You can't do without diesel: The new OM 654 four-cylinder diesel engine marks the start of a new engine family at Mercedes-Benz. The development goals were already set more than four years ago. The result is a modular concept with a special focus on integrating the drive into different models, as well as a number of innovations in the basic engine.
"In our view, diesel engines in trucks and cars are indispensable if traffic-related CO2 emissions are to continue to fall", says Professor Dr. Thomas Weber, Daimler Board Member for Group Research and Head of Mercedes-Benz Cars Development. The bottom line for the new engine is around 13 percent lower fuel consumption. CO2 emissions while at the same time again increasing power – 143 kW instead of 125 kW.
The objective for the new engine generation was to reduce the number of variants as far as possible. The compact dimensions of the engine should allow even more flexibility in adapting to different types of vehicles. The interfaces between the drive unit and the vehicle have been standardized across all series. In particular, all elements of the exhaust aftertreatment system are now located directly on the engine itself, rather than on the vehicle.
The main innovations of the new engine:
-All-aluminum construction for the first time in a four-cylinder diesel -Steel pistons with stepped recess combustion, Nanoslide cylinder coating, fourth-generation common-rail injection -All exhaust gas purification technologies directly on the engine -Lighter and more compact: 168.4 vs. 203.8 kilograms (-17 percent), two liters of displacement instead of 2.15 liters, cylinder spacing 90 mm vs. 94 mm -Lower noise level and improved vibration comfort
The current four-cylinder engine OM 651, built since 2008, is the engine with the highest number of units in the history of Mercedes-Benz. It is available in models ranging from the A- to the S-Class, in the V-Class and in the Sprinter van. This is another factor that makes the new development so significant, as the efficiency improvements thus have a direct impact on Mercedes-Benz fleet consumption. The new four-cylinder OM 654 will celebrate its world premiere as the 220 d in the new E-Class in spring 2016. After that, it will gradually flow into the Mercedes-Benz vehicle range in various performance levels and variants, both longitudinally and transversely mounted.
The first all-aluminum diesel four-cylinder engine from Mercedes-Benz weighs 168.4 kilograms in the 143 kW variant, 35.4 kilograms (17 percent) less than the predecessor unit with 125 kW. If the running engine with all ancillary units is compared instead of the DIN weight, the weight saving is as much as 46 kilograms.
Key factors in reducing weight were the reduction in displacement, the transition from two-stage to single-stage turbocharging, the aluminum crankcase including Nanoslide-coated cylinder liners, and the two engine mounts made of plastic.
Compact dimensions: Important design goal
The main dimensions of the basic engine with bore, stroke and cylinder spacing largely determine the engine length as well as the engine build height. The cylinder spacing has been reduced from 94 to 90 mm compared with the predecessor engine. Bore (82.0 mm) and stroke (92.3 mm) result in a single-cylinder volume of just under 500 cm³ and are intended to ensure an optimum connecting rod ratio in terms of combustion and friction. The aluminum crankcase is designed for maximum performance (peak pressure capability up to 205 bar).
To reduce the overall height, the camshafts are driven from the rear, gearbox-side position, as on the predecessor model. There, in the crash-protected area, is also the high-pressure injection pump on the left side of the engine; it is driven via the timing chain.
In order to accommodate the engine as deeply as possible in the vehicle, the Lanchester balancer shafts are not located below but to the left and right of the crankshaft. Likewise, the oil pump is positioned next to the crankshaft, as in the previous engine, which facilitates installation in various vehicle architectures.
According to Daimler engineers, the compact dimensions of the engine allow even greater flexibility in adapting to different vehicle types and vertical installation of the unit. Additional installation space on the right-hand side of the vehicle has been created by setting the engine at an angle: The vertical axis of the cylinders is offset by twelve millimeters to the left towards the intake side compared with the center of the crankshaft. This also leads to reduced friction of the pistons in the cylinder bore.
Less friction, better combustion
Installed in a comparable vehicle, the new engine is said to consume around 13 percent less fuel than its predecessor. In addition to optimized air routing on the intake and exhaust side and the use of fourth-generation common-rail injection with pressures of up to 2050 bar, the reduction in internal friction power by around 25 percent is responsible for this. This was achieved by flat steel pistons with stepped recesses and long connecting rods, Nanoslide coating of the cylinder barrel tracks, engine restriction, reduction of displacement and a wide range of detail measures, for example in the camshaft drive.
Aluminum housing and steel pistons
At first glance, the combination of aluminum housing and steel pistons seems unusual. Steel expands less under heat than aluminum, conducts heat more poorly and is heavier. This is why aluminum pistons have been used up to now. However, the Stuttgart engine designers report that they were able to turn the apparent disadvantages into advantages. For example, the lower expansion of steel at rising operating temperatures ensures increasing clearance between the piston and the aluminum housing, thus reducing friction by 40 to 50 percent. The higher strength of steel compared with aluminum also allows very compact, lightweight pistons, which even offer additional strength reserves. Finally, the lower thermal conductivity of steel leads to higher component temperatures and thus improves thermodynamic efficiency with higher ignition readiness and reduced combustion duration.
The flat steel pistons allow the connecting rod to be extended to 154 mm. Together with the engine's cross-setting, the piston side forces could thus be reduced by up to 75 percent – depending on the operating point.
According to Daimler, the combination of the steel pistons with the further developed Nanoslide track coating is expected to result in fuel consumption and CO2 emission benefits of up to four percent.
The stepped trough combustion process
The new OM 654 uses the Mercedes-Benz step trough combustion process – named after the shape of the combustion pocket in the piston. The combustion process has been completely redeveloped. Daimler engineers explain that the stepped recess has a positive effect on the combustion process, the thermal load on critical piston areas, and the amount of soot entering the engine oil. The increased combustion speed compared with the previous Omega recess increases efficiency. The special coordination of bowl shape, air movement and injection nozzle is characterized by very good air utilization and enables operation at a very high air surplus. This enabled particulate emissions to be reduced to a particularly low level.
Designed for RDE
The new diesel engine is designed to meet future emissions legislation (RDE – Real Driving Emissions). Development work also focused on the WLTP cycle (Worldwide harmonized Light vehicles Test Procedure), which, compared with the NEDC measuring cycle, aims to ensure that the values for standard and real consumption are close to each other in the future.
All components relevant for efficient emission reduction are installed directly on the engine. Supported by insulation measures and advanced catalytic converter coatings, engine-side temperature management in cold-start and low-load operation could be completely eliminated, as Daimler explains. In addition to the benefits in terms of emissions, this also results in fuel savings, especially for short distances. Thanks to the arrangement close to the engine, the exhaust gas aftertreatment system has low heat loss and optimum working conditions.
The new engine has multi-way exhaust gas recirculation (EGR). It combines the cooled high-pressure and low-pressure EGRs. In this way, the engine's raw emissions could already be significantly reduced over the entire map with a consumption-optimized position of the combustion center of gravity.
The exhaust gas from the exhaust gas turbocharger first enters a diesel oxidation catalyst. It continues through the downdraft mixer, where the AdBlue liquid is added with the aid of a water-cooled metering module. A specially developed mixing section makes it possible to evaporate the AdBlue liquid in the exhaust gas stream over the shortest possible distance and distribute it very evenly over the surface of the following sDPF (particulate filter with coating for reducing nitrogen oxides). Behind the sDPF there is another SCR catalyst for further catalytic reduction of nitrogen oxides. Only then does the purified exhaust gas enter the exhaust system.
Variants are easier to represent
In the last 25 years, the number of performance, vehicle, emission and country variants of diesel engines used by Mercedes-Benz has increased dramatically due to different legal requirements and technical conditions, such as the quality of the available fuels – from significantly less than 100 to currently over 1000.
The aim of the new engine generation was to reduce the number of variants as far as possible. Two solutions are used to achieve this goal with the new diesel, while at the same time enabling flexible production in which fluctuations in the number of individual derivatives can be taken into account at short notice depending on market requirements: On the one hand, the engine family is modular. By simply exchanging individual modules, variants can be derived without having to develop completely new units. On the other hand, the interfaces between the drive unit and the vehicle were standardized across the series. In particular, all elements of the exhaust gas aftertreatment system are now located on the engine itself, no longer on the vehicle.