Nine Things You Must Know about the New Mercedes Inline-Six

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The self-appointed smart set think the internal-combustion engine is on its last legs, but not Mercedes-Benz. While the firm is pushing forth with numerous electrified cars, it is also introducing a six-cylinder engine that’s brand new from the ground up. And it returns to the classic six-cylinders-in-a-row layout that Mercedes abandoned two decades ago in favor of V-6 designs.

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While the new inline-six was launched in Europe in the revised-for-2018 S-class, Mercedes sent the United States a fresh twin-turbo V-6 in that car instead. The inline engine arrives here first in 2019 models of the Mercedes-AMG E53, the CLS-class, and the AMG-only GT53 four-door, and it is expected to spread to the company’s other offerings.

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Of course this new straight-six measures 3.0 liters, in keeping with the now standard 500-cc-per-cylinder displacement. Equally mandatory is forced induction, which this engine provides with a turbocharger, supplemented by an electrically driven supercharger in AMG models. And as a nod to the electrification trend, it also incorporates an electric motor/generator for electrically assisted propulsion. Here are some of its most interesting details.

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1. Mercedes’s Nine Decades of Six-Cylinder History

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Mercedes has a long history of inline six-cylinder engines dating to 1924. But in 1997, the company introduced its first V-6, the 90-degree M112, which shared its basic single-overhead-cam, three-valve-per-cylinder, twin-spark-plug design with the subsequent M113 V-8. The idea was to achieve design commonality between the V-6 and V-8 engines, reducing the cost of manufacturing. By 1999, the existing M104 24-valve inline-six was phased out.

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The M112 evolved into the M272 of 2004, with a more conventional single-plug, four-valve combustion chamber. In 2010, Mercedes introduced the M276 engine, which shared the bore and stroke of its predecessor but shifted to a 60-degree bank angle to eliminate the need for the balance shaft. By this point, the commonality with the V-8s was much diminished.

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2. Back to Six in a Row

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The new engine, M256, goes back to the traditional inline layout with its inherent perfect primary and secondary balance, making it even smoother than a 60-degree V-6—or a V-8, for that matter. Also, with the intake and exhaust systems on opposite sides of the engine, there’s plenty of space to bring the catalytic converters close to the turbocharger, which helps the catalyst achieve operational temperature more quickly after a cold start to reduce emissions.

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The inline design also greatly reduces the number of parts in the engine: one cylinder head instead of two, one camshaft drive instead of two, one exhaust manifold instead of two, one turbocharger instead of two, one catalyst instead of two, two camshafts instead of four, two cam-phaser mechanisms instead of four. Reducing the number of camshafts and cam drives also reduces friction.

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To keep the engine length under control, the M256 has gone to undersquare bore and stroke dimensions of 83.0 and 92.4 mm. This contrasts with the 88.0-by-82.1-mm dimensions of the current 3.0-liter V-6 (one version of the V-6 had a bore as large as 97.0 mm). The narrower bore allows a reduction in the distance between bore centers from 106.0 to 90.0 mm, saving more than three inches overall.

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Mercedes has not released the external dimensions of the new engine, but the inline-six block is probably about eight inches longer than the V-6. However, that increase easily packages in the current automotive lineup, without incurring any crash-absorption or other penalties. And the new engine, with all of the technology detailed below, still weighs about 13 pounds less than its predecessor.

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3. A 48-Volt Integrated Starter/Alternator

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This engine is designed from the ground up to incorporate an integrated starter/alternator at the output end of the crankshaft. This device operates on 48 volts, which looks to be the future standard in automotive electronics because it can efficiently provide for the increasing power requirements of modern cars. For a given power level, increasing the voltage by a factor of four reduces the current by the same factor, which means that the thickness of the wiring used to carry the power can be reduced, saving cost and weight.

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This motor can also develop 20 horsepower and 162 pound-feet of torque, so it can provide a useful assist to the engine, particularly at low rpm, reducing the need to downshift when the driver calls for more acceleration. It can also recover energy during braking to charge its 0.9-kWh lithium-ion battery. And such a powerful motor can quickly and effortlessly restart the engine during idle-stop mode.

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In addition to this 48-volt system, there is a separate 12-volt electrical system—with its own small battery—to operate the numerous legacy 12-volt devices on the car. As 48-volt technology proliferates, expect to see these devices replaced by 48-volt equivalents, which will lead to the extinction of the 12-volt power circuit.

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4. An Electric Supercharger

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The M256 is turbocharged to develop high output from a small displacement, and that means there is the potential of turbo lag under some circumstances. To avoid this problem, the AMG version of the M256 has an electrically driven supercharger to provide boost before the turbo spools up.

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This supercharger, which is located downstream of the turbocharger, uses a centrifugal flow compressor—like the compressor side of a turbo—driven by a 48-volt electric motor. It spins to 70,000 rpm and can develop a peak boost pressure of about 6.6 psi. That’s enough to provide strong engine response while the turbo gets up to speed, at which point a check valve closes and the turbo takes over from the electric supercharger.

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5. A Large Turbocharger

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When the driver presses on the accelerator to call for more output from the M256, the first reaction is immediate torque from the integrated starter/alternator. Then the electric supercharger spins up in 0.3 second to deliver useful boost. With these two quick responses, there’s no need to fit a tiny turbocharger to minimize turbo lag.

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Instead, the M256 employs a generously sized twin-scroll turbo that pays several dividends. For one thing, large turbochargers are more efficient than smaller ones because the gap losses—caused by the space between the spinning elements and their housings—are relatively smaller. For another, a larger turbocharger flows better and creates less back pressure at high rpm, extending the engine’s powerband.

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Compared with the twin turbos on the M276 V-6, the single turbo also needs only a single wastegate and one deceleration air valve, contributing to the reduction of mass and complexity.

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6. No Accessory Drive

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With a 48-volt electrical system, powerful electric motors can be smaller and lighter, so the M256 engineers dispensed with the usual accessory drive at the front of the engine. Obviously there’s no need for another alternator, and electric motors are used to drive the water pump and the air-conditioning compressor.

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An electric water pump can be precisely controlled to provide the engine’s exact cooling requirements, facilitating rapid warmup. And the electric A/C compressor can also deliver the car’s cooling needs efficiently while maintaining cold air during extended idle stops.

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Finally, having no accessory drive slices about two inches from the length of the engine, largely offsetting the thickness of the integrated motor/generator.

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7. Particulate Trap

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Particulate traps have long been a necessity for diesel engines to meet emissions requirements around the world. Now they are starting to appear on gasoline engines as well. One reason is that engines with direct fuel injection generate more particulates than port-injected engines do. This is even true with the very precise piezo injectors in the M256, which operate at up to 2900 psi of injection pressure.

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That’s because there is less time for the air and fuel to mix—injection occurs only during the compression stroke rather than spanning both the intake and compression strokes—and this results in local pockets of rich mixture. When these burn, they produce particulates.

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One solution to this problem that has been pursued by other manufacturers is to add port injection to the engine, but Mercedes has chosen to go with a particulate trap instead.

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8. Part of a New Mercedes Engine Family

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The M256 engine’s dimensions—83.0-mm bore, 92.4-mm stroke, and 90.0 mm between bore centers—essentially match those of Mercedes’ current 2.0-liter inline-four and 4.0-liter V-8 engines (the latter two have a 92.0-mm stroke).

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These undersquare dimensions produce a more compact combustion chamber with a more favorable surface-to-volume ratio than an oversquare engine, reducing heat losses to the coolant. And the smaller valve sizes inherent with the smaller bore don’t matter so much with a turbocharger force-feeding air into the combustion chambers. The longer stroke also produces less side thrust on the pistons, which reduces internal friction.

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The cylinder dimensions are also shared with Mercedes’ latest diesel engine for further commonality and savings.

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• Why 0.5-Liter Cylinders Will Soon Dominate Automotive Engine Design
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• First Drive: 2019 Mercedes-AMG CLS53 and Mercedes-Benz CLS450
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• 2019 Mercedes-AMG GT 4-Door: The More-Door AMG
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9. Power and Fuel-Economy Payoffs

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The payoff for this technology is an output of 429 horsepower at 6100 rpm and 384 pound-feet of torque at 1800 revs in the version of the engine used in the AMG 53 models. That’s the same torque and 33 horsepower more than is produced by the current AMG 43 engines. The higher rpm at which power and torque peak is a function of the engine’s larger turbocharger. And keep in mind that the additional 21 horsepower and 184 pound-feet provided by the integrated motor/generator at low rpm don’t figure into these peak outputs. This is accompanied by a fuel-economy improvement of roughly 15 percent over the previous V-6 powertrain, although EPA figures are not yet available. Non-AMG Mercedes-Benz models will employ a lower-output version of the M256, without the electric supercharger but with the motor/generator and 48-volt systems. It’s rated at 362 horsepower as installed in the CLS450.

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