Sitting up front is BMW’s turbocharged 2.0-litre, four-cylinder petrol engine. It’s tuned to deliver 177bhp and 236lb ft of torque.
This unit is supported by an electric motor mounted in the standard eight-speed automatic gearbox. It provides an additional 94bhp and 184lb ft of torque.
All up, that means this 3-series offers a combined output of 242bhp and 295lb ft of torque in hybrid mode.
These generous outputs are channelled through a reworked version of BMW’s ZF automatic gearbox, which has been modified to improve fuel efficiency and manage the transition between power sources.
The electric motor draws energy from a lithium ion battery mounted relatively high up in the boot, shrinking overall capacity slightly over the 480 litres offered by more conventional 3-series models.
BMW makes some pretty impressive fuel economy claims for the 3-series eDrive, although, like all new plug-in hybrids, the claimed figure arrived at is not truly representative of real-world driving. For what it's worth, though, the 3-series eDrive is said to return 131mpg, giving it an average CO2 rating of just 50g/km.
The hybrid system operates in five different modes, with two new ones specific to the eDrive model, called Max eDrive and Save Battery.
The eDrive mode provides all-electric running. The claimed electric range is a slightly disappointing 22 miles, achieved at speeds limited to around 75mph.
To drive, this early prototype is convincing in terms of overall operation but lacks the final layer of drivetrain refinement to make it truly impressive yet.
Still, with a year of development left before the final version is due, it would be disappointing if it wasn’t on the money by the time it reaches showrooms.
Unlike some recent plug-in hybrids we’ve driven, there’s no extra cockpit drill to study up on before setting off. In fact, the BMW is entirely straightforward. You simply climb in, belt up, place your foot on the brake and hit the start button, at which point the instruments spring to life to indicate it is ready.
There’s no firing of the petrol engine when the ignition catches, merely silence as the electric motor waits for you to draw the gear selector back to select Drive.
With sufficient charge on board and the hybrid system set in its default mode, the new BMW moves off on battery power alone.
In the initial couple of miles of running in eDrive mode, the 3-series eDrive is predictably smooth and, given the relatively modest power, surprisingly swift as it accelerates up to typical urban speed limits. Still, it doesn’t take much of a push on the throttle before the system draws on the engine for more power.
As you up the pace, there is a nice, flexible nature to the delivery and plenty of encouragement to press on. Together, the electric motor and petrol engine provide thoroughly convincing in-gear performance. It may not sound quite as good, but it accelerates with all the urgency of the 335i.
However, the interaction between the two power sources isn’t quite as seamless as we’ve witnessed in some more recent plug-in hybrids. In the prototype we drove, there was a brief pause as the petrol engine kicked in and a distant whine from the electric engine on a loaded throttle.
BMW hasn’t provided any official performance data yet, but with a good deal of low-end torque and impressive traction, the finished version of the 3-series eDrive should crack 0-62mph in less than six seconds.
The 22-mile electric range might not sound much, but most commutes are less, and it’s also 19.5 miles further than the ActiveHybrid 3 achieves.
Dynamically, there’s not much to fault. The big battery is likely to endow the eDrive with a kerb weight of more than 1700kg, but it handles with distinction, even on the modest 225/50 R17 tyres used by the prototype.
The new BMW steers in a direct manner, displays impressive body control and possesses strong levels of grip. It’s only when you push hard in tight corners that the added weight really becomes an issue; the front end relinquishes purchase and understeers a little earlier than a conventionally powered 3-series, and the weight of the battery pack in the boot drags the rear axle around in a mild pendulum effect if you’re really going for it.