All parts rotate consistently in one direction, as opposed to the common reciprocating piston engine, which has pistons violently changing direction. In contrast to the more common reciprocating piston designs, the Wankel engine delivers advantages of simplicity, smoothness, compactness, high revolutions per minute, and a high power-to-weight ratio. This is primarily because three power pulses per rotor revolution are produced compared to one per revolution in a two-stroke piston engine and one per two revolutions in a four-stroke piston engine. Although at the actual output shaft, there is only one power pulse per revolution, since the output shaft spins three times as fast as the actual rotor, as can be seen in the animation below, it makes it roughly equivalent to a two-stroke piston engine of the same displacement. This is also why the displacement only measures one face of the rotor, since only one face is working for each output shaft revolution.
The engine is commonly referred to as a rotary engine, although this name also applies to other completely different designs, primarily aircraft engines with their cylinders arranged in a circular fashion around the crankshaft. The four-stage cycle of intake, compression, ignition, and exhaust occur each revolution at each of the three rotor tips moving inside the oval-like epitrochoid-shaped housing, enabling the three power pulses per rotor revolution. The rotor is similar in shape to a Reuleaux triangle with the sides somewhat flatter.
The design was conceived by German engineer Felix Wankel. Wankel received his first patent for the engine in 1929. He began development in the early 1950s at NSU, completing a working prototype in 1957. NSU subsequently licensed the design to companies around the world, who have continually added improvements. The engines produced are of spark ignition, with compression ignition engines having only been built in research projects.
The Wankel engine has the advantages of compact design and low weight over the most commonly used internal combustion engine employing reciprocating pistons. These advantages have given rotary engine applications in a variety of vehicles and devices, including: automobiles, motorcycles, racing cars, aircraft, go-karts, jet skis, snowmobiles, chainsaws, and auxiliary power units. The power-to-weight ratio has reached under one pound per horsepower in certain engines.
In 1951, NSU Motorenwerke AG in Germany began development of the engine, with two models being built. The first, the DKM motor, was developed by Felix Wankel. The second, the KKM motor, developed by Hanns Dieter Paschke, was adopted as the basis of the modern Wankel engine.