When Mercedes-Benz showed its new inline-six engine at this year's Shanghai auto show, it was assumed the engine would reach the United States in the freshened 2018 S-class sedan. It will not.
Three months later, we have details on the rebirth of the Mercedes straight-six. We can tell you it's packed with new technology, including one of the auto industry's first 48-volt electrical systems, and even more power than initially reported. What we can't tell you is when it's coming to North America, or in what.
For decades, the inline-six was the mainstay at Mercedes, from the E class to the SL to the G-Wagen, to the point where old-time Mercedes enthusiasts revered the straight-six as Porsche geeks revered air cooling. Mercedes' turn toward V-6s began with the launch of the U.S.-built M class in 1997, and by the time the W210 E class went out of production in 2002, the straight-six had disappeared. The old-time enthusiasts rued its demise.
The inline-six -- designated M256 in the holy halls of Stuttgart -- in the S class at Shanghai starts a transition back in the other direction. Old-time enthusiasts may be salivating, but understand that the switch back has little to do with the inherent smoothness, torque characteristics or aural splendor of an inline-six. It has everything to do with managing costs and promoting production efficiency.
In the mid-1990s, as Mercedes began its shift to V-6s, the market was moving toward more displacement and more V-8s. The V-6 allowed assembly with V-8s, modular style, on the same production line. Now, as the market and regulatory climate drive displacement back toward the smaller end of the spectrum, the inline-six allows the modular concept to be tied to four-cylinder engines.
"We need the efficiency and flexibility of building two configurations on the same line," says Oliver Vollrath, director of powertrain development for the S class. "We've chosen the inline architecture, and in certain respects, we've burned some bridges [if we're wrong]. And now the 50 percent of our engineers who favor the inline-six are happy again, and the half that prefer the V-6 are not so happy."
The M256 has two camshafts, 24 valves and direct injection. It's cast entirely of aluminum and displaces 2,999 cc, or about .5 liter per cylinder; subtract two pots and you have a 2.0-liter four. With a bore of 83 mm and a stroke of 93 mm, it's undersquare (or long stroke), as straight-sixes often are.
What else? Where to start? How about the electric compressor, mounted on the intake side of the M256. In concept, it's similar to the belt-driven supercharger on Volvo's most powerful turbo fours, which supplements the turbocharger as it builds boost. Yet because it's electrically operated, the M256's supplemental compressor goes from off to full boost in .3 seconds, according to Vollrath.
There's a more conventional twin-scroll turbo on the other side of the M256 to do most of the heavy lifting, with peak boost of 29 psi. The electric compressor fills the gaps with about 7 pounds of boost as the conventional turbo builds steam. The trick is when to use either or both compressors, and that's left to some complicated software developed by a staff of 10 in-house engineers, according to Vollrath. The result is a steady supply of forced air with virtually no gaps from idle to redline, generating forced-air power and torque with improved efficiency and no lag.
A new 48-volt electrical system helps enable that electric compressor. The M256 is the first automobile powertrain we've seen that generates electricity at 48 volts DC. The few 48-volt appliances in existing combustion automobiles, including Bentley's motor-driven roll-control system, rely on current up-converted from conventional 12-volt DC and stored in sufficient quantity in supercapacitors. The M256 generates at 48 volts, then down-converts to 12 volts through a power-electronics pack for conventional systems such as the stereo or engine ignition. It has both a conventional 12-volt battery and a 48-volt lithium ion battery with 1 kilowatt-hour capacity.
The current comes from a motor/generator -- Mercedes calls it an Integrated Starter Generator -- packaged in the bell housing between the engine and transmission. Its location smack in the middle of the driveline has value in itself.
The ISG manages the engine's idle speed at a hair over 500 rpm, and it damps the low-frequency lope one might expect at that speed. It engages the stop-start feature in milliseconds, imperceptibly, and it allows the engine to stop while the car is coasting down. In similar fashion, the ISG allows what Mercedes calls GlideSlope -- the ability to turn the engine off at high road speeds, such as when the car travels down a long descent, as if the driver put it in neutral and switched off the engine. That's because the ISG has enough torque to restart the engine and immediately get it to appropriate revs to safely re-engage at the high road speeds. Meantime, the ISG is recapturing kinetic energy from the coast-down and dumping it as electricity into the battery. Finally, the ISG can augment the engine's motive power, hybrid-style, with its additional 22 hp and 180 pounds-feet of torque in short bursts (based on the lithium battery's 1 kWh capacity).
Yet none of those features is the key driver behind the 48-volt ISG. The key driver is the 48 volts, which means four times the electromotive force of a conventional 12-volt system. There's just a lot more juice to power more things with electricity. The electric supercharger, for example, draws 5 kilowatts, or nearly 7 hp. Vollrath says that if it ran on 12 volts it would have to be much smaller, with less boost and slower response time.
The same applies to the M256's electric AC compressor, which can run full bore when the engine is off or when it's idling. And to the electric water pump, which generates twice the power and flow of a conventional belt- or gear-driven water pump. That increases cooling capacity, safely allowing greater power density in the engine, and it allows the pump to be operated (faster, slower) irrespective of engine speed. There isn't a belt anywhere on the M256, which means reduced noise and vibration.
Then there's the M256-equipped vehicle's 48-volt electric defroster, which might clear a frozen windshield in seconds, as opposed to minutes with a conventional forced-air defroster. Lastly, there's the opportunity to fit M256-equipped vehicles with 48-volt chassis systems, like Bentley's roll control or true drive-by-wire electric brakes.
The advantages of 48 volts have nothing to do with the M256's inline configuration, to be sure. The 48-volt charging system could just have easily been developed on a V engine. Yet there's one more advantage to the straight-six that helps trim development and assembly cost. Vollrath calls it the "hot" and "cold" sides.
On the hot or exhaust side, Mercedes has packaged the M256's turbo and a close-coupled catalyst that achieves light-off temperature as quickly as any in production, according to Vollrath. The electric compressor, charge cooler and most accessories go on the cool side. Vollrath says the electric compressor couldn't have gone on the hot side without water cooling or other serious heat mitigation because it isn't robust enough. The same applies to some other components and sensors.
Now the bottom line: The M256 debuts in two variants in the S class -- one generating 367 peak hp and 369 pounds-feet of torque, and the other 435 hp, 384 pounds-feet. The latter delivers a stout 145 hp and 128 pounds-feet per liter. It's only 6 percent less peak power than a new, 4.0-liter twin-turbo V-8 headed to the States in the updated S560. Add the quick burst from the ISG, and the M256's peaks surpass those from the V-8. Yet the straight-six delivers at least a 20 percent improvement in specific fuel consumption and a 20 percent decrease in CO2 emissions.
Only there's no M256 for North America, for now. We're getting a 3.0-liter turbo V-6 for the bottom-rung 2018 S class, with less power and torque than either M256, and we cannot explain why, except to say that it appears to be purely a marketing decision. Vollrath says there are no federal or California certification issues and no capacity limitation for the M256. It's hard to fathom the basis for that marketing decision, given that a straight-six was the S class's bread and butter for much of its 45-year run. Perhaps there isn't enough power separation to justify the more expensive V-8.
Whatever the thinking, Mercedes development director Ola Kaellenius says we will see the M256 in the not-too-distant future. It just won't debut in the S class.
E class? GLE? GLS? Kaellenius isn't saying, and we can only guess.
J.P. Vettraino is senior editor at large at Autoweek, a sibling publication of Automotive News.