The long-awaited Mercedes S400 hybrid has finally been officially unveiled, and the results are somewhat surprising. While the S400 BlueHYBRID is still billed as the first series-production car to market with a purpose-built lithium-ion battery, it's not as impressive a hybrid system as may have been anticipated.

By combining a 279hp (205kW) 3.5L V6 engine and a 20hp (15kW) electric motor, the S400 BlueHYBRID is what most would call a 'mild' hybrid. That's enough to get fuel consumption down to 29.7mpg (7.9L/100km) with emissions of 190g/km CO2 - lower than the S350 it's based on, but still not in a league with 'full' hybrids or even some diesels. Nevertheless, it's a good result for a full-size luxury sedan, and a 21 % improvement over the standard S350.

The lithium-ion batteries contribute enough energy to push the car's model number up from its natural displacement of 3.5L (as in the S350) to the equivalent of a 4.0L engine. Equipped with the 7G-Tronic seven-speed automatic transmission with special adjustments for the hybrid model, the car is also capable of the 0-62mph (100km/h) dash in 7.2 seconds. Top speed is electronically limited to 156mph (250km/h).

Fuel efficiency gains are made through a combination of technologies, including stop-start functionality, which helps to cut fuel use while standing still at traffic lights or jams. But improved efficiency isn't the only effect the hybrid system has - it can also boost performance for brief periods. By kicking in the electric motor's full 118lb-ft (160Nm) of torque as the fuel-intensive acceleration phase kicks in, it speeds take-off while minimizing fuel use. Regenerative braking helps to maximize efficiency and recapture energy that would otherwise be lost. All of the operation of the hybrid system is displayed for the driver's convenience in a single display in the instrument cluster.

Due for launch in Europe in June of 2009 followed by China in August and the U.S. in September, the S400 BlueHYBRID hasn't yet been priced, but can be expected to command a premium over the S350.