tough question...

[v10_viper]

I know the formula for cubic displacement of a piston engine, but does anyone know what the forumla is for the rotary engine??

[bowtiebandit]

May find what your after here..alot of good info

http://www.rotaryengineillustrated.com/index.html

[Scott 02]

Damn it, I just did a report on the rotary engine"Wankel Engine" this week for my Algebra 2 class.

[v10_viper]

Went to that sight, doesn't help, and I'm not quite sure there really is a formula. Because in the rotor it has areas ground in on the face of it to increase displacement. Quite a bit of calculus and trig, stuff I'm not into yet. But with the eccentric shape or w/e it's gotta be a hard formula.

I'm also doing a report on it right now Scott, I'll post the report when I'm done with it, suppose to have somewhere in the vicinity of 1000-1200 words, after that we use the report to make a Powerpoint presentation.

btw, I get the emails sent to me when someone replys, and I got one that said ivymike replied, bowtiebandit and scott replied, but for some reason ivymike doesn't have a post in this thread, is this some mistake by AF, can anyone tell me?

[Scott 02]

i'll look around tomorrow and see what i can find.

[Steel]

well basically its BDC swepped area * width of rotor + volume of the indent * how many rotors.

So yeah, complicated calc required. Have fun


edit: p.s. as a hint, the combustion volume of one face of one rotor is 654cc's, *2 for the engine 1308 cc's, hence 13b engine

[ivymike1031]

I 'm a bit curious how the volume of the indent changes displacement - does the indent get bigger and smaller during the cycle?

[flylwsi]

the indent is like having a dished piston. it would lower compression... it's adding volume to the chamber

[ivymike1031]

are you suggesting that a dished piston affects displacement?

[ivymike1031]

Originally posted by v10_viper
btw, I get the emails sent to me when someone replys, and I got one that said ivymike replied, bowtiebandit and scott replied, but for some reason ivymike doesn't have a post in this thread, is this some mistake by AF, can anyone tell me?

I replied, but then decided that I didn't like my original response, so I deleted it.

[ivymike1031]

well, I think I've got a formula figured out for the change in "cylinder" volume from "bdc" to "tdc" for one face of a rotor:

let t = crank throw
let a = radius from rotor center to an apex
let w = the distance between the sidewalls

Q is an angle
eqn1 = sqrt(a^2 + t^2 -2*a*t*cos(2/3*Q + 60deg))

Area1 = integral from 0deg to 360deg with respect to Q of eqn1
Area2 = integral from 270deg to 630deg with respect to Q of eqn1
Area3 = 1.732*(0.5*a^2 - a*t)
Area4 = 1.732*(0.5*a^2 + a*t)

then the volume change for one face from min vol to max vol is

D = w * ((Area1-Area3) - (Area2-Area4))

Unfortunately, I have been unable to perform the symbolic integral of eqn1. Eqn1 is easy enough to integrate numerically, however, so if you have some figures to punch in for t, a, and w, you can (hopefully) calculate displacement.

I'm tempted to say that since there are three faces per rotor, and each rotor rotates once for every three crank revs, total engine displacement is (number of rotors) * D. I haven't really given it much thought, though.

[flylwsi]

technically yes it does.

your displacement is the area inside the combustion chamber x # of cyls.

if you run a piston that's dished, it will give x cc's.

if you run a high comp piston that intrudes into the combustion chamber, you'll get a smaller cc #...

make sense?

to give a bad analogy...
a pie pan is a low comp piston... put something in it (mound shaped in the middle, and the volume drops...

it's miniscule, and not worth changing your 5.0 to a 4.5 or something like that, but it makes a difference.

[ivymike1031]

Originally posted by flylwsi
technically yes it does.

your displacement is the area inside the combustion chamber x # of cyls.

if you run a piston that's dished, it will give x cc's.

You might want to research this a bit further. Hint: displacement is not related to the volume of the combustion chamber (assuming that when you say combustion chamber, you mean the volume above the piston at TDC).

[flylwsi]

and how would you measure your displacement then, if that's not the case?

(and yeah, i meant at full compression as you assumed... )

[ivymike1031]

...by the amount of volume that is displaced by the pistons: the BDC volume minus the TDC volume times the number of cylinders.

It's calculated as follows:
pi * (bore diam)^2 / 4 * stroke * number of cylinders

The compression ratio, (BDC volume)/(TDC volume), does depend on the combustion chamber volume, as I believe you mentioned earlier...