Mercedes-Benz OM 471 – the second generation

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.

The OM 471 series engines have now built up a proven track record in practical operation in both the trucks and the buses and coaches from Mercedes-Benz and Setra. Including the closely related engines for overseas markets, production numbers have now reached the 250,000 mark – no other heavy-duty engine platform surpasses this figure or has proven itself so often.

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 Mercedes-Benz OM 471 is now available in the following basic versions:

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 NO_x 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.

To summarize, this means that: The second generation of the Mercedes-Benz OM 471 combines noticeably improved performance with considerably lower consumption.

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 NO_x 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.

Both the EGR rate and the exhaust gas flow to the turbocharger can be controlled in accordance with the engine's operating characteristics thanks to the flap's new position. This has therefore eliminated the need to monitor exhaust gas recirculation using a sensor in the venturi tube (the exhaust gas recirculation bypass) as well as a downstream EGR control system.

The repositioning of the infinitely adjustable EGR flap furthermore opens up a whole new spectrum of asymmetry: the quantity of exhaust gases directed from the three donor cylinders to the combustion process can be varied exactly as required between zero and 100 percent – something which is unprecedented in engine manufacturing. The EGR flap thereby controls not just the flow of exhaust gas for exhaust gas recirculation, but also the turbocharger.

The asymmetric injection is another new feature. During normal driving, the injectors for all six cylinders in the OM 471 engine are supplied with an identical quantity of fuel. If regeneration of the diesel particulate filter is required at low loads, a high EGR rate of up to around 50 percent is set in order to raise the exhaust temperature.

In order to prevent incomplete combustion in this situation which would result in a higher proportion of soot particles in the exhaust gas, the fuel quantity is smoothly reduced in cylinders one to three as the EGR rate rises and increased in cylinders four to six. In extreme cases, the injection quantity for the first three cylinders can be zeroed, while the other three cylinders operate as if at full throttle. Neither power output nor fuel consumption are affected by this reduction in fuel quantity down to complete cylinder shut-off, but emissions quality improves and soot particle levels drop.

The asymmetric shift in injection is imperceptible to the driver. The same applies when the engine load is increased by depressing the accelerator and fuel injection becomes uniform for all cylinders again.

A new asymmetric turbocharger is partly responsible for the swift and substantial increase in power delivery at low rev speeds. It was developed by Mercedes-Benz and is manufactured in the Mannheim engine plant. An in-house turbocharger ensures customised adaptation to the requirements of the OM 471. The turbocharger excels for its outstanding efficiency. Very tight production tolerances are a guarantee of supreme quality and durability.

As before, the asymmetric turbocharger features a fixed-geometry turbine. From a technical standpoint, this variant is far less complex and thereby less prone to faults than a VNT turbocharger, for instance. It also does without a wastegate valve, further simplifying the design. This measure reduces the turbocharger's complexity and avoids a potential source of faults, such as leaks – another plus point as far as the robustness and durability of the OM 471 are concerned.

On the second generation OM 471, Mercedes-Benz dispenses not only with the wastegate valve for the turbocharger, but also with the boost pressure control. Together with the elimination of the EGR sensor and EGR control, this means that operation of the engine with its meticulously optimised thermodynamics is purely pilot controlled.

Controlling the engine in this way allows it to run even more efficiently. Precision pilot control of all variables results in an optimum efficiency factor without the added complication of individual control systems and their combination. The ingeniously devised emission control strategy, for example, replaces the individual control of exhaust gas recirculation and boost pressure that was customary in the past, and works far more effectively in this way.

At the same time, the engine has become even more robust owing to the omission of numerous components and parts. The fundamental idea here is perfectly simple and makes perfect sense: anything that's not fitted in the first place can't cause a malfunction.

Instead of using sensors with a linked system of control, Mercedes-Benz relies on various modes to assist with operation with, for instance, a cold engine, cold intake air or a cold exhaust aftertreatment system. Suitable modes have likewise been programmed for operation at medium and high altitudes, as well as for passive and active regeneration or regeneration when idling. With the exception of active regeneration, all the control modes are continuously variable, allowing the engine to be operated in an optimum efficiency range at all times.

When it comes to the matter of TCO, fuel consumption and emissions, Mercedes-Benz considers every single vehicle component. The Actros and its siblings have been demonstrating this since 2011 with their regulated ancillary units and innovative fuel-saving air compressors. The latest optimisation measures include a new air conditioning system. Its refrigerant compressor works more efficiently, while the air management system impresses with its needs-based control of fresh air and air recirculation. The air conditioning thereby also helps to achieve the fuel savings in the heavy-duty trucks from Mercedes-Benz.

The progress is impressive: Since 2011, average fuel consumption has sunk 13 percent compared with the successful predecessor Actros thanks to the introduction of the new Actros in accordance with the Euro VI emissions level, the Predictive Powertrain Control (PPC) cruise control and the second engine generation. And it is important to note that the trusted Euro V Actros already ranked as one of the most economical trucks in Europe.

By contrast, long-term improvements in consumption normally average out at around 1.0 to 1.5 percent a year in the commercial vehicle sector, which further underlines the exceptional technical expertise and innovative strength of Mercedes-Benz.

Back in 2011, Mercedes-Benz eclipsed the competition when it brought out the first engines with a Euro VI emissions rating. This is the case again with the second generation of the OM 471. The second engine generation combines optimum economic efficiency with exceptional performance. For the time being, they represent the fascinating pinnacle of development. Yet things will keep evolving, as the No. 1 in the heavy-duty truck segment and the No. 1 for fuel consumption never stands still.

The second generation of the Mercedes-Benz OM 471 is also available for EvoBus touring coaches. In the three-axle versions of the Mercedes-Benz Travego as well as the Setra ComfortClass 500, the version with an output of 350 kW (476 hp) and torque of 2,300 Nm is used optionally. As standard they are equipped with the more compact six-cylinder in-line Mercedes-Benz OM 470 engine with a displacement of 10.7 l. This power band of the OM 471 even comes as the standard version in the case of the top-of-the-range Setra TopClass model series. In these super-high-deck touring coaches, the output can be set at 375 kW (510 hp) and 2,500 Nm of torque on request.

The Setra double-deck S 431 DT impresses not only with its height but also with the outstanding capability of its engine: here the Mercedes-Benz OM 471 with an output of 375 kW (510 hp) and torque of 2500 Nm is the current production configuration.

The descriptions and information contained in this press kit apply to the international Mercedes-Benz model range. Country-specific variations are possible.