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.
