The four-cylinder OM 651 diesel engine is a completely new design. It only shares its overall displacement of 2.15 liters with its predecessor (to be precise: 2143 cc, predecessor 2149 cc). It has a completely different geometry, however. While the bore and stroke were previously roughly square, the new engine is a long-stroke unit with a smaller bore of 83 mm and a stroke of 99 mm. We call this undersquare in design terminology.
The compression ratio has been reduced from 17.5:1 to 16.2:1. The advanÂtages include smooth idling even when cold, despite the undersquare confiÂguration.
The pistons are of aluminum. Owing to the long-stroke design, an omega shape was chosen for the piston crown. This wide, flat crown works well with the long injection spray lengths of an undersquare engine. An optimized combustion process significantly reduces the carbon footprint.
Owing to the long stroke, the forged, weight-optimized connecting rods are short, which is an advantage in terms of strength. The forged crankshaft with eight counterweights rotates in five bearings. It is very low in vibrations, thereby contributing to the smoothness of the engine.
Camshafts are driven by gear-wheels and chain setup. The DOHC camshafts operate a total of 16 intake and exhaust valves using roller-type tappets with hydraulic valve adjustment. The camshafts are driven by a combination of gears and a short duplex chain. Great attention to detail has reduced the louder noise normally associated with gear-driven camshafts.
As before, a common-rail system is used to inject the fuel. This design ensures highly precise and therefore economical fuel metering for each cylinder, as well as smooth running. The maximum injection pressure has been increased 1800 bar in the new engine generation.
The new injectors are free of leaks, therefore no additional line is necessary to return fuel. This improves thermal management of the injection system and makes it unnecessary to cool the fuel. A compact dual-plunger injection pump generates the necessary pressure in the rail, while a throttle valve regulates the charge level.
The developers put great attention to the turbocharging. This is not only of decisive importance for the rated output and torque, but also for power delivery and responsiveness. In the basic 70 kW (95 hp) version, the engine is aspirated by a single-stage turbocharger with variable turbine geometry. This ensures a high level of efficiency and rapid response.
In the other output versions of the Mercedes four-cylinder engine, a further development of the two-stage turbocharging installed in a van for the first time in the preceding model is used. In this case a small high-pressure turbocharger works together with a large low-pressure turbocharger. The two turbines are connected in series. At low engine speeds only the compact high-pressure unit is active, which means that a high charge pressure can already be built up at very low engine speeds.
Thanks to specific improvements to all these units, this combination achieves both very good responsiveness from low engine speeds and excellent operating characteristics at high engine speeds, as well as top performance across the whole engine speed range. The driver does not notice the switchover from one to two-stage operation and back, and the engine characteristics correspond to those of a large-displacement diesel engine.
This so-called downsizing – high output and torque from a comparatively small displacement – is a precondition for low fuel consumption, the resulting, low level of emissions and low weight.
Lets hope the downsized Mercedes diesel engine with additional horsepower does not have the problem Ford has had with downsizing the 7.3 L diesel to 6 Liters.
It is important for you our readers to know what the future will bring us in engine technology. Even though we don’t expect to sell many of these new design Mercedes engines, GotEngines.com blog continues to bring the latest in new technology to you.
