In some ways, EVs are the same as other cars. Four wheels, doors, seats, windows…all basically the same thing, but pushed around by a different drive system. But, under the sheet metal and behind the other covers, they can be quite different. Porsche Group engineers are developing advanced concepts of brake force distribution to ensure optimal recuperation without compromising comfort for electric vehicles. These new chassis demands require substantial research and innovative solutions in order to provide a smoother, safer ride.
As chassis developers transition to more electrified vehicles, they are challenged on two fronts: batteries add weight but also bring better driving dynamics. To accommodate the increased power from an electric motor, hydraulic wheel brakes must be added; this gives a negative effect since it reduces efficiency and range due to its additional weight and rising consumption.
The Porsche Taycan is designed to be particularly economical: as soon as you press the brake pedal, its electric motors switch into regeneration mode, not only stopping the vehicle but simultaneously creating electricity that can be used to boost battery power. Not needing a larger brake system allows for increased driving dynamics without compromising range due to added weight or size of this component.
In ordinary driving conditions, the Taycan’s electric motors are capable of providing 90% of its braking power. Only at speeds under 5 km/h does the hydraulic system need to intervene due to low deceleration from the electric motors. Additionally, if more powerful brakes are needed for full stops during higher speed driving, then friction brakes will become active to ensure complete slowing down and safe stopping.
The Taycan Turbo S is able to maximize the power of its regenerative braking system with up to 290 kW, allowing it to generate enough electricity in two seconds of deceleration that would be equivalent for driving 700 meters. Through this recuperation process, the range increases by an impressive 30%.
When engineering the chassis of battery-powered electric vehicles (BEVs), a primary difficulty lies in blending regenerative and hydraulic brakes. “The driver must not feel the transition between the systems,” emphasizes Martin Reichenecker, Senior Manager Chassis Testing at Porsche Engineering.
Ensuring a consistent transition demands much from the technology, given that electric and hydraulic braking systems operate differently: While an electric motor outputs a similar amount of torque each time, its hydraulic counterpart may vary in response to environmental factors like temperature or humidity. This means it’s conceivable for the power generated by both technologies to differ at the point of transfer – something drivers often experience as a jolt.
Programming Blended Brakes To Get This Right
Porsche has crafted algorithms for the Taycan to ensure a smooth transition from acceleration mode to regeneration. To do this, it monitors its hydraulic system like clockwork and use brake calibration during each charge process. This allows them to determine how much power will be delivered when the vehicle brakes next time, ensuring that there is no sudden change in performance due to mismanagement of energy recuperation levels.
When it comes to braking power in vehicles, two-thirds are typically generated by the front axle and a third is derived from the rear. The new Porsche Taycan electric system operates under this same principle: with its larger back motor, two-thirds of the stopping force come from its front engine while one third originates from the backend even though more potential energy could be recovered.
By altering the distribution of braking force between the axles, we could unlock great potential. A crucial consideration to bear in mind is that driving stability must be maintained; thus it is necessary to limit maximum rear-axle contribution depending on the situation at hand so as to ensure a reliable reserve level of steadiness.
“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.
For a seamless transition between the hydraulic and generative brakes, it is essential to ensure optimal driver and passenger comfort. To this end, Porsche proposes using two algorithms simultaneously: The first algorithm analyzes driving conditions in order to determine an ideal braking force distribution across both front and rear axles — these calculations are based on previous test bench data.
An algorithm developed by Traut is designed to select the most efficient corridor and apply it accordingly in order to guarantee an optimized deceleration which would result in a noteworthy increase of range.
Until recently, the brake in automobile engineering was a relatively solitary system. But with electric vehicles emerging on the market, deceleration now requires collaboration from various vehicle components: powertrain, battery, and power electronics — all of which necessitates an increased amount of interdisciplinary work for chassis developers. What’s more remarkable is that there even exists a dedicated display panel to manage braking activity within the instrument cluster.
In the future, engineers working on the brake must collaborate more intently with those developing transmission because recuperation also involves an electric motor and thus two-speed transmission (as seen in Porsche’s Taycan).
Porsche’s Different Approach To The Pedal’s Role
Producers of electric cars are mostly focusing on one-pedal driving. By simply taking your foot off the pedal, you can immediately start regenerating energy and in some cases experience such extreme braking that the brake lights will come on automatically. This means that most scenarios allow drivers to operate their car with only one pedal. By contrast, Porsche utilizes coasting: a smoother method of allowing the vehicle to travel forward with no external power. The recuperation process only begins when you apply pressure on the brake pedal.
“This is a more efficient way of driving, because it keeps the kinetic energy in the vehicle,” says Reichenecker. One-pedal driving, on the other hand, recuperates first, and only then converts the recovered energy back into propulsion. “That results in twice the losses.”
So, Porsche’s approach could prove to be more efficient, but only if driven efficiently by the driver.
Reduced Brake Wear, But Occasional Brake Use To Keep Them Clean
Another positive effect of recuperation is that there is less wear on the hydraulic brakes. “We expect that brake pads will have to be replaced due to aging in the future rather than wear,” as Traut surmises. 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.
Featured image provided by Porsche.
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