The ActiveHybrid relies on a 20-horsepower, 155 pound-foot electric motor integrated with an automatic transmission to enable fuel efficiency gains of up to 15 percent over a new nonhybrid 750i. Motivated primarily by the 5.0-liter direct injection twin-turbo V-8 seen in the new 750i and current X6, the electric motor eliminates the the starter motor and alternator, increases the efficiency of the stop/start function seen on an increasing number of BMWs, and uses regenerative braking to more than recoup the parasitic power losses from items like air conditioning and infotainment systems.
Eliminating the alternator, especially in a luxury vehicle like the 7 Series, makes a significant contribution to energy savings. The more comfort/convenience items used in a vehicle, the more drag a traditional alternator places on the engine, consuming more gasoline in the process. By placing these electrical loads solely on the vehicle's battery, which is now recharged during coasting and braking, as well as driving the air conditioning compressor electrically instead of off the crankshaft, significant gains in fuel economy are realized. Energy that used to literally go up in the air as heat during braking is now recaptured and used to power many items in the vehicle.
BMW was a participant in the group that worked on incorporating electric motors into a vehicle's transmission for hybrid operation. GM, Chrysler, and Mercedes also participated, and some products (such as the Chevy Tahoe Hybrid and Dodge Durango Hybrid) are already on the road. Mercedes' forthcoming S400 Hybrid appears to use the same transmission-mounted electric motor approach.
Energy saving technologies such as these found on the 7 Series ActiveHybrid Concept will be necessary if luxury carmakers expect to continue selling powerful gasoline vehicles in significant numbers in the United States. The U.S. market, having enjoyed some of the cheapest gasoline in the world, has become the largest market for big-engined gasoline luxury vehicles, and automakers face engine downsizing, diesels, or fuel-saving technologies like gas/electric hybrids if their products are to remain competitive.--Colin Mathews