Wankel Engine

The Wankel radial engine is a fascinating beast that features a very clever rearrangment of the four elements of the Otto cycle. It was developed by Felix Wankel in the 1950s.¹

In the Wankel a triangular rotor incorporating a central ring gear is driven around a fixed pinion within an oblong chamber.

The fuel/air mixture is drawn in the intake port during this phase of the rotation.
The mixture is compressed here.
The mixture burns here, driving the rotor around.
And the exhaust is expelled here.
The rotory motion is transferred to the drive shaft via an eccentric wheel (illustrated in blue) that rides in a matching bearing in the rotor. The drive shaft rotates once during every power stroke instead of twice as in the Otto cycle.

The Wankel promised higher power output with fewer moving parts than the Otto cycle engine, however technical difficulties have apparently interfered with widespread adoption. In spite of valiant efforts by Mazda, the four stroke engine remains much more popular.