Monday, August 01, 2011
BMW has been hyping up its “Megacity Vehicle” for a couple of years now, and despite teasers about innovative tech features and full carbon-fiber body structures, we’ve withheld judgment, even when we spied one in the wild over the winter.
But as of now, the shroud of mystery has been lifted about all the details except how the thing actually drives. Like a BMW? Like nothing before it? Maybe both?
The i3’s story begins with its LifeDrive architecture. According to BMW, experience with the Mini E and BMW ActiveE (the electric 1-series coupe) has proven that creating an electric car out of a structure designed to house an internal-combustion engine results in wasteful excess weight and imperfect packaging. As a result, the i3 (as well as the i8 sports car also being announced now) is made up of two separate modules. The “Drive” portion houses a large array of lithium-ion batteries, plus the suspension, transmission, and electric motor. For a low center of gravity, all of these heavy components are low to the ground, with the batteries spread within the wheelbase for optimal handling. The main structure is all aluminum, with an additional aluminum barrier around the batteries for crashworthiness.
Mod Squad
The “Life” module sits atop the drive components and houses passengers and luggage. As BMW has already announced, this structure consists of carbon-fiber reinforced plastic, or CFRP, a first for any vehicle this side of a few exotic supercars. While the aluminum Drive module weighs 30 percent less than a similar steel structure, the weight reduction in the CFRP Life module is closer to 50 percent. As a whole, the i3 weighs in at 2756 pounds, according to BMW. When speaking about the safety of CFRP, the company points to Formula 1 cockpits as proof of security. While metals require large crumple zones, carbon can be designed with special deformation elements that result in large amounts of energy being consumed in a very small space. A further benefit of the LifeDrive architecture is the deletion of a driveshaft tunnel, allowing more space to be dedicated to the cabin. BMW says each set of seats (front and rear) is connected so that every passenger can leave from one side in a tight parking space, but we think someone at the company just really digs bench seats. There’s approximately seven cubic feet of space left over in the rear for luggage, as well as a small compartment up front where the gas engine would usually be for stowing a charging cord or other small items.
Back inside the Drive structure, the i3 uses an electric motor that’s 40 percent more compact than that of a Mini E, allowing it to fit right above the rear axle that it powers through a one-speed transmission. Yes, one speed. That means the car will never shift on its way up to a computer-limited top speed of 93 mph, which BMW has determined to be the point at which the batteries drain too quickly. The company says the i3 will run 0 to 62 mph in 7.9 seconds. In terms of passing power, accelerating from 50 to 75 mph should take just six seconds.
Peak power is 125 kW, or 168 hp, and 184 lb-ft of torque. Range is estimated at 80–100 miles, and a standard charge will take about six hours. With a special high-speed charger, the i3 is said to be capable of reaching an 80-percent charge in just an hour, meaning medium-range drives can be done with just a long coffee break in the middle. But wait, there’s more, as the infomercial folks might say.
Keep Going and Going and Going
Feedback from Mini E showed that 90 percent of drivers are content with a driving range of under 100 miles, but for the outlying minority, BMW has (somewhat surprisingly) decided to offer the i3 with an optional range extender BMW calls REx. The company is mum on details for the time being, simply saying it’s a “small, very smooth-running and quiet gas engine” that drives a generator, both located alongside the electric motor on the rear axle. Sounds like perhaps BMW is borrowing the idea of using a Wankel rotary from Audi’s A1 E-Tron concept car. The engine meets strict SULEV standards and features auto stop-start capabilities.
Among the i3’s other energy-saving details are familiar concepts like the “air curtain” first seen on the updated 2011 1-series, which smoothes airflow around the front wheels, as well as narrow drag-reducing tires. Like the accelerator in a Tesla Roadster, the i3’s long pedal is tuned so that the car will still coast if the driver lifts just slightly, while lifting entirely off the pedal will engage regenerative braking based on pedal position. BMW predicts that, in around-town driving, this will allow the brake pedal to go untouched 75 percent of the time. Learn More...
Source : http://www.caranddriver.com
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