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March 14, 2012

New medium-duty engines from Mercedes-Benz


With its Blue Efficiency Power range, Mercedes-Benz is now introducing a completely redesigned engine series for light and heavy-duty trucks, as well as for buses and coaches. The medium-duty engines of the OM 93x series, with four and six cylinders and a displacement of either 5.1 or 7.7 litres, span the output range from 115 kW (156 hp) to 260 kW (354 hp). Their outstanding properties serve to underline Mercedes-Benz's expertise in the field of diesel engines.

The new engines set a new benchmark in their class. They are uncompromisingly designed with environmental compatibility, economic efficiency and performance in mind. This is also the first commercial vehicle engine series in which every engine meets the Euro VI emissions standard right from the outset. Daimler Trucks is thus once again leading the way when it comes to environmental issues



Exceptional economic efficiency, in the case of the new engines, means longevity of service life, low consumption of fuel, AdBlue and engine oil, plus long maintenance intervals. Powerful means that the new OM 93x engines offer a pleasing immediacy of response, together with impressive power delivery. Such characteristics are the result of some sophisticated engine technology. One of the highlights: the new engines are the first series-production diesel engines in the world to feature an adjustable camshaft.

OM 93x: successor to the 900 series
The new engine family is made up of a four-cylinder in-line engine (designation OM 934) with a displacement of 5.1 litres, and a six-cylinder in-line unit (OM 936) with 7.7 litres. Each of them is available in various output categories. These two engines take over from the 900 series that was launched in 1996. The engines of that series were considered technically cutting-edge right from the start and still represent the benchmark in their segment in their Euro V versions.

Their successors are now setting a new milestone among medium-duty engines. The engines of the OM 93x series are based on a modular system and share a significant number of common parts. With a total of nine output categories, ranging from 115 kW (156 hp) to 260 kW (354 hp), the range is very finely graduated.

Engines to suit all requirements of medium-duty trucks as well as buses
Manufactured in the Mannheim engine plant of Daimler Trucks, the new engines are customised to meet all light to heavy-duty short-radius distribution needs as well as for use in light and medium-duty applications on construction sites and in long-distance transport operations. The six-cylinder engine, both as a vertically installed unit and in a specially developed horizontal configuration, is also designed for use in urban and inter-city buses.

Further stages are planned that will see the engine used in additional brands and models from Daimler Trucks on other continents. The same applies to its use as an industrial engine. Series production will begin this year in the Mannheim engine plant. Daimler Trucks has invested the sum of around 500 million euros in the development of the new OM 93x engine series and the facilities for its production.

Uncompromisingly eco-friendly: Euro VI from the outset in all output categories
With the new engines in its OM 93x series, Mercedes-Benz is once again leading the way when it comes to environmental issues: they are uncompromisingly designed with the preservation of the environment in mind. As the first diesel engines to do so, all variants meet right from the outset the forthcoming Euro VI emissions standard, which does not become legally binding in the EU until 1 January 2014. The protection of the environment is a hugely important factor for medium-duty engines, since these are primarily used in vehicles operating regularly in inner-city and suburban areas that are particularly sensitive to environmental impact.

The first factor ensuring clean exhaust gases is the highly efficient combustion process used in the new OM 93x-series engines. With ignition pressures of more than 200 bar and an injection pressure of up to 2400 bar these therefore punch above their weight and well beyond the medium-duty class. The engines also feature a cooled exhaust gas recirculation system.

When it comes to exhaust gas aftertreatment, the new OM 93x series uses a technology that was successfully premiered a year ago in the heavy-duty engines of the new OM 471 series and has since proved its worth in everyday practice. A closed-loop particulate filter, and the Mercedes-Benz BlueTec engine technology with AdBlue injection that has already been tried and tested over so many years, complement each other perfectly. BlueTec technology works by injecting AdBlue to break down harmful oxides of nitrogen into harmless components of air in the downstream SCR catalytic converter.

Uncompromisingly economical: outstanding fuel efficiency, maintenance intervals and service life
Cleanness and economic efficiency complement each other perfectly in the new OM 93x engine series. Although the fuel consumption of the previous series had until now been considered the benchmark among medium-duty Euro V engines, that of the new engines is even lower, despite the significantly more stringent requirements of the Euro VI emissions standard. The specific fuel consumption even achieves figures that are on a par with those for heavy-duty engines.

Low fuel consumption is not only a key criterion for economic efficiency as far as the customer is concerned, but also makes more sparing use of resources and reduces the emission of greenhouse gases.

Dramatic reductions in consumption of engine oil and AdBlue
The same is true of the consumption of oil, which has also been reduced still further. The engineers have managed to bring the level down dramatically to around half that in the existing engines. As part of the transition to Euro VI, the consumption of AdBlue has also been reduced to just 2.0 to 2.5 percent of the fuel consumption. This is less than half the amount used with Euro V – and another factor with a positive impact on a company's operating costs.

Long maintenance intervals to match bigger models
The same is true of the longer maintenance intervals. These can now extend to as much as 120,000 km for an OM 93x in long-distance transport operations, even at full-capacity use, and are thus around 20 percent longer than hitherto. The oil filling capacity of the four-cylinder engine has been increased only negligibly to achieve this, while the six-cylinder unit actually shows a reduction of 12 percent compared with the largest predecessor engine. The maintenance interval for the particulate filter can be up to 240,000 km.

Further development work has also been undertaken to improve the functionality of the maintenance monitor. The maintenance intervals therefore now reflect the actual operating conditions of the specific vehicle far better. In short-range distribution use, above all, this leads to longer maintenance intervals. Such long intervals do not only reduce the immediate servicing costs, but also bring about a corresponding increase in the vehicle's availability.

Longevity is another of the new engines' key advantages. With a forecasted service life of 750,000 km without major overhaul in long-distance transport operations, the new medium-duty engines also manage to achieve figures that until a few years ago were the exclusive preserve of heavy-duty engines. The average service life of these engines is around 20 percent longer than that of the previous series.

Uncompromising performance: dynamic response, excellent pulling power and powerful exhaust brake
For all their robust build, the new engines deliver impressive performance. A specific output of just under 34 kW (46 hp) per litre of displacement takes the engines into output categories that until now would only have been reached with larger-displacement engines.

Exceptional performance thanks to 'downsizing'
The four-cylinder engine, with a maximum output of 170 kW (231 hp) and up to 900 Nm of torque, is thus able to operate in areas that were until now reserved for six-cylinder units. By the same token the six-cylinder engines, with an output of up to 260 kW (354 hp) and 1400 Nm from a displacement of 7.7 litres, find their way into an output class that could previously often only be achieved with a displacement of more than 10 litres. This phenomenon of 'downsizing' is one of the key factors in the new engines' excellent performance in terms of exhaust emissions, consumption and power-to-weight ratio.
Excellent driveability, dynamic response
Quite apart from the nominal data quoted, the engines impress with their driveability. From an engine speed as low as 1600 rpm and up to around 2500 rpm, some 90 percent of the maximum output is ready for use. The engines thus get close to achieving a constant power curve across an extremely broad engine speed range. The result is excellent driveability, even in vehicles with fewer gears and thus a bigger distance between gears, as often experienced in short-radius distribution operations.

At the same time the new engines demonstrate dynamic torque, even at low engine speeds. Maximum torque is available at engine speeds as low as 1200 rpm and remains constant up to 1600 rpm in the main driving range. But the engines also demonstrate excellent performance at engine speeds below 1000 rpm.

In practice, the immediacy of the engines' response to accelerator pedal movement is also surprising. In this respect they are noticeably more agile than their predecessors. The vigorous power delivery of the four and six-cylinder engines is, like so many of their other characteristics, reminiscent of engines with much larger displacements.

High-performance exhaust brake, as on a heavy-duty engine
The exhaust brake of the OM 93x series is similarly effective. Its surprisingly powerful performance enhances safety and increases the average transport speed, whilst at the same time reducing brake pad wear as a result of the foot brake being used less. It thus also plays an important part in the overall economic efficiency of the vehicles.

The exhaust brake works as a charged decompression brake similar to that used with the heavy-duty engines of the OM 47x series.

The performance data are remarkable for this size of engine. While the standard version of the exhaust brake in the six-cylinder engines can achieve 235 kW, this figure rises to an impressive 300 kW in the premium version. This sort of figure could, until recently, only be attained by considerably larger engines in the heavy-duty class.

The brake power of the four-cylinder engines, at 145 kW or 170 kW respectively, is likewise quite remarkable. It is thus clear that the downsizing approach has been successfully implemented during the development of the new series in respect of both the performance of the engine and the effectiveness of the exhaust brake.

The engines reach maximum brake power at engine speeds of between 2700 and 3000 rpm – which not only improves driveability as a result of the extended useful speed range, but also serves to demonstrate the consistency and stability of the engines in the face of fluctuating engine speeds. The excellent performance of the exhaust brake at low engine speeds is also impressive in practice: in the premium version this is more than twice as good as in the engines that preceded these.

The brake is controlled via a two-stage (OM 934) or three-stage (OM 936) steering column lever, whereby the actuation itself is a hydraulic process. Each cylinder has its own exhaust brake module, which transmits the movement of the separate cam on the exhaust camshaft to the exhaust valve when the exhaust brake is activated. The advantage of this system is its improved efficiency, with no friction losses, in normal operation.

Uncompromisingly versatile: family of four and six-cylinder engines, including one horizontal variant
The new family of engines is customised to suit all types of applications involving medium-duty commercial vehicles. Along with short-radius distribution, these include use in light to heavy-duty long-distance transport and construction site vehicles, as well as in urban and inter-city buses. Thanks to its modular system with a high proportion of common parts, the OM 93x engine series covers a wide spectrum of outputs and applications.

The engine series comprises the four-cylinder in-line OM 934 with a displacement of 5.1 litres and the six-cylinder in-line OM 936 with a displacement of 7.7 litres. The bore and stroke, at 110 mm and 135 mm respectively, are identical in both variants, while the cylinder spacing of 128 mm is also the same in both.

Horizontal six-cylinder in-line engine for buses and coaches
The six-cylinder in-line unit in the output categories with 220 kW (299 hp) and 260 kW (354 hp) is also available in a horizontal variant, specifically designed for use in urban buses. The engineering design work involved is considerable, since the space available for its installation in the rear of a bus is extremely restricted. The basic engine can remain the same, due to the fact that the horizontal variant was already taken into account during the concept phase. Nevertheless, all peripherals have had to be specially developed for this configuration.

Peripherals include the fuel module, oil/coolant module, oil sump, turbocharger unit with air and exhaust pipes, valve cover with oil mist separation and all ducting. Indeed, behind the specialised production of the horizontal engine lies a complete development and testing process all of its own.

The OM 936 and OM 936 h (horizontal engine) in the two output categories of 220 kW (299 hp) and 260 kW (354 hp) are available in addition for installation in Mercedes-Benz urban and inner-city buses. The output curve and torque figures remain as for the truck engines.

Uncompromising engineering: only the best for the medium-duty engines
Development of the new engines began with the proverbial blank sheet of paper with a line down the middle of it. Nor was there any requirement to take existing production facilities into consideration. The focus throughout the development process could therefore be on delivering a product with exemplary qualities, and thus on delivering maximum benefit for the customer.

Crossflow cylinder head with four valves per cylinder
Sixteen years ago it was the innovative three-valve technology of the series 900 that caused a stir when it was launched. Upping the ante this time around, in its successors, are four valves per cylinder. Intake and outlet are arranged in parallel pairs in the crossflow cylinder head, an arrangement that keeps the intake and outlet ducts as short as possible and thus flow losses to a minimum – one of the principles ensuring that the fuel consumption of the engine remains low.

Sophisticated and high-grade engineering is used in every aspect of the engine, in order to be able to offer the customer an engine that is the most economical and most robust in its class.

Cylinder head and crankcase robustly joined
The cylinder head is made out of grey cast iron with lamellar graphite (GJL). A special cast-iron alloy, developed by the company's own foundry at Mannheim, gives it exceptional strength. The use of this material also ensures optimum thermal management of the component in those areas directly exposed to the combustion process.

The crankcase and cylinder head are held together by six bolts per cylinder, an unusually high number in this segment. Since the materials used for the cylinder head and crankcase share the same coefficients of expansion, there is no warping between the compo­nents. The design of the connection is so robust that not a single instance of damage to the cylinder head gasket was reported at any point during the testing period.

Two composite overhead camshafts
The actuation of the engine's intake and exhaust valves, which are arranged in parallel, is governed by two overhead camshafts. These are fitted in such a way that the overall height nonetheless remains low.

The camshafts, manufactured in a patented process at the Mannheim plant, are what is known as "composite" in design. They comprise a hollow tube, onto which the cams are shrink-fitted. This lightweight yet solid design was first seen in a commercial vehicle engine a year ago, in the OM 471. The camshafts control the intake and exhaust valves via a low-friction, wear-resistant roller-type rocker arm.

Premiere for diesel engines: adjustable camshaft
One of the most technically refined features of the new engine series is its VCP (variable camshaft phaser). This is the term used for an adjustable exhaust camshaft – the very first camshaft of this type to appear in any diesel engine. The adjustment supports the regeneration of the particulate filter. If regeneration is needed, the timing can be adjusted as necessary by up to 65 degrees to "early": in this case the exhaust valves open and close earlier, so the exhaust gas released from the cylinder is hotter. This technology makes regeneration of the particulate filter possible under practically any operating conditions and at outdoor temperatures as low as -30 degrees Celsius.

The adjustment is made hydraulically via a vane piston on the exhaust camshaft, acting upon a signal from the engine control unit. If an adjustment is required, engine oil flows into the vane piston. This then turns, so influencing the position of the camshaft relative to its drive gear.

Rigid crankcase, rigid crank assembly
A key element of the new engines is their rigid crankcase with a supporting spar structure made out of the same material as the cylinder head. The rigid design makes high combustion pressures possible, while at the same time reducing noise emissions. The contact surfaces of the dry cylinder bore are given a final plateau-honing finish. This smooth surface, which thus nevertheless has high oil-retention properties, helps to reduce friction losses and also plays its part in lowering oil consumption.

The connecting rods are made out of forged steel and split at the eye in a process known as "cracking". Bolting the parts together again creates a particularly strong and close-fit join.

The crankshaft is extremely rigid and, like all the other components of the basic engine, is designed to be robust enough to cope with high ignition pressures during combustion, yet also light in weight.

Like the cylinder head and the camshafts, the cylinder crankcase, connecting rods and crankshaft are manufactured in a new state-of-the-art production facility at the Mannheim plant.

The pistons are made out of aluminium and feature an oil-spray cooling system with cooling duct. The geometrically optimised two-stage combustion chamber in a shallow recess at the base of the piston is designed to facilitate the perfect combustion of the fuel in conjunction with the exhaust gas recirculation system. This is one of the key factors behind the outstanding fuel efficiency of the engine.

The bore/stroke ratio of 110/135 mm has been chosen as another way to help reduce low fuel consumption. At the same time the long-stroke configuration also ensures excellent pulling power at low revs.

Robust gear drive
The crankshaft drives the camshafts via a compact and rigid gear drive located on the back of the engine. The arrangement of the gear wheels on the flywheel side of the engine helps to reduce noise emissions, whilst also making it possible to drive the compactly arranged auxiliary units of oil pump, air compressor and high-pressure fuel pump, including feed pump. At the same time the gear drive provides the basis for the optionally available live engine power-take-off units at the rear, with up to 600 Nm of net torque.

The flanks of the gear wheel teeth in the gear drive have all been hardened and ground. As well as giving them a high degree of fatigue durability, this keeps gear noise down to an absolute minimum.

Tremendous injection pressure of up to 2400 bar
The high-pressure fuel injection system is based on the common-rail principle with an oil-lubricated high-pressure pump to deliver the fuel into the pressure reservoir, or rail. Centrally positioned injectors controlled by solenoid valves inject the fuel into the combustion chambers. The very high injection pressure, up to a maximum of 2400 bar, ensures that the combustion of the diesel fuel is extremely efficient and that particulate emissions remain low.

Up to five individual injections per injection process
The highly flexible injection strategy allows up to five separate injections, including pilot, main and post- injections. Depending on the specific operating situation, for instance normal warm running, cold starting or cold running, the system will apply a different strategy, using a combination of either some or all of the possible injection stages in each cycle. This allows optimum adjustment of the combustion process in order to achieve the specific objectives at each operating point, e.g. efficiency, noise reduction or the heating-up of the exhaust gas aftertreatment system.

This ingenious injection system is also the basis of the engines' outstanding cold-start capability, enabling them to start reliably and without preglow, even at temperatures well below freezing point.

Customised turbocharging for each output category
The turbocharging of the new engines is customised in each case to the specific output category. In the four-cylinder OM 934, the pressure for output up to 130 kW (177 hp) comes from a one-stage exhaust gas turbocharger. Two-stage turbocharging is then used for the higher outputs. The six-cylinder OM 936 uses an asymmetric exhaust gas turbocharger with double-flow turbine for outputs up to 220 kW (299 hp). Two-stage charging with twin turbochargers is used once again for the two output categories above this level.

The high level of investment in the turbocharging systems has certainly paid off, resulting in exemplary fuel consumption, power delivery and response characteristics for all four output categories.

All four types of turbocharging system use an electronically controlled wastegate valve to regulate the charge pressure and further improve the engine response during acceleration as well as when using the exhaust brake.

Meticulously designed coolant circuit
Considerable attention was paid by the engineers to the coolant circuit in the new engines. The cross-sections and flow geometry of the water jacket in the crankcase and cylinder head have been optimised to ensure the best possible longitudinal as well as cross-flow distribution of coolant. This thorough cooling of the components ensures that any loss of power that might affect the drive unit of the coolant pump is kept to a minimum.

In order to improve fuel consumption still further, each of the various engine models and variants is fitted with a different coolant pump during its assembly in the Mannheim plant. These vary in their delivery rates and are selected on the basis of the engine output and specification and of the vehicle's cooling system.

Compact dimensions, favourable power-to-weight ratio
Thanks to a compact design, with a water pump that is now mounted on one side, the length of the engines remains much the same as that of their predecessors, in spite of the larger displacement. In actual fact the OM 936 six-cylinder unit is 25 mm shorter than its predecessor, the OM 926 LA.

The power-to-weight ratio of the new engines is also very favourable. The dry weight of the OM 934 is 495 kg, while that of OM 936 is 650 kg (both according to the DIN 70020-A standard). In comparing these engines with their predecessors, account needs to be taken of the additional design complexity required to achieve the Euro VI emissions standard, the larger displacement and the significantly improved performance. In many cases the four-cylinder OM 934 is therefore likely to replace the previous six-cylinder unit; while the six-cylinder OM 936 will take the place of engines that had much larger displacements and were therefore heavier.

Uncompromisingly state-of-the-art and reliable: exhaust gas aftertreatment technology
In the development of the exhaust gas aftertreatment systems, one objective remained paramount at all times: that the driver and operator of a truck or bus should be as little aware as possible of having one on board. The specifications book demanded it should take up a minimum of space and have a low weight, low AdBlue consumption, long maintenance intervals and a high level of reliability.

The fact that this specification has resulted in systems that not only deliver the perfect combination of these properties but also reduce nitrogen oxide and particulate emissions by around 90 percent compared with the current Euro V products, is extremely pleasing in terms of the environment as well as for customers.

The metering system for the injection of AdBlue into the SCR catalytic converter needs no compressed air regulation and, by reducing the requirement for compressed air, so also reduces the vehicle's fuel consumption. The system has been adopted as it stands from the exhaust gas aftertreatment systems used in the OM 47x range of heavy-duty engines.

Similarly adopted from the "heavy" models is the metering system for the diesel fuel (HC doser), which is necessary for the regeneration of the particulate filter. This complex solution was chosen quite consciously. Metering the fuel via the separate HC doser at the exhaust outlet of the engine, rather than inside the engine via the injection system, reduces the impact on both the engine oil and the engine components and helps to increase robustness.

The basic structure of the exhaust system matches the systems used by Daimler Trucks North America since 2009 to meet the EPA 10 exhaust emission regulations in conjunction with the company's own DD13 and DD15 engines from Detroit Diesel. These comprise, in the order of flow through the vehicle, an oxidising catalytic converter (DOC), a diesel particulate filter coated in precious metal (DPF) and an SCR catalytic converter for the reduction of nitrogen oxides with a final AdBlue slip catalytic converter. In contrast to the exhaust gas aftertreatment systems used in the OM 47x engine series, the unit used in the medium-duty engines is single-flow in design.

Intelligent integration of engine and exhaust gas aftertreatment system
Right from the beginning of the concept phase, development work on the engine and exhaust system was closely interlinked and highly integrated to an extent previously unheard of. In relation to each property of the combined product, the search was never for the optimum in terms of the engine or the exhaust system, but for an overall optimum. As just one example, the consumption figures for diesel fuel and AdBlue were never considered in isolation.

The outcome is an optimum level of operating costs that combines the two consumption figures, for diesel and AdBlue, and as a result delivers the maximum benefit to the customer.

A further example of this integration work concerns the regeneration of the diesel particulate filter, in other words the regular process of burning off the particulates filtered out of the exhaust gas. During this process, the engine briefly produces extremely hot exhaust gases, into which the HC doser injects fuel, downstream of the engine. In a catalytic combustion process, this fuel generates heat in the oxidising catalytic converter, which in turn increases the temperature of the exhaust gas even further to the level necessary in order to burn off the filtered particulates. In order to achieve this, the measures in the engine (camshaft adjustment with VCP plus multiple injections to increase the exhaust gas temperature) and in the exhaust gas aftertreatment system (HC doser and mixture with the exhaust gas, plus harmonisation of the surface and precious metal coating of the oxidising catalytic converter) had to be perfectly coordinated with one another. The outcome: even at outdoor temperatures of minus 30°C, an automatic regeneration of the particular filter is possible in almost any operating mode imaginable. This almost completely eliminates the likelihood that a forced particulate filter regeneration should be necessary on a stationary vehicle with its engine running – an invaluable benefit, particularly for vehicles used for inner-city short-radius distribution purposes and for buses.

MCM and ACM control units: everything under control
The impressive efficiency, eco-friendliness and performance of the new engine series is closely related to the electronic control of both the engine and the exhaust gas aftertreatment system. Two 'superbrains' work together as a strong team here: the Motor Control Module (MCM) and Aftertreatment Control Module (ACM).

The electronic systems are constantly using their tremendous computing power to calculate the ideal injection point, the correct charge pressure and the best operating conditions for the aftertreatment system – and thus to deliver the best possible traction, the best possible economic efficiency and the best possible environmental performance. And they do so in hot and cold weather, up in the mountains or down at sea level.

The progress that has been made is impressive: when launched in 1996, the previous engines, in the 900 series, were among the first commercial vehicle diesel engines to have full electronic controls. In those early days, the control unit had to factor in around three hundred different parameters, performance characteristics and mapping data. The new engine series takes advantage of the progress made in the field of electronics: control and regulation in the MCM control module now involves around ten thousand such data fields, with a further seven thousand in the ACM control module.

But the control of the operating functions is not the only area in which such big strides have been made. Self-monitoring of all status data and subsystems has also entered new realms. In other words: the control system reacts with extreme sensitivity in the interests of consumption, longevity and reducing environmental impact, constantly monitoring the complete system for its own protection.

Uncompromisingly tested: extensive testing under extreme conditions in both short and long-distance transport operations
Longevity and robustness have traditionally been considered among the outstanding features of Mercedes-Benz commercial vehicle engines. And it is a tradition upheld by the new OM 93x engine series. Development for series production began in 2005. By the time the engine actually began to take shape in iron and steel, it had already been thoroughly tested – virtually, as it were - in a series of optimisation steps. This was a process that took into account all the experience gained since the very start of development of the OM 47x heavy-duty engines.

Four hundred engines tested at test stations and in vehicles
All in all, some four hundred engines were tested, either at test stations or in vehicles. The endurance testing at test stations and in road-going vehicles alone represents more than 18 million kilometres of operational use with customers. The engineers also invested more than 100,000 hours of running time on engine test stations into the development and testing of the functionality of the engine and aftertreatment system, and of all subassemblies.

In the meantime, tractor/semitrailer combinations with 40-tonne towing weights spent years travelling to and fro along topo­graphi­cally challenging autobahn routes with constant variations in uphill and downhill gradient – putting extreme strain on these compact engines. The other extreme of the application spectrum was also thoroughly tested: highly dynamic applications involving low loads and low temperatures were used by the testing department for tests of the exhaust gas aftertreatment system in particular.

During several stints of low-temperature testing in Lapland and Alaska, as well as during high-temperature and altitude tests in Spain and the Rocky Mountains, the engines were subjected to the same demands as the engines in the heavy-duty class – the most extreme challenges to be faced by any engine.

Methods of torture rarely seen in real life
Further methods of torture practised by the test engineers at Mercedes-Benz included on-road endurance testing under overload power with a raised water temperature, as well as tests on sloping sites or at engine overspeed. It is certainly reassuring for future users of the new engines in either a truck or a bus that they mastered with ease the sort of conditions that are hardly ever provoked in real-life.

The results of such stringent testing deliver the proof: the new engines meet the defined objectives in terms of reliability and durability to virtual perfection. Despite the added complexity involved, the development team were determined to do everything in their power to ensure that the new engines match the exceptional reliability and robustness of the 900 series, itself already legendary in this respect. In terms of durability the new engines even manage to raise the bar: over the challenging reference route between Stuttgart and Hamburg the new engines achieved, with a figure of 6.5 kilowatts per tonne in long-distance transport operation (which corresponds in the case of the OM 936 with 260 kW to a towing weight of 40 tonnes) a B10 service-life value of 750,000 km. This represents an improvement of 20 percent over the predecessor engine. The B10 value indicates that at least 90 percent of the engines will cover at least this distance before needing a major overhaul.

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