Driven: 2010 BMW ActiveHybrid 7
By Rex Roy
December 24, 2009
—Los Angeles, California
Automotive analysts knows that we're driving toward a transportation future powered by multiple fuels. Only a politician blinded by party loyalty or major donor paybacks could dispute this reality. Over the next ten to fifteen years we'll see battery-electric powered vehicles sharing the roads with diesels, multi-fuel-capable internal combustion engines, and a variety of hybrids. Surprising to some, many cars will still run only on plain, old gasoline.
BMW's 2010 product line exemplifies the early stage of this trend. Munich's gasoline engines feature efficient direct injection and turbocharging. Diesels are available across the board in Europe and in several U.S. models. BMW recently introduced its ActiveHybrid X6, the crossover that uses a powerful dual-mode hybrid system to improve city and highway fuel economy. Most recently at the LA Auto Show, the U.S. version of the ActiveHybrid 7-Series appeared in something other than a press release.
If one counts the battery-powered Mini E, BMW currently offers five vastly different powertrain solutions. The mild-hybrid approach to the ActiveHybrid 7 is the newest alternative from BMW.
ActiveHybrid 7 is the car's name, whether it's the standard- or long-wheelbase body. BMW eschewed naming that would have resulted in a 750ih or some such derivation in order to create more awareness of its carbon-reduction efforts. Badging inside and out, as well as the car's Bluewater Metallic paint, politely calls attention to this 7-Series' greenness. The car's nineteen-inch aerodynamically tuned ten-spoke wheels are also unique to the model.
These superficial changes overlay a heavily modified 750i and 750Li. The ActiveHybrid 7 achieves 15 percent better fuel economy than its donor chassis and delivers faster acceleration. BMW claims a 4.8-second 0-60 run, 0.3 to 0.4 seconds faster than the non-hybrid 750i and 750Li. On the streets of LA where we drove the car, the Big 7 certainly felt fast.
The direct-injected, twin-turbo, 4.4-liter V-8 gained 40 horsepower partially due to drops in parasitic losses. There is no alternator or air-conditioning compressor for the engine to spin, helping the hybrid put out 440 horsepower.
An additional twenty horsepower comes from the ActiveHybrid 7's compact electric motor. The motor is sandwiched between the 4.4-liter's flywheels and the ZF eight-speed automatic transmission's torque converter. This new rear-mounted gearbox is sourced from the 760Li, and provides an additional two gears over the 750i's six-cog gearbox. Additionally, the gearbox accepts being back-driven (absorbing power from the drive wheels) so that the electric motor can also function as a generator to facilitate regenerative braking duties.
Helping offset the additional weight that comes with this mild-hybrid system, the motor ahead of the transmission also acts as a traditional starter and alternator.
Total horsepower for the mild-hybrid package is 455, not the 460 your non-engineer author expected. BMW's Dave Buchko explained that because the peak horsepower for the engine and motor occur at different rpm, the two peak horsepower figures cannot be added together to arrive at the final sum.
As is the case with all electric motors, they add much more torque than horsepower. In this application, torque is up 165 pound-feet compared to a standard 7. The ActiveHybrid 7 uses the differential assembly from the V-12 760Li to handle the added twist 516 pound-feet represents. To satisfy the curious, the V-12 makes 552 pound feet.
The motor is not powerful enough to allow for electric-only driving, but does add enough oomph to help the 4800-pound car move briskly off the line with literally no hesitation. The assist is so significant that it allowed BMW engineers to change the transmission gearing and raise final drive ratio considerably.
The result? Across the board fuel savings, regardless of speed. The bump in economy comes from lowering the engine's RPM at any given road speed. For example, in top gear at 1000 rpm, the ActiveHybrid 7 loafs along at about 50 miles per hour. At the same speed, the V-8 in a non-hybrid 750i is turning 1600 rpm. The gearing change made possible by the additional torque of the electric motor. Like most other BMWs, top speed is limited; the electronic nanny on this model steps in at 150 mph.
Another fuel-saving characteristic of the ActiveHybrid 7 is the engine's auto-start/stop. Pull to a red traffic light and the engine smoothly shuts down. Of course, all interior amenities are maintained. Because BMW retained the standard 7-Series hydraulic steering, the wheel goes stiff from loss of boost. A tug on the rim causes the engine to instantly re-fire, making the car ready for action. Releasing the brake pedal causes a similar reaction from under the hood. During the miles we drove, we never caught the car wrong-footed.
The ActiveHybrid 7's lithium-ion battery pack (stored between the rear wheels in a position that maintains the car's 50/50 weight distribution) keeps the car alive when the engine is not. The 120-volt/400 watt-hours battery is about the size of a large encyclopedia and weighs barely 60 pounds. Storing the battery causes a net loss of only one cubic foot from the 7-Series' trunk.
For brevity’s sake, what follows is not a complete driving experience of the ActiveHybrid 7, because much would be replicated from our experience in the all-new 2009 7-Series. We'll instead focus on the differences between the hybrid 7 and the gas-only car, allowing us to sidestep discussions of items like the improved iDrive and the Rube Goldberg transmission lever.
Every time a driver starts the ActiveHybrid 7, the engine fires. It settles into a barely audible and hardly felt V-8 cadence that your author finds melodious, relaxing, and dripping with performance potential. Pulling away from a standstill is simply effortless. Unlike a typical luxury car, there's little perceptible torque-converter slippage. The sense is a more linear thrust.
The ZF eight-speed quickly upshifts through the gears. Engineers from BMW note that the ActiveHybrid 7 spends most of its life in just two gears, second and eighth. First is rarely called for, and the other gears above second are simply transitory. Regardless, the tall gearing of the rear axle maintains a relaxed engine speed barely above idle.
Even though the ActiveHybrid 7 weighs more than a conventionally powered 7-Series, it's tough discern the extra heft. The chassis easily handles the poundage. Given the artificial feel of so many electric power-steering units, we're happy BMW stuck with a great feeling hydraulic rack in this case.
The immediate torque delivery and the quiet burble of the V-8 give this refined luxury sedan some gentle muscle car overtones—kind of like those secondary and tertiary tastes one enjoys in a good red wine.
After driving the ActiveHybrid 7, your author remembered his first drive in a mild-hybird, the Saturn Vue Greenline. While fascinating from the standpoints of its technology and engineering, the Greenline was comparatively miserable to drive. Slow. Noisy. Unrefined. The Vue's view of the future wasn't an exciting one, but as usual, technology has triumphed.
One drive in an ActiveHybrid 7 proves it.
2010 BMW ACTIVEHYBRID 7
Engine: Twin-turbo V-8, 4.4 liters, 32v
Output: 455 hp / 516 lb-ft
Transmission: Eight-speed automatic
0-60 MPH: 4.8 seconds
Top Speed: 155 mph (electronically limited)
Weight: 4509 lb
Base Price: $103,125