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Wednesday, October 7, 2009

Supercar Showdown: Mercedes SLS EV Vs Audi e-Tron

Two German big guns have set up a dual to see who will be the first to market with an AWD EV sports car. Audi stole the show in Frankfurt with an exaggerated, headline grabbing claim of 4500 Nm of torque for the eTron. While Mercedes didn't have their weapon on show in Frankfurt they did make a preemptive strike by leaking news of their SLS EV supercar prior to the show.

While details are a bit sketchy on both vehicles, they are each reportedly headed for series production and are expected to hit showrooms around 2013-2015, so lets compare what we already know.

Both the Mercedes SLS and Audi eTron are AWD with individual motors driving each wheel. These aren't in-wheel motors, they are mounted in-board on the chassis with power transmitted to the wheel via a reduction gearbox and drive shaft.

Mercedes claim 878 Nm (649 ft/lb) for their 392 KW (532 hp) SLS while Audi claimed 4500 Nm (3319 ft/lb) for their 230 Kw (313 hp) eTron. The hocky bit is where along the transmission line they measure the torque.

When a normal road going vehicle is rated for power and torque the figures are measured at the engines flywheel. Mercedes seem to have followed that convention and measured torque at the output shaft of the electric motors. In an effort to take advantage of ignorant media, Audi have decided to ditch convention and measure torque at the wheels, after the torque multiplication of the reduction gearbox. Without knowing their gear ratios we can only speculate how much torque the eTron motors really output, but it will be a fraction of their claim.

We'd have to take all performance estimates with a grain of salt at this stage. Audi estimate 4.8 seconds 0-60 mph which sounds very conservative and even under 4 seconds for the Mercedes sounds shy of the mark. Considering the SLS will have twice the power and twice the traction of a Tesla Roadster we calculate sub 3 seconds may be possible, putting the car in the same league as the 1000 hp AWD Bugatti Veyron.

It will be very interesting to see what they each come up with as far as vehicle dynamic control is concerned. Having an individual motor on each wheel hasn't been put into road car production before, despite the fact that a very young Ferdinand Porsche made his name by putting wheel motors on a battery electric car as long ago as 1897.

Some of the most advanced systems used in World Championship motorsport can be replicated using wheel motors. For example, active differentials are now banned from World Rallying on a cost reduction basis but they are still used in Formula One. An active differential is a computer controlled hydro-mechanical diff center that can vary the percentage of lock up going into and out of corners.

In Rallying where AWD systems required 3 active diffs, the general requirement was for all 3 diffs to open on the approach to a corner, so the driver could throw the car at the scenery, then to lock up on exiting a corner for maximum AWD traction. In Formula One the requirements are slightly different due to the different style of driving required. The diff is fully lock when braking into a corner, released on turn in, then locked again on exit. Currently active diffs are hydro-mechanical-electronic systems. When using wheel motors these feature can be replicated in software making the system cheap enough for series production.

Advanced braking features are another area with huge potential. Audi has said they plan to use a brake-by-wire system in the eTron that will retain ceramic friction brakes on all four wheels in combination with actively variable brake regeneration. They still sound like they're undecided on which technology to go with as they say they will use hydraulic brake calipers on the front wheels, servo operated electric brake calipers on the rear and four wheel motors in alternator mode all operated by a single by-wire control system.

Neither Audi or Mercedes made mention of ABS or stability control features being developed based on their wheel motor systems but considering Mercedes is an industry leader when it comes to automated and advanced braking systems, they were the first by a long way to introduce semi-auto braking systems in production cars with their DISTRONIC system in 1999, we can be sure they have something interesting in the pipeline.

Considering some of the performance cars these two companies have produced, the 450 Kw (612 hp) 6.0 liter V12 BITURBO SL 65 AMG currently produced by Mercedes or the 373 Kw (500hp) Audi Sport Quattro S1, it's almost a torment to have to wait 3-5 years to see these next generation EV supercars hit the road.


Mercedes SLS EV

Audi e-Tron

Launch date






Front/Rear Power

98 KW (133 Hp)

57.5 Kw (78 Hp)

Total output

392 Kw (532 Hp)

230 Kw (313 Hp)


878 Nm (649 ft/lb)

682 Nm (501.5 ft/lb)


1900 kg

1600 kg



53 kWh


150 – 180 km (93 – 112 Miles)

248 km (154 Miles)

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