Most of the Renesis pics I have seen show lightweight rotors that were hollow with supports... the same rotor as the RX-8 CD that Mazda sent out. Looking at the engine on the following page:

http://www.auto-web.co.jp/NEW_CAR/RX-8/08.html

The Renesis has rotors just like the older engines... the inflated triangles.

When was the change made, and why? Perhaps the rotors are different between low and hi-power models?

 

actually, all (i'm pretty sure) of the rotors in Mazda wankel engines have been hollow-cast, and those photos are just of cut-aways because they look cool...

the difference between the high and low power versions of the RENESIS is all in the length of the induction runners (just two longer ones, instead of three long-medium-short ones), and maaaaybe slightly less aggressive ports (just a theory of mine...)
these differences are there only to increase the volumetric efficiency of the engine (how effectively air and fuel fill the chamber on the inspiration stroke) at lower or higher rpm... because the low power engine is lower powered because of the lower rpm it has to run (made for an auto tranny), they would adjust the tuning of the engine to maximize torque at lower rpm where it would be more useful.

 

The shape of the hollow rotors is much different. I have my little CD right here... the difference is pronounced at the apex.

The hollow rotor is on Mazda's Japanese RX-8 site:
http://www.rx-8.mazda.co.jp/?official_icon

Damn, this is a cool little car. The more I look at that interior, the suicide doors, the thoughtful everywhere touches, the more turned on I get. That has got to be the coolest interior I have EVER seen.

I just hope the driving experience is not a letdown. It may not be as quick as a 350Z, but I hope it spanks it in terms of feel. (I drove a 350Z last week and was not impressed... it does not feel like a G35. Feels stiffer, less fluid. I'd rather drive my old GTI 1.8T than a 350Z....) Automobile has been full of themselves and missing the mark lately... hopefully their RX-8 writeup is just another example of their poor journalism..

 

ahhhh, i see what you mean, i think... :D
yes that image looks really really weird, and quite unrotoresque because that too is the silohuette (SP, bad... sorry) of a cut away rotor... the reason it looks like it's three arrows pointing away from the inner gear is because the rotors themselves don't have smooth, curved faces when they're cast: they've got the combustion chambers cast into them.
this "bathtub" shaped combustion chamber is what's know as "Centered Deep Depression" (or something similar to that... Boogs, help me out. i know you know that one)... the depression on each of the three combustion faces is to facilitate the flame front and determine its direction (the depression is around the spark plugs), and also to determine the compression ratio of the engine. in the SAE paper Buger sent me on the R26B (the engine of the 787B) this depression was of a shape closer to a tear drop, called "Trailing Deep Depression" (or something like that), which allowed the third spark plug (which is in a super-trailing position) to use "the squish" effect during the combustion phase...

anyhoo, back on topic... the reason that those cutaway rotors look so funny is 'cause they really highlight the depression in the combustion face of the rotor, as they've been cut (about) down the middle, whereas normally one cannot precieve the bathtub when veiwing the rotor from the side...

i don't know why they did that... maybe some crackhead webdesigners thought that the shape of the cutaway rotor (highlighting the bathtubs) was simply more interesting to look at, or (more likely) they just didn't know.
:D in any case, it's not a big deal.
the rotors in the RENESIS are very similar to those used in the S5 13B and 13BREW...

 

Still, I saw a cutaway engine animation that separated at the side plates, and the rotor looked like a hollow rotor... in fact, I remember reading about the new, lightweight rotors. I have seen the depression before... it isn't as big as the entire rotor face. In fact, I just visited Mazda.com and under "History of the Rotary Engine" the rotors for the hydrogen powered rotary look almost exactly like the hollow rotors I am discussing.

I don't think the rotors are the same... I'd bet the farm in it, LOL. Which ones does the RX-8 have? Based on Mazda's Japanese site, I'm thinking it's the advanced design...

 

well, then i'm a big idiot... :D
sorry, i can't tell the difference or help you on this then.

 

Here is a good closeup of the renesis engine. Like Wakeech mentioned, Mazda decided that it would be interesting for people to see inside the rotor so they "cut off" the side of it.

If the renesis was truly hollow with no sides, you should be able to see the intermediate housing through the rotor right? What you are seeing in this picture is the inside of the other side of the rotor.

The renesis uses a MDR (stands for Middle Deep Recess I believe). Wakeech was right about the reason for the "bathtub" and the volume of the recess is used to control the compression ratio.

You can also clearly see the 3 holes in the peripheral housing in this picture.

DonG35Miata, now how much is your farm worth? :D jk

Brian

 

riiiiight, that's what i meant...

 

Boy, is my face red...

Thanks for the close-ups. The three circular indentations show that the rotor looks the same extrenally as previous rotors.

Damn! I thought my future rotary was going to have some exotic, high-tech rotors in it! Just SOS, different engine...

 

Not quite SOS, the rotors will be approx 8.2 pounds which is quite a bit lighter than any previous Mazda production rotors.

From: http://www.mazda.co.nz/technology/1020.html

"Among the many internal upgrades are a thin wall cast-iron rotor with fully machined combustion recesses to ensure uniform combustion. Apex seal slots have been hardened as well to resist wear. Modifications have also been made to the aluminium rotor housing around the "hot spot" spark plug area for more coolant flow. The engine's induction, exhaust, cooling and lubrication have also been modified or redesigned when compared to the series V RX-7 engine."

 

All this talk about the rotary has brought back fond memories of my grandfather, (God rest his soul- Thanks, Pop) who probably infected me with the car bug as U read through all his car magazines. I remember when the RX-7 came out he mentioned the rotary engine... I asked him what it is was, and he said, "They say it's the engine of the future. It must be, because it isn't here yet!" referring to the lack of adoption by other automakers.

I have to wonder what rotaries might be doing today if Daimler-Benz and GM continued their development efforts. With only one small Japanese automaker devoting a small portion of their engineering resources to it, unlike conventional piston engines, their has to be a lot of potential left in it. It will be interesting to see real-world results to see if it really delivers much better real-world mileage, or just in an EPA test cycle. I'm looking forward to magazine roaad tests and reports from fellow posters.

 

I wonder if this is really the case. Since piston engines have so many moving parts and are fairly complex, it's easy to see that there's lots of ways to improve them. Wankels are so simple in design I wonder just how much better they can get in terms of power efficiency.

 

plenty... think of F1: how the f*ck do they CONTINUALLY come up with improvements in performance when the number of avenues to explore to improve performance is being contricted all the time?? how does BMW exceed the previously thought invincible rev ceiling of V10 configured engines at 186** rpm (some number just short of 19K) for their designated crankshafts (which are relegated to be nearly all ferrous), and go right past 19K rpm??? HOW??? goddamit, that's what i'd like to know...

there are tons of ways to improve the rotary... i can think of a few off the top of my head:
how would one go about creating a continually variable inspriation/expiration PORT?? how would one improve ROTOR COOLING (there's an "easy" one)?? how could one incorporate a direct injection system (to localize the rich fuel/air mixture in the combustion chamber)?? how about a FICHT-esque oiling system for the apex seals (or is it already like that??)??
and so on, and so on, and so on...
of course, it's difficult to try and see where they're going when looking from the outside, and of course the way they determine thier direction of progress is to exploit all of the potential of the engine, look at its inherent flaws in design, then try to come up with a fundamental reason it is incabable in that manner, then fix that fundamental problem. we simply aren't familiar enough with the engine's mechanical workings (and the engineering data...) to know...

 

I would think there's a lot left.

In port shape and location there is so much research left to be done - rotary shops are coming up with new port shapes all the time that continually improve performance of even the old engines.

With the shift to side exhaust ports, which is as monumental as the movement to overhead valves in piston engines in the 40s and 50s, there's a whole new horizon in terms of playing with exhaust port sizes and shapes.

Improvements in materials and processes will lead to even higher engine speeds, and with increased flow even greater power can be gleaned from the motor. Unlike a piston engine, the rotary engine will find the limits of gearbox and clutch technology before reaching its own engine speed limits. I suspect that in the next few years we'll be seeing 10,000+ RPMs from the rotary from the factory which may end up requiring motorcycle-style sequential gearboxes electronically shifted to stand up to the revs.

The rotary engine is deceptively complex - it may only require three moving parts but it also requires the use of calculus to determine its displacement.

 

Since I can't read Japanese, what kind of material are the rotors for the high-power RENESIS made of? They appear to be cast-iron, but I'm not sure. If they are, I hope that aftermarket companies start making lighter rotors, maybe aluminum sprayed with Cermet. Unfortunately, the trade-off may be the compromising of low-speed driveability. The thought, I believe, is similar to lighter flywheels. But, since the rotor exhibits planar motion, which means that

translation + rotation = planar motion

there may be some slight differences.

Also, after examining the exhaust ports, I believe designing a good turbo header may be more difficult than it seems. I believe Mazda switched back to moving the exhaust port to the side housing to lower emissions. Unfrotunately, the side exhaust creates a lower fluid velocity than that of a peripheral port. Why? My theory is that since exhaust gases have to be routed to the side housings, they are forced to change direction. The speed, which is the magnitude of the velocity, is the same, but, with a side-exhaust, the gas velocity has two components. When fluid changes direction, or anything in that matter, the velocity is going to decrease. In short,

FLUIDS HATE CHANGING DIRECTION

What does this mean in terms of designing a turbo exhaust header? Since it is necessary to have an adequate, constant velocity to run the turbine, the side exhaust may not be able to provide that kind of energy. Peripheral ports, in my opinion, only exhibit a one-dimensional flow. This means that the exhaust velocity is going to be theoretically equal to the speed; this is a good thing, as it provides the means to run big turbines. However, I think that despite this shortfall, turbo kit designers will find a way to get around this. Perhaps I am exaggerating? BTW, what ever happened to Buger?

 

Well, the main reason for switching to side exhaust ports was to get better control of when it's open and to stop cross chamber contamination. In previous designs the exhaust port allowed gases to flow from one rotor face to the next. Yes, this affected emissions, but also limited compression ratio (by causing uncertainties as to the composition of the gas to be ignited - leading to nervous engineers being conservative.). In fact, the original design called for a side exhaust port, but due to the lack of knowledge of the apex seal material requirement - leading to lots of oil injection, the side exhaust ports promptly plugged up!

I wonder if they will try direct oil injection to the rotor housing face. Suzuki used that in their 2 stroke MCs with great success.

I don't think you could make the rotor from aluminum, as it softens too much with heat. The reason pistons can be made from aluminum is a combination of small surface area and large heat removal area (sides in close proximity with block). Maybe titanium....lets see, if I mortage my house maybe I could afford one titanium rotor.

To get back to the original topic, the renesis rotors are quite a bit lighter than the older designs. You don't hear much about older engines being operated up past 9K RPM for a reason! They go "BANG" if done so for long. Well, maybe "BOOM".

 

Wouldn't titanium be too abrasive for the rotor housing?

 

The rotor -NEVER- touches the housing. Not shouting, just making sure the engine knows that . The apex seals and side seals are what rub on the housing. Technically, even they don't touch the housing, they ride on an oil film (in a perfect world). In the real world, they do touch the housing from time to time. That's what wears the engine parts.

Titanium does develop a very tough oxide layer, and this actually makes it -less- abrasive - unless the layer is damaged. Aluminum is similar in this respect, just not as much toughness. That's why you can buy titanium coated drills. The titanium protects the steel during rubbing. When you machine titanium, you have to cut through this oxide layer, and that's what makes it abrasive.

 

Oops :o. I forgot the rotor never touches the housing. How about aluminum rotors sprayed with Cermet? But, when I think about it, won't the weight of the aluminum rotor kill off low-rpm power?

 

fun with photoshop...

 

Lots of people get confused about rotating mass and low end power. They really have no connection. High rotating mass can make low RPM operation smoother. Low end power is higher IF:
Intake and exhaust tracts are tuned for that - longer intake, more restriction on exhaust (though the restriction thing is due to the engine being a compromise - if it was designed for low RPM operation 0 restriction would work better). The renesis adjusts the intake length for RPM (in a blocky way).
Engine port timing is set up to allow time for the full combustion cycle to occur - at high RPM it has to occur quickly, and the port opening/closing becomes compromised because at low RPM you have lots of time to burn the air/fuel mix but at high RPM it becomes time constricted.

I suspect plasma coated aluminum was tried. I don't know why it wasn't used. Remember too, the rotors are "filled" with oil - for cooling. Perhaps this oil at what, 250 to 350 F would weaken aluminum too much.

Remember too, the rotors turn at 1/3 eccentric shaft rate - so at 6000 RPM the rotors are turning at 2000 RPM.

 

So, rotor weight is about smoothness, and not about power. I must have gotten (grammar?) the notion from lightened flywheels. Thanks, David . You're right. Shaft power is determined by the airflow characteristics. The rotors move in such a way as they compress and expand the gases, right? The weight of the rotors shouldn't affect the shaft power.

I believe the reason Mazda didn't decide to plasma coat the rotors in the first place is cost.

Also, I don't quite understand the last statement.

 

I was just emphasizing how "slow" the rotors turn. If you look at the neat thumbnail (click on it) that GiN posted above, you will see the gear in the centre of the rotor. The eccentric shaft has turned 3 times when the rotor has completed one turn.

Not coating the rotors may have been a cost thing, although they are treated anyway (low friction coating on the sides, hardening of the apex seal grooves, and probably more I haven't heard of yet or forgot). All that costs too.

 

I see what you mean. Thanks RX-8 Friend .

 

Going back in the thread regarding the emission concerns of the earlier rotaries and port overlap. I can attest to that one big time.

Back in my high school daze a buddy of mine pulled the Thermal Reactor from his 79 RX-7 12A and put on a header. As I remember, he told me the power jumped from 100hp to about 130hp. Regardless of the number, the HP increase was significant the car dropped a set of serious B@lls. It was amazing.

However, there was one serious down side. The exhaust was wicked. It would burn your eyes when idling in traffic. That car was running nasty-dirty—no visible smoke, just a real sinus burner. I mean stock there was supposedly a 30% HP loss from meeting emissions at 1979 standards. That's huge! Anyway it was still big time fun. Still my 68 Mustang didn’t dirty up its exhaust even close to the RX when I put the Holly 4 barrel and manifold on it…

Now to try to figue out the oil flow inside the rotor for cooling purposes...