An under-the-radar, unofficial product debut took place out on the frigid streets in front of Cobo Hall during the 2009 North American International Auto Show, when brief drives of an ML350 Bluehybrid prototype were offered to a select few. Slated for introduction in the second half of 2009, this will be the first "offshore" application of the two-mode hybrid system that was jointly developed by General Motors, DaimlerChrysler, and BMW. (For the record, the hybrid transmission will be assembled in Europe, with final vehicle assembly in Alabama.)

It differs from the Tahoe/Escalade and Aspen/Durango applications in some fairly meaningful ways. Those first applications of the rear-drive two-mode system employ an identical hybrid transmission, mated to different pushrod V-8s, each with cylinder-deactivation technology, running unique powertrain control technologies with tuning by each company. Mercedes-Benz and BMW will share a different transmission, tailored to suit the packaging, noise/vibration/harshness, and torque specs of the German brands.

While the American's placed a high priority on preserving high towing capacities, BMW will position its X6 hybrid as a performance machine, fitting the twin-turbo V-8 and Mercedes-Benz seeks to maximize fuel economy. Hence this will be the only two-mode employing an Atkinson-cycle engine, with an adaptation of the base ML350's 3.5-liter V-6. To keep costs down, a 40-kilowatt nickel-metal-hydride battery pack was chosen, instead of the lithium-ion unit powering the S400 Bluehybrid, and it fits completely within the spare-tire well so as not to compromise utility (that is, unless you suffer a blowout that's too extreme for Fix-a-Flat to handle). The battery pack is water cooled for better performance in extreme hot-weather.

System tuning seeks merely to preserve existing acceleration (the system adds 530 pounds) and top-speeds while boosting fuel economy by 25-30 percent. A sport/economy (S/E) button on the dash switches between 8-speed stepped gearing and a more economical continuously variable shift strategy. Full electric operation is possible up to 34 mph, and a topped off battery will provide about a mile and a half of silent running. The controller will allow the battery to discharge to 35 percent before forcing recharge, which seems a bit lower than usual.