Heavy brake re-gen feel dominates i3 driving experience

  • 15-Apr-2014 04:13 EDT
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With a 32.3-ft (9.85 m) turning circle, quick acceleration, and agile nature, the i3 is a good fit in the city. The base MSRP is $41,350 ($45,200 when equipped with an optional range-extender internal-combustion engine used exclusively to charge the battery).

More noticeable about the BMW i3 driving experience than anything else is the heavy braking effect when lifting off the accelerator pedal, even when going downhill. The effect is intentional; the boldly styled electric vehicle typically is brought to a stop without use of the brake pedal. The primary “brake” is the car’s electric motor, which also acts as a generator that converts the energy expended to slow the vehicle into charge for the 22-kW·h lithium-ion battery pack.

Brake-energy regeneration of this type is not a new concept, but how it’s deployed in the i3 was one never experienced by this Automotive Engineering editor until he test-drove the compact car in the varied terrain and traffic conditions of Los Angeles. (For a more in-depth article overviewing the i3's various technologies, visit http://articles.sae.org/12056/.) It might not have the heaviest re-gen effect of any car, but it is very heavy—especially at lower speeds (BMW engineered it to be speed-sensitive with less or no retard at higher speed). Seldom was the brake pedal commanded into service.

It will take time for buyers of the i3 to get used to the brake re-gen feel. Dealers will, presumably, explain to potential customers before they get behind the wheel for their own test drive what to expect when they lift off the accelerator pedal.

Not expecting such a heavy effect, I was taken aback. The driver of the car following behind my i3 very early in the test drive most likely was too, as the i3 slowed abruptly in front of him. I had to re-apply the accelerator pedal to reach the redlight toward which I initially had begun what in a conventional car would have been a smooth and leisurely coast of about 30 yards. More than a few times did the slow-too-quickly-then-accelerate-to-the-redlight scenario play out during my roughly one hour behind the wheel.

The re-gen system is engineered such that it does not always come into play—for instance, when the driver “eases off” the accelerator pedal, the car free-coasts; toward the end of the pedal travel, near full release, a detente can be felt when free-coast engages. Road grade, driving mode (Comfort, Eco Pro, or Eco Pro Plus), battery state of charge, and driver aggressiveness are among variables that dictate whether re-gen or free-coasting is engaged. The instrument panel display indicates when the car is in re-gen, free-coast, or drive mode.

BMW cites maximum re-gen braking deceleration of 1.6 m/s² (5.2 ft/s²).

Heavy though the re-gen effect is, it’s neither counterintuitive (all cars slow when the accelerator pedal is released) nor aggravating. It’s just different—more extreme—and there’s no reason to think i3 buyers will have much trouble adapting to it.

Through its multi-year EV evaluation program using the Mini E and BMW ActiveE demonstration vehicles in real-world conditions, BMW found that “drivers adjust quite quickly to the re-gen effect,” said company PR representative Dave Buchko. ”In fact, with the Mini E we found that not only did drivers adapt to it quickly, they actually came to enjoy it. We expect BMW i3 drivers to adapt quickly.”

It could be that adapting to the gauges, interfaces, and functionality of the cockpit will be harder for new i3 owners—at least for the digitally challenged ones—although the layout and operation of gauges and controls make sense and are no more challenging that those of other cars.

One of the more unusual cockpit features is the gear selector. The i3’s is a somewhat balky yet simple-to-use steering column stalk with a knob that rotates forward for drive and backward for reverse. The stalk’s location makes for an open center floor area that allows the driver to easily make his or her way across the front of the car for passenger-side exit. This “slide-across” feature is useful in locations for which the i3 is optimized in many ways: the city.

The interior features many different types of environmentally friendly materials with different textures and appearances.

From a driver’s perspective, next in noteworthiness behind re-gen is the rear-wheel-drive i3’s quickness and smoothness getting up to speed. The electric motor/generator (made in-house by BMW) delivers 125 kW and 250 N·m to propel the car from 0 to 60 mph (0-97 km/h) in 7.2 s.

A multilink rear suspension gets some of the credit for the car’s exemplary agility, as does the 50-50 weight distribution of the stiff, carbon-fiber-bodied EV. The 450-lb (204-kg) battery pack is fitted below the front and rear seats for a low center of gravity.

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