Braking & Hydraulic system
The late 20th century Rolls Royce hydraulic system, which supplies both brakes and suspension, shares a lot architecturally with the Citroen hydropneumatique system from which it was derived. In true Rolls Royce fashion however, it was 'improved', most notably by duplicating the main brake circuit to create effectively two completely separate brake systems in one car (indeed early Shadows had a third, surely taking redundancy to an extreme). An overview of a typical system is shown below (courtesy Marinus Rijkers and and Rob Wilde):
The hydraulic systems are driven by two engine driven hydraulic pumps that are integrated into the engine upper casing, one for each brake circuit ( one also supplying the rear suspension air springs). These pumps supply hydraulic fluid from the reservoir to nitrogen gas filled accumulators at a pressure of about 170bar.
TeslaRR replaces these two engine driven pumps with electric hydraulic pumps which feed the hydraulic accumulators in the normal way. These pumps are supplied by the 12V subsystem maintained by the DC-DC converter. The electric pumps are mounted, together with their associated accumulators, in the space vacated by the automatic transmission. They are fed by the existing reservoirs and are only active when the system hydraulic pressure is below nominal. In the event of failure of either pump, the hydraulic system alerts the driver in the same way as in the standard car. ABS, which was fitted to the later cars, operates, unmodified, in the same way as before.
Regenerative Braking
The traditional hydraulic braking is complemented by regenerative braking, which substantially reduces the amount of wear on the braking system.
Regeneration operates on the principle that the electric motor can also operate as an electrical generator. This places a load on the motor, which in turn provides an additional braking effect.
In addition, regen recovers a significant amount of the kinetic energy acquired by the vehicle during acceleration, which is why the weight of the vehicle is less important than its aerodynamics as far as range is concerned.
The amount of regen torque generated is proportional to vehicle speed, battery SOC, and the accelerator pedal position. The pedal provides maximum available regen torque when fully released, then proportionally less as the pedal is depressed. The motor delivers zero regen torque when the accelerator pedal reaches its neutral torque position, which varies depending on driving conditions.
The drive inverter converts this torque command into the appropriate 3-phase voltage and current waveforms to produce the commanded torque in the motor in the most efficient way. The torque command can be positive or negative. When the torque is used to slow the vehicle, energy is returned to the battery to create regenerative braking.
The maximum regeneration braking profiles take speed and corresponding drag into account to aim for a target total deceleration rate. Regeneration braking profiles are tailored to everyday driving conditions, and typically provide higher deceleration rates at lower speeds.
The amount of regenerative braking can be selected using the Driver Control Panel from one of 2 levels:
Standard: Provides the maximum amount of regenerative braking. When the accelerator is released, TeslaRR slows down, reducing the need to use the brakes.
Low: Limits regenerative braking. When the accelerator is released, TeslaRR takes longer to slow down and coasts further than if set to Standard.
Parking Brake
The Rolls Royce automatic transmission incorporates a 'parking pawl'. This pawl is a large metal piece that engages a notch, usually on the output shaft of the transmission, and prevents the output shaft, and therefore the vehicle, from moving when Park is selected.
Since the Tesla drive unit does not have a parking pawl, TeslaRR uses a 'cable puller' type electromechanical parking brake (EPB). This is driven by an EPB actuator attached to the rear crossmember, which acts via Bowden cables connected directly to the parking brakes on the rear wheels.
The EPB is activated by the Vehicle controller when in Park; once the parking brake has been activated, the brake pedal must be depressed before another gear can be selected, at which point the parking brake is automatically released.
In case of a flat battery the EPB can be released using the manual release handle in the luggage compartment.
Electro-mechanical Parking brake actuator