The OM 654: Mercedes’ Newest, Smallest, and Quite Powerful Diesel Unit
The new four-cylinder diesel unit OM 654 marks the debut of a ground-breaking family of engines from Mercedes-Benz. The development goals were defined more than four years ago. The result is a modular concept with a special focus on integration of the drive into different models as well as a series of innovations on the basic engine. Exemplary efficiency and emissions ensure that the premium diesel is future-proof while underlining the key role to be played by the diesel engine in achieving the challenging global climate targets. The first all-aluminium four-cylinder diesel engine celebrates its world premiere in the new E-Class.
“The new family of engines embodies over 80 years of Mercedes-Benz diesel know-how. The new premium diesels are more efficient and powerful, lighter and more compact – and they are designed to meet all future global emissions standards,” says Prof. Dr. Thomas Weber, member of the Daimler Board of Management with responsibility for Group Research and Head of Mercedes-Benz Cars Development. “In our opinion, the diesel engine is indispensable in trucks and cars if we want to further reduce the CO2 emissions from traffic.”
Defined in 2011 in the technical specifications for the new Mercedes powertrain architecture, the demanding development goals were implemented in design and development. The bottom line is that the OM 654 delivers around 13 percent lower fuel consumption and CO2 emissions along with a further increase in output (143 kW instead of 125 kW).
The most important innovations of the new OM 654:
- 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. 203.8 kg (-17%), two-litre displacement instead of 2.15 litres, cylinder spacing 90 mm vs. 94 mm
- specific output up to 90 kW
- lower noise level and top vibration comfort thanks to a host of measures
Manufactured since 2008, the current four-cylinder unit 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. This, too, makes the new engine so significant, because the improvements in efficiency have a direct impact on Mercedes‑Benz’s fleet consumption. The new four-cylinder engine OM 654 will celebrate its world premiere as the 220 d in the new E-Class in spring 2016. In various output ratings and variants, for both longitudinal and transverse mounting, it will thereafter be gradually introduced into the Mercedes-Benz vehicle range.
Systematic lightweight design: all-aluminium construction
In its 143 kW variant, the first all-aluminium four-cylinder diesel engine from Mercedes-Benz weighs 168.4 kg, i.e. 35.4 kg (17 percent) less than its 125 kW predecessor – a new benchmark in its output class. If, instead of the DIN weight, one compares the ready-to-run engine with all accessories, the weight saving comes to even 46 kg – a clear benefit in terms of agility and fuel consumption. 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.
Compact dimensions: a key design goal
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.
Lower consumption: less friction, better combustion
Installed in a comparable vehicle, the new engine consumes around 13 percent less fuel than its predecessor. Alongside optimised 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 percent reduction in internal friction loss. This was achieved by 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
- various detail measures, such as on the camshaft drive
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 flat steel pistons of the OM 654 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 percent.
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.
World premiere in a passenger car: the stepped combustion bowl of the OM 654
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.
Exhaust emissions: all set for the future
The new OM 654 is designed to meet future emissions legislation (RDE – Real Driving Emissions). Development work also focused on the WLTP (Worldwide harmonized Light vehicles Test Procedure), which, in contrast to the NEDC measurement 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. Supported by insulation measures and improved catalyst coatings, there is absolutely 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 reduce the untreated emissions from the engine across the entire engine map, with the centre of combustion being optimised 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. Thanks to 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.
Noise level: quieter with less vibration
Diesel engines in passenger cars impress with their low fuel consumption and high torque. Also in terms of comfort, they are moving ever closer to their petrol counterparts. Already in the concept phase of the new diesel engine, optimisations were made to, among other things, the engine block structure and the connection to the transmission; there were also improvements to the gear wheels and air ducting. The new plastic engine mounts of the OM 654 also make an important contribution to the new level of comfort.
Reduced complexity: variants easier to produce
In the last 25 years, there has been a dramatic increase – from significantly less than 100 to currently over 1000 – in 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. One of the objectives behind the new generation of engines was to reduce the number of variants to the greatest possible extent. The new OM 654 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 standardised across all model series. More especially, all the elements of the exhaust aftertreatment system are now configured on the engine itself and no longer on the vehicle.
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