rx-7 obsessed

i have a strange buzzing noise......






























hahaha just kidding but i do have a question for someone with a better enginering mind then my own. so the other day i was playin in the garage and one turn of the ecentric shaft is = to only 1 rotor face (per rotor) so when your tach is at 7000 rpm is the rotor only turning at 3500 rpm and the eccentric shaft turning at 21000 rpm. i know it sounds like a funny question but its just something thats been bugging me and i need to know

calpatriot

It's a ratio of 1:3. The rotors turn at 1/3 the speed of the eccentric shaft; thus at 7000 rpm on the tach the rotors are actually turning at 2333 rpm.

Grip

iTrader: (9)

interesting.

s2kguy17

iTrader: (5)

wow just imagine a pp engines rpms

jjwalker

Yeah, the rotors aren't moving all that fast at redline, considering the shaft speed.

Hell a reciprocating engine sees every rotation of the shaft, which has always made me wonder why the wankel engine couldn't turn faster. I do however believe that it all has to do with the stationary gears and apex seal velocity, not that the rotor, housings and eccentric shaft can't handle it.

rx-7 obsessed

haha well thats neat....but i think the rpm would have more to do with sealing. because of the angle of the seals at certian points in the cycle it would be hard to make a seal that would work at that speed(i think)

TrboSpdAnt

iTrader: (5)

From my experience, it's the ignition pick up that can't keep up with the need for spark... Once you get somewhere the 10k range, it starts to miss, HORRIBLY...

I think I read someplace that a plane (OBVIOUSLY Rotary-powered) was taching at, like 13-15k.... Under cruising speed... Not Spool up.

Wish I could remember where the *** I found that.

TrboSpdAnt

iTrader: (5)

I think OVER 10, on the stock ignition system, was what I meant. *shrug*

Seen a couple of vids with guys taching pretty close to the 10-range.

Both my FC and FD have A/M ignition-systems and happily tach around there... If I wereen't concerned about a high-velocity disassemble, I get the ignition and fuel system they make for kit planes....

Heheheheh.

jjwalker

Like I said before, I believe the apex seal velocity as it rotates around the housing is the biggest factor. The big epic fail of reciprocating engines is the fact that they reciprocate. They have to come to a stop twice and reverse direction for every rotation of the crank shaft. Now, everyone seems to think the rotary doesn't have this issue, which is mostly true, but not totally. If you look at the housing, you'll notice that between the trailing and leading plugs, and also above the exhaust port, there is a protrusion. When an apex seal hits this protrusion it actually slows down as the apex seal ahead is speeding up. Once the seal clears this point, it will speed up again.

Mazda has a diagram somewhere that shows specifically what i am talking about, and the velocity of the seal as it goes around the housing.

Keep in mind, the apex seal is taking a lot of strain due to its contact with the housing. As the seal moves forward through the housing, the seal is being pushed backward in its groove (mazda has a diagram for this too). Also, the seal is being forced outward and against the housing surface by centrifugal force, which becomes rather strong when you are rotating at such speeds. My point is, the seal is trying to be torn out of the seal groove as it rotates around the housing by centrifugal force and friction.

Now we come to the second point I made earlier about the stationary gears. All rotary race cars that I know of that run high RPM have hardened gears to allow better shear strength. The gear teeth experience shear as the rotor gear teeth starts the leave the stationary gear teeth (try and visualize this). The higher the rotor speed, the higher the shear stress presented to the gears.

Let's throw another variable in here shall we?

The eccentric shaft is a very strong part but look at how it is built and how power is transmitted to it. The eccentric shaft experiences a large amount of torsional load from the power stroke, especially in the center of the shaft where it has no bearing support between the 2 rotors. When the front rotor is providing power to the shaft, you have resistance on the rear rotor portion of the shaft since it is in its compression phase.

I could go into the shear forces presented to the oil in the plain bearings...but won't.

My whole point is, yes the rotary is capable of spinning much faster, but it requires extensive modification not to mention balancing. It wasn't economical for mazda to do it, as much as I wish they did.

TrboSpdAnt

iTrader: (5)

^+1 No doubt.

I wasn't figuring in that Mazda was going to do this as an OEM move, by any stretch...

I think that the fact the reciprocating engine stops twice in a single, 360* rotation is it's downfall, yes, but since we're looking at the 1:3 rotational ratio, so it's not really that much... 7000 > 2333 rpm... That's not **** in reference to something like a standard crank whipping around at over 5k.

LOL... Of course, the LAST thing you want to is chuck an apex seal at any relevant speed... Say 3k ACTUAL rotational speed.

Ouch for the housing.

Agreed.