I am still trying to learn the fundamentals of what all occurs inside the rotary engine, and sorry if this has been asked before...

But in my searching today, I came across this pic:



It looks like long sides of the cylinder come in a bit on each side at the midsection, is this an optical illusion or are they doing this? Are all wankel motors shaped this way? I was under the impression that they were perfectly oval shaped. And if the latter is the case, what is the purpose of this?Larger combustion chamber?'

Thanks for any help

 

Yes.

http://en.wikipedia.org/wiki/Wankel_engine

S

 

Whew, that was a close call.

 

http://www.youtube.com/watch?v=oGrD7FTFLJc

 

There's allot of math in the workings of the rotary engine. Really interesting stuff, ultimately producing incredible output per volume and very smooth turbine-like operation.

Many of us got RX-8's primarily due to its Wankel-based engine. Good to see your 'how it works' interest.

 

So far I am guessing I have stumped you... Can anyone supply a valid answer? I know the basics of how a rotary works, the vids werent much help, although I did stumble onto this pretty cool vid of a 4 rotor making 1600 Hp. It got pretty cool when at the 2 minute mark teh exhaust manifold got so hot it burst into flames. http://www.youtube.com/watch?v=0BGGB...eature=related

But still no one able to answer the question...

 

Thanks for the reply. I have been trying to find out more and more about them, I am pretty interested in learning everythign that is going on inside the motor. I like how simple the concept is

 

it is not an oval. it is an epitrochoid.
i'm no expert and I may be wrong, but a perfect oval would yield no compression. Seals would not be in contact at all times, as it does with an epitrochoid.

 

I see . All the other images/videos I had seen I havent noticed the shape being that noticable. I thought that the motion of the rotor kept them in contact, This would make more sense though, almost sealing the chamber into two overlapping circles.

 

what was your question? it seemed to be "are they all shaped like this?" which was answered by stealth.

was your question "why are they shaped like this?"

if so then its because that is the path traced by the tips of the rotor as it revolves around the eccentric shaft. if it wasn't shaped that way the tips would not be able to keep in contact with the wall for sealing the chambers.

 

I'm proud of you guys. No wankel stroke jokes.

 

Stealth already gave you the basic answer, but since you're really looking for a greater understanding, it wouldn't hurt to pick up a good books on the rotary. But I'll give you a start.
Not an illusion, the epitrochoid shape is real, also called a nephroid, or kidney shape. The shape of the housing is necessarily related to the shape of the rotor and it's eccentricity. If the housing were to become more rounded, less kidney shaped, the rotor also would be more rounded with less eccentricity. Conversely, the rotor could be less rounded, even an angular triangle with a more pronounced kidney shaped housing. The biggest factors in choosing between varying design parameters has primarily been compression ratios and seal angles. Theoretically, you could also have more than three sides to the rotor, which would require different shapes in rotor housing. Pictures are much more helpful, hence my suggestion to pick up a book. Norbye's 2nd chapter on Wankel geometry is available on the web somewhere.

Edit: Here it is:

https://netfiles.uiuc.edu/ro/www/Soc...elgeometry.pdf

 

Pretty interesting information . On an unrelated note, what is the limiting factor for a rotary engine as far as RPM's are involved? Is it simply the rotational inertia of the rotor? It seems like it wouldnt have any or many parts really limiting it from having a nearly endless redline, unlike a traditional motor which is limited by the connecting members and valves.

 

Oh but we were all thinking that.

 

Breathing. Mechanically, I think it's the eccentric shaft and stationary gear.

 

Pretty much yes. Inertia vs. strength of materials.

Keep in mind that the 9k rpm redline is as shaft measured. The rotors themselves are only going 1/3 that speed (3k).

btw: the rotors are steel and the combustion housings aluminum, so impossible to seize the engine, which is a nice innate-materials advantage.

The upcoming 16X (1.6L vs. current 1.3L) has a slightly different rotor "orbit" in its housing, due to a physically larger internal capacity (same external size) , to facilitate a larger combustion chamber and thereby greater power. It will also have aluminum end housings (current iron) to maintain the same weight as the current engine at that greater power. Brilliant!

 

Not long after I asked that, I remembered reading somewhere that the motor stops "breathing" after about 7500 RPM. This is something else I dont quite understand... I am guessing that after reaching a certain rotational speed it isnt able to efficiently expel the spent gasses and inhale the needed combustibles?
I was confused at first about the shaft rotational speed and the pistons rotational speed, but got that sorted out after seeing a few vids . I would be interested to see the components of the 16X compared to the 13B to see how the orbit is different.

 

Yes, the ability to move air is limited by the available size of the side ports, but the limit is higher than 7.5k. The new orbit is not really that different--you wouldn't notice it by looking at it--just a little greater eccentricity for improved torque, lesser loss to chamber surface area, and a little more room to increase the size of the side ports.

 

Part of it is materials, of course, but a major consideration is the speed of the flame-front in the air-fuel mixture. As you've seen in the animations/movies -- as the fuel-air charge ignites and expands it is also moving around the periphery, AND changing shape. We may tend to think of combustion as an instantaneous event, but it does take a finite amount of time for the "flame-front" to propagate. If the rotors moved too far too fast, it wouldn't be able to keep up. As it is, we already have a trailing spark plug to mitigate the issue somewhat.

The flame-front issue makes wankel RPM largely dependent on the physical size of the engine. Large rotaries were made to power natural-gas pumps, with rotors 1 meter in diameter. These were limited to 1200 rpm precisely because of the flame-front issue. Likewise, very small wankel engines could run at much higher rpm.

http://www.me.berkeley.edu/~liepmann...02.IMECE01.pdf

 

That is another aspect that I had not thought about

Interesting info in here. Thanks for all the help . I am sure I will come up with some more questions, but I think my brain is full for now haha.

 

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

They're rotors, not pistons. Using the term "pistons" around here is a no no!!

 

Actually, they're both. Think of a rotor as 3 pistons. Felix Wankel first called it a circular piston motor. With Walter Froede's major change in design it became known as a rotary piston motor.

 

^ technically correct of course, but we still don't like the term 'round he'ah.

When explaining our Wankel to the uninitiated I do say it's comparable to a 6-cylinder piston engine.

 

LOL, I'll go back into my German bunker now.

 

lol. Sorry, I forgot step one - "take everything I know about traditional motors, and promptly forget it all".

Wont happen again