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Developers at Porsche Group are Working on New Concepts for Brake Force Distribution

Updated: 6 days ago

Battery electric vehicles place new demands on the chassis, particularly when it comes to braking and recuperation. Developers at the Porsche Group say they are working on new concepts for brake force distribution which they hope will enable better recuperation without compromising comfort.


Chassis developers are challenged by electrification on two fronts: Batteries make the vehicles heavier, but the cars often exhibit better driving dynamics. Both of these factors usually necessitate a more powerful hydraulic wheel brake. However, this reduces efficiency and sacrifice range because the weight increases and consumption rises.


The Porsche Taycan gets by without a more extensive brake system—thanks to recuperation: As soon as the driver presses the brake pedal, the electric motors switch to generation mode. Once they do, it is no longer the motors driving the wheels but the other way around. This breaks the vehicle and, at the same time, generates electricity that can be used to charge the battery. What’s crucial for chassis developers is that recuperation does not require the brake to be made larger despite the increase in driving dynamics. The brake, therefore, has no negative impact on the range.


Brake Force Distribution


In the Taycan, 90 % of all time the driver brakes in everyday situations, this can be done using electric power only, i.e. without the involvement of the hydraulic system. The latter is only used at speeds below 5 km/h, when the electric motors barely develop braking power. In addition, the friction brake steps in when the electric motors do not have sufficient deceleration power, for example, during full braking from high speeds. The Taycan Turbo S (Electric power consumption* combined (WLTP) 23.4 – 21.9 kWh/100 km, CO₂ emissions* combined (WLTP) 0 g/km, Electric range* combined (WLTP) 440 – 468 km, Electric range* in town (WLTP) 524 – 573 km) can generate up to 290 kW of electric power during braking. At this power level, two seconds of deceleration are enough to generate electricity to drive around 700 meters. Overall, recuperation increases the range by up to 30 %.


One of the significant technical challenges in chassis development for battery electric vehicles (BEVs) is blending, which combines regenerative and hydraulic braking. “The driver must not feel the transition between the systems,” emphasizes Martin Reichenecker, Senior Manager of Chassis Testing at Porsche Engineering.


Guaranteeing a smooth transition places great demands on the technology because the braking systems operate differently: While an electric motor always delivers the same braking torque, the torque from its hydraulic counterpart may vary each time due to environmental influences such as temperature and humidity. The hydraulic braking power likely differs from the electric braking power at the transition point. The driver feels this as a jolt.


Porsche has developed algorithms for the Taycan that prevent this from happening. First, they monitor the hydraulic system continuously: During each charging process, the brake is calibrated to determine the current ratio of brake pedal travel to brake pedal force. This allows the algorithm to estimate how much power the hydraulic system will deliver the next time the vehicle is braked, and deploy it precisely so that the transition to recuperation mode remains smooth.

In vehicles, braking power is usually unequally distributed: Two-thirds of it is provided by the front axle, one-third by the rear axle. The exact ratio applies for the electric system in the Taycan: The front electric motor provides two-thirds of the braking power, and the rear one offers one-third—although the rear motor is larger and could theoretically contribute (and recuperate) more. This potential could be leveraged by varying the distribution of the braking force between the axles. In this context, it is essential to note that, for driving stability, the maximum rear-axle contribution must be limited according to the situation to ensure a sufficient stability reserve. “The electric motor that can absorb the most energy would then deliver the greatest braking torque,” explains Ulli Traut, Function Developer and Integration Engineer Regenerative Braking at Porsche AG.


As with the interaction between the hydraulic and generative brakes, the shifts in forces must not compromise driver or passenger comfort. One solution would be to have two algorithms operating simultaneously: The first one analyzes the driving situation and suggests a ‘corridor’ in which the braking force is optimally distributed between the front and rear axles—based on test bench data. A second algorithm selects a distribution that suits the current driving situation from the most efficient ‘corridor’. According to expert Traut, this solution would guarantee ideal deceleration and bring a “significant gain in range”.


Until now, the brake in automotive engineering has been a relatively isolated system. However, this has now changed in electric vehicles because many more parts are involved in deceleration: Powertrain, power electronics, and battery. Moreover, the brake has its display in the instrument cluster. All of this requires more interdisciplinary work from chassis developers. For example, the engineers working on the brake will have to confer more closely with their colleagues working on transmission in the future, for instance, because recuperation also involves the electric motor and, therefore the transmission (the Taycan has a two-speed transmission on the rear axle).


Brake Force Distribution


This creates new demands on its load-bearing capacity and offers new opportunities, as Reichenecker points out: “Developers have completely new degrees of freedom.” The potential ability to make the distribution of braking force between the front and rear axles variable is the best example of this, he says. In addition, Reichenecker expects that technology for the chassis and drive components will continue to merge. “In future architectures, most software functions will presumably be united in a single control unit.”


Regarding driving, some manufacturers of electric vehicles are concentrating on what is known as one-pedal driving. The principle is that when the driver takes their foot off the pedal, the car starts recuperating energy immediately—and in extreme cases, brakes so hard that the brake lights come on. This means that in most situations, the car can actually be driven with one pedal.

On the other hand, Porsche makes use of coasting, which is the more natural process of allowing the vehicle to continue to roll unpowered. Recuperation only starts when the brake pedal is stepped on. “This is a more efficient way of driving because it keeps the kinetic energy in the vehicle,” says Reichenecker. On the other hand, one-pedal driving recuperates first and only then converts the recovered energy back into propulsion. “That results in twice the losses.”


Another positive effect of recuperation is less wear on the hydraulic brakes. “We expect that brake pads will have to be replaced due to ageing in the future rather than wear,” as Traut surmises. In addition, a feature has been developed for the Taycan to keep the brake discs clean now that they are being used less often: The vehicle brakes at regular intervals using the hydraulic system only, and without the electric motors, to remove dirt from the discs. This could be a considerable advantage in the future because the EU is planning for brakes to emit fewer particulates. The new Euro 7 emissions standard, which is due to come into effect in 2025, will be the first time that limits are set for brake abrasion. This will then put electric vehicles like the Taycan, which only uses electricity nine out of each ten times it brakes, in a good starting position.

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