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Mercedes-Benz showcasing new 7.7L Euro VI natural gas engine for medium-duty commercial vehicles at IAA; replaces two earlier models

8 August 2014

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Relative (percent) CO2 emissions benefit of the new natural gas variant as compared to the 220 kW diesel engine. Benz et al.. Click to enlarge.

Among its trucks on display at the upcoming IAA in Hanover, Mercedes-Benz will showcase an Econic equipped with its new Euro VI medium-duty natural gas engine (M 936NGT), which is derived from the 7.7-liter Euro VI M 936 inline six-cylinder diesel. This engine will replace the current natural gas engines based on 900 and 400 model series engines, with 6.9 and 12 liters of displacement respectively. These are lean-burn engines and fulfill Euro V EEV emissions specification.

As a mono-fuel engine, the new engine runs on compressed natural gas (CNG) and has an output of 221 kW (301 hp) while delivering maximum torque of 1200 N·m (885 lb-ft)—the same performance as the single-stage turbocharged diesel model. CO2 emissions are up to 22% below those of diesel (during high-load conditions), even with the lower efficiency of an Otto-cycle SI combustion engine relative to diesel. Using biogas further improves the carbon footprint.

Compared to the 12-liter M 447hLAG natural gas engine, the new M936NGT shows a significant benefit in fuel consumption, with CO2 reduction of up to 50% at low loads.

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Placement of the M936NGT in the Daimler commercial vehicle engine portfolio. Click to enlarge.   Placement of M936NGT in the FEV scatter band for large bore engines. Click to enlarge.

As described in a paper presented at the 35th International Vienna Motor Symposium in May, with the advent of the new Euro VI emissions standard, Mercedes-Benz made a decision to develop a new engine program based on the new model series of the OM93x diesel engine. By doing so, Mercedes-Benz has replaced the two predecessors (which differ in displacement by more than 5 liters) with one single natural gas engine.

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Characteristics of the earlier generation of natural gas engines and the new M 936NGT (outlined in green); absolute and relative change.

Compared to the 6.9-liter 906LAG engine, the displacement of the new 936 NGT increases by 12% while torque increases by 14% and engine power by 7%. Compared to the larger 12-liter 447hLAG engine, the new 936 NGT offers a reduction in displacement of 36%.

Benz et al. Click to enlarge.

The development program of the new natural gas engine focused on offering the same performance (power and operational characteristics) as the diesel engine while maintaining the same installation space. Mercedes-Benz carried over as many components from the diesel parent as possible in an effort to reduce costs and standardize interfaces and exterior dimensions. The dimensional envelope of the original OM936 was retained to enable the natural gas engine to fit in all vehicles that already use the diesel model.

The engine is a stoichiometrically operated Otto-cycle engine with a three-way catalytic converter aftertreatment system. The combustion system utilizes EGR, follows the Miller cycle, and has a high degree of charge movement to prevent the carryover parts from exceeding the limit temperatures associated with diesel operation while delivering high operating efficiency together with low knock tendency.

The engineers did not change the basic geometry of the cylinder block and head, but adapted them for CNG operation; all diesel-specific components were removed. This included the high-pressure pump, common rail, and injectors. Mercedes-Benz also had to adapt the components required for air charging, supplying, and mixing the air.

Since both predecessor engines were lean-burn, Mercedes-benz evaluated the potential of a Euro VI lean-burn engine. However, stoichiometric operation enabled the use of a simple, robust three-way catalytic converter exhaust aftertreatment system. The system for stoichiometric operation requires no additional actuators or sensors as compared to an active SCR system for NOx aftertreatment in conjunction with a lean-burn concept.

The downside to λ=1 operation is higher exhaust gas and component temperatures; the engine this features externally cooled, high-pressure exhaust-gas recirculation.

Combustion system. Design of the combustion system was driven by the conceptual requirements of stoichiometric operation and the structural characteristics of the diesel engine base. Mercedes-Benz engineered the cavity of the piston so that the piston blank used for the diesel could also be used in this application. The geometry in the induction tract was also carried over and the combustion system was optimized based on the flow conditions in the diesel cylinder head.

To realize a high power density with moderate exhaust temperatures, the engineers opted for the Miller cycle; this is possible without torque reduction only because in the stoichiometric combustion no excess air is needed compared to the diesel, the engineers explained.

The turbocharger is designed for high boost pressures and the pre-compressed air is cooled in the charge air cooler. For identical compression end pressure this leads to a lower temperature at the end of compression but also to a lower combustion peak temperature and exhaust temperature downstream of the engine. The lower temperature level not only improves the durability of the components when it comes to thermal loads, but also has a positive effect on engine knock.

The piston cavity was optimized on the single-cylinder engine in multiple variations to increase the EGR compatibility of the combustion system while optimizing engine knock and exhaust temperature characteristics. Integrating the EGR system made it possible to considerably reduce the resulting exhaust temperature.

… A high level of turbulence leads to faster combustion speeds and safeguards the high EGR compatibility required with respect to low exhaust temperatures. Limiting or restricting the exhaust temperature in regards to a long service life and customer satisfaction in terms of component durability was the most important objective in developing the combustion system. At the same time, the piston was designed to minimize engine knock under high loads as far as possible. By integrating the EGR system, the lower peak temperatures and cylinder wall heat loss encountered not only improve combustion efficiency, but also reduce the heat exposure of the components.

—Benz et al.

Mixture formation. Air and CNG are mixed in a device prior to the subsequent downstream intake of recirculated exhaust. The engineers optimized the homogenization of the air-fuel mixture comprising fresh air, CNG, and recirculated exhaust in CFD simulations.

The new engine uses a central intake gas injection system featuring multiple valves and that is in use on the M447hLAG predecessor engine.

Turbocharging. As in the diesel, the turbocharger for the gas engine was designed with asymmetric turbine geometry. Because the mass flows of a stoichiometric natural gas engine are significantly smaller than those of a diesel, the turbocharger was designed to yield considerably lower flow parameters for the turbine and compressor.

Of note is that the response time of the turbocharger in the presence of small mass flows in conjunction with the volumetric efficiency characterize the dynamic acceleration performance of the overall engine. This is especially noticed at low operating speeds, because an engine with much bigger displacement was replaced.

Resources

  • Michael Benz, Kai Hoffmann, Marko Weirich, Hans-Otto Herrmann (2014) “The New Euro VI Natural Gas Engine for Mercedes-Benz Medium Duty Commercial Vehicles,” 35th International Vienna Motor Symposium

August 8, 2014 in Engines, Natural Gas | Permalink | Comments (0) | TrackBack (0)

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