Regenerative Braking With ERAD
Brakes
The brake system on the Twin Engine is adapted for hybrid operation and is slightly different from the XC90 (16-) with conventional driveline.
The Twin Engine has larger diameter brake discs front and rear. The front brake discs have dimensions of 366 mm x 30 mm, and the rear brake discs 340 mm x 20 mm.
| 1 | Brake pedal angle sensor | 3 | Return spring |
| 2 | Pyrotechnic, Active Decoupling | 4 | Pedal simulator |
The brake pedal in the Twin Engine is the "Brake by wire" type, which means that the driver's braking request is transferred electronically from the pedal assembly to the hydraulic brake system.
The aim of a system of the "Brake by wire" type is that the brake pedal movement shall be disengaged from the brake hydraulics. This enables the use of ERAD (Electric Rear Axle Drive) in order to achieve part of the braking performance and at the same time charge the car's high-voltage battery through regeneration. If the "Brake by wire" system does not work for any reason, the friction brakes will still work as in a conventional brake system but with further pedal travel. The driver must press past a play of approx. 40 mm before the friction brakes are initialized.
The pedal simulator (4) on the Twin Engine is fully mechanical. Normally, when the "Brake by wire" system is working, a "natural" brake pedal feel is achieved within the working range of the smaller coil spring (1). The thicker coil spring (2) then acts as a natural stop.
If the "Brake by wire" system does not work then the braking effect will not be obtained until the thicker coil spring starts to be compressed.
The pedal angle sensor consists of a hall sensor with two independent signals that measure the brake pedal angle. The signals are used by VDDM to register the pedal position for activation of regenerative braking.
For safety reasons, a redundant system is used in which the pedal sensor sends two signals that are independent of each other.
On the front of the internal combustion engine, where the alternator is located on the XC90 (16-) with conventional driveline, the Twin Engine has an electric vacuum pump that operates as a complement to the mechanical vacuum pump. The electric pump shall ensure that there is also sufficient vacuum available during electric operation while the internal combustion engine is not running. The brake vacuum sensor is fitted in the brake booster and communicates with VDDM. When the vacuum falls below a predetermined level, the electric vacuum pump starts. If the vacuum is still insufficient then the internal combustion engine will be started.
Regenerative braking using ERAD (Electric Rear Axle Drive)
ERAD can be used to return kinetic energy back to the high-voltage battery, either by means of regenerative braking or simulated engine braking. ERAD can be used at speeds between approx. 10 km/h and 170 km/h, and for a deceleration of up to approx. 0.3 G depending on the car's load.
Brake control
Regulation of the brake function with ERAD in relation to the friction brakes is controlled by VDDM. During braking, the pedal angle sensor sends information to VDDM about how much the driver depresses the brake pedal. VDDM recalculates the angle to a retarding torque. The request for regulation is sent via Propulsion CAN to ECM, which in turn calculates how much of the requested retarding torque can be generated by ERAD without compromising the car's stability. ECM communicates the results back to VDDM which, if necessary, tops up with the friction brakes. If ECM cannot apply the requested brake torque (e.g. because the battery is fully charged) then VDDM regulates the torque by applying the whole retarding torque with the friction brakes. In the event of heavy braking, only the friction brakes are used.
ReSC
ReSC is a function in VDDM whose purpose it is to monitor and prevent the rear wheels from losing traction during regenerative braking.
During braking where regenerative braking via ERAD is used to a large extent, an additional source is introduced, which may lead to wheel lock-up and instability. Wheel lock-up can be avoided by limiting the torque on the rear axle. In the event of wheel lock-up, VDDM sends a signal to ECM to reduce or stop the braking force torque for ERAD. ReSC can reduce the whole torque to zero on the rear axle within 30 ms. The purpose of the function is to optimize brake regeneration via ERAD and, at the same time, avoid wheel lock-up and instability.
Regenerative braking
During normal braking, the system attempts to optimize the braking to use regenerative braking via ERAD as much as possible. During lighter braking, braking is applied in at least three stages.
- The friction brakes are applied.
- Regenerative braking is started via ERAD.
- The friction brakes are released, while regenerative braking with ERAD continues.
- In instances where braking continues down to stationary, from approx. 10 km/h the braking force changes over to the friction brakes again. This is in order to achieve a smooth transition at the end when the friction brakes are easier to control.
Simulated engine braking
So that the driving characteristics in the different drive modes shall be as similar as possible, during pure electric operation, ERAD can generate an engine braking force equivalent to engine braking by the internal combustion engine at speeds up to 70 km/h.
Since heavy braking leads to higher utilization of the friction brakes, the driver should slow down in good time in order to recover as much brake energy as possible.