Hybrid power-trains save fuel. But the added cost means that they only begin to make financial sense if applied to luxury models, which typically attract well-to-do customers for whom fuel efficiency is not an overriding priority.
With the Active Hybrid 7, BMW is shifting the focus to performance enhancement. It gives potential customers something they can experience each time they get behind the wheel as well as saving them the odd trip to the fuel pump.
BMW's first-ever hybrid endows the 7-series sedan with the sort of straight-line speed and relaxed cruising attributes normally associated with a car running the 6.0-liter V12. This, despite the fact that it runs a reworked version of the twin-turbocharged 4.4-liter V8 found in the 750i. That it also returns rather impressive fuel-economy figures is a bonus.
BMW has developed two different hybrid systems. The first, used by the Active Hybrid 7, is a mild system that provides boost to the gasoline engine via an electric motor but won't run on electric only.
The second, used by the upcoming X6 Active Hybrid, is a full hybrid system mated to a gasoline engine with two electric motors and a larger battery. It can go for short distances on electric power only.
The basis for the Active Hybrid 7 is the new 750i. Its engine has been reworked, with more boost pressure and other changes returning an additional 40 hp and 30 lb-ft of torque. That takes output up to 440 hp at 5,500 rpm and 480 lb-ft at 2,000 rpm.
The engine gets an assist during launch and under light to moderate acceleration by a disc-shaped DC electric motor sandwiched in the housing of a new, ZF-engineered eight-speed automatic gearbox. The motor draws energy from a lithium-ion battery. The combo takes the Active Hybrid 7's overall output to 455 hp and 515 lb-ft.
Other changes include the adoption of the front axle from the four-wheel-drive version of the 7-series for added front-end rigidity and the beefed-up differential from the 760Li to cope with the added torque, said Oliver Walter, the Active Hybrid 7's project leader.
Despite the advanced technology and various modifications, which add 221 pounds to the curb weight, the driving experience is much like any other 7-series. It feels more muscular than the 750i, thanks to the added power and torque. This is a car that wafts along serenely around town and can be coaxed into a rapid cruise at ridiculously low revs.
The electric motor has been engineered to complement the V8, not replicate it. So where the engine is lacking, say, at less than 1,500 rpm, the electric motor is at its strongest. Further up the rev range, around the 5,000-rpm mark, it is the engine that is at its best. There are no discernible interruptions as the electric motor cuts in and out, merely a seamless surge of power. In fact, if you didn't have the central display to show you just how the hybrid system is operating, you'd never know the gasoline engine was being helped along at all.
BMW's claim that the Active Hybrid 7 boasts the performance of a V12 is backed up by the claimed 0-to-60-mph time of 4.7 seconds, which makes it 0.5 second faster than the 750i. It also places it in a different performance league from its main hybrid rivals, the Lexus LS 400h and the Mercedes-Benz S400 Hybrid, whose respective 6.3 seconds and 7.8 seconds reflect a milder approach.
EPA ratings have not yet been released, but European figures indicate a 15 percent improvement in fuel economy compared with the 750i at 25 mpg.
When pushed hard over winding roads, it is clear some intimacy has been lost through the adoption of a fully electric steering system--something Walter says is set to become standard on all 7-series models within its existing model cycle. There's sufficient weighting to provide confidence but a lack of feedback. Otherwise, there's not much to fault the dynamic properties of the Active Hybrid 7.
The biggest compromise in the overall dynamic character is the action of the brakes, which operate in three stages and can be difficult to modulate.
The first few degrees of brake-pedal travel engage only the regeneration system, which provides no real sense of deceleration. It is only when you push harder and the brakes are operated hydraulically that you receive proper stopping power. When that's not enough, the third stage brings serious levels of stopping power and, when required, an additional booster.
As with most hybrids now finding their way onto the market, the initial sales focus with the Active Hybrid 7 is on the North American market, which is expected to account for almost 50 percent of total volume once sales get underway in March.
Right now, we can't think of a better way of being seen to be green. (7CNJBERVUG6A)
source: autoweek
