BMW i3 Electric Car: ReX Range Extender Not For Daily Use?

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The upcoming 2014 BMW i3 electric car is generating a lot of quiet excitement--especially for its optional "ReX" range extender, a tiny two-cylinder engine that fits under the rear deck to power a generator.

The i3 will be only the third high-volume dedicated battery electric car on the market, after the Nissan Leaf and Tesla Model S.

And it's the first one that will offer the option of a gasoline-powered range extender like the one built into every Chevrolet Volt.

The company has shown prototypes of the BMW Concept i3 five-door hatchback (with no center pillar and rear-hinged rear doors) and an i3 Concept Coupe more recently.

The production version will debut at the Frankfurt Motor Show this September. First deliveries will begin in Germany around November, with U.S. deliveries following early in 2014.

Defined as zero-emission

The BMW i3's 21- to 22-kilowatt-hour lithium-ion battery pack is expected to provide a range of 70 to 100 miles.

Combined with a gas tank of 2 to 3 gallons, the ReX engine and generator almost double the i3's range--though a tank that small offers less range than the battery itself provides.

That's deliberate, because (pending final verification) it will keep the BMW i3 designated as a zero-emission vehicle under California's arcane emissions laws.

Buyers seem interested in the idea of an electric BMW with up to 100 miles of electric range that also happens to offer an optional range extender for longer trips.

And BMW has promised the i3 will perform and handle as well as any other "ultimate driving machine," regardless of its electric powertrain.

BMW i3 Concept MkII

BMW i3 Concept MkII

Electric vs gasoline

But those potential i3 owners need to understand a critical point: The BMW i3 may not operate like a Chevy Volt.

That is, its performance in range-extending mode may be more limited than when it is running on battery power.

One of the core principles behind the design of the Volt was that its performance should be identical whether in battery-powered or range-extending mode.

BMW says the range extender of the i3, on the other hand, is designed not for long-distance travel but purely as a short-term stopgap to get drivers to the next recharging location.

Will performance vary?

And while the BMW i3 is expected to have an electric motor producing 125 kilowatts (170 hp) of peak power output driving its rear wheels, the little range-extending inline twin (derived from a motorcycle engine) is likely to produce considerably less.

BMW has not yet given the precise displacement or power output for the ReX, nor the capacity of the generator it will drive.

BMW i3 Coupe concept

BMW i3 Coupe concept

But a good metric will be to see whether the generator develops at least half the output power of the traction motor.

The Volt's 83-hp 1.4-liter four-cylinder range extender runs a generator with a peak output of 55 kW (74 hp), which is half the peak power of the 111-kW (149-hp) traction motor that turns its wheels.

The Volt "buffers" power draw through the battery pack, so except under the most extreme circumstances, the performance of the Volt with its engine running a generator to power the traction motor is essentially the same as one running on battery power alone.

BMW's highest-performance 800cc vertical twin motorcycle engines carry ratings of 85 to 90 horsepower, so it's possible that a ReX range extender adapted from one of those engines could provide the same ratio.

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