Fifteen years ago, Mercedes-Benz abandoned its inline-sixes in favor of V-6s. “Half of our engine guys were sad when we switched from the inline-six to the V-6 layout,” one of Daimler’s engineers remembers. “Now it’s the other way around.” Mercedes is pulling an about-face, returning to the straight-six configuration for the same reasons that originally had it embrace the V-6.
-Back then, a common parts and production strategy dictated the switch. Daimler had developed a generation of V-6 and V-8 engines with common parts. (The last remnant of that era is the loosely related 6.0-liter V-12 that sits under the hood of the 600 and AMG 65 models.)
-The desire for commonality now means combining four- and six-cylinder engines. V-8s have become something of a niche product, and that’s why their development—and that of V-12 engines—is being delegated to AMG. That’s true even for V-8 engines that are used in non-AMG cars.
-Henceforth, the inline-four and inline-six engines will be manufactured on the same lines. BMW, by the way, does a similar thing, and the Bavarians have expanded their approach to three-cylinder engines, in which Mercedes has no interest.
-The engine that interests us most is the 3.0-liter straight-six known as M256 (pictured above). It’s presented in a high-output version that, according to Daimler engineers, makes “more than” 402 horsepower (the output, coincidentally, that was delivered by the naturally aspirated V-12 in the 600SEL in 1990). Maximum torque is said to be “greater than” 369 lb-ft.
-This six needs some help getting to that output level. Specifically, it uses a large turbocharger plus a BorgWarner electric supercharger to fill the gap between stepping on the gas and harvesting full turbo assistance. The supplemental booster spools up in a third of a second to provide strong, seamless acceleration.
-This electric supercharger is powered by a 48-volt electrical system, which facilitates two side benefits. The extra electrical energy is used to drive the coolant pump and the A/C compressor via chain instead of a belt from the crankshaft. A combined starter-generator adds a regen function during deceleration, improving efficiency. As with BMW and several other makers, the M256 is a long-stroke design with a half-liter-per-cylinder displacement.
-The new accessory-drive arrangement and relatively small 3.3-inch (83 mm) bore diameter yield a block length that’s not much greater than the retiring V-6. Daimler also claims gains in smoothness and sound quality attributable to the new engine layout.
-Look for the new straight-six in Mercedes-Benz models from the E-class upward, starting with the S-class next year. We suspect this engine also will form the basis for a six-cylinder AMG engine to replace the 3.0-liter turbocharged V-6 in the 43-model cars, and in that form it eventually will migrate to the C-class and the GLC as well.
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There’s another inline-six that will come to the United States: The 3.0-liter OM656 turbo-diesel, which will make “more than 309 horsepower” but also will be offered in a version rated at 282 horsepower. Look for it in the next-generation GLE.
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