S400 Hybrid Update:

The Mercedes-Benz S400 BlueHybrid will launch in the United States one year from now, marking the German automaker’s first production foray into hybrid technology.

Mercedes started with the S350’s 3.5-liter V-6 and added an electric motor powered by lithium ion batteries – the first in a production vehicle. The battery pack is small enough to fit in the engine compartment (the conventional battery was moved the trunk), while the 20 hp motor sits between the V-6 and the 7G-Tronic seven-speed automatic transmission. The entire package adds only 165 lbs. Engineers also squeezed more power and efficiency out of the gasoline engine by modifying the valve and camshaft timing while adding new cylinder heads and pistons. The changes net 7 hp, combining with the electric motor to make 299 hp and 284 lb-ft of torque.

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In both form and function, the S400 skews toward the mild end of the hybrid spectrum. The electric motor never powers the car by itself, and is tuned to provide more power in addition to economy. Mercedes said the motor adds a “boost” effect during hard acceleration, projecting a 0-62 time of 7.2 seconds and a governed top-speed of 155 mph. Inside, the S400 eschews a prominent hybrid status screen and instead displays the information in the instrument cluster. A stop/start function shuts off the gasoline motor when the vehicle slows to less than 9 mph, and uses the electric motor to ignite the engine again as soon as the driver lets off the brakes. Like most hybrids, the batteries recharge during braking.

The result is a combined fuel economy of roughly 30 mpg, an improvement of about 4 mpg over the S350, with little to no sacrifice in performance and luxury. Carbon emissions are also down by 21 percent. Mercedes said the gains are enough to make the S400 the most efficient car in its class. It should hit U.S. streets in September 2009.