The Mazda may go "Hummmm" again. After years of hints, promises and denials, Mazda finally appears to be moving towards the re-launch of its once-familiar rotary engine, with a concept model said to be in the works for 2017, and a production model to follow three years later. But the purpose may have changed a bit: it would be part of a plug-in hybrid that would take advantage of the compact engine’s advantages while offsetting its traditional design limitations.