Rotary Cycle Engine

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Rotary engines had a radical departure from traditional Otto cycle engines and the Wankel design was a paradigm shift in the internal combustion engine design. Unfortunately rotary engines did not gain in popularity.


Traditional otto cycle engines have pushed rotary engines into the background but it is worth having a look.

  • Felix Millet showcased a 5 cylinder rotary engine at the Exposition Universelle in 1889 after patenting it in 1888 and his engine powered la vehicle in the Paris-Bordeaux-Paris race in 1895.
  • Stephen Balzer developed a rotary engine in New York in the early 1890s with the intent of using them in aircraft. A 3 cylinder engine powered a car in 1894.
  • Adams Farwell came up with a 3 cylinder rotary design designed by Fay Oliver Farwell in 1898 and used in their cars. More powerful models powered aircraft in or around 1910 and his design inspired Gnome.
  • Meanwhile, Louis, Laurent and Augustin Seguin, 3 brothers, were working on a single cylinder stationary rotary engine. The engine took its name, Gnome engine from the company that Louis formed, Societe des Moteurs Gnome but the engine was licensed from a German company.
  • Their own development consisted of a 5 cylinder design with the cylinders arranged radially and refined it into a 7 cylinder model, Omega. Their Gnome engine was used by Roger Ravaud to his hydrofoil/aircraft and by Henry Farman in his aeroplane to fly 180 kilometers, a record at that time. Their engine was widely used in German aircraft during World War I.
  • They further developed a twin row engine named the Double Lambda with 14 cylinders fitted to Fokker E.IV and Fokker DIII fighter planes. By the end of the war these engines became obsolete as traditional in-line piston powered engines delivered better performance.
  • By 1929 Felix Wankel, a German Engineer, patented a design for a rotary engine with a revolutionary concept that did away with pistons and cylinders altogether. The workable engine was developed in 1957 with NSU’s help and the organization licensed it to companies for use in their vehicles.
  • It was widely used in motorcycles, marine vessels and it shone in Mazda cars. Mazda’s President, Tsuneji Matsuda signed the licensing agreement with NSU Motorwerke in Germany and launched the Cosmo Sport in 1967 followed by various models.

Mazda’s 26B racing car won the Le Mans 1991 competition using a Wankel engine. The RX 7 became a cult model followed by the RX-8 but emission norms made Mazda put a stop to their production though development continues.


For practical purposes the rotary engine can be classified into two types:

  • The traditional piston-cylinder rotary engine with a rotating crankcase and cylinders and pistons fitted with valves.
  • Wankel engine with an epitrochoid cylinder in which a triangular rotor rotates on an eccentric shaft.


Main Parts

  • The piston-cylinder rotary engine comprises of the inlet valve, piston, cylinder, spark plugs with an associated ignition system, fuel delivery, spinning crankcase, crankshaft and related parts
  • The rotary Wankel engine has
  1. an epitrochoid combustion chamber,
  2. a flattened oval, a triangular rotor,
  3. a central shaft with ring gear driven around a fixed pinion,
  4. a sparkplug,
  5. fixed gear engaging the ring gear connected to the eccentric shaft and other associated components.
  • The inlet and outlet ports in the epitrochoid chamber are diametrically opposite the spark plugs. The spinning rotor transmits power to the eccentric shaft connected to the gearbox.
  • The rotors have apex seals to seal or isolate parts of the chamber designated for various stages of the combustion cycle.

The eccentric gear in the rotor also serves as the internal timing gear. The fixed pinion gear is the fixed timing gear, both of which are connected to ignition.

How it works

  • Functioning of a traditional piston-cylinder rotary engine:

The intake stroke is when there is a vacuum in the cylinder that opens the intake valve to let in fuel-air mixture. Next is the compression stage when the fuel air mixture is compressed as piston moves up in the cylinder, just a little bit before the top dead center position. The spark plug ignites the mixture, and the resulting explosion is the power stroke, during which the outlet valve also opens. The last stroke is the exhaust stroke in which piston pushes out the exhaust burnt gases.

  • Functioning of a Wankel engine:

The triangular rotor assembly is fixed around a ring gear that meshes with a gear attached to a fixed pinion. The rotor moves in an eccentric pattern within the epitrochoid chamber in which there are intake and exhaust ports as well as the spark plugs. As the rotor moves, it uncovers the inlet port and lets in the fuel-air mixture. Rotor pushes the air mix in the lobe to the next stage of compression while sealing the intake port. As the rotor turns, the space in one lobe is greatly reduced, resulting in compression of the fuel air mixture and, in the next stage, when fuel is fully compressed, ignition occurs in the chamber. The moving rotor pushes the burnt gas mixture to the exhaust port from where it is expelled.

Effectiveness and Challenges


  • The piston-cylinder rotating crankcase engine’s main advantage was its power to weight ratio in those days, especially beneficial in aircrafts. However, these engines were quite fuel inefficient and oil consumption was high.
  • Some engines used castor oil for lubrication and emission levels were high. Fuel is drawn through the crankcase as in two strokes. In some designs only one valve doubled as inlet and exhaust, resulting in valve burning. The rotating mass caused a gyroscopic effect that made maneuvering an aircraft quite difficult. The power output was limited and all these disadvantages eventually resulted in this type of engine going into oblivion.

Wankel Engine:

  • The Wankel rotary engine’s main advantage is its simplicity of construction with fewer parts than that in a four stroke piston-cylinder engine. These engines have higher power output for a given engine size.
  • Wankel Rotary engines are not prone to failures such as piston seizing or piston rings breaking or valves problems. They produce power even though there may be issues with the seal.
  • The disadvantages are that this type of engine consumes more fuel and burns more oil since lube oil is injected on the seals in the combustion chamber. This causes higher emission and higher pollution—the chief reason why these engines have been phased out.
  • Since so few engines are in use, parts are expensive and repairs are only possible at an authorized garage.

The challenges are to make the Wankel more fuel efficient and less polluting.


Maintenance Tips

Apex seals of the rotor in the Wankel Rotary Engine are the main problem areas at times. These seals need to be kept well-lubricated with oil. A regular check on the oil squirting system helps keep the engine in check.


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