[maigret]

"why don't they" -> Remembers me of a recent HN discussion ;)

But... Actually you are not far from something. This is actually an approach from Audi: http://green.autoblog.com/2010/03/01/geneva-preview-audi-a1-... From what I read though this approach was not approved in the mother company, VW. Let's see who wins at the end.

[Gravityloss]

That's very interesting, what Audi was planning! The Wankel engines are quite expensive though.