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.
Superior technology: two-mode hybrid for the most efficient drive system
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 (340 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 four clutches are used to distribute the driving force generated by the electric motor and combustion engine via four fixed ratios and two electric variable ratios to all four wheels on the 4MATIC permanent all-wheel-drive system, depending on the driving situation. Various strategies ensure the best possible fuel consumption and performance, depending on the traffic situation or the driver's requirements:
When moving off or at low speed up to the second fixed gear, the Mercedes-Benz ML450 Hybrid operates in ECVT 1 mode (Electric Continuously Variable Transmission), delivering a very comfortable, smooth ride.
Compact 288-volt battery with high power density
The electrical energy in the Mercedes-Benz ML450 Hybrid is stored in a compact, liquid-cooled high-voltage nickel-metal hydride battery.
Battery data at a glance:
Type: Liquid-cooled high-voltage nickel-metal hydride battery
Sophisticated management system computes the most fuel-efficient operating mode in real time
Operation of the two external rotor magneto motors and the management of the entire high-voltage onboard network requires complex control electronics housed in the engine compartment of the Mercedes-Benz ML450 Hybrid. Waste heat from the electronics is dissipated via a separate cooling circuit to increase efficiency. An integrated Auxiliary Power Module enables power to be exchanged between the 288-volt battery and the 12-volt onboard network.
A powerful computer controls the hybrid system with up to 50 million computing operations a second. The electronic control unit calculates the most efficient operating mode 160 times a second, thus ensuring the lowest possible fuel consumption. The management system also supports new functions and comfort/convenience features:
The transmission's coordinated shift strategies ensure judder-free manoeuvring
The modified 3.5-litre V6 petrol engine has been optimised in accordance with the Atkinson process, delivering substantially better fuel consumption and emissions figures with an output of 205 kW (279 hp) and a maximum torque of 350 newton metres. The development engineers utilised 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 minimising 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.
Operating strategies of the two-mode hybrid drive
The Mercedes-Benz ML450 Hybrid offers the full range of extra functions offered by a hybrid drive. These include variable hybrid mode for city and motorway driving, silent, purely electric moving off and motoring, the "booster function" for fast acceleration, recuperation and the start/stop function. These functions are activated depending on the operating state and traffic situation:
While at a standstill, the combustion engine is switched off. The air conditioning and power steering functions are fully retained thanks to the electric drive of the air conditioning compressor and the steering servo pump.
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.
The choice is yours: Economy or Sport mode
To ensure you can experience the extraordinary agility of the Mercedes-Benz ML450 Hybrid even more emphatically, apart from Economy mode, the model comes with a Sport mode, which is activated using the "E/S" button in the centre console. This operating mode provides the handling of a conventional automatic transmission with eight fixed ratios and very short shift times. The response is even more spontaneous thanks to the decidedly sporty engine and transmission control unit and the direct through-drive in the powertrain. Sport mode also recognises the driving characteristics of the driver or the particular road profile and adapts the system management function accordingly.
Classic benefits of the M-Class remain unchanged
The Mercedes-Benz M-Class models rank among the most successful premium SUVs and offer 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. These attributes naturally also apply to the Mercedes-Benz ML450 Hybrid, which in these respects offers performance on a par with the conventionally powered variants. The hybrid version meets the same crash requirements and offers outstanding active safety thanks to the dynamic handling control systems ESPÂ®, ABS or BAS and the 4MATIC permanent all-wheel-drive system. The hybrid technology has been integrated perfectly, without taking up any additional installation space which would compromise comfort or practicality. The space in the passenger compartment is unchanged; the boot capacity is as large as ever. The only external clues on the Mercedes-Benz ML450 Hybrid are the redesigned bonnet, the BlueEFFICIENCY label on the wings and the HYBRID lettering on the rear.
The Mercedes-Benz ML450 Hybrid is produced at the Mercedes-Benz plant in Tuscaloosa, Alabama, in the United States.
Technical data (US figures)