uhhh.... you're telling me that steelies and ceramic seals have spring pressures in the thousands of lbs/sq.in. ??

side seals and corner seals still use gas sealing, not to mention piston rings.

 

With that said, does anyone know what the spring constant(s) are the for RENESIS' apex seals? My guess is a spring spec'd around at least 1 ksi is over-engineered.

 

Why would I want a rotary engine with no springs?!

 

Apex seal springs are not only effective at startup and low rpm's. They are very effective everywhere. Inertia from the engines rotation assists the seals in sealing at higher rpms but they are not the only thing responsible for it. You always have spring tension against the seals. There is never a time you don't. If people think that the seals themselves are only held against the housings at high rpm's by inertia then I'll throw this little tidbit in. The springs have mass too and are being forced against the seals. This is in addition to their normal tension against the seals.

 

You guys have got me thinking.....
The forces on the seal at the sealing contact surfaces will be
1 - Inertia, a radial force which is a function of the housing profile. It will force the seal outwards where the housing profile is concave and inwards where the profile is convex.
2 - Drag, a tangential force at the housing/seal contact and a radial force at the rotor/seal contact.
3 - Pressure Forces, radial and tangential forces due to chamber differential pressures which vary widely during the cycle.
4 - Spring force, essential constant radial force (ignoring housing profile error, temperature effects, and spring aging).
At any point during the cycle the actual sealing forces will be the sum of the instantaineous values of all of these.
If you want a uniform, predictable housing seal force then less inertia and stronger spring force seems an obvious way to go. The lighter, stiffer ceramic material will have much higher resonant frequencies for the same size & shape
of seal.
That said, do the deficiencies of the iron seals relative to ceramic have any impact on someone like me driving the stock motor on the street at no more than beep speed?

 

don't take it the wrong way MarkJ, it's not like i think you're completely mistaken, i just find it really really hard to believe.

and of course we both know that the springs are entirely necessary when there is no gas pressure to speak of (like, when you start the engine).

 

It's all good MarkJ.

 

The nice thing about ceramic seals is that they are essentially self lubricating. Don't get this confused with not needing oil injection. What I mean is that they are slippery compared to ordinary seals. They have a lower coefficient of friction. This is one reason why they make more power. Their light weight actually lets them use less spring pressure and this too contributes to less wear.

 

MakJ has obviously built and dynoed more than a few stock and race engines, those who haven't please stop arguing.

 

I got a little confused over the spring tension comment so I just went and did a little digging to make sure which it was. This is a quoted passage from Scuderiaciriani.com about the benefits of ceramic apex seals.


"The advantages in any engine are that the ceramic material used has a lower coeficient of friction than the more conventional ductille iron seals, and the material is a lot harder. The lower friction, means less wear to the surrounding parts (apex seal grooves, rotor housings, and side housings), and less wear to the seals due to both the lower friction, and harder material. The wear characteristics have been described to me as negligable, but initially enough for them to break in correctly.

Other advantages, are that the ceramic material has less physical mass, so requires less spring pressure to keep them in contact with the rotor housing surface at a given rpm, plus due to the lower coeficient of friction you can use more spring pressure without causing an appreciable increase in wear. Either one of the above means the engine can run at higher rpms without the seal floating, which can cause hot exhaust gasses to travel back into the intake cycle of the other rotor chamber and cause pre-detonation which is one of the major concerns in any non diesel engine.

Stock apex seals can overheat, and warp which is not a concern with the ceramic seals. The ceramic seals although not indestructable, can take a lot more detonation than stock seals. They can also stand up to a lot more boost/power output.

Now for why people stray away from them: ceramic 2 piece 2mm or 3mm seal set costs $1770, plus in the case of the 3mm seals you have to buy double the number of apex seal springs. You could go for the single piece 2mm or 3mm seals for $1200, but will have less compression under 4000 rpm over the 2 piece seals.

After getting past the 'you don't want those!...why not?...because they cost a lot'....banter, I have heard from several rebuild places, and Franchesco Ianetti himself that when it is time to rebuild the engine, you can re-use the ceramic seals over and over again (the gift that keeps on giving?)."


At least I know I was right as far as spring pressure and lower friction coefficient. I haven't been the one arguing with MarkJ about anything else but this part I am absolutely sure of. I'm just glad I could confirm it elsewhere.

 

Just how much is the apex seal friction reduced with ceramic vs. stock seals?

All other considerations aside, the reduced friction, times the rotor "radius" times the number fo seals engaged at any given time (it would seem that spring compression of two seals should reduce regardless of seal material at the instant of a "flat" on a rotor being at its closest point to the long face of the housing) would equate to a direct increase in torque at the crank, even if all other considerations of the seals are assumed to be constant.

If the interface friction is a small force (or if the actual difference from stock to ceramic is small), then this could mean a relatively small gain due to this effect, but if the change in force is significant.....

 

still, i'd be curious to see spring pressure numbers, and compare them to theoretical gas pressure numbers. i'm still quite sure that we'll see that during operation gas pressure contributes the vast majority of the sealing pressure.

 

none, and how many times do i have to reiterate that springs alone ain't going to do it?? like i said before, it's not like i think you're being dumb, but i'm not convinced.

lets go absolutely nuts and say that it's 80lbs/mm compression. it's 160mm^2 surface area (80mm length, 2mm width), and is under 5 mm of compression when at TDC right at the point of a combustion event.

80 * 5 / 160 = 2.5 lbs/mm^2 * 625mm^2/si = 1562.5 psi crush pressure

now i dunno about you, but even with that level of outrageous spring pressure, it won't get the job done when it needs to stop five times that much.

 

Isn't the combustion pressure mostly coming at the seal from the side?

Spring pressure in a rigid seal would act perpendicular to the housing wall, from the look of it, so there probably isn't going to be a 1:1 corelation in terms of blow-by at TDC, and the relationship between the two would appear to get more comlez as the chamber moves around and the angle of the seal to the housing wall (as well as the spring compression distance) changes.

In terms of seal geometry, doesn't using contained pressure to help close a seal generally require a softer, "wiper blade" type of seal?

Then again, I might be way off base, since I'm pretty much a newbie when it comes to the nuts and bolts of these kinds of engines....

 

...when did i say they did?? i said or rather asked if, from post one, that springs do not do most of the sealing, and gas pressure does.

gas pressure is theoretically equal everywhere in the volume. yes, it does change, being at its maximum sometime very soon after the event of combustion near TDC on its way to BDC (how close varying upon rpm, i suppose), which is where you need the most sealing. the apex seal groove allows the high pressure gas to push the seal up, out, and into the rotor housing (in the same way gasses press the side seals, corner seals, or piston rings out against the block) creating a crush pressure equal to the pressure in the chamber, allowing for theoretically no leakage.

high spring pressures contribute a little force to this, sure, but their importance is in locating the seal in the groove when there is no gas pressure to crush them into the rotor housing, and to make sure that they stay up against the rotor housing on the way back out of the groove (where chatter occurs). low seal mass and high spring rates would create the best sealing for any application, allowing for the best sealing possible... i don't know why you're talking about this "no spring" and "soft spring" stuff...

 

ya, i have.

you know as well as i do that the seal needs to be located vertically and longitudinally in the groove, 'cause they're two peice, as without the spring any seal would lose contact with the chamber wall and cease to function.

 

This is kinda dumb to argue about still, its been printed how the gas seals in a rotary work. they are not going to change the entire operating principle of the seals. they work on gas pressure and allways have. spring pressure is mainly for starting the car as has been stated numerous times. there is no way in hell spring pressure alone would seal the chamber, especially when you accout for the fact that the springs are at there LEAST compressed state during the peak of combustion pressure, thus they are at there minimum spring pressure at maximum chamber pressure doesnt sound to effective to me. Ceramic seals are great but you guys are buying into venders marketing BS. I'm not denying that the springs are stiffer, as ive never been in the same room with ceramics. But i have bulilt a motor and the spring tension isnt very great at all, especially at TDC.

I still dont understand why you guys think that the entire operating principle of the rotaries gas seals has changed for ceramics,.. thats just asinine. Especially when your proposeing a new working principle with no documentation what so ever based on a few ounces of spring pressure... Post something documented and you might have more of an argument, opinions dont really work for me.

 

Be very careful if all or the majority of your experience in a given field comes from things you read; especially on the internet. If you aren't SURE about something, please don't act like an authority on it. It's okay to speculate and to ask questions but it would be prudent not to go to far beyond that if you don't have hands on experience.
I think I'm going to start a thread on rotary engine experience: How many have you built, how many have you tuned, how many have you run on the dyno, how many races or championships have you won, how many types have you worked on ( 10A through R26b and renesis), have your engines ever run at LeMans with Mazdaspeed as your main sponsor?
Now that may sound silly, and although my personal experience is decent, I have the privilege of sucking information from someone who has covered everything mentioned above many times over. I've seen Japanese engineers from Mazda reverence him even admitting that he knew more than them in some regards. He has met Mr. Yamamoto and is reponsible for a lot of the design and specification on the best of the ceramic seals available.I asked him his opinion on gas pressure keeping the seals in contact with the housings and he smiled as he replied that it may to some extent but spring pressure was the key factor since 1974 when the materials changed.
For those who still don't believe this because they haven't read it in a book consider that Mazda has not printed anywhere as yet (To the best of my knowledge) that the side seal clearances on the renesis have totally changed. When I first had a discussion about this on this forum people said we were full of it because they hadn't seen from their typical sources.
I've probably accomplished nothing here today but I pray that some may pause and consider a different approach, if not to life, at least to posting on this forum.

 

Hello there,

We have had a lot of R&D and testing with highly modified renesis rotors, we have developed 2 piece 3 mm twin spring ceramic seals for the rx8 rotors or you can use 1 piece 3mm carbons, the reason we have gone to a 3mm and not a 2mm is because we still use the old style apex seals meaning they a deeper and require a deeper apex seal groove, we have tryed to extend the original 2mm groove of the rx8 rotor but after 6 new rotors in the bin we decided to cut a whole new 3mm groove to solve the problem, and that problem is we cannot match the original 2mm groove to the deeper 2mm cut we are making. And if you are every planning to run rx8 rotor with a conventional large bridge port you will soon find out that the original 4mm deep rx8 seals will fall down the bridge as they begin to sweep the bridge as they are normally held up by the corner seal and lower part of a 8mm deep earier seal..

We have also been working along side our seal maker to come up with a 3mm apex seal that has a 2mm tip which like the prevous poster has mentioned has a better sweeping surface and less seal chatter.. and the ceramic seals do offer better torque at lower revs and also better surface contact at higher revs

Thanks

 

Thanks Charles,

With our sales at work, 70 % of Internet sales are over seas and about 20% of our phones sale are two. We deal a lot with overseas in the last two weeks we have done 2 motors in Perto Rico. We are know for our Rare parts and Engineering and especially our Race Cars, and soon our Drag Cars(sorry guys Iam advertising myself here just establishing who and what we do) So we can help anyone over seas even more so with success of our rx8 motor conversation, into older motors and other.

Thanks,

 

Another important thing to mention and this from experience, On people who have dynoed there car even with exhaust mods, where has the peak power been, even more so the peak torque in relation to the HP, Lets guess 9000 rpm, well unless you really needed to why would you rev it another 2000 rpm beyond that, your not running short manifolds or exhaust systems that are really setup for that setup, tell me where does the torque start to fall off, Iam interested because with using rx8 rotors and gear in older motors we have seen peak power at about 9600 rpm in race trim, including quad throttle body setups and twin exhaust systems all the way pass the diff and very expensive cold air intakes giving a 5 kpa positive at 200ks per hour.. Tell me if we are onto something compared with other motors as we deal with rx8 motors not cars...

Thanks