We’ve heard the expressions ‘fly by wire’, ‘drive by wire’ and now ‘brake by wire’. Generically, what do they mean?
The use of the term ‘by wire’ generally means that an actuator, be it on the ailerons of an aircraft, the throttle of a car, or, in this case, the brakes, is actuated by a device that is under the control of a computer.
Drive-by-wire throttles have been with us for many years and are an easy way of explaining the technology. In this example, a potentiometer is connected to the throttle pedal. The potentiometer is a simple electrical sensor that measures the exact position of the pedal. This position is fed to the computer in the ECU, which calculates what torque the driver is demanding with the pedal and provides it in the best possible way. Of course, the primary means will be to open the engine throttles, and this is done by an actuator, which may be electrically or electro-hydraulically driven. The electrical wiring connection between the pedal and the ECU and that between the ECU and the actuator gives rise to the expression ‘drive by wire’.
Aren’t electronic systems inherently more unreliable than mechanical systems?
No. They may be more complex and therefore have more potential points of failure, but they are actually more reliable in the long term.
They also allow for smarter control. With an old-fashioned throttle cable, the position of the throttle was determined by the position of the driver’s foot. With modern systems this can be overridden during, for example, a gear shift. In terms of reliability, if a cable broke, the car stopped. With electronic systems, an element of redundancy can be easily built in by having two tracks on the potentiometer. Software then automatically detects if one fails and switches to the second with no loss of performance.
Why has ‘brake by wire’ come in for 2014?
The 2014 powertrains’ complex hybrid systems rely heavily on energy recovery through braking. Essentially, an electric motor and an electric generator are the same thing. If the armature of an electric motor is turned, it produces electricity; if electricity is applied to the armature, it will rotate. This principle is what the energy recovery and energy release of a hybrid vehicle is all about. However, the recovery of kinetic energy and conversion to electrical energy doesn’t come for free. As a generator produces electricity it produces a drag that acts like the brakes on the car. If this drag were constant then there would be no problem, but the electricity produced by the generator is stored in a battery.
This battery must not be overcharged, so as the battery reaches full charge, the generator is shut down. Since this is indirectly connected to the rear wheels it has a major effect on the braking capability of the rear axle. In addition, there is a hard limit as to how much energy can be recovered by the system and when this limit is reached then the recovery is also switched off.
The brake-by-wire system is designed to ensure that, as the electrical energy recovery is switched in and out, the apportioning of braking energy between the electrical ‘brake’ and the hydraulic brake is managed in a way that does not disturb the car or driver.
Why wasn’t this necessary with last year’s kinetic recovery system (KERS)?
It would have helped last year, but since the 2013 KERS was only 60kW it was not powerful enough to cause major problems. For 2014 this power was increased to 120kW, which represents a signicant amount of braking power.
So how does the new system work?
It is a complex system but, in essence, the pressure in the front brakes is controlled entirely hydraulically in response to the driver’s pressure on the brake pedal, while the rear brake pressure can be modied by the control system to achieve a constant brake balance irrespective of the state of the rear-axle energy recovery system. Safety is always paramount, and even though the rear brakes are subject to electronic control, they always have a direct hydraulic connection so they will always slow the car even in the event of a failure of the electronics.
As the energy recovery system switches on to recover energy, the brake-by-wire system will decrease the pressure in the rear brakes and, as the recovery system switches off, the brake-bywire will allow the full hydraulic pressure to be applied to the rear wheels.
Are there changes to the brake system itself?
Yes. Since the energy recovery system is doing so much more of the work of braking, it has been possible to t smaller rear brakes. The large six-piston rear callipers that were ubiquitous in previous years have now been replaced by small four-piston callipers. In addition, the complex mechanical systems that used to be employed to shape the brake balance curve as a function of brake pressure have now been replaced with much more simple electronic mapping.
How does the brake-by-wire system affect the feel of the brakes?
The simple answer is that if it is working perfectly, the driver should feel nothing as the control switches in and out. In fact, the system should improve stability under braking and a pleasant by-product of the layout is that the brake pedal now has much greater stiffness, because it is not exposed to the compliance of the rear brake lines.
Brake-by-wire manages the brake effort of the rear wheels and compensates for extra drag caused by the recovery systems as they harvest energy. Some drivers, such as Kimi Räikkönen (pictured) are reportedly struggling to get to grips with it