Under the microscope: New family of premium diesel engines: More economical and powerful, more lightweight and compact
The new OM 654 four-cylinder diesel engine marked the debut 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 also include the combination of aluminium engine block and steel pistons as well as the further-improved NANOSLIDE® coating of the cylinder walls. The in-line six-cylinder OM 656 followed in the summer of 2017.
The new four-cylinder OM 654 engine had its world première as the 220 d in the new E-Class in spring 2016 (fuel consumption, combined: 3.9 l/100 km, combined CO2 emissions: 102 g/km). 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 savings are also 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 the NANOSLIDE® coating of the cylinder walls
- offset of the crank assembly
- reduced displacement
- systematic lightweight design
Unusual combination: aluminium engine block and steel pistons
At first glance, the combination of an aluminium engine block 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. For example, the lower expansion of steel as operating temperatures rise ensures increasing clearance between piston and aluminium engine block, 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 thermal 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 premiere in a passenger car: Stepped-bowl combustion process
Making its debut 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 entrainment 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 the 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). It 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 map, with the centre of combustion being optimised for fuel economy.
Six-cylinder: most powerful diesel engine in Mercedes-Benz history
The new six-cylinder in-line diesel engine was premiered in two output classes: The Mercedes-Benz S 350 d has an output of 210 kW (286 hp) and 600 Nm (combined fuel consumption: 5.1 l/100 km; combined CO2 emissions: 134 g/km). The S 400 d with 250 kW (340 hp) and 700 Nm (combined fuel consumption: 5.2 l/100 km; combined CO2 emissions: 135 g/km) is the most powerful series production car diesel engine ever offered by Mercedes-Benz. Both variants are also available with 4MATIC all-wheel drive.
Although the output has increased noticeably compared to the preceding model, the new engine consumes up to six percent less fuel. The special traits of the top-of-the-line engine in the diesel family include the stepped-bowl combustion process, two-stage turbocharging and, for the first time, the use of CAMTRONIC variable valve-lift control. Its design features a combination of aluminium engine block and steel pistons as well as further improved NANOSLIDE® coating of the cylinder walls.
The new six-cylinder diesel engine is designed to comply with future emissions legislation (RDE – Real Driving Emissions). All the components of relevance for efficient emissions reduction are installed directly on the engine. The integrated technology approach combining the new stepped-bowl combustion process, dynamic multi-way exhaust-gas recirculation and near-engine exhaust-gas aftertreatment, for the first time combined with variable valve-lift control, makes further reduced consumption with low emissions possible. Thanks to the near-engine insulated configuration, exhaust-gas aftertreatment has low heat loss and extremely favourable operating conditions.