|New medium-duty Euro VI engine. Click to enlarge.|
Mercedes-Benz unveiled a new generation of medium-duty engines with the series designation OM 93x. Together with the new heavy-duty OM 470 engine, Daimler is expanding its “BlueEfficiency Power” engine portfolio into a comprehensive range complying with the impending Euro VI emissions standard. The OM 471 engine, presented in 2011 and also part of the portfolio, is already in service on long-haul operations in the new Mercedes-Benz Actros. All the engines comply with the future Euro VI emissions standard, which will be mandatory in the EU from 2013/14.
One of the technical highlights of the new OM 93x is the variable valve timing, which features for the first time in a diesel engine, Mercedes-Benz said. The engines form part of a modular system with a high degree of shared components.
The medium-duty OM 934 and OM 936 engines, with four and six cylinders and a displacement of 5.1 and 7.7 liters respectively, cover a power range from 115 kW (156 hp) to 260 kW (354 hp). In the long term, the two engines will replace the engines of the 900 series launched in 1996, with a production volume to date of almost one million units.
The heavy-duty OM 470 engine with six cylinders and a displacement of 10.7 liters spans the power range from 240 kW (326 hp) to 315 kW (428 hp); it follows the OM 471 with a displacement of 12.8 liters.
The new engines are designed for light- to heavy-duty distribution transport and for service in light- and medium-duty construction and long-haul transport. The OM 936 is also designed for deployment in urban and rural-service buses in vertical and rural-service buses in vertical and horizontal configuration.
The new OM 93x-series engines feature ignition pressures of more than 200 bar and an injection pressure of up to 2400 bar. The engines also feature a cooled exhaust gas recirculation system.
The new OM 93x series uses exhaust aftertreatment technology that was premiered a year ago in the heavy-duty engines of the new OM 471 series: a closed-loop particulate filter and BlueTec engine technology with AdBlue SCR catalytic conversion.
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.
The new engines offer specific output of just under 34 kW (46 hp) per liter of displacement. The four-cylinder engine, with a maximum output of 170 kW (231 hp) and up to 900 N·m of torque is 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 N·m from a displacement of 7.7 liters, find their way into an output class that could previously often only be achieved with a displacement of more than 10 liters.
The new engines features a crossflow cylinder head with 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.
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 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 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.
The new engine series features a variable camshaft phaser (VCP)—the first adjustable exhaust 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.
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 advanced 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 optimized 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.
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, while 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 N·m 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.
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.
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 injection system is also the basis of the engines’ cold-start capability, enabling them to start reliably and without preglow, even at temperatures well below freezing point.
The turbocharging of the new engines is customized 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.
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.
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 optimized 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.
The efficiency and performance of the new engine series is closely related to the electronic control of both the engine and the exhaust gas aftertreatment system, managed by the the Motor Control Module (MCM) and Aftertreatment Control Module (ACM). The electronic systems calculate the ideal injection point, the correct charge pressure and the best operating conditions for the aftertreatment system.
Installation in other brands and models of Daimler Trucks on other continents is planned in due course. It is also planned to use the four- and six-cylinder engines in off-highway applications as industrial engines. Series production of the urban bus variants at the Mannheim engine plant will start shortly.
The Mannheim plant operates according to the “synchronous factory” principle. This means that the three production areas of foundry, machining and assembly function as an integrated system as part of a continuous production flow. Global supplier management is also coordinated in Mannheim.