|Mercedes-Benz SLS AMG Coupé Electric Drive. Click to enlarge.|
At the Paris Motor Show, Mercedes-Benz introduced the production of the SLS AMG Coupé Electric Drive (earlier post). The battery-electric supercar goes on sale in 2013. The price in Germany (incl. 19% VAT) will be €416,500 (US$535,898). (The new, conventional V8 SLS AMG Roadster going on sale next month carries a price in Germany, including VAT, of €213,010 (US$274,074).)
The AMG high-performance EV features four electric motors producing a total output of 552 kW and a maximum torque of 1000 N·m (738 lb-ft). The gullwing model is the fastest electrically-powered series production vehicle, according to Daimler: the SLS AMG Coupé Electric Drive accelerates from zero to 100 km/h in 3.9 seconds. Top speed (electronically limited) is 250 km/h (155 mph).
The Li-ion battery pack for the SLS AMG Coupé Electric Drive is the result of cooperation between Mercedes-AMG and Mercedes AMG High Performance Powertrains in Brixworth (UK). This is an area in which the British Formula 1 experts were able to contribute their know-how with KERS hybrid concepts.
The four compact permanent-magnet synchronous electric motors, each weighing 45 kg, achieve a maximum individual speed of 13,000 rpm and in each case drive the 4 wheels selectively via a axially-arranged transmission design. This enables the unique distribution of torque to individual wheels, which would normally only be possible with wheel hub motors— which can have the disadvantage of generating considerable unsprung masses.
The 60 kWh battery pack in the SLS AMG Coupé Electric Drive offers an electric load potential of 600 kW and weighs 548 kg. The liquid-cooled lithium-ion high-voltage battery features a modular design and a maximum voltage of 400 V. Advanced technology and know-how from the world of Formula 1 have been called on during both the development and production stages: the battery is the first result of the cooperation between Mercedes-AMG GmbH in Affalterbach and Mercedes AMG High Performance Powertrains Ltd. Headquartered in Brixworth in England, the company has been working closely with Mercedes-AMG for a number of years.
The battery pack consists of 12 modules each comprising 72 lithium-ion cells. This optimized arrangement of a total of 864 cells has benefits not only in terms of best use of the installation space, but also in terms of performance, according to the company. One feature is the intelligent parallel circuit of the individual battery modules— this helps to maximize the safety, reliability and service life of the battery. As in Formula 1, the battery is charged by means of targeted recuperation during deceleration whilst the car is being driven.
A high-performance electronic control system converts the direct current from the high-voltage battery into the three-phase alternating current which is required for the synchronous motors and regulates the energy flow for all operating conditions. Two low-temperature cooling circuits ensure that the four electric motors and the power electronics are maintained at an even operating temperature. A separate low-temperature circuit is responsible for cooling the high-voltage lithium-ion battery.
In low external temperatures, the battery is quickly brought up to optimum operating temperature with the aid of an electric heating element. In extremely high external temperatures, the cooling circuit for the battery can be additionally boosted with the aid of the air conditioning. This also helps to preserve the overall service life of the battery system.
Ideally the SLS AMG Coupé Electric Drive is charged with the aid of a wall box. Installed in a home garage, this technology provides a 22 kW quick-charge function. A high-voltage power cable is used to connect the vehicle to the wall box, and enables charging to take place in around three hours. Without the wall box, charging takes around 20 hours. The wall box is provided as an optional extra from Mercedes-AMG in cooperation with SPX and KEBA, two suppliers of innovative electric charging infrastructures for the automotive industry.
The SLS AMG Coupé Electric Drive makes use of an eight-stage safety design. This comprises the following features:
all high-voltage cables are color-coded in orange to prevent confusion;
comprehensive contact protection for the entire high-voltage system;
the lithium-ion battery is liquid-cooled and accommodated in a high-strength aluminium housing within the carbon-fibre zero-intrusion cell;
conductive separation of the high-voltage and low-voltage networks within the vehicle and integration of an interlock switch;
active and passive discharging of the high-voltage system when the ignition is switched to “off”;
in the event of an accident, the high-voltage system is switched off within fractions of a second;
continuous monitoring of the high-voltage system for short circuits with potential compensation and insulation monitors; and
redundant monitoring function for the all-wheel drive system with torque control for individual wheels, via several control units using a variety of software.
The AMG Torque Dynamics feature is permanently active and allows for selective distribution of forces for each individual wheel. The intelligent distribution of drive torque greatly benefits driving dynamics, handling, driving safety and ride comfort. Each individual wheel can be both electrically driven and electrically braked, depending on the driving conditions, thus helping to optimise the vehicle's cornering properties; reduce the tendency to oversteer/understeer, increase the yaw damping of the basic vehicle; reduce the steering effort and steering angle required; increase traction; and minimize ESP and ASR intervention.
The AMG Torque Dynamics feature offers three different transmission modes:
- Comfort (C): comfortable, forgiving driving characteristics
- Sport (S): sporty, balanced driving characteristics
- Sport plus (S+): sporty, agile driving characteristics
The body shell structure of the SLS AMG Coupé Electric Drive is part of the AMG Lightweight Performance design strategy. The battery is located within a carbon-fibre monocoque which forms an integral part of the gullwing model and acts as its “spine”. The monocoque housing is firmly bolted and bonded to the aluminium spaceframe body. The fibre composite materials have their roots in the world of Formula 1, among other areas. The advantages of CFRP (carbon-fibre reinforced plastic) were exploited by the Mercedes-AMG engineers in the design of the monocoque.
These include their high strength, which makes it possible to create extremely rigid structures in terms of torsion and bending, excellent crash performance and low weight. Carbon-fibre components are up to 50% lighter than comparable steel ones, yet retain the same level of stability. Compared with aluminium, the weight saving is still around 30%, while the material is considerably thinner.
The carbon-fibre battery monocoque is, in addition, conceived as a “zero intrusion cell”; it protects the battery modules inside the vehicle from deformation or damage in the event of a crash.
The purely electric drive system was factored into the equation as early as the concept phase when the super sports car was being developed. It is ideally packaged for the integration of the high-performance, zero-emission technology: for example, the four electric motors and the two transmissions can be positioned as close to the four wheels as possible and very low down in the vehicle. The same applies to the modular high-voltage battery. Advantages of this solution include the vehicle’s low center of gravity and balanced weight distribution.
The additional front-wheel drive called for a newly designed front axle: unlike the series production vehicle with AMG V8 engine, which has a double wishbone axle, the SLS AMG Coupé Electric Drive features an independent multi-link suspension with pushrod damper struts. This is because the vertically-arranged damper struts had to make way for the additional drive shafts. As is usual in a wide variety of racing vehicles, horizontal damper struts are now used, which are operated via separate push rods and transfer levers. With this front-axle design, which has already been tried and tested in the world of motorsport, the agility and driving dynamics of the SLS AMG Coupé Electric Drive attain the same high levels as the V8 variant.
Another distinguishing feature is the speed-sensitive power steering with rack-and-pinion steering gear: the power assistance is implemented electrohydraulically rather than just hydraulically.
The SLS AMG Coupé Electric Drive is slowed with the aid of AMG high-performance ceramic composite brakes, which boast direct brake response, a precise actuation point and outstanding fade resistance, even in extreme operating conditions. The over-sized discs— measuring 402 x 39 mm at the front and 360 x 32 mm at the rear— are made of carbon fibre-strengthened ceramic, feature an integral design all round and are connected to an aluminium bowl in a radially floating arrangement.
The ceramic brake discs are 40% lighter in weight than the conventional, grey cast iron brake discs. The reduction in unsprung masses not only improves handling dynamics and agility, but also ride comfort and tire grip. The lower rotating masses at the front axle also ensure a more direct steering response— particularly noticeable when taking curves at high speed.