Audi’s e-tron: four motors, all-electric

Audi announced the e-tron at the Frankfurt motor show in September.  Here’s Autoblog with the full press release, and Audi’s US Web site has some impressive graphics.  It’s a concept car, so it’s unlikely it will ever be made in this form, but the press release throws in all manner of detail, from the lightweight construction to LED headlights, low-drag brakes and the like.

But the core of the car is four electric motors, one for each wheel.  This has to be the best design premise for an all-electric car:  it allows fully-independent traction and braking control, while removing the need for prop shafts, differentials and a central gearbox.  Some have taken the concept a step further, with wheel-hub motors.  But Michelin’s Active Wheel (pdf) has not yet reached production, and the Hi-Pa Drive must be in doubt since PML Flightlink entered bankruptcy in late 2008, apparently emerging as Protean Electric, so perhaps Audi is threading the needle with futuristic but attainable technology. 

Audi gets a lot of credit for not just dropping an electric motor and battery into an existing sports car body, but using the opportunity to re-think the entire design.  One could argue it’s an opportunity to bring back every hair-brained scheme to come out of auto design houses for the last decade, but if today’s big car companies are to prosper, they must use their strengths: financial scale, ability to engineer complex systems across many disciplines, and of course supply chain and manufacturing prowess.  An electric drive train offers the opportunity for a radical departure from traditional design constraints, and conventional wisdom must be suspended while we discover what works and where the engineering tradeoffs find a new balance.

The bare specifications for the motors are reasonable.  Power of 230 kW combined (assume that’s peak output) takes the e-tron from 0 – 60 mph in 4.8 seconds, a little slower than the Tesla (and bear in mind the Tesla can prove it, while the e-tron doesn’t yet roll under its own power) from its 185 kW motor.  Top speed limited at around 125 mph, like the Tesla, which would require about 140 kW continuously from the motor.  Audi claims a light weight , although the overall mass is 1600 kg:  perhaps this really is a low figure, if one considers the large battery pack is 470 kg, almost a third of the total.

Audi doesn’t spend much time on the battery pack.  It’s Lithium-ion, no vendor named, and relatively large at a usable 42.4 kWh, which is supposed to take it 154 miles on a charge.  That’s about 275 Wh per mile, about the same as this Tesla owner’s report.

All in all, an impressive concept, and hopefully a harbinger of evolving thinking on mass-produced electric vehicles.  Here’s one train of thought: once there’s a motor on each wheel for traction and braking, all control can be by wire, and intelligence moves into software. Here it can be combined with many more sensors such as road conditions, traffic sensing and avoidance, even route-learning for a regular commute, so the control intelligence knows when an uphill or acceleration phase is coming and can plan for optimum battery use.

What would it cost?  So much depends on the battery pack:  today’s retail price for a 53 kWh pack alone would be around $20,000.  But electric motors can be mass-produced at a low price, power electronics are not overly expensive, and consider the number of complex mechanical components that can be omitted, and it shouldn’t be ridiculously expensive.  Today we have to compare the mass-produced petrol-engined car, with its high volumes and optimized supply chain with the prototype-quantity, un-optimized electric equivalent.  Once cars like the Nissan’s Leaf get into volume production, we will be able to gauge the cost gap in apples-to-near-apples terms.

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