New generation OM 471. Click to enlarge.

In spring 2011, in anticipation of the Euro VI emissions standard, Mercedes-Benz unveiled a generation of engines developed completely from scratch. The first member of the new engine family was the OM 471. (Earlier post.) The 12.8-liter in-line six-cylinder unit featured a number of technical innovations including twin overhead composite camshafts; common-rail injection system with X-Pulse pressure booster; 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 existing OM 471 series engines have now built up a track record in practical operation in both the trucks and the buses and coaches from Mercedes-Benz and Setra. The engines also operate in the trucks from Freightliner and Western Star in North America and Fuso in Asia. Including overseas markets, production numbers for the OM 471 family have reached the 250,000 mark.

The engine development team for this new generation built on the qualities of the original OM 471 with a range of individual measures. The overriding goal was to ensure the engines are systematically geared towards low operating costs.

The latest generation of the OM 471 delivers about a 3% reduction in fuel consumption, while improving the engines’ already proven robustness. 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 N·m of torque is available 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 helped by the fast torque build-up at very low rev speeds, which has been optimized again in the latest generation of the engine.

Peak output of all the new-generation engines is available virtually constantly over a wide rev range between 1450 and 1800 rpm. Maximum torque likewise stays at a nearly constant level from around 900 up to 1450 rpm. The result is excellent drivability with an exceptionally wide usable engine speed range.

Latest generation X-Pulse. A key component of the new generation of engines is the latest-generation X-Pulse injection system —the common-rail system with pressure booster in the injector and unrestricted flexibility for modeling the injection system. Maximum rail pressure has been increased from 900 to 1160 bar, resulting in a maximum injection pressure of 2700 bar.

Injector for latest generation OM 471. Click to enlarge.

The injection nozzle is an eight-hole nozzle (previously seven holes), increasing the maximum flow rate by around ten%. Additional modifications include the piston bowl geometry; the sizeable increase in compression ratio from 17.3:1 to 18.3:1; and 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 5% of fuel consumption.

Top-of-the-range 390 kW engine. The latest-generation X-Pulse system with its far higher injection pressure is a prerequisite for the new top-of-the-range version of the OM 471 engine. Even from its idling speed of 600 rpm, the new engine produces a mighty 1600 N·m 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, with 30.5 kW (41.4 hp) per liter of displacement and the torque of 203 N·m per liter of displacement. The new flagship engine is 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.

High torque from the bottom of the rev range. The new injection system benefits not just the new range-topping version, but all other output variants of the OM 471 as well. 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 N·m 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 favorably. The rated output rev speed is now 1600 rpm, yet even at the previous speed of 1800 rpm, output is just one% below maximum. Depending on the output variant, 95% of maximum output is already being generated at around 1300 to 1400 rpm.

What this means in practice is outstanding drivability under all 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.

Longer final-drive ratio lowers engine speed. Mercedes-Benz has capitalized 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 3%. When fitted with 315/70 R 22.5 tires, this results in an engine speed of around just 1150 rpm at a speed of 85 km/h (53 mph).With the new output characteristics, this is not accompanied by any loss of performance.

This extra power is not only apparent when driving on the highway, but also on trunk roads and country highways. This is clearly exemplified by the popular Actros 1845. When travelling at a speed of 65 km/h (40 mph) 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 N·m 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 N·m was already sloping off.

Exhaust gas recirculation. The asymmetric turbocharger has been one of the special features of the OM 471 engine from the beginning. 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. 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%—something unprecedented in engine manufacturing. The EGR flap thereby controls not just the flow of exhaust gas for exhaust gas recirculation, but also the turbocharger.

Asymmetric injection optimizes emissions quality. 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% 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.

Proprietary asymmetric turbocharger for new generation OM 471. Click to enlarge.

New asymmetric turbocharger. 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.

As before, the asymmetric turbocharger features a fixed-geometry turbine. From a technical standpoint, this variant is 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.

Controls. Mercedes-Benz has also dispensed with boost pressure control in the latest generation of the OM 471. Together with the elimination of the EGR sensor and EGR control, this means that operation of the engine with its optimized thermodynamics is purely pilot controlled.

Controlling the engine in this way allows it to run 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 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. Instead of using sensors with a linked system of control, Mercedes-Benz relies on various modes to assist with operation with, for example, 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.