• What is it?: The first-ever hybrid version of the big Mercedes-Benz sedan
• The basics: All the S-Class virtues, much better gas mileage
• On sale: This fall
• Price: $87,950

This weekend, GreenCarReports.com will be one of the first media outlets in the States to test the new 2010 Mercedes-Benz S400 Hybrid. We'll offer our impressions, and the gas mileage we get, next week. Meanwhile, we wanted to preview the car for you.

Unlike Asian carmakers, European manufacturers have been slow to adopt hybrid-electric technology. As recent comments from German executives show, many feel hybrids make no sense against the more fuel efficient diesels now fitted to half the new cars sold in Europe.

But it's not clear that US buyers will pay the higher prices of clean diesels, so German carmakers are now hedging their bets.

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

S400 Powertrain

S400 Powertrain

The two-mode hybrid, developed along with GM, was the main fruit of the partnership

The two-mode hybrid, developed along with GM, was the main fruit of the partnership

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

2010 Mercedes-Benz S400 Hybrid

Experiments beyond diesel

Mercedes-Benz, among others, is experimenting with various hybrid technologies, from "full hybrids" that permit some degree of all-electric running to simpler "mild hybrids" that switch the engine on and off when stopped, and add some degree of electric boost.

The first product of those experiments is the 2010 Mercedes-Benz S400 Hybrid, launched last fall in Europe, which goes on sale in the States this fall.

Hybrid help, but no electric running

It adds a 15-kW electric motor between the 275-horsepower, 3.5-liter V6 engine and Mercedes-Benz's standard 7-speed automatic transmission.

The electric motor contributes torque under heavy engine loads, restarts the engine, and even provides a small amount of initial torque to move the car away from stop to 2 or 3 miles per hour. The S400 Hybrid cannot run solely on electric power, however.

The EPA rates the S400 Hybrid's gas mileage at 19 miles per gallon city / 24 mpg highway. That compares to 14 mpg city / 21 mpg highway for the S500, the next model in the S-Class lineup.

World's first lithium-ion battery pack

It's the battery that gives the S400 its laurels; it's the first production hybrid car to use lithium-ion cells in its battery, which hold roughly twice as much energy in the same weight as the nickel-metal-hydride packs used in other mild hybrids like the 2010 Honda Insight.

The pack itself, using cells built in France by JCI-Saft, contains 0.7 kilowatt-hours of energy, or about the same as the 2010 Insight's.

Mercedes-Benz was able to fit the entire pack into the same space (at the right-hand base of the windshield) that used to hold the standard lead-acid 12-Volt starter battery. The designers didn't have to make a single change to the body structure.

To keep the battery temperature below 25 degrees C, however, the engineers had to plumb the S-Class air-conditioning coolant pipes right through the pack.

Invisible hybrid

Absent the chrome lettering that says "HYBRID" on the trunk, you'd never know this full-size S-Class sedan is a hybrid. The performance won't give it away either; the 0-to-62-mph time of 7.2 seconds is similar to non-hybrid V6 versions sold in Europe. (In the States, all other S-Class models come only with V8 engines, the smallest of which is 382-horsepower, 5.5-liter V8 in the $91,600 S550.)

While the company is proud to claim the first lithium-ion hybrid, "the cost is much higher than we would like," says Christian Mordieck, who led the battery development.

The S400 Hybrid, however, is the least expensive S-Class sold in the States, perhaps reflecting its V6 engine and less aggressive performance. It's worth noting that the S-Class model range goes all the way up to the hot-rod S65 AMG, which has a twin-turbo, V12 engine delivering 604 horsepower--almost the antithesis of green.

Atkinson Cycle V6

As in most hybrids, the S400's 3.5-liter V6 engine was redesigned to operate on the Atkinson (or five-stroke) cycle, which uses valve timing to simulate a cycle in which the piston moves through strokes of different lengths, extracting more energy from the fuel than a conventional engine.

The Atkinson cycle's characteristic low torque at lower engine speeds neatly complements the high torque available from the hybrid's electric drive motor when starting from rest.

Two-Mode full hybrid too

Mercedes-Benz will also release the ML450 Hybrid, a "full hybrid' version of its ML sport utility vehicle that uses the Two-Mode Hybrid system co-developed with General Motors, Chrysler, and BMW. The 2010 BMW X6 ActiveHybrid uses the same system.

The Two-Mode hybrid system, in fact, is offered not only in full-size GM sport utilities (the 2010 Chevrolet Tahoe, GMC Yukon, and Cadillac Escalade) but also the company's hybrid full-size pickup trucks (the 2010 Chevrolet Silverado and GMC Sierra).