Quick Spin: 2011 BMW ActiveHybrid 7
by Sam Abuelsamid (RSS feed) Mar 1st 2010 at 11:01AM
2011 BMW ActiveHybrid 7 - Click above for high-res image gallery
BMW used the most recent Los Angeles Auto Show to display the production version of its new Activehybrid 7, the first product of its collaboration with Mercedes-Benz to produce a new mild hybrid system. While BMW and Mercedes partnered with Chrysler and General Motors on the more sophisticated two-mode hybrid that is now launching in the X6, the German's have also created an alternative lower cost and modular system. This mild hybrid will likely become the primary type used going forward for both of the German companies.According to BMW, the point of the mild hybrid system was to create a balance between performance, weight and efficiency. A typical strong hybrid system adds about 300-400 pounds on to the mass of the base vehicle, much of it attributable to the battery pack as well as the complex power-split transmission. The mild hybrid 7 weighs just 165 pounds more than a 750i. We had a chance to take a short drive around downtown Los Angeles in an ActiveHybrid 7 and you can read about it after the jump.
Photos Copyright ©2010 Sam Abuelsamid / Weblogs, Inc.BMW told us that they developed the mild hybrid system to avoid some of the trade-offs inherent in other other systems. Among those are a significant loss of cargo space due the need to package the battery. In the case of the 7, the use of a new lithium ion battery has allowed the engineers to stuff it into a corner of the trunk. The hybrid only sacrifices 1.4 cubic feet of volume and reportedly can still accommodate four standard golf bags. The smaller battery only adds 62 pounds over the rear axle.
Like many automotive lithium ion applications, the unit in the 7 is actively cooled in order to maintain the battery at its optimum operating temperature. The BMW engineers have routed refrigerant from the air conditioning system through the battery to prevent it from overheating.
The only other hardware that takes extra space in the car is the power electronics. These are bolted directly to the side of the block near the transmission. Putting the electronics adjacent to the transmission allows for shorter connections which helps to alleviate some of the potential electromagnetic compatibility problems that can crop with these complex drive systems.
Aside from the obvious efficiency advantages of less weight, mitigating additional bulk certainly helps to maintain BMW's dynamics. Since our drive loop only consisted of about a half hour of driving in downtown LA we didn't really have the opportunity to exercise the handling of the 7, but from what we could tell, it didn't really feel any different from a standard 750i, which is to say it's outstanding for its massive size.
The big difference between the mild hybrid and the two-mode strong hybrid is in the transmission. The mild system is based on the new ZF 8-speed automatic transmission. BMW's powertrain engineers have been able take advantage of the electric motor's instant low-speed torque to make changes to the gearing on the hybrid, thus lowering engine speeds and improving efficiency. The 15 kW motor is sandwiched between the engine and torque converter. Because the electric motor provides much of the torque multiplication normally achieved through the fluid coupling, the engineers were able to use a smaller torque converter.
With 55 horsepower and 65 pound-feet more than the 750i, the hybrid also gets the stronger rear axle from the V12 powered 760 in order cope. Other changes include a modified fuel tank, one inch larger and more aerodynamic wheels and an electric air conditioning system to maintain temperatures during engine shut-off. Interestingly, the 7 did not get electric power steering (EPAS). BMW claims this was because there was no EPAS available with a large enough motor for the weight of the car. This seems dubious since the hybrid X6 (as well as GM's two-mode hybrids) is more than 600 pounds heavier and has just such a system.
One interesting characteristic brought about by the absence of EPAS is apparent when the engine shuts off and you turn the wheel. Tugging the wheel causes the engine to re-start immediately to drive the hydraulic assist pump even if the brake is still applied. When the wheel is first turned, it feels very heavy as would be expected without any boost, and then it quickly lightens up as the engine starts.
At startup, the ActiveHybrid 7 feels pretty much like any standard 750i since there is no silent start (engine off) mode. The auto stop functionality doesn't kick in until the engine has warmed up and the car has actually moved. Once we had pulled away from the LA convention center, and then stopped at a traffic light, the engine shut itself down silently and seamlessly.
While driving the lower quadrant of the tachometer features an automatically reconfiguring LCD gauge. When accelerating or cruising, it shows an analog display of the instantaneous fuel mileage. Lifting off the accelerator switches this to a blue arc indicating the amount of regenerative braking. When the big V8 switches off, it simply pulses blue. Many hybrid vehicles exhibit an inconsistent relationship between brake pedal apply force and actual deceleration as regenerative and friction braking are blended. Fortunately we didn't experience any of this kind of behavior during our jaunt.
Releasing the brake pedal causes the engine to fire up instantly. BMW also provided a couple of ActiveHybrid X6s for comparison so we took advantage of the opportunity. The more powerful two motor hybrid system in the X6 turned the engine over absolutely seamlessly until it fired. The starts in the crossover were completely imperceptible. The single 15 kW electric drive unit in the 7 clearly had to work considerably harder and the engine re-start was not nearly as seamless. It was nowhere near what we had experienced in many older hybrid systems, but it was not up to the standard set by the vehicles with the two-mode system.
When accelerating from a stop, the car we drove actually felt slightly sluggish right off the line in spite of the fact that the motor's instant torque should have had it feeling even more aggressive. The feeling was similar to turbo-lag and may have been an issue with this pre-production model. On the other hand accelerating to pass when already in motion provided all the thrust you would expect of a car with 455 hp.
Just as others have noted when evaluating high end hybrids like the Cadillac Escalade, Lexus LS600h and BMW's own X6, the purpose of putting a hybrid system in a luxury sedan, especially one with a $100,000+ price tag is questionable. Undoubtedly, BMW could have paired the system with a smaller less powerful engine instead of the big twin-turbo V8. That's precisely what Mercedes did with the S400 hybrid which uses the same mild hybrid system with a 3.5-liter V6. The ActiveHybrid 7 does accelerate to 60 mph 0.5 seconds faster than the 750i and gets 2L/100 km lower fuel consumption.
The final EPA numbers are not ready yet, but the EU combined mileage goes from 20.6 mpg to 25.0 mpg. BMW claims that the reduced engine speeds facilitated by the eight speed transmission and the electric boost allows a 2-liter reduction across the board. If that holds true here the 7 should go from the current 17 mpg combined to about 20 mpg. That's not an insignificant amount for a car with this size and performance.
The ActiveHybrid 7 goes on sale in the late spring of 2010 in the U.S. and this market is expected to absorb about 45% of the worldwide volume. While the U.S. is the main market for the hybrid it is still likely to only account for a tiny number of sales, amounting to about five percent of the total. The hybrids will be available in standard and long wheelbase forms with a starting price of $103,175 and $107,075 respectively.
Photos Copyright ©2010 Sam Abuelsamid / Weblogs, Inc.
BMW (Black Mans Wagon). Brothers love the Beamers this 7 series is no exception.
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