In the search for the optimum hybrid system for the premium auto sector, one thing was absolutely clear to development engineers right from the very earliest stages of the concept phase – a Mercedes-Benz SUV with a hybrid driveline will have to possess the same brand characteristics as every other vehicle produced by the brand. Core elements such as the highest possible levels of active and passive safety, the best in driving and user comfort, uncompromising operational safety and reliability over a long period of time and high- quality workmanship with select materials must all be at the same level as a comparable, conventionally-driven series production vehicle – combined with the highest possible efficiency.
At the same time, the hybrid powertrain system should emphasize the innovative strength and leading technological excellence of the company. These objectives necessitated a full vehicle development program – as would be required for an entirely new model range. In bringing it to production, engineers and technicians covered a total of 2.5 million miles – many of them in regions with extreme variations in climate conditions – in order to guarantee the operational safety of the system in everything from freezing arctic temperatures through the dry heat of the desert to the extreme humidity of the tropics. This work was complemented by extensive test cell development and detailed simulation using a “digital prototype”. One important objective of the development process was the optimum matching of the various sub-systems integrated within the driveline. Thanks to this “system matching” process, the user experience of the different drive modes is seamless and intuitive.
The ML450 HYBRID has a powertrain architecture that enables both purely-electric and purely-combustion engine operation, as well as a combination of both. Its two-mode hybrid technology is based on the fundamental work carried out by the “Global Hybrid Cooperation”. This industry grouping brings together the research and development efforts of Daimler AG, the BMW Group, General Motors and Chrysler.
Superior technology – two-mode hybrid is the most efficient driveline
Thanks to its two-mode hybrid technology, the ML450 HYBRID possesses considerable benefits; its performance figures are comparable to those of a classic eight-cylinder SUV with permanent all-wheel drive, while fuel consumption and emissions are up to 60 percent lower. The ML450 HYBRID, which was conceived specifically for the North American market, achieves city gas mileage of 21 mpg and highway mileage of 24 mpg. Classified as a SULEV (Super Ultra Low Emission Vehicle), the SUV currently complies with the most stringent of all U.S. emissions legislation for vehicles with internal combustion engines. At the same time, the ML450 HYBRID delivers a system performance of 335 hp and an overall torque of 381 lb-ft to guarantee a refined driving experience combined with increased powertrain comfort. The powertrain is controlled fully automatically, meaning that the driver can concentrate on the traffic and enjoy the driving experience of the ML450 HYBRID without additional operational effort. The functionality of the 4MATIC all-wheel drive also remains fully intact.
At the heart of the two-mode hybrid driveline is the transmission unit with two compact electric motors, three planetary gear sets and four clutches. This configuration enables the drive to be split through one electrical and one mechanical route. Conventional one-mode hybrid systems generally have no mechanical gearing. These systems must therefore transfer the majority of their power through the electrical driveline with higher electrical output and subsequently reduced efficiency. Their most efficient operation takes place largely at lower loads and speeds. Should power transfer also be required at higher loads and speeds, then a larger electric motor would be required. This, in turn, would use more energy and take up more space, to the detriment of efficiency. In contrast, as load and speed increases, two-mode hybrid technology transfers drive in a variable fashion from the electrical line to the mechanical driveline. In so doing, the system functions significantly more effectively under all conditions ranging from stop-and-go city traffic to rapid highway driving. It also enables the use of more compact electric motors. The “two-mode” technology also derives its name from its increased efficiency in both city traffic and on the open road.
Through the activation of the four clutches, the electrical and internal combustion engine drive is split between all four wheels of the 4MATIC permanent all-wheel drive via four fixed ratios and two variable electrical ratios in accordance with driving conditions. A range of strategies – defined by traffic conditions or driver preference – deliver the best possible figures in terms of fuel economy and performance.
- In pulling away or at low speeds up to the second fixed gear, the ML450 HYBRID drives very smoothly in ECVT 1 mode (Electric Continuously Variable Transmission).
- The ECVT 2 mode takes over at higher speeds starting at the second fixed gear ratio. Depending on the optimum operating point, either a combined or constantly variable mode will be selected.
In the first and third fixed gear ratios, both electric motors operate together either to provide additional energy for acceleration (boost) or to store the braking and drive energy (recuperation). - In the second and fourth fixed ratio, one of the electric motors is switched off in order to increase efficiency. The other electric motor is used for boost and for recuperation.
- Both compact electric motors have a high power density and are fully integrated into the transmission casing. The limited packaging space called for a different set up for each of the two motors. The first is positioned closer to the internal combustion engine, generates 83 hp and 173 lb-ft of torque and is set up specifically for performance. The motor located at the output end of the transmission generates 80 hp and 192 lb-ft of torque and is dedicated to pulling away under electric power and to provide boost.
Compact 288 volt battery with high power density
In the ML450 HYBRID, electrical energy is stored in a compact, liquid-cooled, high-voltage nickel metal hydride battery. Summary battery data:
- Battery type Liquid-cooled, high-voltage nickel metal hydride battery
- Location Below rear cargo area
- Weight (lbs) 183 Modules / cells (no.) 24 / 240
- Electrical output (kW) 45
- Capacity (kWh) 2.4 voltage (V) 288
A battery management system monitors and controls the varying charge flow during electric driving, boost or recuperation and ensures a stable charge condition. The entire high-voltage network on board the vehicle is equipped to the highest safety standards.
Sophisticated control system calculates most efficient operating mode in real time
The operation of the two electric motors and the control of the overall high-voltage network demand a sophisticated system of power electronics, which is located under the hood of the ML450 HYBRID. In order to increase its efficiency, waste heat is channeled through a separate cooling system. An integrated auxiliary power module facilitates energy exchange between the 288 volt and 12 volt vehicle networks.
A powerful computer controls the hybrid system with up to 50 million computations per second. The central electronic control unit calculates the most efficient operating mode 160 times per second, thus achieving the lowest possible fuel consumption. The control system also facilitates the control of new comfort and functionality features:
The coordinated transmission shift strategies allows for exceptionally smooth maneuvering
The “chill down” mode enables battery recovery under excessive load or high temperatures*
An adaptive driving program learns the road profile and the driving characteristics of the person at the wheel and uses this information to optimize the agility of the driving program.
“Power Launch Control” facilitates the best possible acceleration figures in boost mode*
The “Traffic Jam” feature enables pure electric driving in stop-and-go operation, even when the battery charge status is low*
* exclusive features of the ML450 HYBRID
Atkinson V6 engine with optimized fuel economy
The redesigned 3.5 liter V6 gasoline engine has been optimized in accordance with the Atkinson cycle and enables considerably improved fuel economy and emissions values at a power output of 275 hp and maximum torque of 258 lb-ft. Development engineers took advantage of some of the benefits of the Atkinson principle, whereby the intake stroke is longer than the compression stroke. To achieve this, the intake valve stays open a little longer between intake and compression, thus increasing the thermal efficiency of the engine while reducing specific fuel consumption and raw emissions. Vehicles with conventional drivelines don’t employ this process, because, aside from their consumption and emissions benefits, Atkinson engines have a tendency to produce lower torque in the lower rev range, thus offering a poorer throttle response, especially at lower engine speeds. In the ML450 HYBRID, this disadvantage is more than compensated for by the use of the electric motors for normal pulling away, acceleration and boost. The M-Class enjoys the highest levels of throttle response and agility.
Compared with the conventional 3.5 liter V6 engine, the cylinder heads, pistons and camshafts have all been modified in the Atkinson version. The camshaft control is also different. The redeveloped, controlled oil pump adjusts its demand to the actual engine requirements, thus minimizing the required feed and contributing to a reduction in fuel consumption and emissions. As the tasks performed by the alternator and starter motor are integrated into the functionality of both electric motors, these components become superfluous and can thus be removed from the ancillary belt drive. It was also possible to remove the air conditioning compressor and the power steering pump from the belt drive. In the ML450 HYBRID, these are powered electrically and can be driven independent of the combustion engine.
Operating strategies of the two-mode hybrid powertrain
- The ML450 HYBRID offers the maximum additional functionality possible from a hybrid powertrain. This includes variable hybrid driving in city and highway traffic, silent electric- only pull-away and drive, power boost for lively acceleration, energy recuperation and automatic start/stop function. These functions are activated in accordance with operating status and traffic conditions:
- At a standstill, the internal combustion engine is switched off. The functionality of the air conditioning and power steering are fully maintained through the electrical operation of the air conditioning compressor and the steering pump.
- Low-speed maneuvering, e.g. during parking, is handled using electric drive only.
- When the driver activates the gas pedal while the vehicle is at a standstill, the ML 450 HYBRID pulls away in electric-only mode. In the event that the pedal is depressed quickly, the system control recognizes the desire for heavy acceleration, activates the combustion engine and supports this with added torque from the electric motor (boost effect) to noticeably increase the responsiveness of the vehicle. The transmission also has a kick-down function, if required.
- During consistent driving, the operating point of the V6 engine shifts in favor of lower specific fuel consumption in order to increase efficiency, particularly under partial load
- Recuperation – the conversion and storage of braking energy as electrical energy – begins immediately following the application of the boost function or during normal braking maneuvers. Under heavy braking, the mechanical brakes are also applied. If required, the ML450 HYBRID provides the same degree of maximum braking power as a conventionally powered M-Class, while sustaining its exceptional handling stability.
- The automatic start / stop function deactivates the V6 engine whenever it is not required, e.g. when rolling up to a red light. As soon as the driver depresses the gas pedal, the ML pulls away under electric power, before then starting the combustion engine.
- The Silent Start function enables noiseless starting of the vehicle.
Display provides information on current operating status
The ML450 HYBRID driving experience delivers an impressive level of agility and balanced powertrain characteristics under all operating conditions. A specially designed display system informs the driver of the current operating mode. On the COMAND system display, the driver can opt to see the energy flow between the high-voltage battery, combustion engine and electric motors with the associated percentage battery charge status, or a diagram indicating fuel consumption and energy recuperation. The battery charge status can also be called-up on the instrument cluster. The conventional rev counter is replaced by a gauge that shows the total current powertrain and recuperation output.
A matter of preference – Economy or Sports mode
In order to make the extraordinary agility of the ML450 HYBRID even more apparent, the model offers a Sport mode alongside its Economy mode, activated by the “E/S” switch in the center console. This operating mode offers the characteristics of a conventional automatic transmission with eight fixed ratios and extremely short shift times. The sportier engine and transmission logic, as well as the direct drive characteristics of the powertrain make throttle response even more instantaneous. The Sport program also recognizes driving style and road profile and adapts the system control accordingly.
Classic M-Class benefits remain fully intact
The models of the Mercedes-Benz M-Class are among the world’s most successful premium SUVs and offer the highest possible active and passive safety, the best in driving and user comfort, uncompromising operational safety and reliability over a long period of time and high-quality workmanship with select materials. These attributes apply also to the ML450 HYBRID, which offers performance in all of these areas equal to that of the conventionally powered variants. The hybrid version fulfills the same crash test requirements and offers outstanding active safety with stability control systems such as ESP©, ABS and BAS, as well as 4MATIC all-wheel drive. The integration of the hybrid technology has been perfectly engineered, and requires no additional packaging space that would in any way compromise comfort or functionality. The space available in the passenger cabin is every bit as generous as before and the trunk capacity remains fully intact. From the outside, the ML450 HYBRID is recognizable by its modified hood, the BlueEFFICIENCY logo on the fender and by the HYBRID badge on its tailgate.
Manufacturing of the ML450 HYBRID is completely integrated into the production system at the Mercedes-Benz plant in Tuscaloosa, Alabama
