Those motors are a modified version of the i3's synchronous unit. A 201bhp motor is mounted up front in the space usually taken up by the torque converter in the car's eight-speed automatic transmission, while another, developing 268bhp, is mounted within the rear axle assembly.
In total, the system is claimed to produce over 670bhp, along with a torque loading beyond 750lb ft – figures that easily top the 453bhp and 530lb ft of the existing naturally aspirated 6.8-litre V12 used by the 11-year-old Rolls-Royce Phantom.
The principle behind BMW’s Power eDrive system is a maximisation of electric motor performance. “The electric motors provide approximately two-thirds of the combined output, with the combustion engine accounting for the remaining third,” said Franz Drescher-Kaden, BMW concept engineer for the new petrol-electric hybrid set-up.
Energy for the electric motors is provided by a 20kWh lithium ion battery mounted both longitudinally in the rear of the 5-series GT’s centre tunnel and horizontally underneath the rear seat in a space ahead of the rear axle.
It can be charged both via plug-in means and on the run using the combustion engine in a steady state mode. The fuel tank has been reduced in size from a standard 70 litres to 30 litres.
As well as acting as a generator to produce electricity, the Power eDrive hybrid system’s petrol engine can also provide a performance boost with direct drive to the front wheels during kickdown, in which all three power sources are used for propulsion.
Drive is nominally channelled to the rear wheels via the rear electric motor, whose reserves are sent through a multi-speed gearbox.
The introduction of the front electric motor, which operates via the eight-speed automatic in which it is housed, provides all-electric four-wheel drive. This is further enhanced by the combustion engine, which also delivers its power to the front wheels.
BMW has not revealed a weight figure for its new hybrid system. However, it does concede that the addition of two electric motors, as well as the lithium ion battery pack and ancillary electronic management system, adds handsomely to the kerb weight.
Despite this, BMW says the straight-line performance of its 5-series GT Power eDrive prototype exceeds that of the 444bhp twin-turbocharged 4.4-litre V8 550i GT, which tips the scales at 2070kg.
Power eDrive is being developed as developed as a scalable system with power outputs ranging from 250bhp to over 650bhp, according to BMW, which says the plug-in hybrid technology will be “a natural choice for use in upmarket vehicle segments”.
Among the new models thought to be in line to run the new petrol-electric system are future incarnations of the Rolls-Royce Ghost and Phantom as well as the firm's planned SUV – currently undergoing conceptual development.
BMW has also indicated Power eDrive could be offered alongside traditional petrol and diesel engine options in its upcoming X7 – a new range-topping SUV due for launch in 2017, as well as the next-generation 7-series, X5 and X6.
Read Autocar's first drive in the new 3-series eDrive prototype
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