AN open top version of BMW's i8 hybrid supercar has been announced as the trailblazing petrol-electric model gets a mid-life overhaul.

On sale in the New Year and priced from £124,730 the new i8 Roadster will be joined by a revised i8 Coupe costing from £112,730 - an increase of some £6,000 over the current model.

Power on both models has been increased and so has range with the cars now capable of zero emission motoring for up to 34 miles, almost twice the distance possible on the original i8.

Like the i8 Coupe - first launched in 2014 - the i8 Roadster has dihedral doors which open upwards and outwards but comes with a fabric roof that can be operated at speeds of up to 31mph opening and closing in 16 seconds at the touch of a button.

A clever Z-style folding mechanism means the roof is stored vertically behind the cockpit and with the roadster being only a two-seater an extra 100 litres of luggage space is created behind the rear seats.

That's in addition to the 88 litre boot which is common to both with additional stowage space under the bonnet.

When the roof is down, the rear window rises by an inch-and-a-half automatically to reduce turbulence in the cockpit - an operation which can be adjusted by the driver while the aluminium connectors used to attach the hood have been created by 3D printing - an industry first.

The new i8 features an aluminium body supporting a carbon fibre reinforced passenger cell and under the skin the i8's electronics have been given a boost with battery capacity increased by 14Ah to 34Ah.

The result is more power with the gross energy rating now up 7.1kWh to 11.6kWh giving the electric motor which drives the front wheels a power output of 143 horsepower.

As before, the electric motor is mated to a 1.5-litre petrol engine which drives the rear wheels making the i8 all-wheel-drive with a six-speed steptronic automatic transmission.

The engine is now boosted to 231hp giving a combined power output to 374hp which gives the i8 performance figures of 0 to 60 in 4.4 seconds for the Coupe and 4.6 seconds for the Roadster. Top speed on both is restricted to 155mph.

In electric-only mode both models can reach speeds of up to 65mph - 75mph when the eDrive boost mode is engaged - and to help on the eco front the engine is now fitted with a particulate filter to cut emissions.

Theoretical fuel consumption is 149.8mpg for the Coupe with emissions of 42g/km and 134.5mpg for the Roadster with a CO2 figure of 46g/km.

In real world conditions BMW says the i8 should average around 56 to the gallon around town and 40mpg on a run.

The lightweight construction means that the Roadster tips the scales at 1,595kg which is only 60kg more than the 2+2 Coupe.

Recharging the batteries takes around 4.5 hours from a domestic supply or less than three hours with an uprated BMW Wallbox home charger.

The new i8 will be on sale in January and the two versions will have their world premiere at the Los Angeles Auto Show which runs until December 10.