BMW ActiveHybrid 7 Review

By Jonathan Spira on 1 July 2010
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BMW’s quest for fuel efficient engines goes back to the BMW 132 aircraft engine of the 1930s.  More recently, for the 1972 Olympic Games in Munich, BMW provided a fleet of orange-colored BMW 1602 sedans that used a battery-powered electric motor.

A few months ago we reviewed the world’s fastest gas-electric hybrid, the BMW ActiveHybrid X6, and that title has now passed to the BMW ActiveHybrid 7.  This may sound like an odd title for a hybrid but, as we found out in our tests, it proves one can have fun and enjoy better fuel economy at the same time.

The previous version of the BMW 7er Series (the 7er Series was all new for model year 2009) was available in a limited-edition, duel-fuel hydrogen powered configuration and the engineers at the company were hard at work to create additional, green options for their customers.

Enter the ActiveHybrid 7.  Unlike the ActiveHybrid X6, which is a full hybrid that has an electric-only means of propulsion, the ActiveHybrid 7 is a mild hybrid, an approach that adds less weight and complexity (and cost) to the vehicle.  The mild-hybrid platform was jointly developed by BMW and Mercedes-Benz as an outgrowth of a cooperation that began in 2005 and Mercedes uses the same platform in the S400 that we reviewed last month.

A mild hybrid is a gasoline-fueled vehicle equipped with an electric motor that allows the engine to be automatically shut off whenever the car is braking, coasting, or stopped. A mild hybrid also uses brake energy regeneration to recover energy that would normally be wasted as heat through the brakes.  The integrated electric motor acts as a generator when coasting or applying the brakes and feeds electric power into the battery.    As a result, they do not require the same level of battery power and also do not achieve the same increase in fuel economy as full hybrids.

The ActiveHybrid 7 has an upgraded twin-turbo direct injection V-8 engine with a three-phase synchronous electric motor, giving it a combined output of 455 hp and maximum torque of 151 pound-feet.  The electric motor is positioned between the engine and the torque converter and power is transmitted via a new eight-speed transmission.

It also has almost the same amount of luggage space as the gasoline-only version as the 120-volt battery occupies only one cubic foot.

When the driver accelerates, electricity (from the battery) is fed back into the on-board power network and provides power directly to the rear wheels, relieving the combustion engine from this task.

All this translates into 0-60 mph in just 4.7 seconds with 15% better fuel economy than the (slower) 750i.

The ActiveHybrid 7’s transmission uses start-stop technology to avoid engine idling when stopped, although other systems such as air conditioning remain operational as power is supplied through the lithium-ion battery.  Similar to the ActiveHybrid X6, it is equipped with light-alloy Aero wheels that reduce drag.

The car’s Central Display (see video below) can provide a detailed real-time view of the interaction between the V-8 engine and the electric motor including the current charge level of the battery and the real-time flow of energy.  A bar graph that shows the current efficiency over the past fifteen minutes is also available.

In terms of safety, the high-voltage system is fully insulated and the battery is protected by a steel housing.  In the event of a collision, the entire high-voltage system is immediately switched off.

After all this, you may wonder how the ActiveHybrid 7 drives.  I spent a week with the ActiveHybrid 7, driving it on twisty mountain roads, highways, and city streets, and I was hard-pressed to tell the difference between it and the V-12 powered 760Li when hitting the accelerator.  The start-stop feature is reasonably inconspicuous although a quick move from brake to gas can result in a bit of a jerk.  Thanks to the tiny battery and mild hybrid system, the 7er’s weight distribution was preserved, which bodes well for handling (this is one of the reasons BMW uses different hybrid systems in the X6 versus the 7er).

The firm and responsive ride was adjustable via the four driver-selectable settings in the Driving Dynamics Control system.  Set for sport, the ActiveHybrid 7 attacked twisties with a vengeance.  On some pot-holed New York City streets, I didn’t notice the bumps go by.

The fuel economy I got during that week, however, served as an excellent reminder that I was, indeed, in a hybrid vehicle.  It consistently got 3 mpg more than the 750i in city driving, averaging 20 mpg.  My overall fuel economy for the week was 24.5 mpg.

Best of all, the ActiveHybrid 7 qualifies for a $900 IRS Alternative Motor Vehicle Credit.

2011 BMW ActiveHybrid 7
Base price/price-as-tested ­­$102,300
Drivetrain
Engine 4.4-liter/435 hp/twin-turbo V-8 and 20 hp electric motor
Transmission 8-speed automatic
Curb weight (lbs) 4795
Wheelbase (inches) 126.4
Length x width x height (inches) 205.3 x 63.4 x 59.3
0-60 mph (seconds) 4.7
City/highway fuel economy (mpg) 17/26

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