One Pedal or Two?
If you’ve ever driven a hybrid or electric car, you’ve experienced regenerative braking, whether you know it or not. Regenerative braking captures energy that would normally be wasted while slowing down and uses it to charge the vehicle’s battery. This helps increase range, and also reduces wear on the traditional braking system. Instead of pads and rotors turning kinetic energy into heat, regenerative braking turns that energy back into electricity.
In the quest to get as much range as possible out of their batteries, many electric cars have, by default, very aggressive regenerative braking. When you lift off the accelerator pedal, the electric motor turns into a generator, and you immediately feel the car slowing down. In many cars, this effect is so pronounced that you barely ever use the brake pedal itself, which is how the term “one-pedal driving” came about; Tesla was the carmaker that first popularized the term. The idea is that by applying braking as soon as the car isn’t accelerating, more energy can be recaptured and put back into the battery. Makes sense, right?
Porsche doesn’t think so. Its electric cars, like the Taycan, default to coasting instead of aggressive regeneration. Lift off the accelerator in a Porsche EV, and the vehicle continues to roll forward, unpowered, using no electricity, and simply harnessing the momentum of the vehicle. Regeneration only takes place if the driver steps on the brake pedal.
More Efficient Freeway Travel
The company says that not only does this feel more natural to drivers coming out of internal-combustion engines – your automatic-transmission gasoline car doesn’t immediately start slowing down when you lift off – but is also a more efficient way of driving. Porsche says that coasting keeps the kinetic energy in the vehicle. On the other hand, the company says, immediate and aggressive regeneration captures the kinetic energy right away – and then converts that energy to propulsion when needed. That results in twice the amount of losses.
Put into practice, Porsche’s theory makes sense. Particularly at highway speeds, when you lift off the throttle pedal of a Taycan, you can coast for a significant distance – sometimes a mile or more – without using any power and with barely any reduction in speed. That’s because objects in motion like to stay in motion. Heavy objects, like an electric car, have so much momentum that if the tires and other components are relatively low-friction, so they can roll on for a significant distance unencumbered.
In the context of a car designed to spend a lot of its life on high-speed autobahns, Porsche’s stance on regeneration versus coasting is easy to understand. In city stop-and-go traffic, though, coasting’s advantage isn’t so clear-cut. One-pedal driving is easy and convenient in the city, and immediate braking in denser traffic situations is probably easier on the driver than the vehicle coasting and having to apply the brakes manually. Fortunately, the Taycan does have a more aggressive regeneration mode that, while it’s not quite one-pedal driving, does start to capture energy immediately; and there’s also an “intelligent” mode that uses the vehicle’s cameras and sensors to decide whether it should coast or brake.
Continuous Improvement
While brake regeneration is not a new technology anymore – the first Honda Insights and Toyota Priuses used it before the year 2000 – there is still a lot of room for development and optimization. Until regeneration, a vehicle’s brakes have been a relatively isolated system. This has now changed, because many more parts of the vehicle are involved in deceleration: the powertrain, power electronics, and the battery.
In the future, more interdisciplinary work between the engineers for all vehicle systems will help regeneration get even better. The engineers working on the Taycan’s braking system, for example, will have to confer more closely with their colleagues working on its two-speed transmission, as well as those working on its electric motors. While more work will be required, engineers will also have more freedom – such as being able to make the distribution of braking force between the front and rear axles variable.
Braking feel is another area where Porsche has been working intensively. The Taycan’s algorithms monitor the hydraulic braking system as well as regeneration. During each charging process, the brake is calibrated to determine the current ratio of brake pedal travel to brake pedal force. This allows the car to determine much power the hydraulic system will deliver the next time the vehicle is braked, and deploy it precisely so that the transition from regenerative braking to using friction remains smooth.
A Boon for Range and EV Maintenance
Coasting or not, brake regeneration is still a key component of any electric vehicle. Porsche says that in the Taycan, about 90 percent of braking can be accomplished using only the electric motor – without the involvement of the hydraulic braking system. The “real” brakes are used at low speeds, when the electric motors develop very little braking power, as well as in situations where the motors need a helping hand – such as during full braking from high speeds.
Regeneration reduces wear on the hydraulic brakes as well. Indeed, it's likely that many electric cars' brake pads will have to be replaced due to age rather than wear. The Taycan brakes at regular intervals using the hydraulic system only, and without the electric motors, to remove dirt from the discs.
Using its sophisticated algorithms, the Taycan Turbo S can generate up to 290 kW of electric power during braking. At this level, two seconds of deceleration are enough to generate enough electricity to drive around 0.4 miles. Overall, the company says that regenerative braking increases range by up to 30 percent.
