Wankel rotary engine powered thousands of aircraft. One of several problems: It spewed castor oil on its pilots.

The rotorcraft engine business, like the helicopter industry it serves, has been notoriously slow to develop new models and refresh its products over the decades. But now, as new propulsion concepts ...

In a world dominated by pistons, the rotary engine was something different for motorists. It was the vision of German engineer Felix Wankel, built on the belief that the up-and-down motion of pistons ...