Rotary SI/CI combustion engines: A thing of the future?

The internal combustion engine enjoys widespread use as an inexpensive and reliable power conversion system. While piston engines date back 150 years, various alternative engine architectures and cycles have been considered.

Today's small piston engines can be inexpensive, and have suitable reliability to serve a variety of applications including scooters, motorcycles, ATVs, boats, lawn and garden equipment, and auxiliary power generation, not to mention small aircraft including UAVs. While piston engines enjoy prolific use, their efficiency is remarkably low.

The Wankel rotary engine has some advantages that make it a formidable contender for some markets served by reciprocating engines. The piston in a four-stroke reciprocating engine momentarily comes to rest four times per cycle as its direction of motion changes. In contrast, the moving parts in a rotary engine are in continuous unidirectional rotational motion. However, the rotary type engine has some drawbacks, including fuel economy.