Wankel engines were first invented in the early 1960s by the German engineer, Felix Wankel. Since its inception, the Wankel engine saw an increase in displacement and even the addition of turbocharging. Wankel engines, specifically the series of rotary engines produced by Mazda, had a reputation for being relatively small and powerful but at the expense of poor fuel efficiency. The engines became popular among automotive circles and were even used in light aircraft because of their low mass, compact size, and tuning potential. It’s popularity was attributed to the inherently high power-to-weight ratio that rotary engines offered. Mazda first put the engine into production with the release of the NSU Ro80 and the Citroën GS Birotor between 1967 and 1977 as part of a partnership among the automakers. Despite the initial hurdle the early cars had with reliability, the rotary design was subsequently used in a slew of successful models under the Mazda name. Like the Cosmo, RX-3 and three generations of the iconic RX-7.
For this reason, the rotary design has been praised as a competitive engine, and even developing a cult following amongst car enthusiasts. And as rotary engines became a common entry into motorsport events, racing authorities ran into the problem of representing each engine's displacement correctly.
This was attributed to the unique design of the rotary engine since they are variable-volume, progressing-cavity systems. Since each rotor has three faces and each face has three cavities of volume per housing. Each face of the rotor “sweeps” its own volume as the rotor orbits within the housing. Each side of the rotor is brought closer to and then further away from the wall of the internal housing; compressing and expanding the combustion chamber.
The rotor itself is comparable to a piston, and much like a reciprocating engine, the rotary design is an internal combustion engine that employs the same four stages; intake, compression, combustion, and exhaust in each cycle. But where the cylinder volume changes as the piston travels up and down in a cylinder. The volume, configuration, and position of the operating cavity changes as the rotor orbits in an eccentric fashion.
And since a rotor has three faces, the same process is repeated continuously. Looking closely at the rotor, this becomes evident. When side A is about to be propelled by the exploding gas, Side B is in the intake and compression phase. While this is happening, side C is in the exhaust phase. When the rotor moves, side A goes into the exhaust phase, and then side B begins expanding. Side C begins the intake and compression phase.
Furthering the frustration on how to represent a rotary engine’s displacement correctly, is that the rotor travels 1/3 the speed of its crankshaft. Subsequently, its output shaft travels faster than that of the rotor’s oscillating parts.
This perceived advantage caused many regulatory bodies in automotive racing to variously consider the rotary engine to have the equivalence of a reciprocating four-stroke piston engine of 1.5 to 2 times the displacement of one cavity per rotor. So rather than enforcing a rule to divide quoted displacements for participants driving reciprocating piston engines. Most racing organizations simply decided to double the quoted displacement of rotary engines.
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