What is it?
This is the Mercedes S 400 BlueHybrid, the company's first production hybrid. Mercedes has developed the drivetrain technology in a joint venture with BMW, which plans to use it to underpin the similarly conceived 7-Series ActiveHybrid.
Visually, little distinguishes the Mercedes S 400 BlueHYBRID from the car it's based on, the three-year old S 350.
There are no aerodynamic tweaks or changes in ride height, only subtle BlueHybrid badges serve to give the game away.
What's it like?
The S 400 uses a lightly reworked version of Merc's existing 3.5-litre V6 petrol engine, with a new cylinder head, lightweight pistons and a reprofiled crank, although it does without the direct injection that Merc uses in other applications.
Output increases by just 7bhp over the S 350 at 279bhp, with torque remaining the same at 258lb ft.
The clever bit is the disc-shaped three-phase AC electric motor mounted within the front of the gearbox housing, and providing additional thrust when required, output swelling to 299bhp and 284lb ft, with drive channelled to the rear wheels via the standard seven-speed automatic gearbox.
The new system does not support pure electric propulsion like that offered by the Lexus LS600h; the efforts of the electric motor are used exclusively to boost those of the petrol engine.
This is also the first mass-produced hybrid to use next-generation lithium ion battery technology. It weighs just 27kg and provides more efficient charge and discharge than the nickel-metal hydride batteries used by other hybrids.
It's also compact enough to fit in the position normally used by the S-class's standard battery at the base of the windscreen, where it's cooled by a patented process linked to the car's aircon system.
The S400 BlueHybrid doesn't demand any particular change in driving style. In fact, it feels a good deal like the S350, only a touch more powerful under acceleration in lower gears.