I've been driving Audi's all-electric supercar. It was huge fun. Well, it's been a long time coming. And after all that, it's not long going.If you've a long memory you might remember a concept from 2009 simply called the Audi e-tron. It looked slightly like an R8 but it wasn't. The body was very different - it was even a different size - and it had pure-electric drive. Each of the four wheels had their own motor. Audi said it would go on sale in 2013.Rash thing to say. During development the concept got watered down, so finally it's RWD only (though more powerful than originally planned), and uses a body the same size and shape as the R8's - although a quarter of it is carbonfibre to cut the weight. And then they decided it wouldn't, after all, go on sale.They've built ten fully-developed prototypes, they're road-legal, and they will be used in trials to gather data on battery performance and electronics. These parts are modular across the whole VW group, so it's good info. Even so, it's not the original pledge.Anyway, let's enjoy it while we can.There are two motors in the middle of the rear, and each drives one wheel. They aren't mechanically linked, so there's no diff. Each motor makes 140kW (190bhp) so that's 380bhp in all. At low speed there's all the acceleration the tyres can use, and 0-100kph disappears in 4.2 seconds, but the acceleration would peter out towards the motors' rev limit at about 240kph, so instead the car is limited to 201kph for efficiency's sake.Overall weight is 1780kg. The chassis is absolutely chocka with exotic components to cut weight versus the standard R8. The heavy engine and gearbox are absent of course, too. But that doesn't offset the mass of the huge battery. At least it sits low and centrally in the car, so it isn't bad for cornering.The battery is so big it takes 14 hours to charge off the normal mains - that's why there's a fast-charger too. It gives a range of 217 kilometers in the standard drive cycle test, but they admit it falls to 160 kilometres in quick road driving. As an extreme test, they went to the Nurburgring, started with it fully charged, did two laps flat out (just over 16 minutes) and didn't have the juice to complete a third.Sure enough, the e-tron scoots away from a standstill with jet-plane thrust - smooth, controlled but very, very strong. The noise is a synthesised sci-fi hum, rising in pitch but not volume as you gather speed. In normal mode, there's not much regenerative braking when you lift the accelerator, but you can dial that up using wheel paddles if you like. Regeneration also gets stronger if you select dynamic mode, which feels like early downshifts into a corner.The braking system is fiendishly complex. There's regenerative braking at the rear, computer-controlled to match the hydraulic brakes at the front. So it goes up to 0.45g. If that's not enough, or if the battery is already full and can't accept regenerated energy, there are also electrically actuated discs at the back. Amazingly, it all feels well-integrated, progressive and natural.In eco mode, cornering is a bit odd. There's no roll to speak of and grip's OK, but it's a strange combination of very high-geared steering and a strong tendency to understeer in the tight corners of the test course, especially in the early part of the bend.Ah, but then you can summon some magic. Four-wheel-drive was discarded by the engineers as too heavy in a car where every gramme cuts the range. Instead they included full torque vectoring between those rear motors. Switch the dial away from the eco mode and feel its magic.The early-onset understeer is gone. Instead the car feels far more eager and lighter as it swivels into the corner. The inside wheel motor is slightly dragging that wheel back, pivoting the car into the bend. On the way through and out of the curve, the outside wheel is given extra torque, kept right at the limit of traction, and the inside one gets torque variation to keep the car on the course you've set with the wheel.It works amazingly well, and yet the car feels natural, easy to predict and understand. Switch the ESP to 'sport' and it'll even drift a bit. It's astonishing how quickly the e-motors can react to changing circumstances\: far faster and more subtly than using brakes or cutting and reinstating the power in normal ESP systems.Even more astonishing is how the character of the chassis is being altered by software as you change settings.Mercedes AMG pulled a similar torque-vectoring trick with the individual wheel motors in the SLS e-cell. And that car has four of them, for 4WD. Hmmm. AMG has built a car that sounds more of an Audi than the non-quattro Audi does. Maybe that's why the R8 e-tron got stopped so near to production.But Audi boss Wolfgang Durheimer gives other reasons. 'We are a premium company. Our customers don't want to miss out on any day-to-day desire.' The desire he refers to is that of arriving at your destination. He means that being stuck by the roadside with a flat battery isn't a premium experience.He also says the R8 e-tron would be too expensive to build - all that carbon, all that battery. This means it would have to sell at a loss. 'If we used the A3 and A4 to subsidise a flagship, well that's not very polite to those customers,' he says icily.