BMW X5 Plug-In Hybrid Prototype: We Drive Future Electric SUV Page 2

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BMW X5 e-Drive plug-in hybrid prototype, test drive, Woodcliff Lake, NJ, April 2014

BMW X5 e-Drive plug-in hybrid prototype, test drive, Woodcliff Lake, NJ, April 2014

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A 70-kilowatt (95 hp) electric motor producing 185 lb-ft of torque is located between the engine and an adapted version of BMW's standard eight-speed automatic transmission.

The electric motor replaces the transmission's torque converter, and has a clutch on either side. A large engagement clutch sits between the engine and motor, plus a smaller "creeper" clutch between the motor and transmission to allow for smooth idle creep in traffic.

Output of the combined powertrain is expected to be 270 hp or higher, and 300 lb-ft of torque or higher.

BMW X5 eDrive plug-in hybrid prototype

BMW X5 eDrive plug-in hybrid prototype

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Mechanical AWD

Importantly for those in snowy climates, the standard BMW xDrive all-wheel-drive stays intact.

The xDrive system electronically controls torque distribution among the four wheels based on driving conditions, but sends power to all four wheels mechanically.

That system was used by the old Ford Escape Hybrid in its 4WD variant, but it differs from other hybrid SUVs.

The current Toyota Highlander Hybrid, for example, employs an all-wheel-drive system that drives the front wheels mechanically but uses an additional electric motor on the back axle to power the rear wheels when needed.

'Max eDrive'

As befits a standard model-line extension, the X5 plug-in hybrid resembles a conventional X5 in every way possible--including the three standard BMW drive modes: Eco Pro (for maximum energy efficiency), Comfort (the default mode), and Sport (for faster powertrain response).

The addition of an electric motor to the powertrain, however, requires the addition of further options.

The X5 plug-in's default operating mode is currently to run in blended hybrid mode, using battery energy to run electrically where possible up to 42 mph, and the gasoline engine when more power is needed.

But BMW provides a "Max eDrive" button on the console, which serves as the equivalent of an "EV" button to keep the plug-in X5 running purely on battery energy.

Even then, as development engineer Dennis Paul noted, a "rescue function" engaged by pushing the accelerator past its normal stop right to the floor will switch on the engine for maximum power in emergency driving situations. We tested it, and it does just that.

BMW X5 eDrive plug-in hybrid prototype

BMW X5 eDrive plug-in hybrid prototype

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'Save Battery'

Running in "Max e-Mode," once the battery pack's roughly 6 kilowatt-hours of usable energy are depleted, the X5 e-Drive reverts to operating like a conventional hybrid again.

There's also a "Save Battery" button, that kicks the car into blended hybrid operation immediately, retaining the battery's state of charge for later use--in a central-city zone that's restricted to zero-emission vehicles, for instance.

BMW has not included a function to recharge the battery using the engine, however--which is highly inefficient, but offered by at least one other plug-in hybrid maker (Honda).

The company did say that it was possible to recharge a depleted battery using regenerative braking if the route included long downhill stretches or other opportunities to recapture energy.

Smooth, decently integrated

For a prototype 18 months or more from its U.S. launch, the X5 plug-in hybrid development vehicle we drove was pleasantly refined and seemed well on the way to showrooms.

There were a few lumpy transitions among engine, motor, and regenerative braking--the control algorithms for blending those disparate mechanical functions in a way that's not disturbing to passengers may be the most challenging part of developing a hybrid vehicle.

But overall, the two electric and gasoline characteristics of the plug-in X5 felt suitably complementary.

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