Not that 9000 RPM is anything to scoff at, but why isn't the RENESIS's redline higher?

One of the rotary's supposed advantages is the ability to rev higher than a conventional engine. With a piston-driven engine, the redline is mostly dependant on the strength of the con rods and valve springs. However, since a rotary engine has neither of those things, what are the major factors contributing to its redline?

 

If I would have to venture a guess, I would say that the redline is set at 9000 RPM for the benefit of the engine, also to ensure reliability and long life.

I also remember saying that even though the redlind is at 9000 RPM, the fuel cutoff is somewhere in the area of 10000 RPM.

Other than that little bit of information, I have no clue.

 

I think the true reason is for power usage. If you needed to shift at 12k RPMs (which undoubtedly is possible) it's very inconvienient to wait that long for power. Thus the engine has been tuned to get higher power from a lower rev, 9k being that. The torque band wouldn't be the same if they changed it around.

 

There were probably several reasons. How about these 3?

1. Looking at the torque curve, the current renesis with the current intake can't breathe effectively enough if the rpms get too high. Note that the torque curve drops after 7500 rpms (see torque curve below). Some overlap will help volumetric efficiency at high rpms but the renesis has none. Perhaps a future renesis can have overlap at high speeds if the the high rpm intake port was on the peripheral housing? Per Mazda: "The advantage of peripheral induction, high power output, compensates for larger overlapping and deceleration unstable combustion".



2. Mazda wanted to have a wide powerband without any major dips in it. Spacing the 3 higher areas of the torque curve over higher rpms would leave bigger "dips". The s2000 was able to achieve a very high hp peak but Honda spaced the 2 cams very far apart to do this and there is a pretty good dip in the torque curve between them. On cars that are geared a lot, variations in the torque curve are more noticable.

3. The renesis should be able to rev a bit higher. Realize that the rotors are only spinning at 3000 rpms when the driveshaft and the tachometer say 9000 rpms. One of the things that Mazda had to do with the renesis is prove that it is reliable since the last rx-7 gave rotaries a bad reliability reputation. Mazda may have been a little more conservative with the (more mainstream) rx-8 that they would been with a rx-7. Also realize that although the rotors have been lightened to approx 8.2 pounds, the rotors do not take a purely circular path and they can exert a good amount of force to the eccentric shaft.

Brian

 

Good explanation, Buger. Thanks.

I need to ask my boss if he'll let me drive his S2000. Maybe I'll bribe him with a chance to drive my WRX.

 

The RENESIS could 'breathe' better at high speed if the induction had 4 ports instead of 3. That would allow the high speed ports to be made more aggresive. So the RENESIS could make like 300 hp @ 10,000 revs, and peak torque would around 175 ft-lbs @ 8000 revs or so. But the torque curve (3D, not just 2D) would be flatter

This would cost more(Rx-7?). One low side port would work from 0-2500, the other low speed side port would open up at 2500, the first high speed coming online at 5000 or so, and the last top end port kicking in at 7500.

This would be pretty smooth in operation, in contrast to the VTEC kick @ 6000 in an S2000. Although i-VTEC could smooth out that transition, are you listening Honda?

The rotors rotate at 2/3 of main shaft rpms, not 1/3, the wankel just wouldn't work otherwise? That is the gearing of the internal/external gearing, isn't it?

 

Hi MikeW,

I wondered why Mazda didn't use a 4 path induction manifold to optimize a larger range or rpms. I too assumed that the cost and complexity were the probable reasons why this never came to be. We would all love a variable length induction system like the one on the lemans winner.

I am pretty sure that the rotors rotate at 1/3 the speed of the eccentric shaft or the wankel wouldn't work otherwise? The ratio of the gears themselves is 2:3, with the small gear having 34 teeth and the larger one 51 teeth.

From (http://www.rotaryengineillustrated.c...mech-cycl.html) :

"In rotary applications, however, the small external gear is fixed and the larger internal gear moves eccentrically. The external gear is appropriately called the Stationary gear, while the internal gear is simply referred to as the rotor's Internal Gear. Since the internal gear is affixed to the rotor, which is orbiting on the eccentric shaft rotor journal, the eccentricity of the internal gear is that of the rotor. As before the two gears are of a 2:3 ratio, with a tooth counts of 34 and 51 for the small and large gear, respectively. "Basically, this results in one rotation if the internal gear (and thus the rotor) requiring three orbits of the eccentric shaft journal (i.e. three rotations of the eccentric shaft)".

Please correct me if I'm wrong however because this does seem to be a subject that many people are confused about. (perhaps even me?)

Brian

 

... is this really such a far fetched idea?
check out the 26b (from the sae paper) and look at what that system did to the torque curve. All it really was, was a fancy manifold with intake trumpets run on a slider controlled by a healthily sized servo.

Admittedly that servo couldn't keep up in 1st and second but really is the hardware THAT expensive? Surely it's a lot cheaper than a turbo option?

-pete

 

I thought I read that we WERE getting a variable length induction system? I'll have to peruse the articles again...

 

Quote from MazdaUSA site: "engine also features a sequential dynamic intake system"

Also, the C&D article said that she gets a variable-volume intake system.

Are these not the same as, or similar to a variable length induction system?

 

Okay, just checked out the mazdaspeed tech page:

"each rotor employs three intake ports, and a variable intake timing mechanism. Under this system, dedicated high-speed intake ports begin to operate when the engine operates at high-rev levels. This makes it possible to use the intake's dynamic effect at high and low speeds to maximize compression efficiency."

So, basically more of the same, but with a fancy new title.

 

Hi B-Nez,

You're right in that it is just a semantics thing. The rx-8 will have 3 different length intake paths that can time the intake "pulses" so that they will push more air into the other rotor at 3 different rpm ranges.

The variable length intake system that rpm_power and I mentioned used a trombone type mechanism that slid in and out to change the length of the intake path as rpms would increase or decrease! This helped the engine "breathe" better at all rpms rather than just at 3 rpm ranges.

When I first heard about the rx-8, I too hoped that it would have some sort of intake piping like the lemans winner. Such a system would add a little cost and complexity to the design (it would also be another thing that could break) but I think the real reason the rx-8 doesn't have it is that the trombone looking intake would probably be sticking up out of the hood at low rpms. I think it would be pretty cool if we had this weird trombone looking thing going up and down with our rpms but the general public might think it might be too weird?

Brian

 

Isn't it true that manufacturers have to put a scatter shield in a car thats capable of spinning over 10k? Or is that just NHRA rules?

 

i'm unaware of any such rules for road cars... and the reason any ruling body would mandate such a protective feature is so that everyone would have them and not loose any sort of competitive edge. i'm sure road cars, regardless of the speed the engine and transmission spin, would be safe as people generally wouldn't buy them if they weren't... not to mention that road cars, for the most part, don't grenade as often as race cars.

Boogs, the system in the RENESIS then is a manifold with three separate solid pathways, which are then used depending on rpm and load, yes?? i suppose a mechanical system would be a bit more expensive (couple of hundred $$ per engine), and they WOULD have to worry about the reliability of such a fragile system in the real world... like road muck, salt, sand, etc, etc, getting into the works... i suppose it could be made robust and impervious, but then it'd be heavier, and probably wouldn't be as adjustable as the solid manifold in such a tight space?? bleh, neat idea... wish it would work :D

 

NO, I KNOW NHRA requires a scatter sheild for engines spinning above 10k, and clutch companies HIGHLY recommend it. I HAVE seen a tranny that was spinning at 11k at which point a peice broke loose and put a 6 inch round hole in the side of the bell-housing. There defenitly is a need if the engine is above 10k...I was just wondering if that was part of the idea of a 9k redline.

Michael

 

Hi Wakeech,

You pretty much have the idea. The different length paths are used depending on rpms (not loads). The way I understand it, air is sucked into the intake port as the rotor turns. The rush of air into the intake port is abruptly cut off when the intake port closes. This causes a pulse of air to go in the opposite direction. This pulse can be timed (depending on the length of the path) to arrive at the exact time the intake port for the other rotor opens. At rpms get higher, the path from the intake ports on one rotor and the next must be shorter to time the pulse.

Below is a picture of the rx-8 3-path intake:

And below is a link to the 4-rotor lemans sae paper:

http://www.mymazdarotary.com/mazda_r...paper_html.htm

Brian

 

thanks for the link...
wowee zowee!! that's all that comes to mind when i think about the blunt simplicity found in the R26B... hooooooly crap...

it was especially neat to see how they employed "the squeeze" with the third plug (the idea hadn't occurred to me), as i'd read a bit about using the squeeze in karting applications!! it's really cool to see that...
man oh man... that engine is SOOOO KILLER SIMPLE!! WOW!! just incredible.

it's really impressive how, looking at the torque curves from the R26B, they can make something streetable less than 10 years later, with the same redline, and "better" (smoother) overall driveability... of course, the RENESIS isn't making 690 hp at 9000 rpm

 

:D ta da!! i'm bored, sorry... read.

 

I read somewhere (sorry, i don't remember where) that the RENESIS was tested to be able to rev to 17000 rpms before breaking.
The rotors on a wankel engine move in a sort-of up and down motion while rotating in the chamber and excert tremendous force on the eccentric shaft. This motion can cause the rotor tips to actually touch the housings after compressing the apex seals' springs if revved too high thus killing the engine.
Also remember that all designs carry a safety factor...

 

actually, the rotors move in an "planetary" motion, which means they orbit a centre point while at the same time rotating... but yes you're right about the rest.

http://www.rotaryengineillustrated.c...hanics101.html

 

If you want to compare the Renesis to a piston engine, you have to compare it to a motorcycle engine, because car engines are too big (displacement).

The Suzuki Hayabusa has a 1.3 L engine and redlines at 11,000 rpm. However, the Hayabusa doesn't even come close to the RX-8's power or torque output.

The Hayabusa generates 175hp and 102lb-ft of torque.

 

but that's a 4 stroke 1.3L... it'd be more comparable to a 1.3L 2 stroke...

 

The Renesis may be more comparable to a 2-stroke in output per Liter, but a 2-stroke can't even be made street legal, so any comparison would be purely academic.

 

so often this is true.
... but yes, that was my only intention was to be fair to the comparison: it's true that a 2 stroke is more comparable.

on (nearly) all accounts though, the RENESIS would win: emissions, fuel consumption, breadth of the torque curve, sound (:p that's subjective, i know, but it's still better)