The perfect hybrid powertrain

• Compact 288-volt battery with high power density
• Sophisticated management system computes the most fuel-efficient operating mode in real time, 160 times a second with up to 50 million computing operations
• Consumption-optimised Atkinson V6 petrol engine developing 205 kW (279 hp)
• Classic benefits of the M-Class such as economy of space, high active and passive safety, ride comfort or the 4MATIC permanent all-wheel drive system are fully retained

In the search for the optimum hybrid system for the premium motor car, 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 ML 450 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 ML 450 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 ML 450 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 ML 450 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 ML 450 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 
ML 450 HYBRID operates in ECVT 1 mode (Electric Continuously Variable Transmission), delivering a very comfortable, smooth ride.
• At higher speed from the second fixed ratio, ECVT 2 mode is used. Depending on the optimum operation point, a combined or continuously variable mode is selected.
• In the first and third fixed gear both electric motors work synchronously, providing either additional energy for acceleration (boost) or storing the braking and coasting energy (recuperation).
• In the second and fourth fixed gear, one electric motor is switched off to improve efficiency. The second electric motor is used for the booster function and recuperation.

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 (85 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 (82 hp) and 260 newton metres and is thus the ideal solution for moving off solely under electric power and providing the booster function.

Compact 288-volt battery with high power density

The electrical energy in the ML 450 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
Mounting location
Vehicle rear
Weight (kg)
83
Modules / cells (quantity)
24 / 240
Electrical output (kW)
45
Capacity (kWh)
2.4
Voltage (V)
288

A battery management function monitors and controls the various charge flows during electric motoring, boost or recuperation to ensure a stable charge state. The entire high-voltage onboard network complies with the most stringent safety standards.