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.