An hour after Mercedes-Benz unveiled its new ML450 hybrid at the New York Auto Show, we got to take our first quick drive in the first German version of a two-mode hybrid. Mercedes-Benz has been collaborating with General Motors, Chrysler and BMW for the last four years to develop the two-mode hybrid system for applications smaller than the buses that GM originally designed it for. GM has now installed the system across the board on all its GMT900 full size trucks and SUVs and Chrysler briefly offered it on the Durango and Aspen before production of those vehicles ended at the end of 2008. In 2010, Dodge will add the system to the Ram pickup trucks.
Mercedes-Benz will be building its own version of the two-mode hybrid transmission at a plant in Germany. Those transmissions will also be supplied to BMW for the X6 hybrid when that debuts. Read on for our first on the road impressions of the ML450.
Photos Copyright ©2009 Sam Abuelsamid / Weblogs, Inc.
The two-mode hybrid transmission for the Mercedes and BMW applications is based on the same mechanisms as the GM variant and in fact much of the innards are common. It features 4 clutches and three planetary gear sets. The main difference for the German variant is smaller motors that allow for a tighter package size so that it fits into the more compact vehicles. The two motors are rated at 83 hp/173 lb-ft and 80 hp/192 lb-ft. Similarly, the power electronics are repackaged but 75 percent of the internals are common with GM.
All four manufacturers did their own calibration of the hybrid system to their applications but most of the control algorithms are common. One area where Mercedes has diverged from the US applications is the addition of a new sport mode in addition to standard economy mode. Like the sport mode found in other Mercedes-Benz models, this one provides quicker response to the throttle as well as different transmission calibrations. Additionally, while the GM/Chrysler applications use four fixed gear ratios during higher speed highway operation, Mercedes has opted for eight closer spaced ratios.
Under hood, Mercedes has modified the 3.5-liter V6 that does duty in the base ML350 so that like other hybrids it now runs on an Atkinson cycle. The Atkinson cycle which uses a late intake valve closing to effectively give a shorter compression stroke than the power stroke is more efficient than the Otto cycle. However, it also looses out on low end torque. The extra torque provided by the electric motors more than compensates for that deficit. The engine itself is rated at 275 hp and 258 lb-ft of torque. The net output of the hybrid system is 335 hp and 381 lb-ft.
The only visual difference between an ML350 and ML450 is a revised hood need to clear the power electronics package that sits on top of the engine. Inside, the passenger and cargo space is unaffected by the hybrid system because the battery is buried beneath the rear cargo floor. The Cobasys supplied nickel metal hydride battery pack is liquid cooled so no extra air ducting is required.
On the roads of Manhattan, the ML450 proved itself to be one of the most refined systems we've driven yet. Alexander Dolpp, the manager of the transmission controls group at the joint hybrid development center in Troy, MI rode along for our drive. According to Dolpp, the collaboration of the four automakers worked to the benefit of all. Lessons learned from development allowed Mercedes and the partners to produce a very seamless control strategy.
In the ML, the transitions between engine on and off were only discernible by watching the power-flow display on the center stack or listening for the barely audible sound of the engine. The system allows the SUV to operate in electric drive mode at speeds up to 31 mph. Among the new control features first implemented on the ML is logic that detects when the vehicle is driven in traffic, allowing it to operate in electric mode to an even lower state of battery charge. On the battery saving side is a "chill-down" mode that allows the battery to cool down and recover after periods of extended hard driving.
The performance of the brake system in the ML was particularly impressive, showing absolutely no signs of transitioning between friction and regenerative braking. Even during light braking at low speeds coming to a stop, the ramp out of regenerative braking could not be detected. The ML450, like all other current Mercedes models, has hill-assist which uses the accelerometer for the stability control system to detect when the vehicle is on an incline. When the vehicle comes to a stop on a hill, the system will hold the brake pressure in the wheels until the driver presses the accelerator to prevent roll back.
The ML450 will go into production and go on sale late this fall. Mercedes-Benz is expecting mileage numbers of approximately 21 mpg in the city and 24 mpg on the highway. For comparison the ML320 BlueTec is rated at 18/24 while the ML350 is 15/20 and V8 ML550 is at 13/18 mpg. Mercedes hasn't set pricing yet, but Dolpp told us that it should be somewhere between the ML350 and ML550.