hydraulic unit, and sent to the Electronic Stability Program control unit (N30/4).
The Electronic Stability Program control unit communicates continuously over chassis CAN 1 (CAN E1) with the ME-SFI [ME] control unit (N3/10) and over the hybrid CAN (CAN L) with the power electronics control unit (N129/1) about the value of the currently available, requested and the applied regenerative braking torque.
The regenerative braking torque is only applied via the drive 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 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 by pressing the brake pedal.
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 ME-SFI control unit 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 to the Electronic Stability Program control unit over chassis CAN 1.
On the regenerative braking system, during a fault-free operation the hydraulic connection between the master brake cylinder and a reservoir chamber of the front axle brake circuit is released through the switching of a valve. After an integrated short free travel, operating the brake pedal causes the primary piston located in the master brake cylinder to be pushed. The brake fluid is shifted here into the reservoir.
At the same time, pressure is built up in both brake circuits until the brake pads are applied to the brake disk without immediately creating a noticeable braking power.
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 of the drive axle can be fully generated through regenerative braking, then the division into an additional hydraulic portion is no longer required. In this case the deceleration is generated solely by the alternator output. As soon as a speed of less than approx. 10 km/h is recorded during a brake application, a switchover takes place from regenerative 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. Regenerative braking is not possible at speeds lower than 3 km/h.
The braking torque actually applied is sent by the power electronics control unit over the hybrid CAN and chassis CAN 1 back to the Electronic Stability Program control unit using the ME-SFI [ME] control unit interface.
In the event of any intervention by the ABS, ESP® or BAS system, regenerative braking at the rear axle is interrupted for this brake application and the braking torque is only applied by the hydraulic service brake on the front and rear axle.
If the high-voltage battery is fully charged, regenerative 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 is able to absorb electrical energy again.
Regenerative and hydraulic brake mode, function sequence
If the driver's requirement for braking torque increases further along with the resulting pedal travel and built-up pressure in the front axle brake circuit (primary brake circuit), hydraulic braking torque is additionally applied to the front and rear axle.
In addition, if driving instability increases the regenerative braking torque at the rear axle is taken back and, if necessary, hydraulic braking torque is generated. For this, the high-pressure and return flow pump in the traction system hydraulic unit are actuated, which generates the required hydraulic 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 when:
- Certain faults occur in the brake control system
- The requested regenerative braking torque is not provided correctly
- A fault is present 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, purely hydraulic braking torque is generated. 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-9999GRH |