Mercedes-AMG goes with the current flow

  • 19-Jul-2010 04:48 EDT

Opening the doors on new technology: the Mercedes-AMG SLS E-Cell has 880 N·m (649 lb·ft) of torque from electric propulsion via four motors.

When it comes to creating advanced technology, Mercedes-Benz can be counted on to occupy the front row of the creative grid. The SLS AMG E-Cell is very much an advanced design, and at a media event in Kristiansund, Norway, full details were revealed of this more environmentally compatible version of the gullwing gasoline internal-combustion-engine version of the SLS, production of which is likely to be limited to as many as 100.

Development of the car is continuing, particularly with regard to extending its range. The company has not officially confirmed figures, but it understood that at present the prototype can cover up to 180 km (112 mi) between charges, although development engineers are believed to be hoping for more than 200 km (124 mi) by production build time in about 30 months to three years’ time. Conventional plug-in recharge time is around 8 h.

When the electric SLS was seen at last year’s Frankfurt Motor Show, the company indicated then that it was a serious R&D project to demonstrate the efficacy of zero-emissions technology applied to a supercar. The E-Cell’s 393-kW power output is allied to 880 N·m (649 lb·ft) of torque to give 0-100 km/h (0-62 mph) acceleration of 4.1 s—only 0.3 s slower than the conventional SLS with a 6.3-L V8 gasoline engine. Top speed of the prototype is understood to be about 200 km/h (124 mph), but further development may see this reaching 250 km/h (155 mph), which would be both a significant technology target but also an effective marketing tool.

“It is our goal to continually reduce the fuel consumption and emissions of new models in the coming years, while at the same time enhancing the core brand value of performance,” said Ola Källenius, Chairman of Mercedes-AMG.

Persuading a sometimes skeptical public that environmentally responsible vehicles are not dull and can achieve high performance levels is part of the auto industry’s challenge as emerging technologies move center stage.

Mercedes-AMG decided that, to achieve the power outputs and weight distribution required to meet its supercar criteria, the E-Cell needed individual electric synchronous motors to drive each wheel but, to avoid high unsprung weight, would not be placed in the wheel hubs. Instead they are positioned inboard. Two transmission systems are fitted.

Compared to the regular SLS with double wishbone front suspension, that of the E-Cell has a multilink setup with damper struts operated via separate pushrods and transfer levers instead of the gasoline car’s vertically positioned struts, which would impede the additional driveshafts for the front axle. The E-Cell design owes much to motorsport applications.

Electrohydraulics (replacing just hydraulics) are used for the E-Cell’s power steering.

While suitable electric motor design is essential for a car of this type, as always it is the battery and power control systems that are at the epicenter of technology priorities.

The E-Cell’s power comes from a liquid-cooled high-voltage-capacity (40 A·h at 400 V) lithium-ion battery comprising 324 lithium-ion polymer cells. Maximum electric load potential is 480 kW, described by Mercedes as being “an absolute best value in the automotive sector.” The system uses an intelligent parallel circuit of the individual battery modules, and brake energy recuperation is used.

Controls include a high-performance system to convert dc current from the battery to three-phase ac, necessary for the synchronous motors. It regulates energy flow for all operating conditions.

Two low-temperature cooling circuits are used to keep the four motors and power electronics at a suitable temperature, while another low-temperature circuit looks after cooling for the lithium-ion battery.

Low ambient temperatures and electric vehicles are often incompatible. To overcome this, the E-Cell has an electric heating element to achieve operating temperature in the required timescale and contribute to battery service life. Battery life (because of the initial high capital cost) is a very significant element of all electric-vehicle economics, and OEMs and suppliers are addressing other possible solutions to offset through-life costs.

Most of the present production electric vehicles are merely adaptations of an internal-combustion-engine vehicle. But Mercedes stated that it planned an electric SLS version from project conception much as it would a cabriolet or station wagon. Therefore, the car’s aluminum spaceframe is the same for both versions.

The design facilitated the four electric motors and the transmissions being positioned as low as possible to improve the center of gravity (cg). Battery modules are positioned ahead of the firewall, in the vehicle’s spine and aft of the seats, again to help both cg and front/rear weight balance.

Driver-information needs are different for an electric vehicle, so the E-Cell’s instrument cluster and center console configuration are not the same as those for the conventional car. Information in the E-Cell cockpit includes battery charge status and estimated range. A 25-cm touch screen provides information on power flow from each electric motor.

Transmission selection has P, R, and D modes, with P selected automatically upon motor shutdown.

The E-Cell has standard AMG ceramic brakes—discs of 402 x 39 mm (15.8 x 1.5 in) front and 360 x 32 mm (14.2 x 1.3 in) rear—that are optional on the conventional car for about 40% weight saving, helping to increase range. ABS and ESP have been modified.

Full LED headlights are fitted to reduce energy consumption.

Aerodynamic modifications to the car include changes to the front grill and air outlets on its hood and flanks. The front apron has been repositioned slightly forward to optimize underbody airflow, reducing downforce and improving aerodynamic efficiency. An extendable—by 7 cm (2.8 in) at 120 km/h (75 mph)—front splitter is fitted. Like the conventional car, there is a rear diffuser but without an engine exhaust system; it is steeper to increase rear axle downforce.

It all adds up to a supercar message that reads: driving exhilaration with responsibility.

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