The Nitty Gritty Details On Audi’s Two-Pedal EV Braking System
The e-tron offers a new flavor of EV braking: sweet vanilla.
On our recent first drive of the all-electric Audi e-tron crossover, we learned how the company wants the driving experience to feel completely natural for first-time EV owners. Yes, there is the brisk acceleration and quiet ride typical of EVs, but most drivers in a 2019 Audi e-tron will forget that its power is supplied by an electric motor or regenerated during braking.
When we wrote about the drive, the specific point that stirred the biggest reaction – some in favor and others strongly opposed – was Audi’s decision to make one-pedal EV driving unnecessary. “One-pedal driving is a thing of the past,” declared Carter Balkcom, Audi’s global product marketing manager for electric vehicles.
To be clear, EV drivers who are devoted to the one-pedal approach – in which lifting your foot off the accelerator pedal brings the car quickly to a stop – can still make it happen in the Audi e-tron SUV. But it requires searching through a couple of menus on the dashboard, finding the “Efficiency Assist” screen, and setting it to Manual. Only then can you use the paddle shifter on the steering wheel to increase the level of brake regen to two different higher levels and have it remain activated to provide a persistent one-pedal feel. And even in that case, the regen level is relatively mild compared to, say, the BMW i3 or a Chevy Bolt in L gear.
Balkcom told me, “If you really like the one-pedal thing, you can have it. But because we have a fully separated brake system, you don’t need one-pedal driving. Driving in the Automatic mode is the most efficient way to drive this car.” The Automatic mode also allows the car to read the terrain and other conditions to help determine the most efficient level of brake-regen level.
If you have the e-tron in the default Automatic efficiency mode, the use of paddle shifters for increased regen is temporary. As soon as you put your foot on the accelerator, the regen level is wiped clean, reverting to the lowest level. (It also times out after a few seconds to let the Automatic function do its thing).
Brake as Normal
The bigger point is that setting a higher level is unnecessary because you get exactly and precisely the same effect by stepping on the brake pedal. “Whether it’s through the paddles or the brake pedal, you are purely using regenerative braking up to a limit of 0.3 g,” said Victor Underberg, an Audi engineer who works on chassis development (but knows about the full gamut of e-tron technology).
By the way, there were some complaints about how the left steering-wheel paddle with the minus (-) sign increases the regen level – and the right side with the plus (+) sign provides a looser, lighter amount of regen. Is that counter-intuitive? Maybe for experienced EV drivers. But Underberg explained to me that Audi was matching what drivers of its gas-powered experience when they downshift, thereby slowing down faster as you do in a lower gear (hence the minus designation). It’s yet another example of how Audi wants its EVs to drive, feel, and operate in a way that’s most familiar to drivers of its conventional vehicles.
“If you drive an Audi A4, then normally on the road if you want to slow down with the engine, then you use the minus to shift one gear down,” said Underberg. “If your second car is an e-tron, you should do the same thing.”
0.3 Is a Magic Number
Audi says that its novel approach creates the industry’s first EV with a pure brake-by-wire strategy. The hydraulic braking is completely subordinated and controlled by the electronic, regen braking. There’s no physical connection between the pedal and brakes. “We are the first manufacturer providing an electric car with an integrated brake system,“ said Underberg. “You also have really progressive braking, which is exactly what you want.”
So, unlike fixed regen settings, the amount of regen pressure can be highly variated and controlled by how hard you apply your foot to the brake pedal – much like drivers have used a car’s brake pedal for generations. But pressing on that same pedal only applies the hydraulic brakes on rare occasion because the threshold of 0.3g is a pretty serious stomp. “If you get higher than 0.3 g, then the hydraulic brake adds to what you need. It doesn’t replace the recuperation,” said Underberg. “If you then release the brake pedal again, then you are once again under the 0.3 g threshold, so you are exclusively using recuperation.”
Underberg also explained that increasing the level to 0.4 g would not contribute much additional benefit. “When do you brake over 0.3g?” he asked. “Very seldom. And if you are driving at speed in a curve and have deceleration in the rear axle, then it can be slippery (and unsafe). That’s why we set the level at 0.3 g.” A meter on the dashboard very clearly shows the driver when the braking approaches or exceeds the 0.3 g level.
The Audi press release says that drivers brake below 0.3 g about 90 percent of the time. It also claims that its EV brake system ensures a smooth transition between electric and hydraulic braking because essentially all of the braking is electric. Audi says its electro-hydraulic brake system comes in the form of a compact module weighing about 13 pounds, which is 30-percent lighter than a conventional brake system. It’s also very responsive because its controller computes the required amount of braking power in milliseconds.
Easy Does It
Based on our time behind the wheel, the feeling of driving an e-tron is noticeably different than other EVs because you coast more. Unless you use the Manual mode and set higher regen, then there’s a very light touch to the pedals. “If you are using one-pedal driving, then have to push through with the accelerator when you don’t need to,” said Underberg. “That’s not as convenient (and comfortable) as you experience with the e-tron.”
Audi claims that its approach maximizes efficiency – with the vehicle reclaiming more than 70 percent of operating energy when you press the brake pedal and otherwise allowing you to efficiently coast until you actually need to stop. The brake resistance also goes away when you are traveling below about seven miles per hour in a parking lot situation. “You don’t want (higher regen) in a parking lot,” said Underberg.
Despite its very deliberate strategy, Audi allows exceptions to its just-like-normal rules when it makes the most sense for comfort and efficiency. For example, unlike its gasoline-powered cars, the Audi e-tron doesn’t have any creep. If you take your foot off all the pedals, the vehicle remains stationary.