It’s been three years since BMW announced the creation of its i brand, a range of so-called megacity vehicles that begins with the hybrid-powered BMW i8 sportscar and the all-electric BMW i3 city car.

The i range is conceived to take motoring to the next level of efficiency, using a combination of lightweight construction, electric propulsion and, as BMW chairman Norbert Reithofer promised in 2010, initiatives that would make it possible for anyone with the simple need to get from A-to-B to get in on the action.

The German car maker’s ideas for battery-based motoring were initially presented on the limited-production Mini E and, more recently, the BMW 1-series ActiveE, both of which have been run on carefully monitored fleets as a means of gathering data on driving habits. So far, these two cars have covered almost 10 million miles in testing and, BMW says, provided valuable information, such an average commute for city dwellers around the globe of 26 miles.

Most significant of all, though, has been the appearance of BMW’s advanced i3 city car and sleek i8 sportscar, initially as conceptual models and more recently as fully functioning prototypes. Together, these two futuristically styled, carbon-fibre bodied models will spearhead BMW i’s entry into the ranks of dedicated alternative drive vehicles. The i3 has been conceived primarily for urban use with an all-electric driveline, while the BMW i8 uses a petrol-electric hybrid layout in the interests of performance.

But before they make their world debut in production guise later this year, Autocar was invited to join a team of engineers in north Sweden as they put the final winter test miles on a series of near-to-production mules. Far removed from the pomp and ceremony of a motor show reveal, it is here on snow strewn roads and under sub-zero temperatures where BMW’s new i cars must prove their mettle before they can be considered ready for the showroom.

Riding in the BMW i3 in the snow

The focus of engineering activities less than a year out from launch is the calibration of electric drive systems and cold start compatibility. As we arrive at BMW’s in Arjeplog, ground central to the European automotive industry’s winter test activities, it is the BMW i3 that exits the workshop and we get to ride in first.

Set to underpin BMW i sales, the compact hatchback has been conceived primarily for city use with dimensions that make it 120mm longer, 81mm wider and 132mm higher than the existing Mini Cooper hatchback. It also boasts 200 litres of luggage space behind the rear seats, or 40-litres more than its British-built sibling.

They’ll be two versions of the i3 on offer in the UK later this year, both running an electric drivetrain similar to that used by the 1-series ActiveE. It consists of a rear mounted electric motor that provides 168bhp and 184lb ft to the rear wheels via a fixed gear transmission.

Energy is provided by a lithium-ion battery mounted in the flat floor. It can be charged both through plug-in means and by kinetic electricity produced on run, with the range put at an official 140 miles on the European test procedure or, as BMW suggests, between 80-100 miles in everyday use.

Alongside the standard version of its new city car, BMW will also offer range extender option. It will use a 650cc two-cylinder petrol engine borrowed from its C650 GT step-through motorcycle. The engine fires when the battery level drops to a pre-specified point, acting purely as a generator to produce electricity and help to extend the range to somewhere in the region of 200 miles on an as-yet-unspecified fuel tank – all without any effect on the drive process. The car is expected to cost about £38,000, which puts it in BMW 330d territory in terms of price.

The tall stature of the i3 is a big departure from existing BMW models, but when you see it up close you’re aware that it has the stance to carry off its height. Even with the disguise that covers the prototype, it appears well planted. Standard 19-inch wheels, chosen in part to provide greater clearance for the battery mounted in the floor of the platform structure, are shod with narrow low rolling resistance 155/70 profile front and 175/65 profile rear tyres. They help endow the new car with a grown-up look. Even larger 20-inch rims will be available as an option.

Owing to the inherent stiffness of its carbonfibre body structure, the i3 does without a B-pillar. All doors are frameless affairs, and as I swing the front one open it becomes obvious that this is a car you climb up in to, rather than climb down in to.

Recent developments not seen on the original BMW i3 concept unveiled at the Frankfurt motor show back in 2011 include coach style rear doors. They look heavy with substantial width to conform to tough side impact regulations, particularly in the US. But with plastic panels and an aluminium frame, they’re considered vital in providing the entry-level vehicle in the i range with the sort of every day practicality BMW deems necessary to make it its new city car a success. They help ease entry to the rear, and unlike the Mini Clubman, there’s one on either side, not just the right.

Inside, the i3 impresses with a clean and uncluttered driving environment devoid of a centre stack. All controls, including a stubby gearlever that sprouts from steering column and a pair of digital screens to rely information, are concentrated within a horizontally themed dashboard, freeing up space between the driver and passenger and helping to provide the cabin with feeling of genuine space.

The raised seating position, upright seat backs, sweeping windscreen that extends back into the leading edge of the roof and generous depth of the near vertical side glass, provides excellent visibility out front and to the side. It instantly reminds me of the original Mercedes-Benz A-class, which is no real surprise given the new BMW i3 is roughly the same size and has been conceived for similar driving duties.

My chauffeur Patrick Mueller, head of BMW i drivetrain development, reveals the i3 is much like the BMW 1-series ActiveE in the overall character of its power delivery, with a heavily sprung throttle feel, instant acceleration and an aggressive recuperation mode aimed at recovering as much kinetic energy as possible. “It’s an approach that will set our cars apart from the electric car competition,” he suggests.

When the driver backs away from the throttle the electric motor acts as a generator, providing sufficient levels of retardation that the brakes are rarely required. It’s so aggressive that BMW has programmed the LED imbued brake lights of the i3 to illuminate when you lift the throttle.

It doesn’t take long to realise the BMW i3 is already at an advanced state of development. The prototype I rode in felt solid, even if there was the odd rattle from the makeshift disguise covering parts of the interior.  What really grabbed my attention, though, was its overall agility and, in trying conditions with those narrow tyres, excellent traction. Watching on as Mueller drove the prototype down slip lanes, over steep gradients and around the car park of BMW’s winter test facility, it appeared to possess all the qualities that will be required to make it suited to city use, including an excellent turning circle of less than 10 metres diameter.

If our time spent in the passenger seat as BMW’s engineers put the tall five-door hatchback through its paces on a frozen Swedish lake told us anything, it’s that the i3 will not only provide a high level of maneuverability but should also prove an entertaining proposition on less-congested roads as well.

New i3 retains BMW's trademark dynamics

With a kerb weight of 1250kg, it accelerates in truly impressive fashion thanks to the ability of its electric motor to deliver an instant and quite sizeable slab of torque. Straight line performance is roughly on par with the Mini Cooper S, with BMW claiming 0-62mph in 7.2sec and 37-75mph acceleration in 6.0sec. The top speed of the production version will, however, be limited to around 93mph to protect the charge of the battery.

The impression of sportiness can largely be traced to a driveline layout that places the i3’s electric motor low down at the rear within the axle assembly. This gives it a much lower centre of gravity than conventional front engined city cars. By providing drive to the rear wheels, the front wheels are also left to do the steering without any corruption from the drive process, as with more traditional front-wheel drive.

Just how entertaining the i3 is set to be is keenly displayed by Mueller, who whips the compact hatchback for two complete laps around a giant skid pan with an armful of opposite lock and wild oversteer without ever backing off. “Despite the electrification of the drivetrain, it remains true to the BMW philosophy in terms of its dynamic character,” he offers with a smile.

The BMW i8 is an alternative sportscar

As intriguing as the i3 is, though, it is the i8 that will headline BMW’s i car line-up when it goes on sale in 2014. Conceived as an alternative to the current crop of conventional combustion-engined sportscars, the sleek new coupé aims to provide the best of both words by offering the sort of straight line performance to see off the Porsche 911 Carrera and combined cycle fuel economy to make even the Volkswagen Up appear thirsty by way of comparison.

This is the image leader for BMW’s push into the alternative drive ranks, and its futuristic styling, complete with its signature fluted rear bodywork, fully reflects it. Like its hatchback sibling, the distinctively styled 2+2 coupé has undergone subtle changes in appearance since it was originally revealed in concept car guise in 2010. Among them are new headlamps and a more formal version of BMW’s traditional kidney grille, and as I prepare to climb down in to the BMW i8 it also dawns on me that the long doors have lost their butterfly style hinges for more conventional front hinges.

From inside, the i8 possesses all the hallmarks of a proper sportscar. You sit low, below the level of the carbonfibre sill, with your legs well out in front. The seats are tight, hugging, hard shell affairs. The deep but low dashboard is very prominent. However, it is the instrument binnacle – whose mesmerising graphics alter depending on the driving mode chosen, going from a calm hue of blue in eco-pro and comfort to a racier orange hue in sport mode – that initially steals my attention as we set off down a slip road and out on to BMW’s test track.

As with the i3, the i8 is based around a carbonfibre structure and hits the scales at 1480kg in production guise. Unlike the new city car, which will only be available with electric drive, however, the more sporting of BMW i cars relies on a plug-in petrol-electric hybrid system for its performance. In less severe weather, it would nominally start in electric mode. However, the electronic system has detected the sub-zero temperatures of northern Sweden and fires on the combustion engine.

At the heart of the new car is a compact, turbocharged 1.5-litre three-cylinder direct injection petrol engine. It's the same unit we tested last year fitted to a BMW 1-series prototype and set for BMW’s first front-wheel drive model, as previewed by the Concept Active Tourer at last year’s Paris motor show, as well as the next-generation of Mini models, the first of which will appear at this year’s Los Angles motor show.

Mounted transversely behind the roomy cabin, the aluminium block unit develops an impressive 220bhp and 221lb ft of torque, all channeled to the rear wheels via a six-speed automatic gearbox boasting a conventional torque converter. It’s a boisterous unit, emitting a distinctive three-cylinder hum that gets deeper and more prominent as engine speeds increase. A glance over at the highly detailed digital tachometer as Jos van As, head of chassis development for all BMW models, fires the i8 down a long straight reveals the redline is pegged at 6500rpm.

But, as the i8’s plug-in hybrid billing suggests, the petrol engine is not the only form of propulsion. An electric motor, shared with the i3, sits up front in the nose, sending an additional 128bhp and 184lb ft to the front wheels. There’s also a second, much smaller electric motor mounted at the rear next to the combustion engine, producing 5bhp and 6lb ft. But while it is capable of providing drive to the rear wheels, its primary purpose is as an alternator and/or generator depending on which driving mode is chosen; Eco-pro, Comfort or Sport.

Altogether, there is 349bhp, giving the i8 a power-to-weight ratio some 16bhp/tonne beyond that of the BMW Z4 sDrive35i, at 236bhp/tonne. And yet, it is the torque, which swells to a peak of 406lb ft which leaves the biggest impression. There’s great urge when BMW’s chassis ace knocks the gear lever to the left to engage Sport mode, in which all three power sources are engaged, and then throws the throttle open. BMW’s claims of 0-62mph in 4.6sec and 37-75mph in 4.0sec are in no way optimistic. In fact, I wouldn’t be surprised if the definitive performance figures (to be revealed closer to its launch next year) are even sharper.

As we rush along snow-covered roads, I notice a button on the centre tunnel marked 'E mode'. Van As obliges, depressing it to alter the drive process from petrol-electric to solely electric, in which energy is provided by a lithium-ion battery pack mounted within the centre tunnel. It is a neat trick – one that will allow drivers of the BMW i8 to undertake journeys of up to 20 miles on battery power alone, allowing them to dodge London’s congestion charge and other similar zero-emission zone charges. It also provides the swoopy coupé with near-to-silent cruising qualities.

Three versatile power sources in the BMW i8

The most impressive aspect of the new car when we leave the BMW test track and head out on public roads is the smooth interplay between the three power sources, the result, Van As reveals, of countless hours spent refining the algorithms of the i8’s so-called power electronics.

“It’s a crucial part of hybrid drivetrain development, and something we’ve put a great deal of effort in to perfecting to keep us in good stead for the future,” he says. “It’s part of the reason why we decided from the outset not to engage an outside partner, but to keep all electronic development in-house, and retain the intellectual property rights for ourselves.”

The way the BMW i8’s advanced drivetrain switches from hybrid mode (in which all three power sources are in use) to pure electric mode (in which just one power source is relied upon) at the press of a button on the centre console, is extremely impressive. In hybrid mode, all four wheels provide drive. In electric mode, only the front wheels channel drive. It all sounds remarkably complex, but you’d never know it, such is seamless interplay.

Further impressions? While it may be billed as a sportscar, the i8 boasts an excellent ride. The overall set-up is claimed to be close in terms of comfort to that set to appear on the upcoming BMW 4-series coupé. “We are aware certain customers will use the i8 every day. It needs to offer sufficient low speed compliance for commuting in combination with the control required at higher speeds," says Van As.

So, is the BMW i8 be capable of taking the fight to more conventional sportscars like the Porsche 911 Carrera? After all, it is expected to cost about £100,000, a similar price to a Carrera 4S. It’ll be another year at least before we get to steer the BMW i brand’s flagship model for ourselves but we now know that it is not only spectacularly futuristic in terms of appearance but also engagingly fast, imminently usable and comfortable enough to be used everyday.