OM 906

Mercedes-Benz OM 906

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OM 906

Daimler-Benz
Damler-Benz OM 906 LA
-
Diesel
Motor	OM 906 LA	OM 906 LA II/1	OM 906 LA III/2	OM 906 LA II/3	OM 906 LA	OM 906 LA III/4	OM 906 hLA	OM 906 LA III/3
Variant	906.940	906.910 906.920 906.922	906.915 906.925 906.927 906.951	906.911 906.921 906.923	906.941	906.916 906.926 906.928 906.952		906.939
Byggeår
Ytelse (HK)	231 @ 2300	231 @ 2300	231 @ 2200	280 @ 2300	280 @ 2300	280 @ 2200		245 @ 2200
Dreiemoment (Nm)	810 @ 1300	810 @ 1200 - 1500	810 @ 1200 - 1600	1100 @ 1260 - 1500	1100 @ 1300	1100 @ 1200 - 1600		1100 @ 1260 - 1500
Vekt tørr (Kg)
Innsprøytning	Injektor
Sylindre	R6
Slagvolum	6374 cm³
Borring	102 mm
Slag	130 mm
Kompresjon	:1	17,4:1	18:1	17,4:1	17,4:1	18:1		18:1
Turbo	Ja
Ladeluftkjøler	Ja
Kjøling	Vann
Ventiler	2 Innsug - 1 Eksos
Innsug åpner	- før ØD
Innsug stenger	- etter ND
Eksos åpner	- før ØD
Eksos stenger	- etter ND
Klaring innsug	mm - kald	0,40 mm
Klaring eksos	mm - kald	0,60 mm
Tenningsrekkefølge	1 - 5 - 3 - 6 - 2 - 4
Dieselpumpe								A 028 074 69 02
Dysespiss
Dieselfilter	A 000 090 15 51							A 000 090 15 51
Oljefilter								A 000 180 17 09
Stempelhastighet	m/s
Stempelprodusent
Startmotor
Produksjonssted
Produksjonstall

OM 906.XXX

OM 906	kW	HK	RPM
.900	205	279
.901	170	231
.903	205	279
.910
.911
.912
.913
.914
.915
.916
.917	170	231
.918	180	245
.919	180	245
.920	170	231

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The new six-cylinder diesel engine OM 906 LA from Daimler-Benz

(From a article in MTZ)

DB OM 906 LA

With the introduction of the new OM 906 LA six-cylinder in-line engine, Daimler-Benz completes the 900 engine series (BR 900), whose four-cylinder variant, the OM 904 LA, has been on the market since 1996. These engines are designed for both light-duty and heavy-duty commercial vehicles. With a displacement of 6.4 liters, a maximum rated output of 205 kW at 2,300 rpm, and maximum torque of 1,100 Nm at 1,300 rpm, the new engine enters a power range previously covered by engines with significantly larger displacement. This article introduces the new six-cylinder engine.

Introduction

The introduction of the new six-cylinder OM 906 LA diesel engine completes the renewal of the engine product range for commercial vehicles, following the introduction of the four-cylinder OM 904 LA and the 500 series (BR 500) engines in 1996. The design of the OM 906 LA took into account the diverse applications of both Daimler-Benz's global vehicle concepts and those of other automobile manufacturers, each with its own unique requirements. With its current power range of 170 to 205 kW, particularly for on-road use, the OM 906 LA enters a performance segment previously covered by significantly larger displacement engines, such as the 11-liter OM 441 LA. This was addressed through the use of modern technologies and systems, as well as specific testing to ensure a B10 service life of 600,000 km in heavy-duty long-haul applications. This article discusses the design, testing, and, in particular, the comparison with the larger engines previously used in this power segment.

Product concept

The new OM 906 LA six-cylinder engine builds on the basic concept and features of the OM 904 LA four-cylinder engine, introduced in 1996. Both engines were planned as a single engine series from the outset. The design considerations for a new engine series in the power segment between the 600 Series, which was further developed from passenger car diesel engines for commercial vehicle use, and the new V-engines of the 500 Series introduced with the Actros, were essentially based on the following basic requirements:

– Low life cycle costs

– Compliance with global emissions standards

– Heavy-duty characteristics in terms of service life, fuel consumption, and maintenance intervals

– Suitable for application in all vehicles used worldwide by Daimler-Benz and other automobile and equipment manufacturers

– Suitable for use as a horizontal engine in city buses

– High engine braking performance

– Low noise emissions.

Another key criterion in the design of the new engine series was that four-cylinder engines had to be used in the power segment up to 150 kW, especially for distribution transport, due to their short overall length and the associated design advantages for the vehicle (low entry, level cab floor, and thus free access).

With a stroke/bore ratio of 1.27, a mean effective pressure of approximately 17 bar at rated speed, and approximately 22 bar at maximum torque, the technical data shown in Figure 2 resulted. The six-cylinder OM 906 LA engine, in particular, with its maximum output of 205 kW, covers a power segment previously achieved by the large 400 series engines with displacements of 11 l and 12 l. Therefore, a guaranteed B10 service life of 600,000 km was a key focus of testing for the OM 906 LA. The choice of injection system had a significant influence on the engine design. The plug-in pump concept still represents the best solution today, even compared to the common rail systems currently under development, for displacements >1 l per cylinder and the exhaust gas test cycles applicable to commercial vehicles, and it still has sufficient potential for further development in the foreseeable future. The key features of the BR 900 engines have already been described in detail in previous publications [1, 2, 3, 4, 5, 6]. The following section discusses the further development measures following the series launch of the OM 904 LA four-cylinder engine.

Injection system with leak-free single-spring nozzle holder

The OM 904 LA was introduced with the dual-spring nozzle holder concept, particularly due to its noise advantages during idle operation. Since then, the injection system has been further developed so that the required idle noise levels can also be achieved with a single-spring nozzle holder combination without leak-through fuel return ("leak-free"). By eliminating the leak-through fuel return, it has become possible to reduce the nozzle opening pressure – and thus the idle noise – of the single-spring nozzle holder to the low level of the first stage of the dual-spring nozzle holder.

The hydraulic pressure built up during the injection process in the closed spring chamber of the leak-free nozzle holder increases the closing force on the nozzle needle, thus ensuring blowback-free operation of the injection system despite the low opening pressure – set via the compression spring.

To reduce the number of variants, a uniform injection nozzle is used for all engine variants of the 900 series. The resulting uniform injection hole cross-section results in maximum injection pressures of 1,800 bar in the most powerful variant of the OM 906 LA. The same nozzle holder was used for this application as in the 500 series, adapted to the increased stress by selecting a higher-quality material and careful machining to avoid notch effects.

Electronics and Diagnostic Concept

The engines are installed in automobiles and work equipment that have different electronic systems. The overall function of the electronics is therefore divided into two parts: an engine-side control unit and a vehicle-side control unit. The control unit mounted on the engine, which is identical to the 500 Series, is subject to high demands regarding vibration resistance and temperature load. When installed in the Atego, for example, this unit is connected to a vehicle-side control unit (FR) via a data bus. For general applications (Figure 3), the engine control unit is supplemented by an adaptation module (ADM), which covers some of the FR functions. Additional components that meet application-specific requirements can also be connected via a system bus.

For engine diagnostics, the onboard diagnostic system in the vehicle is initially available. A significantly more powerful system, Star Diagnostics, is used worldwide for service. For customers who do not have access to Star Diagnostics, a more cost-effective device with comprehensive diagnostic quality for this application is being developed.

With these diagnostic systems, the rapid and targeted detection of faults in electronically controlled engines during service is significantly easier than with conventional engines with conventional injection systems, which have become very complex due to emissions regulations.

Use of Plastic Components

The main incentive for using plastic components lies, on the one hand, in the weight savings and, on the other hand, in the lower noise emissions with appropriate design. Another advantage is the improved sealing function, since, for tooling reasons, plastics require significantly higher sealing elements with significantly greater working capacity than aluminum die-cast parts, Figure 4. Thermoplastics with varying glass fiber content are used for the cylinder head cover, the charge air housing, and the cover on the injection pump side. The lower thermal conductivity and thus lower heating of the cooled charge air is an additional advantage for the charge air housing compared to an aluminum housing.

A duromer with 30% glass fiber content is used for the oil pan. The 25 mm long glass fibers make the component more resistant to stone chips than an aluminum pan. In the event of serious damage, the oil pan is not completely punctured, but rather a leaky, yet still coherent structure remains, from which the oil only slowly escapes. This gives the driver significantly more time to prevent consequential engine damage.

By using these plastic parts, the overall noise level at a distance of 1 m was reduced by 1.3 dB(A), Figure 5. With the widespread use of large plastic components, the new 900 Series sets future standards in the commercial vehicle engine sector.

Engine Braking System

The six-cylinder engines of the 900 series are equipped with the constant-throttle engine braking system, similar to the four-cylinder engines. The system consists of an exhaust flap – similar to conventional engine brakes – and a constant-throttle valve located in the cylinder head in addition to the gas exchange valves [6].

Due to the gas-dynamic processes in the exhaust tract of the six-cylinder engine, the specific engine braking power increased by approximately 35% compared to the OM 904 LA, so that a brake mean effective pressure of 11 bar and a braking power of 160 kW are achieved at the permissible engine braking speed of 2,700 rpm. The higher brake mean effective pressure of the OM 906 LA also requires greater actuation forces for the constant-throttle valves compared to the OM 904 LA, especially during opening and closing. In order to reliably prevent uncontrolled slamming of the valves caused by gas forces – associated with high landing speeds and high wear – the six-cylinder engines are equipped with hydraulic control of the constant throttle valves instead of the pneumatic control of the four-cylinder engines.

Fuel Consumption and Emissions

The fuel consumption map (shell diagram) of the high-performance variant (205 kW/1,100 Nm) shows the engine values. This map is shown in Figure 6; it clearly demonstrates that a specific fuel consumption of less than 200 g/kWh was achieved over a relatively large range of the map. In terms of distance-based fuel consumption, endurance tests supplemented by dynamometer tests with Road Load Simulation (RLS) show that the OM 906 LA achieves fuel consumption advantages of 2 to 6% compared to the previously used 200 kW OM 441 LA engine at approximately the same average speeds (Figure 7). As expected, this fuel consumption advantage of the smaller engine increases further on lighter routes or during constant-speed driving.

Since the Euro 3 emission limits are currently only a proposal and therefore cannot yet be certified, the engines are currently only offered with Euro 2 settings. The engines used in Mexico are already certified according to the US 98 limits (NOx < 4 g/HPh, PM < 0.1 g/HPh) without exhaust aftertreatment.

Since the Euro 3 emission limits are currently only a proposal and therefore cannot yet be certified, the engines are currently only offered with Euro 2 settings. Extensive studies have shown that the plug-in injection system meets the proposed Euro 3 emission limits with lower fuel consumption than is currently possible with the adaptation of a common-rail injection system. Common-rail injection systems cause very rapid heat release in the initial phase of combustion. Compared to the plug-in injection system, this results in higher NOx emissions, based on the same ignition timing. The earlier injection timing possible with the plug-in injection system leads to lower and therefore more economical fuel consumption.

Turbocharger Design

The use of the OM 906 LA with permissible gross vehicle weights up to 40 t requires a turbocharger design specifically tailored to this displacement and power class. Optimal start-up performance in challenging operating conditions is ensured by a relatively small turbine. This allows the turbocharger to generate rapid boost pressure build-up during transient processes.

The combination of a relatively small turbine with a compressor optimized for high airflow rates necessitates a wastegate in the high-performance variants of the 900 series. The OM 906 LA with 205 kW output achieves a maximum boost pressure of 2.85 bar even at a low engine speed of just 1,300 rpm and an effective mean effective pressure of 21.7 bar. Comparable vehicle acceleration tests show that the dynamic behavior of the smaller-displacement OM 906 LA engine is equivalent to, and even superior to, the OM 441 LA after further gear changes (Figure 8).

The high excess air on the combustion side results in excellent high-altitude performance. The engine can be operated at full power without reducing the injection quantity up to an altitude of 3,000 m. Provided the turbocharger-specific limits are observed, further high-altitude operation above 4,000 m is possible with automatic reduction of the injection quantity by the electronic engine control unit. Targeted optimization measures on the compressor side ensure a sufficient safety margin from the turbocharger's pumping limit in all operating conditions.

Cold Start Behavior

The electronic engine management system opens up new possibilities for fine-tuning cold start and white smoke behavior. Combustion chamber pressure analyses of cold starts have shown that targeted optimization of the injection timing and fuel quantity during the start-up phase can significantly improve cold start and warm-up performance. Additional cold start enhancements, such as the glow plug, are therefore only necessary in very low ambient temperatures. Extensive winter testing has demonstrated very short start times and low white smoke emissions at low ambient temperatures. Figure 9 documents the start times of various vehicles with 900 series engines without a glow plug as a function of the measured ambient temperatures.

At an ambient temperature of -15 °C, the 900 series engines start in significantly less than ten seconds. In very critical operating conditions and in Nordic countries, the glow plug, available as an optional extra, can further improve cold start performance.

Service Life

Service Life

For the universal application of the engine, not only in automobiles but also in the equipment sector (for example, cranes and agricultural machinery), service life requirements had to be met that had previously only been achieved by engines with a displacement of more than 10 liters. During testing on the test bench and in vehicles, it became apparent that the piston ring/cylinder liner combination is significant among the service life-determining wear pairings of the OM 906LA.

The wear of the chromium-ceramic coating of the first ring increases the so-called end gap. This can be easily measured as a characteristic parameter, representing the not always uniform wear around the circumference of the ring. The wear tests also showed that the measured values ​​can be plotted in a scatter band when they are assigned to the fuel flow rate, Figure 10. The total coating wear with the 100% limit was used as the ordinate. Wear is linear in the area under consideration, so extrapolations are also permissible. This type of representation allows for the specification of service lives for the conceivable, different application areas, which also take into account the engine's utilization, measured by fuel consumption.

The diagram shows that with a total fuel consumption of 190,000 liters, only 50% of the potential wear has been reached. For long-haul truck operation with a consumption of 32 liters per 100 km, this corresponds to 600,000 km. Similarly, heavy or lighter operating conditions can be considered and their service life determined. The specification requirement of a B10 service life of at least 600,000 km has been met and offers the potential for future higher service life requirements.

Engine Maintenance

In heavy-duty long-haul transport, despite the very high specific load on the engine, oil changes are only necessary every 100,000 km. To achieve this, it was important, among other things, to minimize soot ingress into the oil through optimized combustion design. Due to the aforementioned low wear of the valve train, valve clearances only need to be checked or adjusted at every other service.

Vehicle Applications

The OM 906 LA engine is already available on the market for a wide variety of installations. The most significant applications are in the Atego, the light and medium-duty commercial vehicles of Daimler-Benz AG. In addition to these, the OM 906 LA is installed in the Econic, the chassis for refuse collection vehicles. The engine is also attracting considerable interest from customers outside the Daimler Group. For example, crane carrier vehicles, snow groomers, and agricultural equipment are already equipped with it (Figure 11).

Series production of the 900 series in North America at the Daimler-Benz subsidiary Freightliner is currently being prepared, particularly for the business class. The horizontally mounted variant, the OM 906 hLA, was developed for the Citaro city bus. The basic engine of the upright version was retained, but the engine peripherals had to be redesigned according to the installation situation and the changed requirements, also with regard to the auxiliary units, Figure 12. Thus, a powerful and lightweight engine is available for this specific application.

Daimler Benz OM 906 hLA

Engine Applications

A gear-driven power take-off (PTO) is available upon request, located at the rear of the control housing. It is designed for an output torque of 600 Nm, which is required for driving concrete mixers. A short-term torque increase to 720 Nm is permissible. The gear ratio to the engine is 1:1.071, and the maximum torque can be drawn within the engine's speed range of 900 to 2,500 rpm. For refrigerated transport vehicles, the engine can be prepared for the installation of a 380 V three-phase generator to power an on-board refrigeration system. This generator is installed directly on the engine in conjunction with a special oil pan and is driven by a poly-V belt drive. Winter service vehicles can be equipped with an engine-mounted, belt-driven tandem hydraulic pump to power the snowplow hydraulics and the salt spreader. These additional special applications, which go beyond the engine attachments already presented for the OM 904 LA, significantly expand the customer benefits.

Following the series production launch of the engines, further work will focus on development to meet the Euro 3 legislation, which is not yet enacted. The main focus is on undercutting the emission limits while achieving the best possible fuel consumption. Environmental protection and customer benefits are of high priority. To accomplish these tasks, intensive work is being done on the further development of the existing plug-in injection system, which still has considerable potential.

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