Review: 2010 Mercedes-Benz S400 Hybrid and Lexus LS600hL

For 2010, the Mercedes-Benz S-class gains its first-ever hybrid powertrain, a clear indication that its maker is shifting from a strict diesel approach to efficiency and finally acknowledging the worth of hybrids as an essential step on the path to electric propulsion. What's more, the Mercedes S400 Hybrid is the world's first production hybrid automobile using lithium-ion batteries (the Tesla Roadster is powered solely by Li-ion batteries). Hybrid propulsion systems aren't anything new, of course, but they're still rare in the luxury class, where the Lexus LS600hL has been standing alone for the past two years.

Mercedes and Lexus systems wouldn't be more different if they had been designed on different planets. The Lexus approach adds some $30,000 to the price of the sedan, not to mention nearly 600 pounds, compared with a conventional LS460L. Included for those extra dollars and pounds are a four-wheel-drive system, a 221-hp electric motor, nickel-metal-hydride batteries, and a 5.0-liter V-8. Loaded to the gills, the LS600hL is Lexus's flagship sedan, designed to offer V-12 performance with V-8 fuel economy.

Mercedes has taken a decidedly different route - one that adds less weight and complexity. Similar in design to Honda's Integrated Motor Assist hybrid powertrain, the Mercedes setup has a compact, ring-shaped electric motor sandwiched between the engine and a conventional automatic transmission. Like Lexus, Mercedes employs braking regeneration and engine shutdown at stoplights to curb consumption. Although the Lexus can propel itself a few feet at very low speeds with the engine off, that function is not available in the S400. Nor does Mercedes offer 4Matic four-wheel drive in this hybrid.

Instead of promising performance similar to an S-class with a larger engine, Mercedes chose to introduce its hybrid model as a more environmentally conscious way of traveling in luxury. The S400 Hybrid should cost about the same as the $90,225 V-8-powered S550 when it goes on sale in the United States this September. It uses the European-market S350 as a starting point, so behind the three-pointed star lurks a six-cylinder engine. The 3.5-liter V-6 produces 275 hp, the same as the base V-8 did in 2006, the last time an S-class was available to U.S. customers with a six-cylinder. And that V-8 didn't benefit from an additional electric motor.

The V-6 is similar to the 3.5-liter unit found in other Mercedes cars and crossovers, but this version has different pistons and cylinder heads. It also runs on the Atkinson cycle, a common hybrid approach. The Atkinson cycle uses late-closing intake valves to stretch the expansion (power) stroke for increased efficiency. The one sacrifice is some loss of torque, especially at low rpm.

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