Recently, Consumer Reports' Eric Evarts spoke about the i3 with BMW chief engineer Ulli Kranz. Evarts asked of the i3's optional range-extending engine and, even though Kranz wouldn't discuss specifics, Evarts seems to have prodded Kranz in such a way that BMW's range-extending solution was revealed.
Evarts suggested to Kranz that one of BMW's motorcycle engines would be ideally suited for range-extending duties and, rather than deny this, Kranz' "eyes lit up when it was suggested that a motorcycle engine might be a perfect fit for such a car," according to Evarts.
Furthermore, Green Car Reports speculates that handling the i3's range-extending duties will be one of BMW's motorcycle engines, perhaps the company's twin-cylinder 600-cc unit. Specific details are, unsurprisingly, not available, but that's what future posts are for.