Overview: Mercedes SLS AMG E-Cell
Perhaps the most interesting thing about AMG’s electric SLS is the fact that it was planned alongside the V8 petrol model. AMG might be the performance arm of Daimler, but the company is clearly following the same route to environmental credibility as more mainstream manufacturers. Perhaps companies like AMG have even more need to, as conspicuous consumption of hydrocarbons looks ever more inappropriate. (All of which leads to one rather obvious question: when petrol-heads become electric-heads, where will all of the renewably-generated electricity come from?)
The SLS wouldn’t be an AMG if it were slow, and the E-Cell gullwing is emphatically not slow. Each of its four synchronous traction motors generates around 133PS and 220Nm. To save you the mental arithmetic, that comes to 880Nm and over 530PS. By comparison, the regular V8 model deploys 571PS and a (relatively) trifling 650Nm. It also delivers 308 grammes of CO2 per kilometre.
AMG quotes a time of four seconds for the spring from a standing start to 100km/h for the E-Cell; if this is accurate, then it’s only a fraction slower than the petrol-powered production version.
AMG describes the SLS E-Cell as a ‘technology vehicle’. It is part of a programme called AMG Performance 2015, which is all about reducing fuel consumption and emissions. Quite how close to the heart of AMG such matters really lie isn’t something we would like to guess at, but the Company has spoken openly about the prospect of a possible small series production run; no timescale has been suggested, though.
The E-Cell’s traction motors are mounted inboard, to the benefit of the unspring mass at each wheel. One single-speed reduction gearbox is fitted at each axle.
The SLS uses a liquid-cooled lithium-ion battery pack, delivering a nominal 400V. Its capacity is 40Ah and 48kWh, its maximum load 480kW. The battery consists of 324 lithium-ion polymer cells. The pack is charged on-the-fly by kinetic energy capture.
Controlling and cooling
An electronic control system converts the direct current from the battery pack into three-phase alternating current which is required for the traction motors. Two low-temperature cooling circuits take care of the drive motors and the power electronics; a separate low-temperature circuit is responsible for cooling the high-voltage lithium-ion battery.
At low external temperatures, the battery is brought up to operating temperature using an electric heating element. This helps to preserve the service life of the battery; at very low temperatures, the pack would deliver now power at all without heating.
At very high external temperatures, the cooling circuit for the battery can be boosted using the air conditioning system.
Body and chassis
The conventional SLS, you will recall, uses a front-mid engine and rear transaxle connected by a torque tube, an arrangement that provides near-as-dammit 50:50 mass distribution and a fairly substantial polar moment. The electric version follows the same righteous path, with its hefty battery pack divided into three modules: one goes in front of the firewall, the second in what would otherwise be the transmission tunnel, the third behind the seats. There is no change to the gullwing’s aluminium spaceframe, though the front suspension has been substantially rearranged.
The addition of drive to the front wheels demanded the abandonment of the double-wishbone front suspension of the standard car. The vertically-arranged damper struts have been replaced by horizontal items, operated by means of pushrods. This is a common arrangement in motorsport. Additionally, the E-Cell’s speed-sensitive power steering is operated electrohydraulically.
Ceramic composite brakes are fitted to the electric gullwing, giving a precise actuation point and very good fade resistance in extreme conditions. The over-sized discs — 402mm x 39mm at the front and 360mm x 32mm at the rear — are made of ceramic material strengthened with carbon fibre. They are connected to an aluminium bowl in a radially-floating arrangement. The ceramic brake setup is 40 per cent. lighter than the conventional grey cast iron brake discs, providing a useful saving in unsprung mass.
The lower rotating masses at the front axle deliver a more direct steering response, particularly at high speed.
The ABS and ESP systems have been adapted for the E-Cell’s all-wheel drive powertrain.
There are some more minor revisions to the SLS incorporated in AMG’s technology vehicle. All of the front lighting is now provided by LEDs, reducing power consumption in comparison with regular bi-xenon light systems. The wider radiator grille is of a slightly different design. The air outlet openings on the bonnet and the vehicle sides have been modified to improve the aerodynamics.
The front apron has been brought further forward, improving airflow underneath the car, reducing drag (and downforce). An extensible front splitter enhances this effect: in parallel with the automatic rear spoiler, it extends downwards by seven centimetres at speeds above 75mph to help accelerate the airflow under the car. When it reaches the area of the rear axle, the air-stream enters the rear diffusor, which is angled more steeply than would be possible if an exhaust system were fitted. This increases downforce at the rear axle. Tyre equipment is 265/35x19 at the front and 295/30x20 at the rear.
Inside, the E-Cell’s new instrument cluster provides information on speed, charge status of the battery and the estimated range. The newly-designed centre console houses a 25cm touchscreen, which the driver and passenger can use to operate all of the audio, climate and navigation functions, and also to view a graphic display of the flow of power from the four electric motors.
The AMG Drive Unit, which is angled towards the driver, houses switchgear for starting the motor, for the ESP functions, the AMG memory function, and the extensible front splitter and rear spoiler.
Three buttons allow the driver to switch between Park, Reverse and Drive. The Park setting is set automatically when the electric drive is shut off.