irresistibo

Ive read somewhere that a rotary engine could basically rev as high as it wants to since everything moves in a smooth circular motion. Of course the red line is probably determined by the parts that are used in the engine, but is the redline as important in a rotary car? In a piston engine i know the pistons can only turn so fast, but the beauty of the rotary engine is that it can turn basically as fast as it wants, so what would happen if you took the Renisis engine to say, 15k rpm? Just curious what would happen.

Grimace

Well, I'm not an expert, but I suspect the apex seals are a limiting factor. I've got some other ideas to what limits the top RPM, but I'll wait until other people chime in, in case I'm totally off the mark.

stan11003

I am not sure piston technology is that much of a limit. If you look at Formula One I think they are doing 18,000 RPM. They are offcourse using some very advance materials to do that however.

wakeech

actually stan11003, BMW just broke the (theoretically impossible) 19K rpm barrier a week-and-a-half ago at the Monza GP with their qualifying engine.
true, piston technology far outclasses the rotary from this point of view... then again, think of how much it costs to R+D this type of thing, build it, and let's not forget, USE it, 'cause F1 engines will run for about 400 km, and then (almost infaillibly) blow up... they rebuild them and use them for testing of course, but that's basically the limit.

The rotary isn't invulnerable to the laws of science. The apex seals always have posed a very serious problem, by virtue of their need for violent reciprocation (as they pass up or down the "sides" of the rotor housing, where they quickly travel out/in/out as they pass over the bump halfway down) and distance they actually run around. But, there are other factors which limit the wankel's operable rpm limits too... architectural things like eccentric shaft strength (the rotor DOES reciprocate too, but just very gently, and in a circular rather than linear way, thus putting "pulling" and "pushing" loads on the shaft), or mechanical things like, yes, the seals. Also, because of the enormous amount of internal friction on the seals (again, notably the apices) could pose warping or failure issues, but also the very tiny (5-peices-bolted-together) block has to deal with enourmous amounts of heat without a very good way of cooling the rotor itself (done only by oil, which obviously isn't as good as water: molar heat capacity, it's tendancy to cook, and all that...), among many many many other mechanical things which must be built to take such high rpms, and still be operable at normal idle speeds (like the alternator, you can't just give them different pulley ratios...)...

so, just some ideas. again, for anyone who doesn't know, i'm NOT an engineer, a student of engineering, and won't ever be (in all forseeable possibilities). i'm just a nut about engineering (hence my love of F1, and the wankel rotary), and pretend to know a lot more than i do...

Sputnik

Picture in your mind a piston going up and down in it's cylinder (the perverts out there can just move on without making a comment on that ). Notice that the Piston/rod combination is stopping, reversing direction, and accelerating twice for every revolution. The higher the revs, the faster it is having to stop, revese, and accelerate, which puts extreme forces on those parts. It's not the wear, its having to control the piston at those forces. Go too fast, and forces will increase so much that the rod will actually pull like taffy.

That's why stronger rods and lighter pistons are used by tuners for getting more power out of an engine, because that "taffy-pull" limit becomes higher.

If I remember correctly, the pistons in F1 engines (almost 20k rpms) are going so fast that they are close to, or are going supersonic. Imagine it, even if I had a bad source and the pistons are only going half the speed of sound, the pistons are accelerating/deceleratiing from 0-350-0 mph in 1/666th of a second. How the heck do they keep those things together?

Anyway, there are still forces that the rotors and crankshaft in a rotary also have to handle, that can also reach extreme levels. Remember, the rotary isn't moving in a perfect circle. Even though it's not having to stop and start completely, it still has to change direction, which does put force on the crankshaft (and seals to a point). That's why you see rotary tuners also putting in lighter rotors, to reduce those stresses. Then you add the limits of the ignition (double the rpms, double the amount of power it has to provide), the limits of airflow, and the "weakest point" will soon make itself evident.

---jps

EDIT: Man, by the time you write a "book" someone else already dun beat you to it.

stan11003

Good points Wakeech, with that in mind 24 hour Le mans races must be really hard on the engineers.

wakeech

:D heh heh, yup... 24 hours of straight hard running is a bitch on motors, even low revving turbos.
but with any kind of motorsport, the trick is only to make an engine as powerful as you can make it, with driveability concerns taken into account (which are totally negated by the rotary's inherent torque delivery characteristics: there's no need to worry), then just detune it until it'll last as long as you need it to... at least that's how they do it in F1... Honda had a pretty quick engine earilier this year, but it would blow up so fast that they just put the redline 2k down, changed the management computer settings (to optimize the lower rev range) just so it'd last most of a race (they still blew up a lot... :p)

but yep, Mazda's engineers came through, and many of the solutions in the 787B made it into the 13BREW ('cause i'm pretty sure the "REW" mean "Race Engineered Wankel", but i've never seen any real official confermation of this...) with teh changes in teh block from the ol' 13B found in the FC (like the larger amount of coolant running past the "hotspot" around the sparkplugs... and other things too... i don't know all of 'em)

73JPS

Don't forget that even gas turbine engines have RPM limits.

wakeech

ya, friction, and centripital forces... often friction first, obviously... :D

Jerome81

I believe balance is a key part to all of this. I read somewhere that the basic limit to how fast a rotary can spin is the balance of its parts.

The other thing about pistons is what is called valve float. It is where the cams are spinning too quickly for the valves to actually have time to close before they need to open again. Basically the valves just never close.

Lastly, I remember a saying from one of the 1st gen RX-7 racers who let a journalist drive his car in one of the SCCA events. Redline on those older engines I believe was 7K. It went something along the lines of: "Shift at 7K if you want to go fast, shift at 8K if you want to have a chance, shift at 9K if you want to win." I might have the limits a bit off, but it was something along those lines.

I think the biggest thing is that if you get a rotary to drive well at super high revs, its low end is worthless. You can modify a rotary to have tons of torque or tons of revs by modifying the "K" factor, which has something to do with the size of the combustion chamber. The different sizes are what controls how the fuel ignites, and what is called a "flame front". That is why on the side port exhaust rotaries, if they stuffed enough fuel in there, and revved it high enough, they could shoot flame... because the car couldn't light the fuel fast enough as the rotor swept past the exhaust port. That is also why Mazda used two spark plugs per chamber firing sequentially.

There might be a few errors in here somewhere, as I'm not a rotary engineer. I'm just trying to remember everything I've read about how the rotary is designed and how problems were overcome.

Hope that helps

MikeW

The earliest limiting factor is the cycloidal gearing. I wouldn't want to sustain anything more than 8500 rpms. Hopefully 8500 rpms will be 155 mph or better.

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

wakeech

no, you hit the nail on the head there...

the LOCAL imbalance of the rotors (net, they balance perfectly, which is why they're nearly vibrationless under acceleration) stresses the eccentric shaft (what i was talking about, with the "gentle" reciprocation in my first post)...

ya, in F1, they've defeated valve float with ultra-hi-tech (and MEGA hi dollar) springs, if the team is still running cams (okay up to about 18K and change...)...
BMW (the factory who broke the previously undefeatable theoretical rev barrier of 18, 8** RPM by going well over 19K) uses nitrogen gas actuation to blow the valves back up, and vent the gas once it's closed to prevent it bouncing... Honda too uses gas, but their system is neumatic.

yes, getting high rev torque kills low end torque, and is true for piston engines too... this is becuase the port size, when made really really really big (ie. perhipheral porting a rotary), will reach it's volumetric efficiency max. way waaaaay up the rev range, but at low RPM that enourmous hole doesn't pass gas though it quickly enough (velocity is too slow) for the chamber to fill efficiently enough for the motor to power itself. this is why F1 engines, and perhipheral ported rotaries too, often IDLE at +2K RPM or so...
and the fuel combustion speed is a big problem too around 10K revs (i think...), and this is why the 787B's 4 rotor had 3 spark plugs per rotor, to further optimize combustion (for power and fuel efficiency)...
the "K" factor i've heard of, and remember almost nothing about... but i do remember that the SHAPE (ratio of depth to frontal area) of the wankel combustion chamber really determined the necessity of (at least) two plugs...

nice post :D

wakeech

oh, that's right Mikey!! jeez, i forgot about that...
but that 8500 rpm limit is 10 years old, and with today's new (and far more expensive) alloys, i could see the inner gear teeth taking on mroe than 11K rpm no problem in competition trim... maybe even over 12K...

one thing is for certain though... if F1 (or CART) gets deregulated, and Mazda decides that'd be a perfect place to showcase their technology (which it would be), i think that the gear teeth would certainly be their first big problem in hitting 20K (which i think they'd need to do to compete...), and next maybe port size and keeping all the seals from shooting out of them... hmm... interesting thought though...

Sputnik

I thought they had all gone onto hydraulic setups by now.

---jps

wakeech

hydraulic?? no, not everyone... many use pneumatics or pressurized gas, but i know for a fact that Renault's "111 degree" (which is closer to 108 i believe) engine still runs cams, and i'm sure only a year or two ago, there were many teams still using cams...
infact, ALL the teams could still be using cams, as they're not the problem... the tricky bit is getting the valves back up, which is where the pneumatics comes into play...

MikeW

What I was getting at was the maximum power 250 ps (247 hp) is at 8500 revs. The unofficial 'official' speed limit on the autobahn is 250 kph (155 mph), so if the 6 speed manual is lifted out of the Miata, and the same real axle ratio is used and 225/45 18 tires are used. Then 8500 in 5th gear would be 162 mph (okay if the axle ratio is shortened to 4.1, then 8500 would be 155)





Is desmodromic valvetrains used by any teams in F1?
(positive valve actuation, cam lobe for opening and closing)

wakeech

oh, i'm pretty sure that they'd shorten the final ratio as to maximize at-the-wheel torque, and shut-up the naysayers of this small displacement gem...
nice observation.

and ya, some still use cams for opening... as for closing, i think it's mostly up to pressureized gas systems (nitrogen, pneumatics, and the like), but again, i couldn't say as i've never LOOKED at one... just heresay and tidbits of insider info from www.f1mech.com forum (that i observe from time to time)

irresistibo

If the shape and size of the rotary dictates the amount of torque/hp the engine can make, do you think that 2 different types of roor shapes in one engine would work? Im totally guessing on this, but i think you can make 2 different shaped rotors spin one eccentric shaft therefore having one rotor for low end torque and one for high revving fun.

do you think this would work?

Quick_lude

I have no clue but if the rotors are different size then the chambers would also have to be different in size too, right? Is that feasible/practical.

Sputnik

No, I don't think so. First, you would just be compromising the heck out of the engine, instead of making an engine that is at least strong in one point. Second, the different "types" of rotors would need different fuel and timing parameters, and different airflow/exhaust setups for them to be efficient.

And most importantly, it would probably throw off the balance of the engine, ruining some of that smoothness (and thus some of the power you would have gained).

Quote:

...and ya, some still use cams for opening... as for closing, i think it's mostly up to pressureized gas systems (nitrogen, pneumatics, and the like)...

With the way that gases can compress and decompress, I don't know why they would use a pneumatic system instead of a hydraulic system. I mean, we bleed brakes for a reason. For all I know, they've found a way to make it work, but there's something missin'.

---jps

MyT13B

No!

this would be like taking a 12A rotor next to a 13B rotor.

The difference in the two is purely in the width of the rotor. 13B is wider and makes more torque. it is also heavier and top rpm is slightly less.

If you want more torque, add more rotors.....