Mercedes-Benz has taken the wraps off of its first hybrid car for passenger, the S400 Blue HYBRID. Based on the S350, this mild hybrid auto pairs an electric motor/generator and the first lithium-ion battery in a production car with a specially tuned version of the company's 3.5L V6 driving the wheels through a 7-speed automatic transmission.

Lithium-ion batteries are ideally suited for use in hybrid vehicles to help reducing fuel consumption and thus also CO2 emissions. At the same time, the Daimler engineers are investigating to what degree this technology can be applied to other vehicle concepts, such as electric and fuel cell-powered cars.

Current hybrid vehicles use nickel-metal hydride batteries, but the lithium-ion alternatives are known to offer greater efficiency, as well as a better overall weight-to-power ratio. The problem with integrating the Li-ion technology was in securing their stability—they became far too hot to be a viable option in a hybrid vehicle. Fortunately, Mercedes-Benz has 25 patents it is confident solves the dilemma of battery overheating, whether they stay on target for 2009 is another matter, but we certainly wouldn't doubt them.

The assist from the electric motor gives the S400 an agile feel and more immediate throttle response, although it doesn't feel as strong as our V-8-powered base S-class, the 382-hp S550. The kick from the electric motor is limited to only about 15 seconds because of the relatively small lithium-ion battery pack, and the electric motor only works up to 4000 rpm. But the boost is useful for low-speed acceleration and even in passing maneuvers. In all other driving situations, the S400 feels pretty much like a conventional S350.

Although the electric motor cannot power the S400 by itself, it does allow the gasoline engine to shut off when slowing to speeds below 9 mph, when coming to a stop, and when idling. When stopped, and with the gasoline engine asleep, the lithium-ion battery powers the accessories such as air conditioning as well as the electro-hydraulic steering. When the driver lifts his foot off the brake pedal, the gasoline engine whirs quickly to life without any hesitation or odd vibrations.

Thomas Weber, member of the Daimler AG Board of Management and responsible for Group Research and Mercedes-Benz Cars Development says: "What we have here is a groundbreaking key technology that is going to be a decisive factor for the future success of the automotive industry. That is a tribute to our intensive research efforts, which we have been conducting in this area since 1992."

With the new S-Class, Daimler has become the first European automobile manufacturer to launch a vehicle with a hybrid engine and the innovative lithium-ion battery. The S 400 Blue HYBRID consumes only 7.9 liters of gasoline per 100 km in the NEDC. This results in very low CO2 emissions of only 190 grams per kilometer, a very low value for this vehicle class and power class, making the S 400 Blue HYBRID the world's most economical luxury sedan

What’s more important to consumers is that the battery pack is installed in the engine compartment, where it replaces the conventional starter battery. This means that the interior space and boot capacity of the S400 remain unchanged. Additionally, thanks to its compact dimensions and modular design, the additional weight of the overall hybrid system including the comprehensive safety systems is only 75 kg or 165 lbs.