bgreene

But, if the reduced equasion is:

V=(3)[(3^(1/2)]weR,

then doubling both e and R would give a 4X increase in V (and thus power), but would maintain a constant e/R ratio value. Similarly, to say that V varies directly with e/R is misleading, since the ratio would increase with a reduction of R and leaving e constant, but would result in a reduced value for V.

I'm not sure ho much more technical I can get, it's been several years since I had to use any thermodynamics (and all of my propulsion education was focused on turbojets/fans, Ram/SCramjets and rockets). For internal combustion engines, my knowledge in this regard is just the basic theoretical/philosopnical level.

shelleys_man_06

You too? My thermodynamics teacher dejected the Otto cycle, and we just learned power plants, and Brayton-cycle analysis, none of which is related to automotive . I think the equation I expressed is very inflexible; I am trying to find a way to make these equations relevant to what IKN wanted to know. My head hurts . I hate to sound redundant yet again, but experimentation may better yet help solve this problem. I don't know. My overall knowledge about ICE only goes as far as the basic engineering equations. The developed concepts that come from the equations, I believe, are most important. Thank you for bringing up that point bgreene. I have yet to plug anything into my TI-89 :o.

bgreene

I think part of it for me was that my propulsion class was through the AE department (since that was my major), and it's been some time since any sort of ICE has been considered the primary power plant for aircraft. Also, all of my fluid dynamics coursework was through AE, and dealt mostly with wind tunnels, aerodynamics, and compressibility (little of which is very useful inside an engine)

We took thermo from the ME dept, but that was just the basic tools and principles (if memory serves, most of the sample stuff actually had to do with steam engine components and before/after states more than any sort of real combustion problems).

On top of that, it's all a bit rusty 'cause I've been doing structural analysis since I graduated in '96, and have had little need to delve into thermo or fluids in that time.

ah well, such is life.....

Kewl

I am beginning to understand, having read this entire thread....and only God knows why I did that.... why cars cost so much. And all this time I was thinking along the lines of push gas pedal, cars goes. Push gas pedal further, car goes faster, when clearly there is far more to it.

I think I said I read the entire thread. Its true, I did. Understanding it however is another matter altogether. Careful, did I say matter? I certainly don't want to get this thread started on a matter/energy discussion. Forgive me for saying that.

I will say that this thread has been most enlightning. I shall check back often to learn more. Thanks guys!

shelleys_man_06

Well guys, I am still crunching numbers to see if there are any correlations between the eccentricity ratio and other engine parameters. It's hard . What I would really like to get my hands on is Kenichi Yamamoto's book, Rotary Engine, to help me further understand how this novelty works . As an ME student, I have to take two thermodynamics classes. I've taken the first one, which is pretty much power cycles. My second one includes combustion, psychrometrics, you know, stuff normal gearheads never have to worry about . My friend has my thermo book so I cannot go back and do anything useful until the fall semester starts in August . I will do what I can to get to the bottom of this. Thank you everyone for supporting this rather complicated thread . And thank you, IKN for bringing it up .

IKnowNot'ing

Apparently, we ALL want that book...

I wouldn't bring the subject if I didn't need the answer! For the moment being, I'm doing all my modeling work using the Renesis e/R ratio as it is confirmed it mechanically works...

IKnowNot'ing

Apparently the R/e ratio is commonly called the K Factor. Does this name ring a bell? I can now re-write my question : how critical is the K factor for a Wankel rotary piston machinery and why (+ recommendations)? Thanks to all.

bgreene

IKN,

I don't want to inadvertantly insult you by saying this (I don't know how technical your background is), but just about anything that's engineered to a significant extent is likely to have some attribute called "K factor" (at least in USA educated engineers, since "k" and "n" are used in notation as sort of catchall variables in many different kinds of calculations where some sort of coefficient is needed that doesn't match with some otherwise defined value).

I did find some interesting things using google to seach for "k factor" and wankel, including a site with some pics of parts from a 25-liter displacement rotary engine. (searching just on "k factor" returned a list the length of the phone book, with very few sites having anything to do with engines).

From the equasions that shellys_man posted, the total engine compression can, but doesn't neccesarily vary with just the R/e ratio (this would depend on how the ratio is modified). However, the ratio could play directly into compression ratio, or flatness of the rotor faces, both of which could affect engine performance/efficiency.


P.S. bookfinder.com has a line on someone selling a used copy of _Rotary_Engine_. The seller (not me) is asking about $220 for it, though. You can check it out going to www.bookfinder.com and do a title search.

IKnowNot'ing

Dear bgreene,

Although I indeed have a sufficient technical background, I did not now about the common use of the K Factor terminology. Thanks for clarifying that to me. My searchon Google did not return any satisfactory hit (not in the first 10 pages anyway - shall try again).

Re the effect of this ratio on CR, please refer to one of my previous posts in this thread showing the variation of CR as a function of e/R ratio, for a given swept volume. This ratio indeed also affects the curvature radius of the rotor flanks in order to ensure proper sealing during operation.

I also know about the book at $220- in a US based library. However, I'm first trying to find another,cheaper, source for the book, considering I don't realy know its exact content and technical relevance. Thanks nevertheless.

bgreene

Looking over this one quickly, I'm not sure if you've got the right two numbers.

You say that e/R of 0.25 is maximum, however one of my google searches (I'll see if I can find the page again and post a link) made reference to a wankel engine with a R/e value > 25.

Obviously "R" refers to the distance from a rotor tip to the centroid of the "triangle". This would define the circle within which the pure rotation of the rotor could occur.

"e" however, is a geometric property of the eccentric shaft, not the rotor shape, and would determine the circle around which the rotor centroid travels in its planetary motion. For a triangular rotor, the limit for "e" would be the distance from the "flat" to the centroid (minus eccentric shaft radius and whatever minimum wall thickness is required by the strength/durability limits of the rotor). However, "e" could be any value from zero to this limit.

In theory, assuming a flat-sided equilateral triangle rotor with face length of "A",

R= A/(3^.5), and e(max)= A*(3^.5)/6 [this rerults in a R/e ratio = 2.0 at e(max)]

These two values are theoretically independent of each other to some extent, although there may well be many combinations for which the required housing shape would not be producable.

Curvature of the rotor faces, beyond influencing the needed shape of the housing, would also increase the maximum theoretical eccentricity of the engine, since the midpoint of each face would be farther from the centroid of the rotor "triangle".

As with any complex mechanism, theory will only get one so far before real test/operation data is needed. It's likely that a big factor in Mazda's choice of the R/e ratio for the Renesis (at or very close to 7.0) is as much due to this value being one that they have a lot of data points for, since it's very close to or the same as the ratio for previous rotaries that they have made, since extensive study of this variable wold get quite expensive in a hurry with all of the engines they'd have to build.

Just to increase the eccentricity at the current rotor size might require more exotic materials to make the rotors or the eccentric shaft (makes the thing more expensive), could cause durability issues with rotors cracking, might require changes to the depth of the "pocket" in the rotor faces, and would alter the way the gearing on the e-shaft and the rotors would interface, possibly making proper meshing impossible. And those are just the possible issues that I can think of......

IKnowNot'ing

R/e = 25 would mean e/R = 0.04, and that's < 0.25 !
e/R of .25 is not an absolute limit. Beyond that point, you get a negative curvature of the rotor flanks. Also, a bit futher beyond that point, the epitrochoid become destorted. Also, well before e/R=0.25, the planetary gear on the rotor is bigger than the rotor itself, wich can lead to design difficulties.


In my documents R refers to the epitrochoid generating radius. But indeed it corresponds to the distance you mention above.

e directly and univocally influences the curvature radius of the rotor flanks. You indeed somehow have to respect a given curvature in order to obtain proper 'squish' of the charge after both TDC.

Yes indeed, so far Mazda seems to have progressed on the rotaries in very careful steps. Geometry of Mazda rotaries have been almost identical for quite a while now.
I'm not intending to design a rotary engine though, but another 'machinery' that would benefit from more freedom in the e/R ratio. However, the e/R criticality, as you mentionned, could have other effects on the rotary design/operation than just the CR. And I'd like to know...