Under the microscope: New family of premium diesel engines: More economical and powerful, more lightweight and compact

The new four-cylinder OM 654 diesel engine marks the début of a ground-breaking family of engines from Mercedes-Benz. It is the first passenger car diesel engine to use the stepped-bowl combustion process – named after the shape of the combustion bowl in the piston. The innovations furthermore include the combination of aluminium case and steel pistons as well as the further advanced NANOSLIDE® cylinder wall coating.

The new four-cylinder OM 654 engine celebrated its world première as the 220 d in the new E-Class in spring 2016. Installed in a comparable vehicle, the new engine consumes around 13 percent less fuel than its predecessor. Alongside optimised airflow on the intake and exhaust sides and the use of fourth-generation common-rail injection with pressures up to 2050 bar, the fuel saving is due to a reduction in internal friction by around 25 percent. This was achieved by

  • flat steel pistons with innovative stepped combustion bowls and long connecting rods
  • extensive measures for friction reduction such as NANOSLIDE® coating of the cylinder walls
  • offset of the crank assembly
  • reduced displacement
  • systematic lightweight design

Unusual combination: aluminium crankcase and steel pistons

At first glance, the combination of an aluminium crankcase and steel pistons appears unusual, because steel expands less than aluminium when hot, conducts heat less well and is heavier. This explains why aluminium pistons have been used to date. Yet the Stuttgart engine designers have succeeded in turning these seeming disadvantages into advantages. As an example, the lesser expansion of steel as operating temperatures rise ensures increasing clearance between piston and aluminium crankcase, reducing friction by 40 to 50 percent. At the same time, the fact that steel is stronger than aluminium allows very compact, lightweight pistons that even offer additional strength reserves. Finally, the lower heat conductivity of steel leads to higher component temperatures, thereby improving thermodynamic efficiency, increasing combustibility and reducing combustion duration.

The combination of innovative steel pistons with further refined NANOSLIDE® cylinder wall coating results in a reduction in consumption and CO2 emissions of up to four percent. At lower and medium engine speeds, which play an important part in everyday motoring, the reduction in fuel consumption is even more pronounced.

World première in a passenger car: the stepped-bowl combustion process

Making its début in a passenger-car diesel engine, the new OM 654 employs the Mercedes-Benz stepped-bowl combustion process - named after the shape of the combustion bowl in the piston. The combustion system has been completely redesigned. The stepped bowl has a positive effect on the combustion process, the thermal loading of critical areas of the pistons and the introduction of soot into the engine oil. The efficiency is increased by the higher burning rate in comparison with the previous omega combustion bowl. The characteristic feature of the specifically configured combination of bowl shape, air movement and injector is its very efficient utilisation of air, which allows operation with very high air surplus. This means that particulate emissions can be reduced to an especially low level.

Exhaust emissions: all set for the future

The new diesel engine is designed to meet future emissions legislation (RDE – Real Driving Emissions). All components of relevance for efficient emissions reduction are installed directly on the engine. Supported by insulation measures and improved catalyst coatings, there is no need for engine temperature management during cold starting or at low load. In addition to the advantages in terms of emissions, this results in fuel savings, especially on short journeys. Thanks to the near-engine configuration, exhaust aftertreatment has a low heat loss and optimal operating conditions.

The new engine is equipped with multi-way exhaust gas recirculation (EGR). This combines cooled high-pressure and low-pressure EGR. This makes it possible to significantly further reduce the untreated emissions from the engine across the entire engine mapping, with the centre of combustion being optimised for fuel economy. The production of the OM 654 takes place at the main factory in Untertürkheim, and at the wholly owned Daimler subsidiary MDC Power in Kölleda.

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