New Mercedes Straight Six Engine To Feature 48V Hybrid Tech As Standard

The facelifted S-Class will be the first model to carry the new inline six petrol engine which will come with 48V mild hybrid tech next year.

The new powertrain will come with an electric motor attached to the crankshaft, acting as both an alternator and a starter.

Codenamed M256, the new engine is the second member of Mercedes’ new family of engines and is closely related to the OM654 four-cylinder diesel that debuted in the E-Class. Mercedes is keen to replace almost all existing petrol V6s in their range with the M256, according to C&D’s report.

The company didn’t release any specs of the new powertrain apart from saying that the electric motor will be able to boost the engine’s performance by up to 18hp and harvest up to 20hp of regenerative braking.

Being bolted on the crankshaft will allow it to start the engine in a fraction of a second but it will not be able to move the car on electric power alone. Mercedes also says that the system will assist when the gasoline engine is running at less than 2000rpm.

Despite the small power gains and the narrow window of operation, the company promises an economy improvement of 10 to 15 per cent. The new 48V system is also going to replace all of Mercedes’ non plug-in hybrid models with a longitudinally mounted powertrain.


  • pcurve

    yesss…. MB goes back to inline 6…..

    • Dennis James

      It’s a cost-cutting measure, but probably beneficial for the end-user. They will probably reuse 90% of components for 3, 4 and 6 cylinder petrol engines, and 50%+ of the components between petrol and diesel engines.

  • smartacus

    they need to just have an electric helper motor to smoothen the engine pulses at low RPM

    thereby enabling a very low 250-400 RPM idle. No need for a heavy starter/alternator.

    • Matt

      It will have one of those (well an electric motor attached to the crankshaft).

      • smartacus

        yeah, but i mean just for maintaining a really low idle

        • Matt

          Well I’m sure if the efficiency gains were there, Mercedes engineers would implement such a system.

          Personally, I don’t think there is any advantage. If petrol engines could be made to run at a bizarrely low RPM while still being efficient and not negatively affecting NVH levels, then automakers would have done it already.

          As it stands, they all decided start-stop technology is a better solution, an engine not running at all saves more fuel than one that is (particularly when it may require more battery power to do so).

          • smartacus

            Actually; yes there is an advantage and no it won’t require more battery power.
            In fact; start-stop requires more battery power.

            Start-Stop requires a beefier, heavier, more expensive starter that will see a lot more use.

            Start-Stop requires a bigger, heavier, more expensive battery that can handle more draws (and heavier draws) from the beefier starter.

            *this is the big one**
            Start-Stop requires you to switch to electric pumps for the oil and the coolant to continue circulating. Especially since the turbos need constant oil and coolant circulation to prevent damage from repeated starting-stopping.

            Start-Stop requires even the transmission to have an electric pump to keep the hydraulic pressure up.

            Start-Stop cycle gets overridden anyway by the air conditioner.

            Start-Stop gets overridden every time you relax your foot off the brake or clutch pedal. Which can happen many times in a day when creeping forward.

            My idea is to forgo all that.
            Just have a small electric motor connected to the crankshaft to smooth out the idling roughness that occurs from 250-500 RPM.

            In fact; just like Start-Stop, it can revert back to normal idle if need be.

            Less Consumption AND Less Complexity

          • Matt

            I see your point. However I think automakers are already utilising electric pumps as it reduces parasitic losses on combustion engines, resulting in better fuel economy. 48v electric systems will be able to cope with these additional pumps quite easily so I think a few of your points are moot.

          • smartacus

            i agree about 48V systems being a long time coming already, but weight reduction is the goal for automakers from here on in. Smaller displacements, less cylinders, etc.

            Sure start-stop is here right now so they are going to use it, but the helper motor config is already being used in F1.

            My off-the-cuff idea is just switching it from high RPM use to low RPM.

            or hey! i gots me another idea. Why not have it help spin the engine up to roughly 1500RPM when turbos usually reach max torque?

  • Merc1

    I hope we get a S400/450 in the U.S. with a 365hp version of this new I6.


  • Vassilis

    I guess this is what all future petrol engines will have. Not bad.

  • Elmediterraneo

    An inline 6 can share more parts with a 4 cylinder than V6. It’s all about cost cutting.
    And they’ll drop the V6 because Mercedes-Benz will no longer produce a V8.
    The 4.0 liter AMG will be the only Mercedes V8 and will be use on the next 500 versions in a less powered version. And if you don’t trust me just look whats beneath the hood of the G500 4×4 squared.

  • Dennis James

    I really like this “mild hybrid” approach from both BMW and Audi. BMW should take notes.

    • Ermal Morina

      You mean from both Mercedes and Audi?!

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