Daimler Trucks launches new heavy-duty, Euro VI-compliant engines; X-PULSE injection system with pressure booster
Daimler Trucks has launched the Mercedes-Benz OM 47x, under the name “Blue Efficiency Power”. The Blue Efficiency Power generation of Mercedes-Benz engines has been specifically developed for use in Europe and is the first of its type to meet, from the outset, the Euro VI emission standard.
The Blue Efficiency Power engines, which share a common basic design, have been under development for five years. Features include the new X-PULSE injection system with pressure booster; an asymmetric exhaust gas turbocharger; emission control with SCR technology, EGR and a particulate filter; and a three-stage exhaust brake.
The core design of the engine is based on the new platform used for Daimler Trucks’ heavy-duty engines used since 2007 in trucks built by the Group’s North American Freightliner brand (manufactured by company-owned engine manufacturer Detroit Diesel) and since last year by Fuso in Japan.
The platform concept used for the new generation of engines involves an identical basic design, which can be varied with different emission-related components or through the addition of regional applications or assemblies for specific markets or customers. However, the different emission standards and usage profiles that apply in Europe, together with the different installation position in the cab-over-engine truck as opposed to the conventional model, means that some essential components of the engine do differ.
These include the injection nozzles, turbocharger, flywheel, control electronics, exhaust system, air compressor and the overall tuning, including that of the various output and torque variants. All together, the engines differ in more than 200 components from the engines produced in North America or for Japan.
OM 471. The first member of the new engine generation is the Mercedes-Benz OM 471, with a 12.8-liter displacement. The OM 471 is the first engine in its class to have received type approval and already be available with the coming Euro VI emission standard (which begins taking effect in two years). The OM 471 has a power output range of 310 kW (421 hp) to 375 kW (510 hp) and maximum torque of between 2100 and 2500 N·m.
Featuring a combination of exhaust gas recirculation, SCR catalytic converter and particulate filter to reduce emissions, the engine has been designed to meet the specific requirements of the European emission standard Euro VI and the particular needs of European customers.
Among the special features of the Mercedes-Benz OM 471 are its broad range of four power and four torque variants, as well as two different brake power configurations. Typical of the specifically European design of the engine are the variants with enhanced power and torque, which are configured to suit typical usage here as well as to the needs of operators and drivers. The basic line-up of the Mercedes-Benz OM 471 comprises the following versions:
|Output kW||Output hp||Torque N·m|
These four basic power variants are complemented by three variants that have been given the name Top Torque, which release an additional 200 Nm of torque in the highest gear of the automatic transmission.
The rated engine speed of the new engines is set at 1,800 rpm for all power variants, with the maximum torque available at 1,100 rpm. As a result of the very steep output curve immediately before the main operating range, most of this maximum torque is already available at an engine speed of 1,000 rpm. Even below 1000 rpm the torque is high, resulting in an extension of the usable speed range downwards—depending on the profile of the route—to around 800 to 900 rpm, with a correspondingly positive effect on fuel consumption. Likewise, at 1400 rpm, the engines deliver almost 100% of their full output. The power and torque curves thus combine to ensure excellent driveability with a high output in all key engine speed ranges.
The design of the Mercedes-Benz OM 471 is based on six cylinders mounted vertically in-line. The compact dimensions of the engine, at 1531 mm long (valve body assembly to fan coupling), mean that it fits neatly underneath the driver’s cab on cab-over-engine (COE) models. The long-stroke design of the Mercedes-Benz OM 471, which has a bore of 132 mm and stroke of 156 mm, gives it excellent pulling power.
To provide robustness and durability, the crankcase includes vertical structures and ribbing to make it very rigid. This design also helps to reduce noise emissions. To optimize weight, the sump is made out of a synthetic material. The oil level is checked by a special sensor linked to the engine control unit.
In order to keep the design compact, the spacing of the cylinders has been kept to a minimum. With a view to a long service life, the one-piece pistons are made out of steel. They have two compression rings and one oil control ring each, along with splash-oil cooling. A protective coating ensures that the engine will perform under load even while it is being run-in. The almost negligible distortion of the pistons and the rigid crankcase keep oil consumption and blow-by losses to a minimum, thereby reducing costs and improving environmental acceptability.
Optimum engine cooling is ensured by wet cylinder liners. The main cooling flow circulates around the upper third of the liner, while a secondary flow cools the lower third, which is not subject to such high temperatures. In general, the distances travelled by the coolant are kept short, making the cooling process here very efficient. The thermostat for the cooling system is located at the input end to ensure sensitivity of adjustment. The fine plateau honing of the cylinder liners reduces oil consumption as well as friction losses.
The connecting rods, also made of steel, are split at the eye in a process called cracking. This involves the connecting rod being broken at a pre-determined point and then screwed together again to form a particularly stable, close-fit join with a larger surface area. The crankshaft is made out of induction-tempered steel. Seven main bearings and a balancing process with counterweights ensure the smooth running of the engine.
The high rigidity of the crankcase, the steel pistons, reinforced connecting rods and bearings were all selected with the high ignition pressures of the engine in mind. These increased from 180 bar to more than 200 bar, so improving efficiency.
The turbocharger, starter motor and crankcase ventilation system are all located on the hot side of the engine. The motor control module (MCM), oil/coolant module including filter and coolant pump, the fuel pumps for the high and low pressure system and the consumption-optimized two-cylinder air compressor are grouped together for ease of maintenance on the cold side.
The crankcase has a very carefully machined surface. This, in conjunction with the cylinder head gasket, results in a smooth and thus close-fit and leakproof join to the cylinder head.
The one-piece cylinder head of the new engine is made of cast iron with vermicular graphite (also known as compacted graphite iron, or CGI). This material is extremely resistant to temperature fluctuations, has excellent damping characteristics and is subject to only minimal expansion at high temperatures. It is also extremely robust and well suited to cope with the high ignition pressures of more than 200 bar in the new generation of engines.
The materials used for the cylinder head and crankcase respectively have virtually the same expansion coefficient. This means that there is no distortion between the components, regardless of the conditions. The water jacket (the cooling ducts) in the cylinder head is arranged in two layers. The basic principle behind the cylinder head cooling is that of cross-flow. In the upper part of the water jacket, this is overlaid by an added longitudinal flow that, amongst other things, ensures an even distribution between the cylinders. The cylinder head cover is fastened in place with 14 screws.
The alternator, cooling-water pump, air conditioning compressor and fan are driven by up to three poly-V belts located at different levels on the front of the engine. The number of levels depends on the specific configuration, whereby the third level can also be used as a power take-off for optional ancillary equipment. There is a choice of two cooling-water pumps. The closed-loop cooling-water pump kicks in as needed to help reduce fuel consumption. Both the closed-loop cooling-water pump and the fan are among the many components that were specially developed for the Mercedes-Benz OM 471.
At the power take-off end of the engine is a very compact and rigid gear drive. The gear drive drives the oil pump, the very fuel-efficient two-cylinder air compressor, the common rail high-pressure pump, the power steering pump and the double overhead camshaft.
Each camshaft controls two intake and outlet valves mounted vertically in the cylinder head via low-friction rocker arms supported on friction bearings. The camshafts are not milled from a solid piece of material but assembled—a premiere for engines of this size—on the basis of a shaft that, for weight reasons, is hollow. They are mounted on seven bearings, without the need for bearing shells, in a diecast aluminium camshaft frame.
Common-rail system with X-PULSE pressure booster. One of the key priorities for the new generation of Mercedes-Benz engines is the clean and efficient combustion of fuel. The system used is based on a fully electronic, flexible common-rail system. Flexible in this case means that not only can the injection pressure, timing and the quantity of fuel injected be varied, but also the injection rate, thanks to the new X-PULSE injection system with pressure booster that has been developed exclusively with Daimler Trucks. In general, high-pressure injections in a common-rail system are comparatively quiet, giving the advantage of a smooth-running engine. In conventional common-rail systems, however, the maximum injection pressure is produced purely via the high-pressure pump in the common rail, which feeds the injectors of each cylinder.
In the X-PULSE common-rail system with pressure booster, the twin-piston high-pressure pump produces a maximum pressure of around 900 bar in the common rail. This pressure is boosted within the individual injectors to up to 2,100 bar. The X-PULSE pressure boost varies according to the engine mapping and adjusts continuously to the actual operating conditions of the engine—for example, to the demand for torque from the accelerator. Control of the injection timing, quantity, rate and number of injections and of the injection pressure is managed for each individual injector separately through the engine control unit. It is thus able to even out any differences between the various cylinders.
X-PULSE not only makes it possible to achieve extremely high maximum pressures but both the pressure and the pressure distribution can also be freely adjusted during the main injection with the help of two solenoid valves. Since all parameters are variable, each individual injection can be precisely adjusted to the specific situation.
Each injection process is made up of a series of separate injections. Up to two pilot injections precede a gradual increase of pressure, so reducing noise levels and ensuring excellent smooth running characteristics. The flexible rate-shaping of the subsequent main injection ensures that fuel consumption is kept as low as possible, while continuing to meet emissions regulations. A post-injection ensures the almost complete combustion of the particulates. A further post-injection can also be made, as necessary, in order to regenerate the particulate filter. The Mercedes-Benz OM 471 has a separate injection valve for this purpose—known as the HC doser—located in the exhaust nozzle. This is used to control the active regeneration of the filter.
Many different forms of injection are possible with the new X-PULSE injection system. Their use depends on various parameters, such as engine load: injection without pressure boost but with just the pressure in the rail; injection with early-stage pressure boost (a “square” injection rate); late boost (“boot” rate); or in between (the “ramp” rate).
All in all, this means that it is possible to control fully the injection sequence at every operating point of the engine. As the highest pressure is only produced actually inside the injectors, injection delivery is exceptionally stable. The result is quiet and easy-running engines with smooth running characteristics, low fuel consumption and minimal exhaust emissions. This first appearance in Mercedes-Benz engines is only the first stage for X-PULSE; the X-PULSE injection system with pressure booster has the potential for an injection pressure of 2500 bar.
The injection process takes place in a geometrically optimized combustion chamber with a shallow piston recess. The X-PULSE injector developed specially for the Mercedes-Benz engines is not the same component as the injectors used in Daimler Trucks’ engines on other continents. As part of the overall injection strategy, it is designed specifically for typical operating conditions in Europe, with a higher proportion of heavy-duty use.
The injector is positioned vertically and centrally between the vertically arranged intake and outlet valves. It features an injection nozzle, also specially designed for Mercedes-Benz, with seven injection orifices, executed here as miniature blind orifice nozzles.
The high maximum pressure of injection and the extremely fine atomization of the fuel in the combustion chamber are the keys to efficient combustion. The shape of the combustion chamber is such that there is no swirl or tumble, so ensuring that the combustion of the fuel/air mixture takes place in an extremely efficient way. The intake valves and load-change behaviour have also been configured especially for Mercedes-Benz to suit European conditions.
The engines feature a comparatively high compression ratio of 17:1.
Asymmetric exhaust gas turbocharger. The turbocharger on the Mercedes-Benz OM 471 is characterized by its asymmetric turbine casing, fixed geometry and charge air cooling system. With the asymmetric turbine, the exhaust gases from the first three cylinders flow directly through the exhaust gas recirculation system to the turbine, without any losses. Only three of the cylinders are linked with the exhaust gas recirculation duct, and the asymmetric design of the turbine means that a higher pressure level can be maintained in them for the recirculation process. As a result, the engine can be operated across a broad range of parameters with an economical, positive scavenging gradient, despite the exhaust gas recirculation.
A wastegate valve is used to limit the charge pressure and further improve engine response during acceleration. This is actuated according to the operating point directly from the engine control unit via a pressure control valve.
Three-stage exhaust brake. In order to improve performance—particularly in the intermediate speed range—Mercedes-Benz dispensed with conventional technology in the form of an exhaust brake with exhaust throttle valve or a constantly open throttle in favour of a turbocharged decompression brake. This has been an integral feature of the engine’s design and control system from the outset. Configured to meet European requirements and quiet in its operation, the system features a short response time of less than 150 milliseconds.
The exhaust brake can be controlled in three stages through a steering column stalk. In the first stage, the exhaust brake is activated on three of the cylinders. The remaining three cylinders are then brought in as a second stage. In the third and highest stage, the EGR valve and wastegate controls are used to increase the charging level of the engine and so achieve the maximum brake power of 2300 rpm at 400 kW (544 hp). In addition to this manual actuation, the exhaust brake is also used in cruise control mode, whereby here the ideal braking torque is adjusted continuously.
As is already the case in the current vehicle generations, the exhaust brake is also used to synchronize the engine speed when the automatic transmission shifts up. As well as shortening the synchronization time, the use of the exhaust brake during shifting ensures that the charge pressure level in particular is maintained, ensuring that the subsequent build-up of torque occurs faster.
Exhaust gas recirculation, particulate filter and SCR technology. Mercedes-Benz has developed a cooled exhaust gas recirculation system (EGR), particulate filter and SCR technology for use in the new Blue Efficiency Power generation of engines. This combination has already proved successful in use in vehicles from Daimler Trucks on other continents. The configuration here has been specifically adjusted to European emissions legislation, while the particulate filter, including the strategy for its regeneration, is a special European development.
The differences between the optionally available Euro V versions and the standard Euro VI emissions-level variant include the omission of the particulate filter, a reduced recirculation rate in the EGR system and a smaller EGR cooler.
In order for them to meet the Euro VI emissions level, the engines are fitted with a sophisticated emission control system. BlueTec engine technology has now been successfully used at Mercedes-Benz for around six years.
Motor Control Module. The Motor Control Module (MCM) is a further development of the MR2 control module. It comes from the 500 series of engines where it, as well as in the OM 457 series, has now been fitted more than a million times. The control module is attached by screws to the cold side of the engine, near the crankcase. The MCM control module is not only responsible for translating the demand for power governed by the driver'’ foot on the accelerator, but also for controlling and monitoring all functions of the engine, from the start and rate of injection through to the actuation of the exhaust brake.
Sensors in the control module are constantly checking factors such as oil level, the position of crankshaft and camshaft, the pressure in the common-rail system and the injectors, the turbine speed of the turbocharger, the temperature of the engine oil, coolant, fuel and charge air, the charge pressure and the exhaust gas recirculation rate. As a result of this extensive monitoring, the engine operates in the optimum range.