Today’s Technology

by Ken Adams, Master Technician
 
The New Generation of Mercedes-Benz Engines
 
The newest generation of Mercedes-Benz power plants serves up some very exciting improvements. These two gasoline engines signal some major changes that are designed for greater reliability, improved gas mileage, and lower emissions, and will represent a large percentage of the engines that will power Mercedes-Benz vehicles in the U.S. market. The new V-configuration six-cylinder M276 replaces the well-known M272, introduced in 2004, and the new V-8 M278 will replace the M273.

Both engines share several new technologies, starting with direct fuel-injection technology. In past fuel-injection engines, the fuel traveled through an injector and was sprayed into the intake runner. The new engines use a newly designed fuel injector that sprays a finely atomized mixture directly into the combustion chamber. This requires higher fuel pressure. A high-pressure pump is driven off the exhaust camshaft and provides up to 200-bar pressure (approximately 200 times the pressure of air at sea level). The fuel injectors are a new design using piezo technology to create multiple ultra-short spurts of atomized fuel.

Design of Fuel Injector

1. Teflon ring
2. Valve group
3. Piezo actuator module
4. Coupler
5. O-ring (leak line)
6. Fuel feed (high pressure)
7. Electrical connection

Piezo technology, incidentally, uses the same scientific principle as most cigarette lighters, where pressure on a crystal produces an electric spark. The process can be reversed; by applying an electric spark to a crystal structure, the crystals expand slightly and then immediately contract, rapidly opening and then closing a tiny valve.

To optimize power and efficiency, Mercedes refined or eliminated any engine-driven components that required operating power. For years, Mercedes has utilized smart alternators that operate when needed. Now, the company is phasing electromechanical steering into production on various models. This eliminates the need for a power steering pump mounted on the engine. And, electrically driven air conditioning compressors will soon make their debut.

Also new for this engine family is an oiling system that operates on two pressure output levels. Oil pumps require significant power to operate, but there was no need to produce full oil pressure on an idling engine or under other operating conditions. A solenoid-controlled valve was designed so the oil pump can switch between 2- and 4-bar output pressures. Additionally, the oil spray nozzles directing oil flow to the bottom sides of pistons now contain a spring-loaded valve that requires a greater-than 2-bar pressure to open; there is no oil flow in 2-bar mode because oil is sprayed to the bottom side of the pistons to cool them. Having the ability to cut flow at predetermined times allows the pistons to warm faster – and the catalytic converters warm faster, too, reducing emissions.

While the V-6 M272 performed well, it was plagued with issues surrounding balance-shaft sprocket failures. Balance shafts are widely used by many manufacturers to calm vibration created from uneven firing strokes on V-6 engines designed with cylinder banks placed at 90-degree angles from each other. The counter-rotating shaft used weights at each end to cancel these vibrations. But these balance shafts came with problems. Soft sprockets wore prematurely, causing the cam timing to retard and trigger a check-engine light. Offset crankshaft journals have been used by other manufacturers with some success to create improved firing stroke; readers who have driven a Buick vehicle with the old 231-cubic-inch V-6 know what I mean.

Mercedes solved the vibration problem by switching to a 60-degree cylinder configuration in the new V-6, which has allowed engineers to eliminate the balance shaft altogether. The 90-degree configuration remained for an extended period because it was built on the same assembly line as the V-8, reducing tooling costs; both shared the 90-degree block configuration. The new 60-degree configuration produces a more favorable firing stroke. This is just the beginning.



Chain-Drive on 8-Cylinder Engine

1. Right secondary drive chain
2. Left secondary drive chain
3. Right guide rail
4. Left guide rail
5. Upper guide rail
6. Lower guide rail
7. Chain drive intermediate gear
8. Primary drive chain
9. Right tensioning rail
10. Left tensioning rail
11. Right sec. drive chain tensioner
12. Left sec. drive chain tensioner
13. Primary drive chain tensioner
14. Oil pump chain
15. Crankshaft

Another advancement is the timing chain drive configuration. The single chain is now replaced with three separate timing chains. The primary chain runs from the crankshaft to an idler gear, with a 2:1 ratio reduction. From the idler gear, separate left and right chains drive the respective camshafts. The benefits are many. Shorter chains require less pressure from a tensioner controlling slack, and chains are less prone to stretching. And because the 2:1 reduction gear is low in the block, the cam sprocket size is reduced, allowing for lower engine height. The hydraulic cam adjusters were redesigned as well, allowing each camshaft 40 degrees of continual adjustment at improved reaction speeds compared with its predecessors.

The M276’s intake manifold was completely redesigned to extract as much torque possible. The V-6 manifold has three possible runner lengths controlled through a selector drum and two resonance flaps. By changing the length of intake runners, the airflow’s kinetic energy through the manifold can be optimized for greater power output with a wider rpm range.

The M278 V-8 engine was also completely redesigned. The V-8 block configuration remains at 90-degrees because the opposing cylinders balance one another as they fire, but the engine has all the design revisions noted above, with the exception of the variable intake manifold runners. And here’s the best: Turbochargers – not one, but two of them, one for each cylinder bank. With intake air forced from the turbochargers, the V-8 does not require variable-length intake runners.

Turbocharging – using an external system to compress intake air to increase engine power – has been around for quite a while. However, previous turbo systems were plagued with power lag when responding to increased throttle, excessive heat, and a host of other issues. Mercedes-Benz did its homework here, in my opinion. With two turbos, each can be smaller in physical size – and with less consequent inertia and increased efficiency in the compressor vane’s design, the dreaded turbo lag is all but eliminated.

Each turbocharger on the M278 is welded directly onto the exhaust manifold. This improves heat dissipation to the exhaust system and allows the catalytic converters to reach operating temperature more quickly. Both turbochargers provide up to 0.9-bar boost pressure, controlled by waste gates that open to prevent excessive force.

The turbos have charged-air coolers to reduce heat produced when the air is compressed. However, these are not the typical air-cooled configurations mounted in the grille. Instead, the charge air cooler is mounted deep in the engine vee, fed by a separate, low-temperature fluid cooling system.

The performance gains with these new innovations are so substantial that the V-8 engine displacement was reduced from 5.4 to 4.6 liters, but succeed in delivering huge gains in torque, horsepower, and fuel efficiency.

I’ve always been partial to the tire-obliterating torque of the supercharged engine in the old E55, but this new M278 is quite impressive. The power comes on strong right off idle, with no turbo lag whatsoever. And the power increases in a quiet, smooth, and civilized manner, straight up to redline. The company has also worked its magic on this engine to produce the AMG M157, producing 664 pound-feet of torque at 2,500-3,700 rpm; this engine is sure to keep Michelin in business.

With so many changes to this new family of engines, it’s important to note that there are some sounds bound to be different from models of the past, especially with the hood open. On both M276 and M278s, the mechanical high-pressure fuel pump makes noise in normal operation that some might find disconcerting, but with the sound-insulated hood down, it’s not noticeable.

On a cold start, owners of vehicles equipped with the M278’s V-8 engine might be concerned with a noise that sounds like a bearing problem. The noise can be heard during cold starts because the idle speed is increased and the waste gates are closed, ensuring increased heating to the catalytic converter’s operating temperature – an emissions-reduction strategy. The noise never lasts for more than 30 seconds and is normal.

I have had the pleasure to drive a few of the new cars with both the new 60-degree V-6 engine and the twin-turbo V-8. I look forward to putting one of them in my garage very soon.

Chain-Drive on 8-Cylinder Engine

1. Right secondary drive chain
2. Left secondary drive chain
3. Right guide rail
4. Left guide rail
5. Upper guide rail
6. Lower guide rail
7. Chain drive intermediate gear
8. Primary drive chain
9. Right tensioning rail
10. Left tensioning rail
11. Right sec. drive chain tensioner
12. Left sec. drive chain tensioner
13. Primary drive chain tensioner
14. Oil pump chain
15. Crankshaft