Diversity among electric cars is rapidly expanding, from the all-electric Nissan Leaf and Mitsubishi 'i' to the upcoming Toyota Prius Plug-In Hybrid, which provides just 9 to 13 miles of electric range.

Now, it turns out, the 2014 BMW i3--the all-electric urban four-door subcompact that BMW showed last week in concept form--will offer the option of a gasoline range extender.

The i3 will run entirely on electricity from a 21- to 22-kilowatt-hour lithium-ion battery pack in the floor. The battery pack powers a 125-kilowatt (170-horsepower) motor driving the rear wheels.

Photo Gallery: BMW i3 Electric Car And i8 Plug-In Hybrid Live Photos

Photo Gallery: BMW i3 Electric Car And i8 Plug-In, Under The Hood: Live Photos

BMW i Launch Event, Frankfurt, July 2011

BMW i Launch Event, Frankfurt, July 2011

Company executives said the gasoline range extender, which they call REx, is intended to "double the range" of the electric car. That would take it from 80 to 160 or more miles, presumably adding more increments of 80 miles with each refueling.

REx is a "small, very smooth-running, and quiet" gasoline engine that powers an electric generator. The electricity it produces maintains the battery level once it's depleted, providing power to the traction motor that moves the car.

BMW explains why this is useful: "In a sense, therefore, the Range Extender is like having a reserve fuel can on board."

More details: BMW i3 Concept: Battery Electric Urban Minicar—Full Details

BMW i Launch Event, Frankfurt, July 2011

BMW i Launch Event, Frankfurt, July 2011

The optional REx engine-generator set will be compact enough that it can sit over the rear axle, in the same space as the electric drive motor. Both components are hidden under the standard rear deck of the i3, whose shell is made of carbon fiber-reinforced plastic.

In multiple interviews at last week's i3 Concept launch event in Frankfurt, BMW executives revealed the following details about the REx engine:

  • It's an inline two-cylinder of roughly 600 cc (0.6 liter) capacity
  • It will be fitted with automatic start-stop to save fuel when the vehicle is at rest
  • It will be rated as a Super-Ultra Low Emissions Vehicle (SULEV), the lowest-emission rating for any combustion engine
  • It will be heavily optimized to minimize noise, vibration, and harshness in operation

Most intriguingly, BMW acknowledged that it's an engine that "you might find in another BMW Group product"--which is to say, most likely derived from a BMW motorcycle engine.

Summary: BMW i3 Electric Car And BMW i8 Plug-In Hybrid: Overview

BMW i Launch Event, Frankfurt, July 2011

BMW i Launch Event, Frankfurt, July 2011

One i3 program executive, however, pointed out that the REx version of such an engine will be "very highly modified" to meet the targets set for it as a range extender.

Motorcycle engines must meet a wide range of power demands, from fast acceleration to high-speed cruising. Range extenders, on the other hand, can be optimized to run at a single speed or within a narrow rev band to operate most efficiently while running the attached generator.

BMW stressed that the REx "isn't for regular use, just to keep the electric system going," and that the i3 was clearly an electric car with a range extender, and not a plug-in hybrid like the i8 Concept mid-engine sports car it showed at the same time.

More details: ­ BMW i8 Concept: Plug-In Hybrid Sports Coupe—Full Details

The BMW i3 (nee MegaCity Vehicle) was always intended to be a battery-electric car, said one i3 program manager. The concept of a range extender "was added a little bit later."

BMW i Launch Event, Frankfurt, July 2011

BMW i Launch Event, Frankfurt, July 2011

BMW didn't release any technical specifications for the REx engine, or even show a prototype. Instead, the i3 Concept's electric drive motor was displayed with a pair of long driveshafts--leaving a space in which you will have to imagine the engine and generator set.

It's worth noting, by the way, that the i3 Concept isn't the only electric car fitted with a tiny range-extending engine-generator set under the rear deck. That's the same layout used by the Audi A1 e-tron concept shown at the 2010 Geneva Motor Show--except the A1 used a tiny Wankel rotary engine.

If once is innovation, perhaps twice is a trend?

BMW provided airfare, lodging, and meals to enable High Gear Media to bring you this first-person news report.


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