Hey, I've been a lurker on this forum for a while now - even helped assemble a 13b-REW and plopped it into a friend's 2nd gen back in the summertime.

From what I understand, this is the non-flame technical forum area, so hopefully this stupid question isn't gonna get shot down.

One question that I can't seem to find answers to is... why are there no larger displacement rotaries? I mean, people add a third (like the 20b) or even a fourth rotor to get more displacement , but what if the entire rotor assembly was just enlarged?

Piston/reciprocating engines don't have the same standard piston size and cylinder bores. Having only one standard rotary size (654cc) for every (modern) rotary engine out there is like having only one piston size for every single piston engine out there. Want more power? Add cylinders/pistons!

There is no replacement for displacement, turbo or otherwise.

 

Mazda currently has the 16x (1.6L) engine in development, which is effectively a 3.2L. Increasing displacement is far more difficult than just boring out a cylinder and adding a large piston. The entire engine has to change shape; namely the housings, irons and rotors.

16X: http://www.mazda.com/mazdaspirit/rotary/16x/

 

after reading that link, the comment i had was invalid.

 

the whole no replacement for displacement thing is a piston engine saying. The rotary make much more efficient use of the combustion cycle. Thats why the rotary was outlawed in many racing circuits with displacement requirement, because a under 4.0 piston engine just cant make the power a small displacement rotary can.
Sure if you up the displacement you will have more power, but when the motor as it is is capable of making plenty of power why change the design

 

Cool, that'll be very interesting. Will it be possible though to simply scale it up? Increase the depth? I mean, companies routinely CNC custom sized (and shaped) pistons, and they also resleeve existing blocks with larger bores and different depths.

No offense, (really, I don't mean to offend at all) but I've heard that argument over on the Subaru boards for a decade. They defended the EJ20 2.0L turbo because it was capable of making as much power as any V8, provided you upgraded the turbos. However, with the bump up to the 2.5L STi, they've now seized on to that as the new standard of "you don't need more displacement than that."

 

you have to think from a production stand point though, a 1.3l rotary is a good size for a small car, like what was said before increase the displacement involves a complete redesign of the entire motor not just putting in a deeper crank and larger pistons, In a piston engine you can gain .3-.5L no problem the rotary just isnt that simple

 

Relatively speaking, if you want to compare a 13b to 4cycle piston engine, the actual displacement would be 2.6L, not 1.3L.

 

I'm so not an engineer and would stand corrected on any of these points. But it seems like on 13b...you are limited by geometry and e-shaft. There are no aftermarket parts available to my knowledge for simply widening the stock dimensions of the housings and rotors. In some limited cases you can switch rotors of different compression ratios, but at least with the 13bREW, they wouldn't be optimized.
First, the shape of the combustion chamber on a rotary is particularly long when compared to a piston's. And the rotation of the rotor makes it longer yet. You have to consider speed of the flame-front during the combustion event. Complete combustion and efficient use of the force produced is different than that of a piston. Simply deepening the "bath-tub" on each rotor lowers the compression ratio and might even change how the force produced by the combustion event is directed. Widening or lengthening the combustion chambers would also effect exhaust and intake characteristics (and timing) in ways that make my small brain spin.
Second, there are no piston rods. The rotors are in direct contact with the e-shaft (crank). So the lobes of the e-shaft would also have to be widened relative to the wider rotors and bearings.

Even if you were able to simply widen the housings and rotors with something aftermarket, cooling might also become an issue. Without separate inlet and outlet ports for each housing (and rear iron), the coolant might be too hot by the time it reachs the combustion side of rear housing to do much good.

It's efficient in that there is no reciprocating mass and fewer moving parts. The nature of the rotary's exhaust pulses also lends itself well to turbocharging. But it's actually less efficient in other ways.

 

Holy mind-boggling rotaries, batman.

The reason I asked is because I like building custom engines, and I was looking into the possibility of trying to do a 'different' rotary. I'm beginning to think that building any rotary itself is a miracle.

If you guys could do something unique, or interesting, to a rotary engine, what would it be?

 

4 rotor. Kind of back to your "no replacement for displacement"....but in a different way.
Bing or Google 4 rotor. It's been done but very different and unique.

 

not to mention very expensive!

 

You won't be "building" a rotary like a piston engine. You will be using 95% of the things Mazda gave you and improving on them. There is nothing really that would benefit from upgrading like you would on a piston engine (i.e. forged pistons, stronger connecting rods, cam, ect.), there are 3 moving pieces, compared to how many on a piston engine?

 

lol