Roadstar Sharjah

[Ermelindo Klatt]

A perfect blend of economy and ecology: in spring 2011, in anticipation of the Euro VI emissions standard and the heavy-duty Actros, Antos and Arocs trucks that were launched one after the other for heavy-duty long-distance transport, short-radius distribution and construction-site transport, Mercedes-Benz unveiled a generation of engines developed completely from scratch. The first member of the new engine family was the OM 471.

With its technical features and the resulting capabilities and qualities, the heavy-duty engine took engine manufacturing for heavy-duty commercial vehicles into a brand new era. The 12.8-litre in-line six-cylinder unit made its mark with a robust design and technical innovations with significant benefits for the customer. Key features included the twin overhead composite camshafts, the singular common-rail injection system with X-Pulse pressure booster, the asymmetric exhaust gas turbocharger, the powerful engine brake and emission control based on SCR technology, exhaust gas recirculation and particulate filter in order to meet the stringent Euro VI emissions requirements.

The impressive fuel savings of the OM 471 caused a sensation from the outset, further helped, amongst other things, by outstanding aerodynamics and optimised auxiliary consumers. The new Mercedes-Benz Actros provided the first evidence of this in autumn 2011 at the Record Run, a fuel economy drive over a distance of some 10,000 km. Shortly afterwards, customer feedback also corroborated the low fuel consumption figures. And in the meantime it has been confirmed by businesses and drivers alike: the OM 471 series sets new standards for heavy-duty truck engines worldwide.

Businesses benefit first and foremost from the low costs: low fuel and oil consumption, extremely long maintenance intervals of up to 150,000 km, as well as the engines' robustness and durability. Drivers, meanwhile, appreciate the spontaneous response from standstill, the tremendous pulling power even from low revs, as well as the drive units' refined operation and powerful engine brake.

In the second generation of the OM 471, the engine developers have taken the good properties and improved them considerably with several individual measures. The overriding goal for the engines' further development was to ensure they are systematically geared towards low operating costs.

As a result, the second generation of the OM 471 once again emphasizes the quality of the engines. It brings about a further reduction in fuel consumption of as much as three percent, while the engines' robustness, which already verged on the proverbial, has been honed too. In addition to this, the engineers have achieved a substantial increase in torque at low rev speeds and expanded the line-up to a total of five output ratings with the addition of a new range-topping engine variant.

The basic output variants delivering 310, 330 and 350 kW are complemented by three "top torque" versions. When these engines are fitted in trucks, an extra 200 Nm of torque is placed on tap as required whenever the highest gear of the Mercedes PowerShift 3 automated transmission is engaged. This strategy has the effect of reducing the frequency of gear changes and increasing transportation speed without any negative impact on fuel consumption. This is further reinforced on the second generation of the engine with the further optimized fast torque build-up at very low engine speeds.

The raw figures for the engines' maximum output and torque only tell part of the story, however: Maximum performance is available almost constantly at a wide range of engine speeds from 1,450 to 1,800 rpm on all second generation engines. Maximum torque likewise stays at a nearly constant level from around 900 up to 1,450 rpm. The result is excellent drivability with an exceptionally wide usable engine speed range.

An important component of the new engine generation is the second generation of the X-Pulse injection system, the unique common rail system with pressure boosting in the injector and free modelling flexibility of the injection system. The maximum fuel pressure has been increased from 900 to 1,160 bar, resulting in a maximum injection pressure of 2,700 bar.

The injection nozzle is an eight-hole nozzle (previously seven holes), increasing the maximum flow rate by around ten percent. Additional modifications include the piston bowl geometry, the sizeable increase in compression ratio from 17.3:1 to 18.3:1, along with a reduced exhaust gas recirculation rate (EGR rate). All these measures add up to a further improvement in efficiency across the entire engine performance map. This in turn lowers fuel consumption significantly. The optimum values in the consumption characteristic map have followed the new torque curve towards lower rev speeds.

Systematically configuring the engine for low fuel consumption means that untreated NOx emissions rise. This is countered by the SCR technology featuring an innovative and efficient SCR catalytic converter. AdBlue consumption is therefore on a par with earlier Euro V engines at around five percent of fuel consumption.

The cost balance is right: The up to three percent reduction in fuel consumption is countered only by a slight increase in the considerably lower-priced AdBlue for exhaust gas cleaning. In other words: For a yearly distance travelled of 130,000 km in long-distance transport and a consumption of approximately 28.5 litres/100 km on a challenging route, fully laden, every single Mercedes-Benz Actros with the second generation OM 471 engine saves approximately 1,100 litres of fuel per year and emits around three tonnes less CO2.

The latest-generation X-Pulse system with its far higher injection pressure is also a vital prerequisite for the new top-of-the-range version of the OM 471 engine. It generates an output of 390 kW (530 hp) while delivering a sensational peak torque of 2,600 Nm. Even from its idling speed of 600 rpm, the new engine produces a mighty 1,600 Nm of torque. This takes the OM 471 into the sort of power output and torque ranges that were still the preserve of far larger engines and eight-cylinder units just a few years ago.

The OM 471 is one of the most powerful diesel engines in its class. Special mention should be made of both the output of 30.5 kW (41.4 hp) per litre of displacement and the torque of 203 Nm per litre of displacement. Thanks to these exceptional figures, the new flagship engine is perfectly capable of handling even very demanding applications with gross combination weights of 40 tonnes and more. On top of this, operators also benefit from substantial weight and fuel savings when compared to larger engines with similar output ratings.

The new injection system benefits not just the new range-topping version, but all other output variants of the OM 471 too. Although the nominal maximum output and torque figures for the engines remain unchanged, the output and torque curves at bottom-end revs rise far more sharply, endowing the engines with quite different performance characteristics. Now, all engine output ratings already muster up at least 2000 Nm of torque at just under 800 rpm. Depending on the output rating, close to peak torque is now already on tap between around 800 and 950 rpm.

Consequently, the output curves of the new engines develop just as favourably. The rated output rev speed is now 1600 rpm, yet even at the previous speed of 1800 rpm, output is just one percent below maximum. Depending on the output variant, 95 percent of maximum output is already being generated at around 1300 to 1400 rpm.

What this means in practice is outstanding drivability under all conceivable conditions across an extremely wide usable engine speed range of about 1000 rpm. At low rev speeds in particular, the already powerful engines now rank at least one output category higher than previously.

Mercedes-Benz has capitalised on these new performance characteristics and used a longer standard final-drive ratio. A ratio of i=2.533 instead of the previous i=2.611 translates into a rev speed reduction of three percent. When fitted with 315/70 R 22.5 tyres, this results in an engine speed of around just 1150 rpm at a speed of 85 km/h. Thanks to the new output characteristics, this is not accompanied by any loss of performance. On the contrary: the engines now have significantly higher power reserves on uphill stretches.

This extra power is not only apparent when driving on the motorway, but also on trunk roads and country highways. This is clearly exemplified by the highly popular Actros 1845. When travelling at a speed of 65 km/h in top gear, its engine speed is not even 900 rpm with the new axle. At this rev speed, the engine already generates close to its maximum torque of 2200 Nm and feels as if it still has plenty of power in reserve. With the previous engine and axle configuration, the Actros 1845 was running at a rev speed of just over 900 rpm when moving at the same speed in top gear, while its torque of just under 2000 Nm was already sloping off.

The asymmetric turbocharger has been one of the special features of the OM 471 engine from the very start. In order to build up charge pressure fast with an equally brisk rise in output and torque, the exhaust gases from cylinders four to six are channelled straight to the turbine without any detours. A defined quantity of the exhaust gases from cylinders one to three, on the other hand, is diverted for the purpose of exhaust gas recirculation. This serves to reduce the NOx emissions.

This fundamental solution has been retained, but the details have been refined to crucial effect. The previous EGR flap in the EGR path has now given way to an EGR flap repositioned much further forwards in the exhaust manifold, some distance before the exhaust gas enters the turbocharger.

Whereas the distribution of the exhaust gases between exhaust manifold and turbocharger was previously partly dependent on the shape of the manifold and the geometry of the asymmetric turbocharger, distribution can now be infinitely and very precisely adjusted throughout the entire range of the engine performance map. This results in effective thermal management and a lower EGR rate in general with advantages for fuel consumption.

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