Mercedes' version has two flywheel-sized electric motors installed where the torque converter normally sits. The front motor does the job of both the alternator and starter motor for the conventional diesel engine up front. If extra power is needed then it can revert into a conventional electric motor offering an additional 34bhp and 147lb ft.

The second electric motor also powers the car when needed, with its 34bhp and 221lb ft, but has a second role of providing braking. It does this by swapping polarity and becoming a generator to recharge the battery pack. A multiplate clutch is controlled by a computer playing the role of the torque converter.

The driving experience is interesting. In town, stop-start motoring is taken care of by the electric motors, until the batteries lose power. Cruise on the motorway and all the power is taken care of by the conventional piston engine. It really gets interesting once you're off the motorway and go for a full-power overtake, as that's when both electric motors kick in to provide an additional 68bhp and 368lb ft.

Manufacturers have two ways of using this technology; either they fit a smaller engine up front and go for economy benefits, or use it to supplement a powerful petrol engine and offer the driver bursts of simply massive acceleration - a sort of Nitrous injection for grown-ups. The big players are teaming up to develop the technology - Mercedes with GM, Ford with BP and Porsche with Toyota.