BMW's first-ever hybrid sedan, the BMW ActiveHybrid 7 launched for 2011, was an odd beast.
Its mild-hybrid system paired a 455-horsepower 4.4-liter twin-turbo V-8 with a small electric motor, making it a bit faster than its conventional counterpart but only marginally more fuel-efficient.
For 2013, BMW has upgraded the powertrain of its largest hybrid luxury sport sedan, making the electric motor bigger, reducing the size of the engine, and giving it the full-hybrid capability of powering the car solely on electricity under some circumstances.
Same hybrid for 3, 5, 7 Series
In effect, the 2013 BMW ActiveHybrid 7 gets the same package of engine, electric motor, and 8-speed automatic transmission already used in the hybrid 5-Series and the hybrid 3-Series sedans that were unveiled last year.
The big V-8 is replaced by a 315-hp 3.0-liter twin-scroll turbocharged inline six-cylinder engine, with a 40-kilowatt (55-horsepower) electric motor fitted between the engine and an eight-speed automatic transmission that has been adapted for hybrid use.
That's significantly larger than the 15-kW (22-hp) motor fitted to the first generation car, and powerful enough to move the car on electric power alone under light loads.
The new model is part of a much larger update for the entire 2013 BMW 7-Series line.
Up to 2.5 electric miles
BMW says the second-generation ActiveHybrid 7 can run up to 2.5 miles on electricity alone, if the driver "goes easy on the accelerator," at speeds up to 37 mph.
The combined output of the engine and motor is 349 hp, and peak torque is quoted at 367 lb-ft. BMW estimates that 0-to-60-mph acceleration will be about 5.5 seconds--down on the previous model's 4.7 seconds, but still quite respectable for such a large, heavy, luxurious sedan.
The lithium-ion battery pack in the trunk uses cells from A123 Systems, which also provides batteries for the Fisker Karma range-extended electric luxury sport sedan.
MPG not yet rated
The EPA has not yet rated the 2013 BMW ActiveHybrid 7's fuel efficiency.
The earlier ActiveHybrid 7 was rated at a combined 20 mpg. We logged 24.0 mpg during a not terribly strenuous road test of the 2011 BMW ActiveHybrid 7 over our usual test route.
BMW notes that on the European cycle, the latest hybrid 7-Series achieves 14 percent higher fuel economy than the updated 740Li sedan with the same 3.0-liter turbocharged six in non-hybrid form--which itself did 20 percent better than its predecessor model on that same European cycle.
In other words, the 2013 ActiveHybrid 7 might come out with a combined EPA rating of around 27 mpg.
Who's the fastest hybrid?
That's lower than the combined 29-mpg rating of the 2012 Infiniti M35h, which its maker has touted as the "fastest full hybrid sedan" on sale.
(BMW in turn claims the first-generation ActiveHybrid 7 is "the world's fastest street-legal hybrid" of any variety, with its 0-to-60-mph time of 4.7 seconds.)
BMW notes that the 2013 hybrid 7 can call on up to 155 lb-ft of extra torque from the electric motor in "boost mode" if the driver needs extra mid-range acceleration for passing or other maneuvers.
At the other end of the economy scale, there's also a "Eco Pro" driving mode that includes a coasting function and more economical settings for accelerator, engine, and transmission mapping.
As with all hybrids, the engine switches off when the car comes to a stop and regenerative braking turns the electric motor into a generator to recharge the battery pack.
The BMW hybrids now all use a single-motor hybrid system, unlike the twin-motor packages used by Toyota and Ford. The system can either power the car or charge the battery, but it can't do both at the same time.
The 2013 BMW ActiveHybrid 7 is available only in the long-wheelbase model of the 7-Series, unlike the first-generation hybrid, which was offered in both regular and long-wheelbase styles.
Pricing hasn't been announced for the U.S. but its long-wheelbase predecessor carried a base price of $101,000.