The new premium diesels are more efficient and powerful, lighter and more compact, and they are designed to meet all future global emissions standards. In our opinion, the diesel engine is indispensable in trucks and cars if we want to further reduce the CO₂ emissions from traffic.

—Prof. Dr. Thomas Weber, member of the Daimler Board of Management with responsibility for Group Research and Head of Mercedes-Benz Cars Development

Another of the objectives behind the new generation of engines was to reduce the number of variants as far as possible. The engine’s compact dimensions allow even more flexibility in adapting to different vehicle models. The interfaces between drive unit and vehicle have been standardized across all model series. Further, all the elements of the exhaust aftertreatment system are now configured directly on the engine itself and no longer on the vehicle.

The new OM 654 delivers around 13% lower fuel consumption and CO₂ emissions along with a further increase in output (143 kW instead of 125 kW) compared to its predecessor, the OM 651. The OM 651 is the most-produced engine in the history of Mercedes-Benz. The engine is available from the A-Class to the S-Class, in the V-Class and in the Sprinter van.

Features of the engine include:

• First all-aluminium construction of a four-cylinder diesel engine.

• Steel pistons with stepped combustion bowls, NANOSLIDE cylinder coating, fourth-generation common rail injection.

• All exhaust treatment technologies configured directly on the engine;

• Significantly lighter and more compact: 168.4 kg vs. 202.8 kg (-17%), two-liter displacement instead of 2.15 litres, cylinder spacing 90 mm vs. 94 mm. Key factors in weight saving were the reduced displacement, the move from two-stage to single-stage turbocharging, the aluminium crankcase including NANOSLIDE-coated cylinder liners and the two plastic engine mounts.

• Specific output up to 90 kW/l (compared to 58.3 kW/l in the OM 651).

• Lower noise level and outstanding vibration comfort thanks to a range of measures.

Dimensions. The main dimensions of the basic engine with bore, stroke and cylinder spacing substantially determine the overall length and height of the engine. The cylinder spacing was reduced from 94 to 90 mm in comparison with the previous engine. Bore (82.0 mm) and stroke (92.3 mm) result in an advantageous individual cylinder volume of just under 500 cc and ensure an optimal connecting rod ratio with regard to combustion and friction.

The aluminium crankcase is designed to cope with an extremely high power output (capable of withstanding peak pressures up to 205 bar).

To reduce the overall height, the camshaft drive is on the rear (transmission) side, as in the previous engine. There, in the crash-protected area, the high-pressure injection pump is also housed on the left-hand side of the engine; it is driven by the timing chain.

To allow the engine to be installed as low as possible in the vehicle, the Lanchester balancing shafts are arranged not below, but on left and right of the crankshaft. Likewise, as in the previous engine, the oil pump is positioned next to the crankshaft, which facilitates installation in various vehicle architectures.

The compact dimensions of the engine provide even greater flexibility in adapting to different vehicle models while allowing vertical installation of the engine. Additional space on the right-hand side of the vehicle was created by the offset of the crank assembly: the vertical cylinder axis is offset from the crankshaft centre by twelve millimetres to the left towards the inlet side. This also results in reduced friction of the pistons in the cylinder liners.

Less friction, better combustion. Installed in a comparable vehicle, the new engine consumes around 13% less fuel than its predecessor. Alongside optimized air ducting 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 an around 25% reduction in internal friction loss.

This was achieved with the use of:

• flat steel pistons with innovative stepped combustion bowls and long connecting rods;
• NANOSLIDE coating of the cylinder liners;
• offset of the crank assembly;
• reduced displacement; and
• various detail measures, such as on the camshaft drive.

Aluminum crankcase and steel pistons. The combination of an aluminium crankcase and steel pistons may appear 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. Mercedes-Benz engineers leveraged these characteristics 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%. 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 flat steel pistons allow the connecting rod to be lengthened to 154 mm. Together with the offset of the crank assembly, this enabled the side forces on the pistons—depending on the operating point—to be reduced by up to 75%.

The combination of innovative steel pistons with further-developed NANOSLIDE liner 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.

Stepped combustion bowl. Making its debut in a passenger-car diesel engine, the new OM 654 employs Mercedes-Benz stepped combustion bowls—named after the shape of the combustion pocket in the piston.

The combustion system has been completely redesigned. The stepped combustion bowl has a positive effect on the combustion process, the thermal loading of critical areas of the pistons and the ingress 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 in the case of a very high excess air factor. This means that particulate emissions can be reduced to an especially low level.

Emissions. The new diesel engine is designed to meet future emissions legislation (RDE – Real Driving Emissions). In contrast to the current NEDC measurement cycle, the WLTP (Worldwide harmonized Light vehicles Test Procedure) cycle is aimed at ensuring that the figures for standard and real-world consumption are close together in future.

All components of relevance for efficient emissions reduction are installed directly on the engine. With support from insulation 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 multiway exhaust gas recirculation (EGR). This combines cooled high-pressure and low-pressure EGR. It makes it possible to significantly further reduce the untreated emissions from the engine across the entire engine map, with the center of combustion being optimized for fuel economy.

The exhaust gas from the turbocharger is sent first to a diesel oxidation catalyst. It next passes the downdraft mixer, in which AdBlue is added by means of a water-cooled dosing module. Using a specially developed mixing area, the AdBlue evaporates over the shortest possible distance in the exhaust gas stream and is distributed very uniformly on the surface of the downstream sDPF (particulate filter with coating to reduce nitrogen oxides). Positioned behind the sDPF is an SCR catalyst for further catalytic reduction of the nitrogen oxides. Only then does the treated exhaust gas enter the exhaust system.

Reduced complexity. In the last 25 years, the number of output-, vehicle-, emissions- and country-specific variants of Mercedes-Benz diesel engines owing to different legal requirements and technical preconditions, such as the quality of the available fuels has increased from significantly less than 100 to currently over 1,000. One of the objectives behind the new generation of engines was to reduce the number of variants to the greatest possible extent.

The new diesel engine achieves this goal on two fronts while at the same time allowing flexibility in manufacturing, which enables the production volumes of the individual variants to be changed at short notice in response to market requirements.

The engine family is of modular design: the simple exchange of individual modules makes it possible for variants to be configured without having to develop entirely new engines.

The interfaces between drive unit and vehicle have been standardized across all model series.