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.

BMW i3 electric car undergoing winter testing, February 2013

BMW i3 electric car undergoing winter testing, February 2013

Fine in most cases

The i3 with the range extender will surely run just fine on level pavement, where only a fraction of total power is needed to maintain a steady speed.

For short bursts of acceleration--on-ramp acceleration to join high-speed freeway traffic, say--the i3 will likely dip into its battery pack temporarily for a bit of extra energy.

The big, and so far unanswered, question will be whether a range-extended i3 will suffer performance shortfalls in other, more demanding situations.

Consider, for example, a heavily loaded range-extended electric car on a 10-mile uphill grade at freeway speeds.

Once the buffer capacity of the pack is depleted, would a 40- or 50-kW generator be enough to keep the i3 at maximum speed on that freeway?

Not meant for daily use

BMW, however, doesn't view the range extender as a way to provide that kinda of extended travel on a regular basis--but only as an emergency fallback.

At the concept introduction in August 2011, the company said, "In a sense, therefore, the Range Extender is like having a reserve fuel can on board."

Even then, executives stressed that the ReX "isn't for regular use, [but] just to keep the electric system going,"

Herbert Diess, global R&D boss for BMW, was recently quoted on the ReX range extender in Plastics News (in an article originally published in trade weekly Automotive News).

Diess explained the company's point of view, reinforcing the viewpoint cited at the launch:

The range extender is not intended for daily use. It's for situations when the driver needs to extend the range of the vehicle to reach the next charging station. Therefore, the i3 probably won't be the choice for customers with a need for an extended range.

BMW i3 Concept MkII

BMW i3 Concept MkII

Diess suggests that a plug-in hybrid is "a more suitable solution" for those customers who frequently need a car with range beyond that offered by the i3's battery pack.

That's in contrast to the marketing approach taken for the Volt, which is promoted as a "no-limits" electric.

While Volt owners cover almost two-thirds of their miles on battery power from the electric grid, one third of their miles are powered by the gasoline range extender--far more than BMW's "not for regular use" approach.

Nonetheless, Diess says that BMW doesn't think the range extender will be needed by a "substantial share" of i3 buyers:

It is more of an issue for those who have not yet had a chance to use an electric car. After a few days, they usually discover that a base range of [100 miles] is sufficient to limit recharging to about two times a week. In most cases where people first think they need a range extender, it actually never is used.

Diess was quoted elsewhere as saying that while the take rate for the ReX might initially be as high as 50 percent, BMW expected it to stabilize at around one-fifth of all i3s sold.

Europeans vs North Americans

We have two thoughts in regard to this.

First, we think Diess may be right that most buyers won't opt for the ReX in Europe and Asia.

But we're not so sure he's right for North American buyers, who travel longer average distances and have little access to viable mass transit for trips of 100 miles or more.

Second, we think that BMW could have a hell of a marketing challenge on its hands.

2013 Chevrolet Volt, Catskill Mountains, Oct 2012

2013 Chevrolet Volt, Catskill Mountains, Oct 2012

The company has to launch a brand-new electric car while simultaneously explaining that, yes, the ReX is a range extender that will almost double the range of the car--but, no, the i3 in range-extending mode shouldn't really be used for that purpose on a regular basis.

One of the selling points of the Chevy Volt is that it's the electric car with "unlimited range" when drivers need it, but that can't be said of the i3.

New kind of range anxiety?

And we wonder if there's a different type of anxiety that will beset i3 drivers in range-extending mode, if their performance isn't up to the standard set by the battery car.

Range anxiety, then, could extend not only to the fear of the car going dead at the side of the road--but to fear of the car losing some portion of its performance capability once the range extender kicks in.

Has BMW made the right decision? Is the ReX designed to provide the best safety net with the least impact on the basic electric-car design?

Leave us your thoughts in the Comments below.


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