New six-cylinder petrol engine (M 256): Fascinating 48 V drive

Mercedes-Benz has developed a new six-cylinder in-line petrol engine that has for the first time been systematically designed for electrification. New, intelligent turbocharging, e.g. with an electric auxiliary compressor (eZV), as well as an Integrated Starter-Alternator (ISG) provide the guarantee of excellent drivability with no turbo lag. ISG is responsible for hybrid functions, such as boost or energy recovery, while allowing fuel savings that were previously reserved for high-voltage hybrid technology. The bottom line is that the new six-cylinder in-line engine offers the same performance as an eight-cylinder machine while being much more fuel-efficient. The new petrol engine (internal code: M 256) launches next year in the new S-Class.

The six-cylinder in-line engine features systematic electrification. For example, there is no belt drive for ancillary components at the front of the engine, which reduces its overall length. The narrow construction, together with the physical separation of intake/exhaust, creates space for near-engine exhaust aftertreatment. At 500 cc, the new engine has the same displacement per cylinder as the premium diesel engine family unveiled last year as well as the family of four-cylinder petrol engines. An additional bonus is the unsurpassed refinement of the six-cylinder in-line engine.

The output and torque of the new six-cylinder in-line engine are similar to the current eight-cylinder machine, i.e. over 300 kW (408 hp) and more than 500 Nm. In comparison with the previous V6, the CO2 emissions from the engine have been reduced by around 15 percent.

Its performance is as dynamic as that of a V8, as the new six-cylinder in-line engine comes with an especially intelligent form of turbocharging: assisted by the ISG at start-off, the electric auxiliary compressor (eZV) guarantees immediate high torque when driving off and accelerating, bridging the time before the large exhaust turbocharger cuts in. The electric turbocharger accelerates to 70,000 rpm within 300 milliseconds, ensuring an extremely spontaneous reaction from the engine. The result is a dynamic engine response with no turbo lag.

The M 256 is the first member of a new family of premium petrol engines that have for the first time been systematically designed for electrification from the outset. The 48 V electrical system serves not only high power consumers, such as the water pump and air-conditioning compressor, but also the Integrated Starter-Alternator (ISG), which also supplies energy to the battery by means of highly efficient energy recovery. The ISG dispenses with the need for a belt drive for these components. This not only reduces the overall length of the engine and its complexity, but also paves the way for new, efficient control possibilities. The still existing 12 V system supplies power to consumers such as lights, cockpit, infotainment and control units.

The ISG is a key component of the 48 V system and not only serves as an alternator, but is also responsible for hybrid functions. This allows fuel savings that were previously reserved for high-voltage hybrid technology. For the first time, the ISG is also responsible for idle speed control. The hybrid functions include:

  • Boost: 15 kW/220 N
  • Energy recovery
  • Shifting of the load point: the engine can be operated in a more favourable area of the engine map; load increase/reduction depending on the state of charge of the battery
  • Coasting
  • 3S start: almost imperceptible reinstatement of the engine

Another feature of the new family of premium petrol engines is near-engine exhaust aftertreatment. The as-standard particulate filter is the only part of the exhaust system that is under the floor. The M 256 is produced at the Untertürkheim plant.

The data of the new engine in comparison with its predecessor:

Engine

 

M 256

M 276

No. of cylinders/arrangement

 

6/in-line

6/V

Displacement per cylinder

cc

500

499

Displacement

cc

2999

2996

Cylinder spacing

mm

90

106

Bore

mm

83

88

Stroke

mm

92.4

82.1

Bore/stroke

 

1.11

0.93

Connecting rod length

mm

140.5

148.5

Rated output

kW/hp

over 300/408

245/333

Peak torque

Nm

over 500

480

Compression ratio

1:

10.5

10.5

 

Loading