by Adrian-Liviu Dorofte

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six-cylinder engine

If an engine could be described as "heavier than heavy duty", the new OM 473 in-line six-cylinder engine from Mercedes-Benz would fit the bill perfectly. The technology of the new range-topping engine for heavy-duty trucks bearing the three-pointed star is based on the same platform as the ultra-modern Mercedes-Benz OM 471. But with an engine capacity of 15.6 l, an output of up to 460 kW (625 hp) and up to 3000 Nm of torque, the OM 473 stands apart from the rest, in a category of its own, characterised by peak performance under harsh conditions and meeting the toughest requirements in terms of transport speed, pulling power and robustness.

Top-of-the-range model of the BlueEfficiency Power engines

As the top-of-the-range engine, the OM 473 crowns the new BlueEfficiency Power engine generation from Mercedes-Benz. These medium-duty and heavy-duty engines now range from the compact OM 934 four-cylinder engine with 5.1 l displacement for short-radius distribution vehicles to the new OM 473 for heavy trucks, providing a complete product line comprising five vertical engines and one horizontal engine with 21 power ratings altogether, ranging from 115 kW (156 hp) to 460 kW (625 hp). All engines already meet the Euro VI emissions standard. Only Mercedes-Benz offers such a wide range of ultra-modern diesel engines with such a range of power ratings.

Maximum performance level for demanding applications

High transport speeds even in demanding topography, large loads in European road traffic through to heavy haulage, top off-road performance and maximum durability – the new OM 473 is extremely effective. This becomes clear with the first glance at its performance figures: the output ranges from 380 kW (517 hp) and 460 kW (625 hp) and is already available in the main operating range. And at 1600 rpm, the rated engine speed is unusually low. In the range of 1500 to 1700 rpm, and even beyond, depending on the variant, these engines have a nearly constant maximum output.

Tremendous torque at just above idle

Just as impressive is the maximum torque of the OM 473. Depending on the version, the figure is between 2600 Nm and 3000 Nm. Just above idle speed, approximately 2500 Nm is available at a mere 800 rpm in all versions. That is a higher torque than the maximum torque achieved by most engines in heavy trucks.

Nominally, the highest torque is available at 1100 rpm. In reality, nearly the entire pulling power is available over a wide range from roughly 900 to 1400 rpm. Immediately thereafter, the engines reach their peak performance.

This characteristic produces excellent driveability over an extremely wide engine speed range. Moreover, all versions of the engine prove impressive with a response to accelerator pedal movements which is as eager as it is powerful.

Systematically designed for low fuel consumption

At the same time, the new Mercedes-Benz OM 473 provides surprising economic efficiency. The most powerful engine in the new engine generation was systematically developed for low fuel consumption. For instance, depending on the version, the engine achieves between 0.5 % and 1.5 % lower consumption than the previous engine, the V8-cylinder Mercedes-Benz OM 502.
Likewise, maximum environmental compatibility is ensured: all versions of the OM 473 meet the future Euro VI emissions standard.

Like all engines in the BlueEfficiency Power generation, the new OM 473 is produced at the Mannheim engine plant. Series production will begin in September of this year. The new OM 473 will be used in both the new Mercedes-Benz Actros and the new heavy-duty Mercedes-Benz Arocs construction trucks.

Power ratings from 380 kW (517 hp) to 460 kW (625 hp)

The new OM 473 is available with three power ratings:

Power / Maximum torque

380 kW (517 hp) at 1600 rpm / 2600 Nm at 1100 rpm
425 kW (578 hp) at 1600 rpm / 2800 Nm at 1100 rpm
460 kW (625 hp) at 1600 rpm / 3000 Nm at 1100 rpm

Thus the OM 473 provides a seamless continuation of the previous top-of-the-range version of the new engine generation, the OM 471 with its maximum output of 375 kW (510 hp), 2500 Nm torque and an engine capacity of 12.8 l.

Technical highlights: turbocompound technology

Amongst the special features of the new Mercedes-Benz OM 473 is a technical highlight, known as turbocompound technology. It is one of the main reasons for the engine's high performance and economic efficiency.

The term turbocompound refers to a second turbine located downstream of the exhaust gas turbocharger. It makes use of the exhaust gas temperature which is maintained after the gas has flowed through the exhaust gas turbocharger, thereby further boosting efficiency. The power is transferred via a shaft and a hydrodynamic clutch to the engine's gear drive and thus directly to the crankshaft.

Instant response, consumption advantage at high load

The driver can feel the turbocompound technology directly by the even more eager response of the engine, even in the low rev range. At the same time, a consumption advantage of approximately 2% is achieved at high load. These demanding conditions are typical for the range of application of the OM 473.

In support of maximum efficiency, a turbocharger with a full-flow radial turbine adapted to the high exhaust gas mass flow is located upstream of the turbocompound axial turbine. It is controlled by a wastegate valve.

Thoroughly rugged, long-lasting construction

Extraordinary ruggedness and longevity are amongst the requirements in the class of heavy-duty, high-powered trucks. The crankcase, made of grey cast iron with an alloy developed and patented at the Mannheim plant, has both horizontal and vertical ribbing and is therefore extremely stiff. The distance between cylinders is small to maintain a compact design.

The single-piece pistons are made of steel to ensure maximum durability. Thanks to minimal distortion of the pistons and the stiff crankcase, oil consumption and blow-by losses are minimal – this lowers costs and improves environmental compatibility.

Efficient cooling with divided air flow

Wet cylinder liners ensure optimal engine cooling. The main air flow flows around the top third of the cylinder, while a smaller air flow is directed at the lower section of the cylinder liner, which is subjected to lower temperatures. Generally speaking, the coolant paths are short, resulting in extremely efficient cooling.

Highly stable cylinder head, distortion-free design

The single-piece cylinder head of the new engine is made of compacted graphite cast iron. It is highly stable and designed to withstand high ignition pressures of over 200 bar.

The material of the cylinder head and crankcase has the same expansion coefficient. This means there is absolutely no distortion between the components. The cooling ducts in the cylinder head are arranged in two levels. A precisely adjusted longitudinal flow complements the primary transverse flow.

Two overhead composite camshafts

The compact, stiff gear drive is situated on the output end of the engine. It is quiet, smooth and highly efficient in its operation. It drives the two overhead cams, amongst other things. These camshafts are composite and based on a hollow shaft – a first for engines of this size category.

Using low-friction plain bearing rocker arms in the crossflow cylinder head, the camshafts each control two inlet and exhaust valves per cylinder; these are arranged vertically in the head.

Variable X-PULSE fuel injection system

As in the other new-generation heavy-duty engines with the three-pointed star, the flexible common rail system with X-PULSE pressure boost is responsible for fuel injection in the OM 473. The maximum pressure of approximately 900 bar in the common rail is increased to up to 2100 bar in the individual injectors.

The X-PULSE system used exclusively by Mercedes-Benz continually adjusts the injection specifically for each cylinder based on the engine's current operating conditions. Alongside the injection timing and amount, the injection quantity and the injection pressure, modulation of the individual injection characteristic (called rate shaping) is also possible. At the same time, the injection is extremely stable.

Combustion chamber and fuel injectors adapted to the engine

The combustion chamber in the piston crown of the OM 473 is shaped like a graduated bowl. Both the shape of the bowl and the geometry of the six-hole injectors are adapted to the special characteristics of the turbocompound system.

The result of this highly variable injection system with efficient combustion is a quiet, smooth-running engine, low fuel consumption and minimal exhaust emissions.

Extremely powerful decompression engine brake

The already familiar, extremely powerful pressurized decompression engine brake is also used in the new OM 473. The driver activates the brake in three stages via the right steering-column lever, and it provides an extremely fast response.

Owing to the significantly larger engine size of 15.6 l, the engine brake for the OM 473 clearly outperforms its high-output counterpart in the OM 471, developing up to 460 kW (625 hp).

Efficient exhaust gas treatment for the Euro VI emissions standard

In the new OM 473, as in the previously introduced OM 470 and OM 471 heavy-duty engines, cooled exhaust gas recirculation is used to achieve low untreated exhaust emissions.

Like all engines in the new-generation BlueEfficiency Power units, the OM 473 also has a highly efficient exhaust gas treatment system based on the Mercedes-Benz BlueTec technology with SCR technology. A synthetic urea called AdBlue is added to the exhaust gas. In the SCR catalytic converter located downstream, harmful nitrogen oxides are converted to nitrogen and water, harmless components of the air. The consumption of AdBlue is very low, at 2-3 % of fuel consumption. This can be attributed to the engine's low emissions. And there is also a particle filter.

All components are specifically adapted to the OM 473. All three performance versions of the new engine meet the Euro VI emissions standard.

Weight commensurate with performance

The rugged new Mercedes-Benz OM 473 is relatively compact, at 1425 mm long (without fans), 1128 mm wide and 1212 mm high. The weight according to DIN 70020-GZ is 1284 kg, an appropriate weight for a heavy-duty engine of this size which already meets the Euro VI emissions standard for the highest of demands.

Systematically trimmed for economic efficiency

Customers will choose the OM 473 primarily due to its performance; even so, its developers paid close attention to maximum economic efficiency. Alongside the comparatively favourable fuel and AdBlue consumption, this includes factors such as long maintenance intervals for the engine of up to 150,000 km in long-distance haulage. The particle filter only needs to be cleaned for the first time at 450,000 km and thereafter only every 300,000 km.

The easily accessible filter module with oil and fuel filters, and the water separator for the fuel system on the cold side of the engine simplifies service – a further cost-reduction factor.

Longevity proven in practice

Extreme longevity is one of the essential points of economic efficiency for the new OM 473. It achieves a B10 value of 1.2 million kilometres. In other words, at least 90 percent of the engines achieve this value without a thorough overhaul. The engines have furnished proof of this not just during testing, but rather in practical applications on another continent.

Millions of kilometres in daily operation

The new Mercedes-Benz OM 473 is specifically designed for European requirements. But the engine benefits from extensive testing in North America, where for the past five years over 100,000 units of the same engine block have already been in use as Detroit Diesel DD 16 and 15 with two different engine capacities (15.6 l and 14.8 l).

This means a kilometre reading to date of many billions of kilometres in total. During this time, a number of engines have covered over a million miles (1.6 million km) of daily haulage in tough real-world conditions. The turbocompound technology amongst others has proven highly successful.

In Europe, only the larger 15.6 l variant is used. It differs from the American powerplants by more than 200 parts. The different manufacturing plants – the units for Detroit Diesel are produced in North America – make the distinctions clear.

Tough, intense testing in overload mode

The European version of the engine was subjected to tough, intense testing on par with any new engine from Mercedes-Benz. Alongside extreme driving cycles in the Spanish Sierra Nevada and cold tests in Scandinavia, this includes endurance runs in South Africa. There, it was tested amongst other things in the overload range with 60 tonnes of gross combination weight in semitrailer/tractor combinations with two semitrailers, in extremely rugged topography. By the start of production, the OM 473 will have covered around 18.5 million test kilometres in total – an impressive number.

In a subsequent step, the extremely durable engine will also be used in heavy-duty tractor units from Mercedes-Benz. It also serves as the basis for an industrial engine in the off-highway segment under the name MTU 1500.

Credits: Daimler AG

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