"Do you want to touch the brake disc?" As far as questions go at an automotive press launch, this was a first for me. Dr. Michael Wein, the manager responsible for the new regenerative braking system on Audi's, leaned in and said, "Go ahead." I admit, the thought hadn't exactly occurred to me.
Mere moments ago, we had descended from the 14,000-foot summit of Colorado's Pikes Peak, pausing part of the way down, at historic Glen Cove Inn (altitude: 11,500 feet), for a mandatory park ranger brake check.
The ranger dutifully popped out of his little wooden guard shack with a pistol-grip infrared thermometer, pointed the barrel at our car's front disc, and crisply announced, "forty-seven-point-nine degrees."
At that point, we had only descended around 2,500 feet, but by then, the ranger assured me, most car brakes are sitting at an easy 150 degrees Fahrenheit, and some are a good deal hotter, with a small percentage already overheated at 500 or worse (at which point they're typically billowing smoke and our park ranger friend orders to pull over and cool).
Our Audi E-tron prototype's discs, by contrast, were sitting just a few degrees above ambient temperature. I know, because I dutifully put my hand directly on the disc on and checked, to Dr. Wein's visible satisfaction.
A steep 2,500-foot descent, and we had barely had occasion to graze the discs.
Welcome to the E-tron's remarkable new by-wire regenerative braking system.
In EVs and hybrids, regenerative braking -- opportunistically using the drive motors as generators to slow the vehicle while pumping kinetic energy back into the battery -- can have a significant positive effect on overall driving range.
This isn't isn't new tech, but Audi's execution certainly is.
The German automaker says that the E-tron will be the first production EV to receive a by-wire braking system. That means that under normal operation, there is absolutely no mechanical connection between the brake pedal and the calipers. (If a failure is detected, a mechanical backup is automatically engaged.)
That's all well and good, but how does this sort of system impact the driving experience? Well, I can't say for sure how the brakes will feel from behind the wheel, as I haven't had the chance to actually drive the E-tron yet. (My Colorado Springs descent was merely an early ride-along opportunity.)
What I can tell you is that Audi claims that the system is active in over 90 percent of typical daily braking scenarios, recouping up to 0.3 g using the motors alone. Most competing systems max out at a rate of about 0.2 g.
In practical terms, that means that in about 9 out of every 10 braking events — whether coasting to a stop using regen or stepping on the brake pedal — the six-piston front- and single-piston rear calipers are unlikely to be doing much work. If any. Instead, the E-tron's two asynchronous motors busy themselves replenishing the 95-kWh battery, decelerating the vehicle and saving wear and tear on its conventional friction brakes.
That's exactly what happened with my test run down the mountain. Over the course of our descent down Pikes Peak Highway -- all 12.4 miles and 156 turns worth -- my test vehicle's regenerative braking system actually added 6.8 kWh to the battery pack.
According to a tablet computer tied into our car's braking system that displayed values in real time, we started at the summit with 56.25 kWh and finished with 63.05 kWh at the base of the mountain. To get more granular, we had recovered around 3.4 kWh under coasting and a further 6 kWh under regen braking. Being an almost constant descent, only 1kWh of energy was expended on propelling the vehicle forward for our entire trip down the mountain. This, from an SUV with over 400 horsepower and 490 pound-feet of torque.
What's even more remarkable is how few times our prototype's test driver actually initiated braking hard enough to require intervention by the conventional friction brakes. As shown on our real-time tablet, the number of times could almost be counted on one hand, and it's likely that with a bit more planning and deliberate use of coasting regen, it might've been possible to complete the entirety of the journey without ever disturbing the brake pistons.
Granted, ours was a Euro-spec prototype, and descending the second-highest paved road in the US is an extreme case. However, it does suggest that significant range gains will be possible via regeneration, even in normal driving. Plus, hilly and mountainous country is where EVs tend to struggle with range the most, because ascents can take a lot of juice out of battery packs.
The E-tron is rated at a range of over 248.5 miles on the Worldwide Harmonized Light Vehicle Test Procedure (WLTP) cycle. That should put it in close company with Tesla's smallerand new on the EPA test cycle.
In fact, because of they way the system is designed, the E-tron's friction brakes are used so sparingly that Audi actually had to engineer a daily self-cleaning cycle, just to make sure the steel disc surfaces don't develop unsightly, performance-sapping surface oxidation, aka rust. While Audi declined to tell me more about the brake pads themselves, they confirmed they are a proprietary blend of fibers and materials specifically designed to cope with the E-tron's unique demands.
As a happy side benefit, it seems all but assured that the lifespan of the E-tron's hydraulic friction brake system will be significantly extended versus those on a normal crossover, if only because they aren't being exercised much. Dr. Wein also confirmed to me that Audi was able to specify smaller discs and calipers because of the regenerative system, but it hasn't yet revealed what size discs the US-spec model will feature.
So how does it all work? The E-tron's braking system is electrohydraulic. As the company says, "the wheel brakes are actuated hydraulically, the reinforcement is actuated electrically, and the activation is actuated electronically."
That sounds quite simple, but the reality has a lot more layers to it. In the main, a powerful electronic brain has to discern driver intent by looking at things like how much force the driver is applying to the brake pedal, as well as the rate at which it's being applied (e.g., a short stab, as in a panic braking event, versus a more gradual application). The system favors recuperation, but if it determines that friction braking is needed as well, an electric spindle drive springs to life and triggers a displacement piston that fills fluid into the brake lines nearly instantaneously.
Because the E-tron's front and rear motors are not identical, the system selectively calls upon the regenerative power generation from either motor depending on which one's operating range is optimal for efficiency on a moment-to-moment basis. Those motors, by the way, are enough to power the Audi E-tron to 62 mph in under 6 seconds.
Audi says that despite the complicated hardware at work beyond the firewall, under braking, the driver will nothing abnormal and be absolutely unable to feel any blending of the electrical and hydraulic systems when such moments occur. A mechanical brake pedal simulator smooths out any inconsistencies to deliver a constant pedal feel -- in fact, even under hard braking, there's no ABS pulsing detectable through the pedal.
And it's not just the brake system itself that's in on the action. The E-tron uses something called Predictive Efficiency Assist, or PEA, which marshals data from the SUV's various systems to determine braking and acceleration needs predictively in order to maximize efficiency and smoothness.
PEA considers everything from planned routes in the navigation system to the proximity of other cars (via intelligent cruise control sensors), plus road sign data gleaned from GPS data and the E-tron's forward-facing camera. It then synthesizes all of this information with data sourced from Car-to-X connected technology, all in an effort to maximize energy recuperation opportunities.
Those of you who have driven modern battery-electric cars like theand know that strong regenerative braking can make driving a one-pedal endeavor under most circumstances, not unlike a golf cart. Many of us enjoy driving this way, but not everyone loves the feeling of heavy regen braking, so the E-tron offers varying degrees of recuperation that can be specified in one of three steps via . The system varies the amount of recuperative drag from freewheeling to Level 1 ("minimal deceleration") to Level 2 ("high deceleration"), the latter of which amounts to one-pedal driving.
In addition to offering better kinetic energy recovery, the E-tron's by-wire architecture looks to be a win from a safety perspective, too. Audi claims the electrohydraulic setup in the E-tron reacts significantly quicker than a conventional hydraulic system, curbing braking distances up to 20 percent. Audi has not yet specified braking distances, but in light of the fact that the E-tron is a full-size SUV laden down with a heavy battery pack, any advantages it can come by to cut braking distances will be particularly welcome.
We won't really know what to think about the E-tron's groundbreaking new by-wire braking system until we get behind the wheel. Will the brakes feel reassuringly predictable and progressive, artificial or wooden? While our progress felt nothing but smooth from the passenger's seat, there's no way to know until we put our foot down and experience it for ourselves.
Fortunately, that opportunity is probably not that far away. Audi has vowed to strip away the QR-code-like camouflage from the E-tronin San Francisco. Roadshow will be there, and we hope to get some wheel time in the all-electric SUV soon thereafter.
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