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Rotary Displacement

As rotary engine enthusiasts, we all know the obvious, intoxicating charms of Wankel wonder. Unique, unusual, lightweight, compact, high revving, what’s not to like? One of the most commonly deliberated subjects is “what is the rotary engine’s displacement”? An excellent question. Here’s our best to make the case for the engine’s displacement.

Conventional reciprocating engines are the standard that most relate to. We all know that these engines are measured by measuring the area of the bore, multiply by the stroke and then multiply by the number of cylinders. Simple as that.

For simplicity, we’ll use the 13B engine as the standard we’re calculating. You can use these ideas and apply them to a 12A or 20B yourself. The rotary engine is obviously unique. The engine has two rotors shaped roughly like a triangle. This makes for three combustion faces per rotor and a total of six for a standard two-rotor engine. Each face has a “swept volume” or displacement of 40ci(654cc) and there are a total of six faces. With this known, the engine displacement should be 40ci(654cc) times six to equal 240ci(3.9L), right? In a way, yes, but that would not be a comparable displacement to the 4-cycle engine.


Thermodynamic cycle breakdown of a Rotary engine.


Thermodynamic cycle breakdown of a piston engine.


The key for comparing the displacement between the 4-cycle engine and the rotary engine is in studying the degrees of rotation for a thermodynamic cycle to occur. For a 4-cycle engine to complete every thermodynamic cycle, the engine must rotate 720° or two complete revolutions of the crankshaft. The rotary engine is different. The engine rotor rotates at 1/3 the speed of the crankshaft. On two rotor engines, front and rear rotors are 180° offset from each other. Each rotation of the engine (360°) will bring two faces through the combustion cycle (the torque input to the eccentric shaft). This said, it takes 1080° or three complete revolutions of the crankshaft to complete the entire thermodynamic cycle. Obviously, we have a disparity. How can we get a relatable number to compare to a 4-stroke engine? The best way is to study 720° of rotation of the two-rotor engine. Every 360° of rotation, two faces of the engine complete a combustion cycle. 720° will have a total of four faces completing their cycle. 40ci(654cc) per face times four faces equals 160ci or 2.6L. That’s a well-reasoned number and now gives us something to be able to compare to other engines. In addition, since four faces passed by in the comparison, it’s like a four cylinder engine.

Now we know, the 13B compare well to a 2.6L 4-cylinder 4-cycle engine.


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