BMW opens its i’s technology wider

  • 23-Sep-2011 02:24 EDT
BMW i cars a.jpg

The first i cars, the i8 and i3, signal a major new direction for the company's design and engineering.

As part of its project i, the BMW Group is currently conducting field trials in everyday conditions with vehicles running purely on electric power that it believes are unparalleled worldwide in their scope. Those trials underline BMW’s determination to introduce the pure electric i3 city car and i8 gasoline hybrid 2+2 sports coupe at the quality and reliability levels achieved with its conventional internal combustion engine (ICE) vehicles when they enter production in 2013. Further i cars are now in R&D.

Although some early information about the two cars was revealed by AEI earlier this year, BMW decided that it needed to show the concepts and provide far more detailed facts and figures despite manufacturing being some two years away. Both concept vehicles are totally purpose engineered and are not mere conversions of conventional ICE models, which would bring weight penalties plus complexities and compromises in the installation of drive and control systems, says BMW.

The 1250-kg (2755-lb) i3 and 1480-kg (3260-lb) i8 each comprises two major elements, together called LifeDrive, respectively denoting the passenger cell (Life) and the powertrain and aluminum-intensive chassis (Drive).

Tested in a pre-production version of the BMW ActiveE, the rear-wheel drive i3’s optimized synchronous electric motor is positioned over the rear axle to help achieve optimum front/rear weight distribution and a low center of gravity. It generates 125 kW and 250 N·m (184 lb·ft) to provide a 0-60 km/h (0-37 mph) time of less than 4 s and to 0-100 km/h (0-62 mph) in less than 8 s. To contain energy consumption, top speed is an electronically governed to 150 km/h (93 mph). The motor drives through a single-speed gearbox.

When the accelerator is lifted, the electric motor operates as a generator to recoup energy. This also creates a retarder effect, which equates to about 75% of all braking operations in typical urban area conditions.

BMW estimates that “intensive use” of energy recuperation can boost driving range by about 20%. The accelerator pedal has a neutral pedal position to hold off recuperation and simply coast without using power. The car’s drive capability also includes ECO PRO mode for added overall efficiency, the accelerator’s mapping limiting power output; HVAC requirements are also minimized.

The i3's lithium-ion battery has an integrated heat exchanger cooling system to maintain operating temperature at around 20°C (68°F) for optimal performance and cell life. Battery recharge time is 6 h from a standard domestic European socket, but a high-speed charger will provide 80% charge within an hour.

BMW has decided to offer the i3 with an optional small-capacity range extender engine to drive a generator. Like the electric motor, it is positioned over the rear axle. It has a stop-start function.

The all-wheel-drive (electric front, ICE rear) i8 is a plug-in hybrid with very strong performance, including a 0-100 km/h time under 5 s and a fuel consumption in the European test cycle of sub 3.0 L/100 km and about 5.0-7.0 L/100 km in typical “real” terms, even when subjected to “a fast driving style,” as BMW puts it.

The car’s electric motor is a variation of that in the i3 but driving the front axle. In that position, it provides maximum brake energy recuperation, as most braking energy is generated at the front wheels.

The ICE driving the rear wheels is a 1.5-L three-cylinder producing 164 kW (220 hp)  and 300 N·m (221 lb·ft). It is expected to drive through a dual-clutch gearbox. Operating in parallel, the two power units give the i8 a governed top speed of 250 km/h (155 mph).

The i8’s lithium-ion battery is positioned midway between front and rear axles. The car has an all-electric range of up to about 35 km (22 mi). Battery recharge time at a European domestic socket is 2 h. The car has a high-voltage generator linked to the ICE, but this does not offer an alternative to stationary recharging from a power socket.

While the mechanical hardware and associated controls and software are very significant for the i sub-brand vehicles’ design and engineering, so are their structures, use of materials, and aerodynamics.

“The LifeDrive concept avoids the additional weight involved in making the necessary modifications to conversion concepts,” said BMW body and equipment specialist, Bernhard Dressler. He elaborated that in both vehicles the extra mass added by electric-drive technology had been canceled out through the innovative use of materials and intelligent lightweight design. BMW regards battery weight as “no longer an issue” with regard to the i sub-brand products.

The Life module (passenger cell) makes extensive use of carbon-fiber-reinforced plastic (CFRP), 10% of it recycled, for high levels of torsional rigidity, although so far no specific figures have been quoted. BMW describes the use of CFRP, developed with U.S. specialist SGL, on what is expected to become medium-volume vehicles as being unprecedented.

Both concepts have a panoramic roof that reveals some of the CFRP structure. As a styling signature, it is also revealed when the doors are opened. Crash test results from pole impacts, side impacts, and rollovers have been impressive, according to BMW specialists.

Specially woven braided-profile CFRP is used in some areas of the structure, such as door sills, doors, and A-pillars, with preforms molded like a “sock over a mold.” Wall thickness can be optimized via adjusting the diameter of the mold. Complex pieces can be produced without the need for bonding or the use of connecting pieces.

Considerable design freedom is achieved with joint count reduced. Flush-fitting connections give added strength.

Cabin packaging of the i3 is particularly effective. It is accessed via four side doors plus tailgate. The rear side doors are rear-hinged, while the i8’s two doors are upward-swinging in a scissor action.

The i3 has no B-pillars, making access to the rear easier. Trunk space at the rear is about 200 L (7.1 ft³), and there is another small load carrying space beneath the front "hood" for stowage of utility items including the charging cable. Battery positioning is in the so-called “energy tunnel” in place of a transmission tunnel on a front engine, rear-drive car. The battery is penned in by aluminum profiles to achieve crash protection.

Passengers sit in semi-command positions in the i3, lower and more snugly in the i8. Bench seats are fitted front and rear in the i3. Driving oriented controls are fitted to the steering column, and transmission selection is via a rotary controller.

Elements of the instrument panel and door paneling use natural fibers and 25% of the weight of interior plastics of the i3 is of recycled or renewable material.

Aerodynamics include total enclosure of the underbody of both vehicles. Wheels are large and narrow to reduce drag and rolling resistance. “Air curtains” are used to control flow around the wheelarches, and aeroflaps are positioned low down behind the front wheels. Air diffusers are fitted.

For the i8, narrow openings around the front apron guide the in-flowing air into two aircurtains—enclosed ducts which channel air along the inside of the front apron to the wheel arches to be expelled via a narrow outlet, hanging over the front wheels to minimize turbulence.

The i3 is 3845 mm (151.4 in) long and 1537 mm (60.5 in) tall, the i8 4632 mm (182.4 in) long and 1280 mm (50.4 in) tall. Wheelbase of the i3 is 2570 mm (101.2 in) and the i8 2800 mm (110.2 in). The i3’s width is 2011 mm (79.2 in) and the i8’s 1955 mm (77.0 in).

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