Mercedes-Benz ML450 Hybrid

In the search for the optimum hybrid system for the Mercedes-Benz ML450 Hybrid, the engineers were already sure of one thing right from the concept phase: a Mercedes-Benz SUV with a hybrid drive would have to share the same typical brand attributes as any other Mercedes vehicle. Essential elements such as the greatest possible active and passive safety, optimum ride comfort and ease of operation, uncompromising reliability over a long period and high-quality workmanship with select materials would have to be on a par with a comparable, conventionally powered standard-production vehicle. And all combined with the greatest possible efficiency. At the same time, the drive system should be testimony to the company's innovative strength and cutting-edge technology. These objectives demanded a complete development cycle - the kind of process any new model family developed from scratch would undergo. Until the Mercedes-Benz ML450 Hybrid reached series-production maturity, the engineers and technicians clocked up a total of four million kilometres, taking in some of the world's most extreme climatic conditions to prove the system's reliability, be it in the sub-zero Arctic circle, the searing heat of arid desert regions or in the sweltering, humid temperatures of the tropics. The work was supplemented by an extensive test-rig programme and detailed simulations as part of the "Digital Prototype". An important aim during development was to optimally match the various subsystems within the powertrain. System matching enables the various drive modes to be easily used while the customer is driving.

The two-mode hybrid technology in the Mercedes-Benz ML450 Hybrid, whose drive architecture supports purely electric operation and purely combustion engine operation or any combination of the two, takes its lead from the fundamental work undertaken by the "Global Hybrid Cooperation". This group brought together research and development work undertaken by the companies Daimler AG, BMW Group, General Motors and Chrysler.

Thanks to two-mode hybrid technology the Mercedes-Benz ML450 Hybrid boasts tangible benefits: its performance figures are on a par with a conventional eight-cylinder SUV with permanent all-wheel drive while fuel consumption and emissions have been reduced by up to 60%. The vehicle designed specifically for the US market delivers 'city' fuel consumption of 21 mpg and 'highway' consumption of 24 mpg, making it the most efficient vehicle in its segment. With its SULEV (Super Ultra Low Emission Vehicle) classification, the SUV meets the most stringent emissions standard for vehicles with a combustion engine currently applicable in the US. At the same time, the Mercedes-Benz ML450 Hybrid with a system output of 250 kW (335 hp) and a combined torque of 517 newton metres guarantees a superlative driving experience combined with an enhanced level of ride comfort. The driver can concentrate fully on the traffic and enjoy the Mercedes-Benz ML450 Hybrid driving experience without having to select additional functions as the drive management system is fully automatic. The functionality of the 4MATIC permanent all-wheel drive system is also fully retained.

At the heart of the two-mode hybrid drive is the transmission unit with two compact electric motors, three planetary gear sets and four clutches. This configuration enables the power to be split into an electrical and a mechanical path. Conventional one-mode hybrid systems tend not to have mechanical ratios. As such, this kind of system transfers the bulk of the power via the electrical path with high electrical output and hence lower efficiency. It works efficiently principally at lower loads and speeds. If power also needs to be transferred at higher loads and speeds, an accordingly larger electric motor would be required. Having to power and find space for this engine would only be possible at the expense of efficiency. In response to increasing engine loads and higher speeds, two-mode technology, by contrast, variably shifts the power transfer from the electrical path to the mechanical drive path. As a result, the system is much more efficient in all situations, both in inner-city stop-and-go traffic as well as on fast motorway journeys, while also enabling smaller electric motors to be used. The efficiency boost - both in urban traffic as well as on motorway journeys - is therefore where "two-mode" technology gets its name from.

The two compact electric motors boast a high power density and are fully integrated into the transmission housing. The limited installation space means the two electric motors have to be configured differently. The first electric motor - positioned nearer to the combustion engine - develops 62 kW (83 hp) and 235 newton metres of torque and has been designed with power output and minimal losses firmly in mind. The electric motor placed at the end of the transmission develops 60 kW (80 hp) and 260 newton metres and is thus the ideal solution for moving off solely under electric power and providing the booster function.

The modified 3.5-litre V6 petrol engine has been optimized in accordance with the Atkinson process, delivering substantially better fuel consumption and emissions figures with an output of 205 kW (275 hp) and a maximum torque of 350 newton metres. The development engineers utilized some of the benefits of the Atkinson principle whereby the expansion phase is longer than the compression phase. The intake valve is kept open slightly longer between the intake and compression phases, which improves the engine's thermal efficiency while reducing the specific fuel consumption and untreated emissions. This process is not used with conventionally powered vehicles because Atkinson engines - apart from the lower fuel consumption and emissions - tend to deliver lower torque at lower revs and thus inferior response particularly at low engine speeds. On the Mercedes-Benz ML450 Hybrid this drawback is more than offset through the use of the electric motors when moving off normally, accelerating or using the booster function. The M-Class thus offers superb responsiveness and agility.

Compared with the conventional 3.5-litre V6 petrol engine, the Atkinson version features a modified cylinder head, pistons and camshaft. The camshaft timing has also been modified. The newly developed regulated oil pump adjusts the flow rate to the engine's actual requirements, thus minimizing the necessary delivery rate and helping reduce fuel consumption and emissions. Since the alternator and starter functions are integrated in the two electric motors, these two components, which are now surplus to requirements, no longer need to be connected up to the belt drive on the combustion engine. The air conditioning compressor and the steering servo pump are another two components that do not need to be powered via the belt drive. These are driven electrically on the Mercedes-Benz ML450 Hybrid and can be run independently of the combustion engine.

The Mercedes-Benz ML450 Hybrid driving experience offers an impressive mix of high agility and balanced ride comfort under all operating conditions. A specially designed display concept enables the driver to keep track of the particular drive mode. The COMAND system display can be used to either display the current energy flow between the high-voltage battery, combustion engine and electric motors including the percentage charge state of the battery, or a fuel consumption and recuperation energy graphic. The battery charge state can also be called up in the instrument cluster. The conventional rev counter is replaced by a display which illustrates the combined drive system and recuperation output at a given moment.

Source: Mercedes-Benz press

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