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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.

And that's where hybridization comes in. The electric motor in the S400 contributes up to 118 lb-ft of torque, more than making up for the Atkinson sacrifice. In fact, total system power is quoted at 295 hp and 284 lb-ft, topping the 268 hp and 258 lb-ft ratings in a C-class fitted with the 3.5-liter V-6.

As a result, the S400 Hybrid, Mercedes says, reaches 60 mph in about seven seconds, but it can't come close to matching the sports-car-like S550, which takes only 5.4 seconds. The payoff in fuel economy, though, is substantial. Mercedes estimates that the S400 will achieve 23 mpg in the city and 33 mpg on the highway. The latter figure sounds optimistic to us, but even if the S400 earns an EPA combined 25-mpg rating, that's a fuel savings of nearly 50 percent over the V-8. And, let's face it, how many luxury sedan owners really need all of a V-8's prodigious thrust?




Lexus would say all of them - and to that end, the LS600hL loses none of the performance of the nonhybrid LS. Total system output is a robust 438 hp, sufficient to slingshot the LS to 60 mph in 5.5 seconds, according to Lexus. That's only 0.1 second slower than the nonhybrid version, but the more expensive Lexus achieves a combined 21 mpg in EPA tests, beating the all-wheel-drive LS460 by 3 mpg.

The Lexus suffers in one area that the Mercedes doesn't - cargo capacity. The large nickel-metal-hydride batteries and the extra air-conditioning system needed to keep them cool reduce trunk space from 18.0 cubic feet to only 11.7. Mercedes was able to locate its smaller and more energy dense Li-ion battery pack, which consists of thirty-five cylindrical cells supplied by the French firm Saft, in the engine compartment, leaving the S-class's 16.3-cubic-foot trunk intact.

On the road, the Lexus somehow manages to be lithe and nimble; it's an uncommonly responsive supersedan despite its supertanker weight. It combines an ultrabrisk throttle response with very strong brakes and a reassuring blend of grip and traction, although it's let down by a somewhat detached chassis, androgynous steering, and a ride that seems to vary almost at random between brittle and cushy.



The biggest benefit of the Lexus hybrid system is its smoothness. The two electric motors and two planetary gearsets allow the big V-8 to loaf except during hard acceleration. Flat-out, the LS600hL might not be any faster than the regular LS, but it doles out power far more effortlessly.

The Mercedes, by comparison, feels like a normal V-6-powered S-class. The Benz and the Lexus behave so differently that you realize this isn't a battle of which car is the better sedan, it's a clash between two philosophies. The Lexus was admirably early to market, but suddenly it seems that Benz's more elegant system reaps nearly the same benefits.



Indeed, rumor has it that this hybrid system will be standard equipment across the board on the next-generation S-class set to debut in late 2012. Mercedes also has a two-mode hybrid system coming this December in the ML450, and we're confident that Lexus has a few more hybrid tricks up its sleeve. So, which is it going to be? Will we see more luxury hybrids with straightforward powertrains, or will the Lexus model prevail? The answer isn't completely clear, but there is one thing you can bet on: hybrids are here to stay.

The Specs

Mercedes-Benz S400 Hybrid
On sale: September
Price: $90,000 (est.)
Engine: 3.5L V-6/electric hybrid, 295 hp (combined), 284 lb-ft (combined)
Battery: Lithium-ion, 0.8 kW-h
Drive: Rear-wheel

Lexus LS600hL
On sale: Now
Price: $106,035
Engine: 5.0L V-8/electric hybrid, 438 hp (combined), 385 lb-ft
Battery: Nickel-metal hydride, 1.9 kW-h
Drive: 4-wheel