The Mercedes turbodiesel straight-six returns
Thanks again, China! The “billions served” market taxes engines on a per-liter basis in half-liter increments, so manufacturers mapping out new engine families are heavily incentivized to bore and stroke their cylinders so as to come in a whisker under 500cc per hole, then add up however many of them are needed to hit the desired performance (and tax) target. Then voila! Instant engine family sharing bore, stroke, and usually cylinder-bore spacing.
What’s more, all manufacturers seem to be coming to the same efficiency-optimized, computer-analyzed realization that this ideal cylinder wants to be slightly under-square (bore smaller than stroke), with a bore/stroke “under-squareness” ratio somewhere in the 0.867-0.902 range. To wit: BMW (82.0/94.6), Volvo (82.0/93.2), VW (82.5/92.8), and now Mercedes (83.0/92.0-92.4). For a given displacement, a narrower bore and longer stroke results in shorter flame-travel distance for more complete combustion, it reduces heat lost to the water jacket, and it enables closer bore spacing, allowing a trade of precious engine length for less precious block height. Finally, designing one ideal cylinder and using it in multiple engines drastically reduces engineering expenses and permits proliferation of a single injector, spark plug and location, set of valves, etc.
Mercedes’ new combustion chamber first appeared in AMG’s potent M178 4.0-liter biturbo V-8 in the GT S (and in its wet-sump cousin M177 in the C63), then made its inline-engine debut in February 2016 with the OM654 2.0-liter turbodiesel in Europe. Now the roll-out continues in 2017 with four new S-Class engines: the OM656 twin-turbodiesel I-6, the M264 turbocharged gasoline I-4, the M256 twin-turbo gas I-6, and the M176 twin-turbo gas V-8. At least initially in the S-Class, we’ll only get the last two, but all the gas engines are interesting because they will each feature exhaust aftertreatment systems that trap particulate matter, and both inline gas engines are of particular interest because they illustrate two different 48-volt approaches toward a stated goal of achieving at least partial electrification of the entire Mercedes-Benz range.
M264 Turbocharged 2.0-liter I-4
In the case of this small engine towing around a big car, the mild-hybridization is a means of reusing energy that would otherwise be lost to braking and deceleration to eliminate the sense of turbo lag. This allows the single twin-scroll turbo to be slightly larger and more efficient than would be acceptable without a14-hp boost of electricity that can be applied even up to 2,500 rpm. In this case that e-boost comes courtesy of a belt-alternator-starter (BAS) device. It looks just like a normal alternator, except with a hefty spring-loaded tensioner that keeps the belt tightly wrapped around more than two-thirds of the pulley so that it never slips when the motor is regenerating or delivering power from the small lithium-ion 48-volt battery. On this application, the 48-volt architecture also powers an on-demand electric water pump. A separate 12-volt lead-acid battery serves the rest of the car’s electrical needs and gets charged via a DC-DC converter. The more powerful BAS motor is able to restart the engine extremely quickly, which means it can be shut off more often, and a new iECO system will even shut off the engine at low speeds (below 12 mph (19.3 km/h)) when a stop is anticipated.
M256 Twin-Turbo 3.0-liter I-6
The Mercedes faithful will surely cheer the return of this iconic and inherently smooth engine layout to the brand. Packaging the longer block is largely enabled by its 48-volt electrification. By replacing the alternator with an integrated starter/generator (ISG) packaged between the engine and transmission and then using 48-volt motors to directly power the A/C compressor and water pump, Mercedes has freed up all the space usually consumed by the accessory belt drive on the front of the engine. This shrinks the overall length of the fully dressed engine to nearly that of a typical four-cylinder with belt-driven accessories. One more important device also takes advantage of the 48-volt power—an electric turbocharger. So when you suddenly floor the accelerator, the powertrain controller instantly signals for the electric turbo to spool up. It takes 0.3 second to reach 70,000 rpm, during which time the ISG fills in with its 20 hp and 162 lb-ft or torque. Shortly after the e-turbo has spooled up, the larger twin-scroll exhaust-driven turbo is ready to provide the boost that will carry the engine to redline. Meanwhile, all the driver has felt is seamless acceleration. It’s only a matter of time before power-sucking comfort/convenience items like window defrosters, seat heaters, and the like tap into this onboard 48-volt power grid.
Output will be rated “at least” 408 hp and 369 lb-ft in the W222 series S450, which is not too far off the old V-8’s output while fuel consumption drops by around 15 percent relative to the former V-6 in S320 models sold elsewhere. To minimize all other emissions, the main catalyst bolts right to the turbo, lighting off in 20 seconds or less. The only aftertreatment device packaged beneath the car is the particle filter.
Mercedes was first to market with a diesel particle filter in the CARB-certified 1985 S-Class, and now it’s pioneering the gas particle trap to burn off the particulates formed in modern direct-injected engines. Instead of the silicon carbide material used in diesel traps, Mercedes uses cordierite, a magnesium iron aluminum cyclosilicate material that is more tolerant of the far greater temperatures reached in a gas-engine exhaust system. The trap gets regenerated (burning the particles off) whenever there’s excess oxygen in the exhaust stream, which typically happens every time you lift off the throttle and coast, so it’s maintenance-free for life. The backpressure added by these filters is said to be just 1.7 to 1.9 psi.
M176 Twin-Turbo 4.0-liter V-8
How closely related is this luxury V-8 to the fire-breather in a C63? The block, heads, exhaust plumbing, and turbos are virtually identical. Even the 17-psi boost pressure is the same, although a far less restrictive air-cleaner/intake system improves breathing on the AMGs. The turbos are also heat-wrapped so as not to melt the plastic engine cover on an S-Class (the AMGs get a metal cover). The bigger news is the fitment of Camtronic valve timing with cylinder deactivation for improved fuel efficiency. All three of the new gas engines get Camtronic cams that can vary intake and exhaust timing over a 40-degree range, but in the V-8s the cam lobes are splined to their shafts in such a way as to enable them to slide fore and aft exposing a zero-lift lobe when shutting down cylinders two, three, five, and eight (that’s the middle two on one bank and the front and rear two on the other). This only happens when operating in comfort or economy modes under light loads at engine speeds of between 900 and 3,250 rpm. This arrangement allows the full exhaust system to remain hot (the turbos even produce some boost in this mode). One of the instrument cluster menus will tell you which mode the engine is operating in, as we’re assured the switchover is all but imperceptible. This is in part thanks to a pendulum mass damper on the flywheel that helps counteract both the fourth-order vibrations in eight-cylinder mode and the second-order vibrations in four-cylinder mode.
Friction-reduction efforts include honing the cylinder bores in a jig that simulates full cylinder-head clamping force to improve cylinder-bore roundness and thereby reduce piston ring tension. This is done after they receive the Nanoslide iron-carbon-alloy twin-arc-sprayed cylinder lining treatment that is said to reduce piston/cylinder friction by 50 percent (this technique has been in use at Mercedes for three years and is employed on the other new engines detailed above).
This engine does not get any 48-volt electrification, but its alternator and A/C compressor are driven by reduced-friction four-groove belts while the water pump is driven by the cam chain drive. It does get a pair of exhaust particle filters mounted under the floor—another differentiator with the AMG M177 and M178 engines. Rated output in the forthcoming S560 (not an AMG model) is 476 hp and 516 lb-ft. That subdivides the base and S level horsepower ratings of the C63 and matches the S-level torque! Fuel consumption is said to improve by 12 percent relative to the S550’s twin-turbo 4.7-liter V-8.
We very much look forward to welcoming these new engines to the family with a test drive in the months to come.