The rotary engine was an early type of internal combustion engine. They were used in the world war I as a replacement for conventional inline engines. These engines proved out to be an efficient solution to many problems of power output, weight, and reliability. The main purpose or use of these engines were in aircraft but these engines saw their early use in a few motorcycles and automobiles.
The rotary engine works in a completely different way than the piston engine. Rotary engines are generally designed with an odd number of cylinders per row in a radial arrangement. During operation, the crankshaft remains stationary. The crankcase and the attached cylinders rotate around crankshaft as an entire unit.