 Mercedes’ M278 4.6-litre twin-turbocharged V8. Click image to enlarge |
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Mercedes’ New V Engine Family
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Stuttgart, Germany – Consumer demands, good environmental practices, and ever tightening fuel consumption regulations – both here and in the European Union – are of particular concern to premium brand automakers like Mercedes-Benz, whose model lines are largely comprised of heavier, more powerful cars.
While Mercedes makes a range of passenger car engines that varies from the diminutive Smart Fortwo’s 800-cc 3-cylinder turbo diesel (a motor no longer offered in North America), all the way up to the 603 horsepower, 6.0-litre twin turbo V12 found in the Maybach 62 S, its core products, particularly in the North American market, are driven primarily by V6 and V8 gasoline engines. Efficiency gains in those motor lines are therefore critically important.
 Prof. Dr. Leopold Mikulic (Vice President Program Management and Development Passenger Car Engines and Powertrain, left), and Gerhard Doll (Project Manager Development Gasoline Engines V6 and V6 Mercedes_Benz Cars, right), with the new 4.6-litre M278 V8 engine. Click image to enlarge |
Mercedes’ current V6 and V8 engine families received their last substantial redesign in 2004, when the previous single overhead cam/three-valve per cylinder head design (with twin spark plugs) was jettisoned in favour of a more efficient double overhead cam/four-valve layout. The switch to individual camshafts for intake and exhaust actuation meant that each set of valves could now be variably timed relative to one another, with commensurate benefits in economy, emissions, and power delivery.
Technology marches on relentlessly, and Mercedes has further improved its engineering; both its V6 and closely related V8 engines have been considerably reworked.
While the 3.5-litre displacement remains, the most immediately noticeable change between the current “M272” V6 engine and the new “M276” V6 that will replace it over the next few years is that the motor’s V-angle has gone from the 90 degrees it presently shares with the V8 to a narrower 60 degree offset.
This seemingly minor change is actually a profound one, because not only does it require significant retooling and major structural design modifications, it also has the costly potential to reduce how many components can be shared between the six and eight cylinder motors, both of which are to be produced together, as before, in the company’s Bad Cannstatt facility.
The benefits however are two-fold, and as a result of some clever planning, outweigh the drawbacks.
 Mercedes’ M278 4.6-litre twin-turbocharged V8. Click image to enlarge |
Most immediately obvious to drivers should be smoothness. Because of the particular frequency and timing of a six cylinder’s combustion events, 90 degree V6 engines have an inherent imbalance that creates undesirable vibrations and resonances; conditions that are not present when that angle is changed to a V6-optimal 60 degrees.
Given the present M272’s occasionally coarse demeanour, particularly at elevated rpm levels or under load, overall refinement should be appreciably better.
Since an engine-driven balance shaft is no longer required to counteract those unwanted forces, its weight, complexity, and frictional losses can all be eliminated. Less drag equates to more power, and – just as importantly – reduced fuel consumption.
As such, initial output figures peg the new V6 at a class-competitive 306 hp and 273 lb-ft of torque, up from 272 hp and 258 lb-ft, while attaining a predicted 24 per cent reduction in fuel consumption in the European cycle.
The newly revised V8, dubbed “M278”, has been reduced in displacement from the current M273’s typical 5.5 litres to 4.6 litres.
The addition of twin close-coupled turbochargers, providing up to 0.9 bar (approximately 13 psi) of boost, has resulted in a jump from 388 hp to a robust 435 hp. Torque grows from 391 lb.-ft. to 516, a 32 per cent increase, with the peak output available at just 1,800 rpm. Despite these impressive gains, Mercedes anticipates a 22 per cent improvement in fuel economy.
 Mercedes’ M278 4.6-litre twin-turbocharged V8. Click image to enlarge |
In the name of simplicity and economies of scale, many of the primary components are interchangeable between the V6 and V8, even though the structures of the two engines now vary considerably.
Both engines have adopted “spray-guided” high-pressure direct fuel injection; fuel is sprayed into the combustion chamber at up to 180 bar (~2610 psi) in the V8, and a maximum of 200 bar (~2900 psi) in the V6. The charge-cooling effect of a direct-injection layout allows for higher compression ratios – 12.2:1 in the V6, and a high-for-a-turbo 10.5:1 in the V8 – which markedly improves efficiency.
Another common feature is a new variable displacement, variable output oil pump, which produces higher pressure and volume only when needed. Below 3,000 rpm, pressure is electronically regulated to 2 bar (~29 psi); above that engine speed or under certain load conditions, engine oil pressure is increased to 4 bar (~58 psi), effectively reducing parasitic drag under most normal driving conditions.
Also shared are fully redesigned two-stage chain drives for the camshafts that utilize a quieter style of chain and sprockets. They drive smaller, more effective cam timing phasers, now capable of up to 40 degrees of phase adjustment at oil pressures as low as 0.44 bar (~6.5 psi). The 30 mm smaller diameter phasers reduce overall engine height, important for providing the hood crush space required by European pedestrian impact standards without resorting to blocky styling or expensive pyrotechnic hood hinge systems.
Mercedes’ M278 4.6-litre twin-turbocharged V8. Click image to enlarge |
The rest of the world will get starter-based stop/start systems that further reduce fuel use; a lack of consumer acceptance on this side of the Atlantic has Mercedes’ reluctant to include it in our cars, though the final decision had yet to be announced.
A unique multi-spark coil-on-plug ignition system fires the plugs up to four times in one millisecond during each combustion event to promote more stable, more thorough burning.
Multi-spark and the ability to provide several precisely-controlled fuel injection events per cycle allows the V6 to operate at times as a lean-burn, stratified charge engine; again, the decision to bring this capability to our market has not been made, partly dependant on our continent adopting the more rigorous low-sulphur fuel quality standards required to do so without creating excessive NOx emissions.
While not as green and sexy as hybrids and electric vehicles – areas that Mercedes is also active in – making such considerable efficiency gains in a technology that has had over 100 years to mature is noteworthy. It’s also more likely to provide quantifiable consumption reductions in the short term.
Look for the V8 to bow first, as it’s expected to be found in the 2011 S class this fall, joined later by the 2012 CLS. The V6 should follow close behind, likely in a variety of 2012 models.
