I made searches and understand the basic combustion cycle of a rotary engine, but what's still unclear to me is that although we all agree the mpg is relatively "poor-moderate".
How does one relate that fact to the 1.3L number? and why?
When talking about the 8 to friends and relatives, everyone seems to think at first that since it is only a 1.3L engine, gas mileage should be awesome which we all know is not the case. But I have been unsuccessful in explaining correctly the matter to them.

 

It's displacement may be tiny, but compare a 1 cylinder piston engine to a 1 cylinder rotary. On the piston engine, the crankshaft has to rotate 2 complete turns to make 1 power stroke. Now in a rotary engine, when the crankshaft makes 1 revolution, there are 3 power "pulses" (?). So when a rotary engine makes 2 complete revolutions, it make 6 "pulses" compared to the similar piston engine's 1 power stroke.
Because of that, the size can't be directly compared. I other countries, they call it a 2.6L though.

 

The rotary has a long combustion chamber. So, thermodynamic effiency isn't as good as a piston and mpg suffers.

 

@dano: Your explanation is correct, but a piston engine usually has 4-12 pistons, whereby a rotary engine has only 2-4 rotors. Also, the crankshaft rotates three times per rotor spin, while 1 rotor spin is equal to three ignitions. As a result, a 4-cylinder engine should produce an equal amount of ignitions (4/2=2) like a 2-rotor (3/3*2=) engine does per crankshaft spin.

I think one point is that the rotary in general revs higher and therefore has more ignitions per timeframe. And maybe the rotary is not developed as far as the piston engine yet.

Besides that, the relation between the displacement, power output and mileage is simply not linear, eg a supercharger can drastically change the numbers. And if you're looking at a jet engine, you will find that it is capable of burning enormous amounts of fuel with a relatively small combustion chamber.

 

Taken from http://www.rx7.com/techarticles_displacement.html

"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."

 

In otherwords, to keep it simple:

At a fixed RPM, your rotary will burn the same volume as a 2.6 L piston engine.

 

As a four cylinder 2,6 l piston engine, to be exact. No turbo, no supercharger.

But the things that happen in a rotary are still miles away from a piston engine and how much more millions have been spent to research the piston engine?

 

RotaryGod wrote a long complicated explanation in a Tech section thread that's worth reading. Also, check out this thread:

Reason for Eating Gas?

 

Gee... I thought the simple explaination was... you keeping the petal to the metal too much!