mark57

Technical correctness is what we're chasing down. A 4-stroke's size is based on its entire thermodynamic cycle of 720* x bore x stroke. A 13B's size has to be 1080* to complete the thermodynamic cycle to acknowledge its entire thermodynamic capacity as MikeC pointed out earlier.

Technically, the FD3S occupant capacity is a driver + 1 passenger vehicle. Technically, the FD3S occupant capacity is a driver + 3 passengers vehicle.....

neit_jnf

iTrader: (17)

^^^ I don't get it

Aviator 902S

Fair enough. But that's still not an apples-to-apples comparison since the rotary gets three rpms to the boinger's two.

Like MikeC, I also have experience around various engines, including turbines. Acronyms like TOT, EPR, Wf, SHP,T1 and T2 are not lost on me. Neither is the term "Brayton cycle."

All three arguments have some merritt and are correct to varying degrees:

The 3.9 crowd is correct in that by the time all three sides of each rotor have fired, 3.9 liters has been displaced. In the minds of some, this is what is required to complete the entire auto cycle in a rotary engine. Ignore crank rotations because "they don't count."

The 2.6 crowd is correct in that the 2-rotor rotary fires the same number of times as a 4-banger per rev, ie: twice. But in 720 degrees of rotation (a complete cycle for a boinger) the rotary fires four times. 4 times 650cc = 2.6L. This of course ignores the fact that 2-cycle boingers of the same cylinder dimensions as a four stroke are still labelled as the same displacement as the 4-stroke. Whether that is proper or not depends on who you ask.

The 1.3 crowd is correct in that there are two chambers in a two-rotor engine, and each one is 650cc of displacement. 2 x 650cc = 1.3L. But that's only through one complete rev of the e-shaft, 360 degrees (like a 2-stroke), not the 720 degrees of rotation required of a 4-stroke.

So the question becomes, "Is it a two stroke or a 4-stroke?" That's a whole 'nother argument. If a 2-stroke boinger can be labelled with the same displacement as an equivalent-sized 4-stroke, so can a rotary vs. a 4-stroke. "Not fair", argues the 2.6L and 3.9L masses. "The rotary still requires four sequences to complete the combustion process, so you have to call it a 4-stroke." Strong argument, but these same four events also happen in a 2-stroke, just not in sequence. And that engine's displacement isn't considered to be doubled because of this characteristic, so we're back to square one.

The other argument revolves around the power output of the rotary per given displacement: "It HAS to be a 2.6 (or 3.9). If it's not, where does all the extra power come from?" Simple--- and there are two reasons for this:

As previously mentioned by me and others, the power sequence lasts a full 270 degrees of e-shaft rotation vs, only 180 degrees for boingers. This of course ignores valve timing and overlap, but the end result is a full 50% more power "stroke" per rev for the rotary vs. the boinger.

The second reason is the lack of a valve train in the rotary. Like the 2-cycle boinger, this job is done via ports. With no power-robbing valve train to be seen anywhere in these engines it's not hard to figure out where all the extra power comes from.

Torque is another matter, and the Achilles Heel of rotaries. With the smaller length of "crank" throw in a rotary, the torque at low rpm can never be as good as that of a typical boinger of similar displacement unless said boinger is severely over-square, ie: shorter stroke in favor of larger diameter bore. Most if not all street-legal boingers of this size are either square or over-square, but not excessively so. Still, the power potential of the rotary far exceeds it's size.

What all of this means is that it is nearly impossible to obtain a true apples-to-apples comparison between rotaries and boingers, so this debate will continue long after we're all in our graves, pushing up daisies, trolling for top-soil, etc.

But this has been a fun thread to watch, what with all the egos clashing, beer spilling, punches thrown and bottles broken over heads. A veritable train wreck if you will. I have to leave now. I'm out of popcorn.

edmcguirk

Nigel: "You see, most blokes will be playing at 10. You’re on 10, all the way up, all the way up...Where can you go from there? Nowhere. What we do, is if we need that extra push over the cliff...Eleven. One louder."

DiBergi: "Why don’t you just make 10 louder and make 10 be the top number, and make that a little louder?"

Nigel (after taking a moment to let this sink in): "These go to 11."

tmiked

BEER ! We spilled BEER ?

***Gets out his straw.***

Aviator 902S

Ah, Spinal Tap... Best Heavy Metal satire movie ever. Of course, it's probably the only heavy metal satire movie ever.

"That's sounds really pretty. What's it called?"

"I call it 'Lick my Love Pump'."

Rogue_Wulff

This whole debate has made me wonder about something.
Many other manufacturers started developing their own versions of rotary engines, yet few ever saw the light of day. Did this same debate interfere with their ability to come up with a working design? Did the engineers at Mazda simply agree to disagree, and make it work?
Had other manufacturers actually mass produced, and sold to the general public, a rotary engine powered car, the whole debate would be a moot point, as the field would be level. (ie rotary vs rotary, rather than rotary vs piston)

Aviator 902S

[QUOTE=Rogue_Wulff]

Many other manufacturers started developing their own versions of rotary engines, yet few ever saw the light of day. Did this same debate interfere with their ability to come up with a working design? Did the engineers at Mazda simply agree to disagree, and make it work?

LOL. Interesting theory. Kinda like the one that sez dinosaurs became extinct because they all turned gay and would no longer reproduce. Or tides being caused by everybody along the coast of China flushing their toilets simultaneously.

Here's what really happened:

First, when piston engines were being developed in the first decades of the 20th century there was not yet an alternative to them. No matter how many bugs and problems with durability and inefficiency were encountered, engineers kept at it until most of the biggest shortcomings were solved. They took a machine that by its very design was hell-bent on self-destruction and developed it into a dependable propulsion device that could last at least as long as the warranty it came with. But it still went "boing." Had the rotary come first, this miserable contraption might never have seen the light of day.

By the time the rotary was first sold in North American showrooms high-performance boingers were all the rage. Muscle cars were still king, and small economy shitboxes (especially those from Japan, a country that was still fresh in every American's mind due to the unpleasantness at Pearl Harbor only 30 years earlier) were laughing stocks and ridiculed by all. Nobody wanted to see these things succeed, but because they delivered excellent fuel economy, the fact that they were gutless wonders was tolerated and many were sold.

But the rotary-powered Mazda shitboxes hauled ***. They wound out to redline and beyond in the blink of an eye, won many races, and were quickly banned from many racing events due to complaints from opponents driving boingers who felt that the rotary constituted an unfair advantage. They were right. Soon, every other manufacturer was stepping up their rotary engine research and development programs. Mercedes and GM lead the way, trumped only by Mazda, who had an obvious head start.

But the rotary still had a couple of issues. First, water jacket seals were beginning to fail due to not enough heat being carried away from the bottom of the engine and in the area of the spark plugs, and poorly maintained rotaries were throwing apex seals. And they were fuel hungry. 20 mpg on the highway was acceptable in a mid-size American car, but everyone knew that shitboxes were supposed to get at least 30 mpg. And the Mazdas were shitboxes--- good-looking shitboxes mind you, but shitboxes nonetheless. Not only were they too small, they didn't even offer decent fuel economy.

Everyone seemed to forget that the performance from a rotary eclipsed anything else coming from the far east, and also many domestic and European cars as well. "May-ud in Jer-pay-un" became the derisive mantra of V8-loving rednecks from coast to coast. Mazda quickly fixed the durability issues, but the damage to the rotary's reputation had already been done. And many wanted to see these and other Japanese cars fail in the North American market anyway. (The kicker is that domestic attempts at building shitboxes (Ford Pinto, Chevrolet Vega) failed miserably, actually making the Japanese imports look good by comparison. But that's another story.)

Then along came the energy crisis, where middle-eastern oil-producing companies, as well as U.S.-based companies like Exxon, Mobil, Gulf and Shell conspired to turn off the taps to force demand to outstrip supply and convince many that the world's oil supply was drying up. As was their plan, gas prices skyrocketed. The side-effect was that everyone wanted to get rid of their large cars and buy a shitbox.

But everyone knew that rotary shitboxes were not exactly fuel efficient, and those manufacturers still developing their own rotaries knew that most people were aware of the rotary's less-than-stellar fuel burn rates. They could have continued developing rotaries but unlike piston engines when they were first developed over 50 years earlier, there WAS a viable alternative to rotaries. So all manufacturers but Mazda dropped them from their R&D programs.

This is probably a good thing, because with the track record of many domestic and European auto manufacturers, it's likely that any rotaries produced by them would have been way inferior to Mazda's. This would have perpetuated the myth that "rotaries are junk." These manufacturers who dropped the rotary have been producing inferior engines ever since. And Mazda lived happily ever after.

Damn, I love happy endings!

MikeC

Very true, this is a *very* important point and it is *the* thing that is causing all the confusion. But it's not the ratio between the shaft and the rotor that is the issue (3:1) it's the ratio between the chambers and the shaft. That might sound a bit confusing at first but if you look at piston motor, if the piston goes from min vol to max to min vol again the shaft will have gone around once, which I call a shaft/chamber ratio of 1:1. If you do the same thing for the rotary and follow a single chamber from min to max to min volume then the shaft will have rotated an extra half a turn which is a ratio of 1:1.5. This can also be seen with the 270 vs 180 degree expansion, 1.5 times longer.

A lot of people think that the 270 degree expansion (or 1.5 ratio as i've stated it) gives the rotary longer to burn its fuel, which is sort of true but misleading. It's really the other way around, the rotary will tend to give itself the same amount of time to burn it's fuel resulting in the output shaft spinning 1.5 times faster.

MikeC

It's not the the crank rotations don't count it's just that the output shaft is geared to run faster than it otherwise would by a factor of 1.5. See my previous post.

The 2.6 crowd fail to take into account the 1.5 factor so are 1.5 times out.

Again they are thinking that the rotations of the shaft are more important than they are. The rotary only fires with the same frequency as a 2 stroke due to co-incidence. Being a 4 stroke it should fire once every 720 degrees, but because there are 3 chambers it would fire every 240 degrees (720/3=240) but because it expands over 270 degrees instead of 180 the shaft rotates 1.5 times longer or 360 (240x1.5=360). Pure co-incidence.

The rotary is definately not a 2 stroke.

Not really. The rotary just revs higher to compensate giving itself the same amount of time to burn fuel and no advantage at all.

Losses due to running valves is not going to account for the difference in power, it's only a small percentage.

If you're an engineer I could prove to you beyond doubt that the rotaries lower torque is due to it's 270 degree expansion andhas nothing to do with it's short stroke. If you've got 100psi of pressure in a chamber in a rotary and measure the torque at the shaft it will be 1.5 times lower than doing the same thing with a piston motor. It's fairly simple maths, if you have the same pressure and allow it to expand over 1.5 times the rotation then the torque will be 1.5 times lower because the same energy is released. As the energy = torque x degrees, if energy is equal and degrees is 1.5 times more then torque must be 1.5 times less.

I disagree, 3.9L is not an equivelent displacement or a comparable displacement it is the actual apples for apples displacement of the rotary engine. If you take into account that the shaft is geared to spin 1.5 times faster then everything fits, every last detail.

edmcguirk

To be honest, I really don't care what labels you use. I just wanted to figure out what size pulleys to use on my supercharger.

thanks - done.

ed

maxcooper

Use 2.6L, since it flows like a 2.6L 4-stroke piston engine, if the formula you are using has a "4-stroke piston" constant baked into it.

It will ingest 1.3L per rotation of the main shaft.

-Max

neit_jnf

iTrader: (17)

To add gas to the fire, it also compares in power to a 4-cylinder 2.6 liter

STI: 2.5L @14psi boost ~300hp

13B: "2.6L" @14psi boost ~300hp

give or take a few horses

hemihead

Ok, im gonna attempt to clear this swept volume issue up once and for all.

First off the swept volume or displacement of a 13b, which we will use as an example, is 1.308L or 1308cc. This is possible due to the fact it has 2 rotors, each with a swept volume of 654cc for EACH rotor face.

Alright then, with that established, now for the tricky bit. To visualize this concept
the best way would be to consider the Wankel as an air pump, just like any other internal combustion engine. A pump, no matter of its design, ie. reciprocating, or rotary, has its performance measured a number of ways. But one of those ways is by
swept volume per revolution. One revolution is 360 degrees. So if we look at the Wankel the same way, for every 360 degrees the eccentric shaft turns, one chamber
will displace 654cc. Thats it. Displacement be definition means to either move a fluid in out out of a chamber in 360 degrees. Its the standard. For every 360 degrees of the eccentric shaft turning, one chamber will sweep 654cc in OR out. A maximum volume occurs every 360 degrees on either the maximum volume exhaust revolution side or every maximum volume intake revolution side. If viewed from the front of the engine on one of the rotors this will be max. volume at the top chamber for intake or the max. volume on the lower chamber for exhaust rotating clockwise of course. One chamber displaces per revolution so saying that a 13b is 3.9L is totally wrong no matter how you view it. A 2 rotor 13b is only capable of moving 1.308L of air and fuel per revolution (360 degrees) no more, no less. Its physically impossible. Now if you consider a 4 cycle reciprocating engine, a wankel is exactly the same. Every 360 degrees there is either an intake or exhaust stroke, disregarding the compression and power stroke. From tdc to bdc and back to tdc will displace the same volume as a wankel in a 360 degree rotation. 654cc in, 654cc out. So for anyone who thinks a 13b is unfairly rated, think again. Its an engineering fact and if im not mistaken the guy who designed this little marvel was an enginner, Felix Wankel. Dont dispute the math if you dont understand it. So to sum it up a 13b is only capable of displaceing 1.308L in a 360 degree rotation. After all each power pulse only burns 654cc of air and fuel. The comparrison to a 4 stroke reciprocating engine is that it produces a power pulse every 360 degrees vs every 720 degress hense the 2:1 comparison for displacement. A 4 stoke would have to do twice the work to produce the same nominal output as a wankel or any other 2 cycle engine.

hemihead

Sorry, i slipped up there on the last posting...it displaces max. volume of one chamber every 180 degrees...thats either intake side or exhaust side...so in 360 degrees it displaces 2 chambers... 654cc in and at the same time 654cc out. Like a pistion engine from tdc to bdc would be intake or 180 degrees...and from bdc to tdc would be exhaust, another 180 degrees...360 degrees in total sweeping in 654cc and sweeping out 654cc. Of course the eccentric turns 3 times faster than the rotor.
So for max. volume to occur every 180 degrees of the eccentric shaft, the rotor turns 60 degrees, which is the angle formed between two rotor faces. This is why you have intake and exhaust occuring simultaneously, intake on one chamber and exhaust on another...hence the displacement of 654cc in and 654cc out in 360 degrees of rotation. If you can find an animation of a wankel engine and start and stop the animation you will be able to see this going on. It'll take a few looks at it to see it and understand.

hemihead

Two Chambers are displaced in 360 degrees of rotation. You have to view one chamber as the intake stroke and the other as the exhaust stroke if you are to compare it to a piston engine for visualizing. The maximum about of air and fuel that can be drawn into one chamber and burned in 360 degrees is 654cc. The other thing you have to consider is that the intake and exhaust events overlap eachother during that 360 rotation.

Monkman33

iTrader: (28)

Comparing it to a 2.6l 4 stroke piston engine is the closest way to approximate any type of similarities.

It doesn't matter that there are 3 faces on a rotor, what matter is that .65 liters of volume is compressed and combusted per revolution of the eccentric shaft, per rotor. thus a 2 rotor will compress and cumbust 1.3 liters o volume per revolution of the eccentric shaft. The eccentric shaft is where the rpm is measured, and thus where you would take your comparison.

A 4 stroke 2.6 liter engine will compress and combust 1.3 liters of volume per rotation of the crankshaft.

The crankshaft and eccentric shaft are comperable.