Mercedes-Benz provides an update on the electric drivetrain of the SLS AMG E-CELL

New rendering of the electric drivetrain of the SLS AMG E-CELL showing modifications to packaging and layout of some components. Click to enlarge.

The drivetrain of the battery-electric Mercedes-Benz SLS AMG E-CELL sports car has been in development since 2010 as a result of the cooperation between Mercedes-AMG and Mercedes AMG High Performance Powertrains in Brixworth. (Earlier post.) A small series-production run of the Mercedes-Benz SLS AMG E-CELL is expected to be launched onto the market in 2013. Mercedes-Benz has now provided an update on the drivetrain.

Four synchronous electric motors located near the wheels provide a peak output of 392 kW and torque of 880 N·m (649 lb-ft). The carbon-fibre transmission tunnel, which also serves as the monocoque housing for the high-voltage battery modules, is structurally integrated into the aluminium body shell and firmly bonded to it. The lightweight fibre composite materials have their origins in the world of Formula 1, among other areas.

2011 rendering of the drivetrain components. Click to enlarge.

The gullwing model accelerates from zero to 100 km/h in 4 seconds – which almost puts it on the same high level as the SLS AMG with 6.3-liter V8 engine developing 420 kW (571 hp), which can accelerate to 100 km/h in 3.8 seconds.

The four compact electric motors each achieve a maximum rotational speed of 12,000 rpm and are positioned close to the wheels. As a result, compared with wheel-hub motors, the unsprung masses are substantially reduced. One transmission per axle transmits the power.

The resulting permanent all-wheel drive of the electric SLS enables torque vectoring via the individual control of the electric motors 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 electrically driven and electrically braked, depending on the driving conditions, thus helping to optimize 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 intervention.

The SLS AMG E-CELL incorporates a liquid-cooled high-voltage lithium-ion battery featuring a modular design with an energy content of 48 kWh. Its development has made use of advanced technology from the world of Formula 1—the battery is the first result of the co-operation between Mercedes-AMG GmbH in Affalterbach and Mercedes AMG High Performance Powertrains (formerly Mercedes-Benz High Performance Engines). Headquartered in Brixworth, England, the company has been working closely with AMG for a number of years.

F1 engine experts have benefited from its expertise with the KERS hybrid concept, which made its debut in the 2009 Formula 1 season. At the Hungarian Grand Prix in 2009, Lewis Hamilton achieved the first historic victory for a Formula 1 vehicle featuring KERS hybrid technology in the form of the Mercedes-Benz KER System.

The high-voltage battery consists of 12 modules each comprising 72 lithium-ion polymer 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. The maximum electric load potential of the high-voltage battery is 480 kW—a best value in the automotive sector. Another technical feature is the intelligent parallel circuit of the individual battery modules which helps to maximize the safety, reliability and service life of the battery. As in Formula 1, the 400-volt battery is charged by means of targeted recuperation during braking while the car is being driven.

A high-performance electronic control system converts the direct current from the high-voltage battery into 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 brought up to operating temperature with the aid of an electric heating element. This helps to preserve the overall service life of the battery. In extremely high external temperatures, the cooling circuit for the battery can be additionally boosted with the aid of the air conditioning system.

The body shell structure of the SLS AMG E-CELL 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 body shell and acts as the gullwing model’s “spine”. CFRP 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” for crash safety. It protects the battery modules inside the vehicle from deformation or damage in the event of a crash.

Through their experience with the SLR, the AMG Black Series vehicles and in motorsport, Mercedes-Benz and AMG have accumulated more than 10 years of expertise in working with carbon-fibre materials. AMG currently makes the propshaft for the SLS AMG, for example, in carbon-fibre. On the SLS Roadster, the supporting structure for the draught-stop is made as standard as a carbon sandwich structure. This component, with extremely short cycle times in an industrially oriented manufacturing process, already demonstrates what will be possible in the future, Mercedes-Benz says.

The purely electric drive system was factored into the equation as early as the concept phase when the gullwing model was being developed. The four electric motors and the two transmissions are 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 centre of gravity and balanced weight distribution—ideal conditions for optimum handling, which the electrically-powered gullwing model shares with its gasoline-driven sibling.

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 E-CELL features an independent multi-link suspension with pushrod damper struts. This is because the vertically-arranged damper struts in the series SLS 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.

Due to this front-axle design, the agility and driving dynamics of the SLS AMG E-CELL 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 vehicle uses AMG high-performance ceramic composite brakes, which feature extremely short stopping distances, a precise actuation point and excellent 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. The ABS and ESP systems have been adapted to match the special application spectrum of the permanent all-wheel drive.

Source: Green Car Congress