Translating a linear motion (pistons) to be rotary is inherently inefficient which is why coolant and lubrication systems are so critical in modern cars. Ultimately, I suspect, we will be replacing modern engines with electric motors which have 1 moving parts and are far, far less complicated and far, far more efficient.

The basic concept of a (pistonless) rotary engine avoids the reciprocating motion of the piston with its inherent vibration and rotational-speed-related mechanical stress. As of 2006 the Wankel engine is the only successful pistonless rotary engine, but many similar concepts have been proposed and are under various stages of development...

While typically larger than the piston of an engine of corresponding capacity, a rotor may perform many strokes per revolution. The Wankel produces twelve strokes per revolution of the rotor (four strokes per chamber times three chambers) (although the spindle rotates three times faster than the rotor or three times over the twelve strokes), as opposed to two strokes for each crankshaft rotation of a single-cylinder single acting piston engine, or four strokes for a double-acting cylinder such as found in some steam engines. The quasiturbine and MYT engine deliver sixteen strokes for every rotor (and spindle) revolution.