Why Do Rotaries Suck?

[GT-R32]

Why is there all this talk of a rotary being closest to a two stroke?

It's a 3.9L Wankel Cycle. There are six rotor faces that are measured in a compression test. The 4 Otto cycles you refer last 1080 degrees, rather than 720 degrees in a 4 stroke piston. And when compared in this manner a 13B be is directly comparable to a 2.6L piston engine (where the the extra strokes should not be ignored, as a 4-stroke cylinder produces a power stroke only every other rotation of the crankshaft which doubles the volume for the four stroke piston and the demand of displacement). CAMS lower this (normal parity as combustion chamber rotor face volume x2) as the engine is not as efficient as a 4-stroke piston for various reasons. Racing and rego bodies run between 1.5-2 times rotor face volume for the Wankel as a 4-stroke equivalent - Mazda quote 1308cc (654x2) for a 13B which is equivalent to a 2 stroke piston.

I don't get why it's so hard to understand?

[mad082]

simon likes this!

except for the 3.9L, it has a capacity of 2.6L... yes it uses 3.9L in complete combustion of all the rotors but it only every has the capacity (volume) of 2.6L

Well there's another person...

At least we've moved on from 1.3L, that's a good start. But it's still not correct.

Your reasoning here would make an RB26 less then 2.6 litres. You have taken a moving item and stopped it, taken a freeze frame of the moment that it would hold (assumedly) the most amount of air at one time. Thanks to the differentiating strokes of the pistons in the RB26 engine (and any other piston engine) this would mean that there is NO SINGLE POINT that there would be 2.6L of air inside the engine. The 2.6L calculation is taken from one full 'cycle' of all the pistions, as is the 3.9L in the rotary.

But I could be wrong.

doofy you are pretty much right on that (you can quote that if you like, LOL)

the capacity of a piston engine is the volume of a single chamber, multiplied by the amount of chambers, reguardless of the amount of rotations of the crank required to achieve a full cycle (hence why a 2 stroke and 4 stroke engine of the same displacement are both stated as such, rather than the 2 stroke being twice the capacity). and this is why a rotary should also be classed as the volume of a single side of the rotor, then multiplied by the number of sides of rotors in the engine. so in the case of the 13b, there are 2 rotors, with 3 sides each, so a total of 6 combustion areas. mazda only calculated 1 side of each rotor.

if you look at a 4 cylinder engine the volume of air in the engine at any time is half it's displacement. when the number 1 piston is at bottom dead centre, number 2 will be at TDC, number 3 will be at BDC and number 4 will be at BDC as well. so you have 2 full capacity chambers and 2 chambers at zero capacity. or if number 1 is halfway, all the others will be at halfway as well, so you still only have 50% capacity.

[SimonR32]

Well there's another person...

At least we've moved on from 1.3L, that's a good start. But it's still not correct.

Your reasoning here would make an RB26 less then 2.6 litres. You have taken a moving item and stopped it, taken a freeze frame of the moment that it would hold (assumedly) the most amount of air at one time. Thanks to the differentiating strokes of the pistons in the RB26 engine (and any other piston engine) this would mean that there is NO SINGLE POINT that there would be 2.6L of air inside the engine. The 2.6L calculation is taken from one full 'cycle' of all the pistions, as is the 3.9L in the rotary.

But I could be wrong.

sorry, my bad... i explained it incorrectly, maximum capacity of a indiviual bank times by the number of banks

a full cycle using you theory of a 2.6L RB is actually 5.2L

[Birds]

RPM, thats a tricky one. I tend to agree that the output shaft is the most appropriate way to measure RPM, but if someone were to put a 3:1 gear inside a piston engine to triple the output shaft velocity, where would you then measure it?

You would measure it from the last static output in the engine, because that is part of the engine. A gearbox is dynamic...you can change gear ratios when you like and the engine can still run without this gearbox attached. Now it doesn't matter what ratio you add to the final output of the engine...if you want to put a 3:1 gear into what is normally a 6000rpm engine, it will become a 2000rpm engine...a very torquey one at that. And your tacho will read to 2000rpm and you'll be none the wiser that any components in the engine are moving at 6000rpm because it is IRRELEVANT. It's no different to changing the bore and stroke of the engine to achieve different speeds of the crankshaft for different applications. Everyone needs to understand that firstly, revolutions per minute refers to something round that can actually revolve...i.e. crankshaft, because pistons go up and down and rotors follow an eliptical orbit. And secondly, engine speed is NOT relevant taken from the pistons or the rotors. It is NOT coincidence that the crankshaft travels at 1:1 with the pistons in a piston engine, it is coincidence that the PISTONS travel at 1:1 with the crankshaft in a piston engine. Any measurement you take from an engine will always be relative to crank/eccentric/output shaft speed. That is true engine speed. IF you want to compare a piston engine with a rotary the closest thing you are going to get for your comparison in terms of equivalence is a 2.6 litre 4 stroke or a 1.3 litre 2 stroke...and this equivalence is based only on engine displacement. This is at the same time acknowledging that rotaries and piston engines are completely different. Yes, we all know they are different, there is no need for people to keep reiterating this tired point that people are saying they're the same. We are talking about equivalence, not saying they are the exact same thing.

The problem here is that Sydneykid has gone from arguing some bullshit points about comparing rotary engines with piston engines...to just plain defining rotaries. It's a nice illusion that makes him appear to have been correct all along...but this is part of what I was talking about before with selective arguing. And there's alot of blind supporters here who haven't really paid attention to the content of the argument thus far...it's just that Sydneykid wrote something so it's SAU gospel. All the worse for it though...if you want to learn about rotary engines without some sort of bullshit about manufacturer lies over decades...then tune in to Ausrotary (as unfriendly as some of the guys over there have been). Because the rest of us who also work with rotaries and understand how they work...acknowledge that some of what he is saying is complete crap. If you want to believe that a rotary engine has an unsprung 3.9 litres that gives it a 1.3 litre displacement advantage over a 2.6 litre 4 stroke...then I guess only your own perceptions will suffer. But that means that everyone else involved in motorsport including the governing bodies got it completely wrong too

*waits for a 9/11 style conspiracy video based on the rotary*

[SimonR32]

the capacity of a piston engine is the volume of a single chamber, multiplied by the amount of chambers, reguardless of the amount of rotations of the crank required to achieve a full cycle (hence why a 2 stroke and 4 stroke engine of the same displacement are both stated as such, rather than the 2 stroke being twice the capacity). and this is why a rotary should also be classed as the volume of a single side of the rotor, then multiplied by the number of sides of rotors in the engine. so in the case of the 13b, there are 2 rotors, with 3 sides each, so a total of 6 combustion areas. mazda only calculated 1 side of each rotor.

capacity (Volume) of a single rotor in a 13B at it's maximum is only ever 1.3L, times that by 2 and you get 2.6L... , you can never actually have 1.95L air by volume at any time in a single rotor like sydneykid suggests

Edited September 23, 2009 by Cerbera

[Smity42]

if you want to put a 3:1 gear into what is normally a 6000rpm engine, it will become a 2000rpm engine...a very torquey one at that. And your tacho will read to 2000rpm and you'll be none the wiser that any components in the engine are moving at 6000rpm because it is IRRELEVANT.

Agreed, mostly. However, it is slightly relevant in that the physical difficulty of getting a piston engine to rev is predominantly that the pistons have to change direction so fast. (There are of course many other factors involved)

If a car manufacturer sold me a car telling me it had a 4 stroke piston engine that reved to 24,000 RPM, I would marvel at the feat of engineering at getting pistons to move so fast. However if i found it was just a plain old 6,000 RPM engine with a built-in 4:1 (or 1:4, depending which way you look at it) gear inside the engine, I would feel cheated. However I would agree with you, it is still a 24,000 RPM engine.

[Smity42]

capacity (Volume) of a single rotor in a 13B at it's maximum is only ever 1.3L, times that by 2 and you get 2.6L... , you can never actually have 1.95L air by volume at any time in a single rotor like sydneykid suggests

I spose much depends on if you consider the rotor the equivalent of a single piston, or of 3 pistons. Either are probably valid arguments...

[GT-R32]

They are unique! You can draw comparisons and rate them in relative terms, but for it to really make sense seeing them in terms of a piston engine need to stop.

[180Roman]

I gave up reading after 7 pages, so sorry if someone mentioned this

i see people arguing that the eccentric shaft is a step up gear with a 3:1 ratio and it just happens that Felix Wankels made it 3:1

why not 1:1? 1:2? or 1:6? why not even 2.5:1 or 3.5:1?

if its a simple step up gear then why i see nobody changing the ratio?

its coz the engine would not work otherwise

if you can design a rotary engine which would use an eccentric shaft with a different ratio

you must be a lot smarter then me, coz i couldn't

[Smity42]

@GT-R32, that is difficult when trying to calculate the capacity/displacement, as basically every definition i can find on these two terms is based on pistons.

@180Roman, I am reasonably certain that any multiple of 3 would work. I am no expert however, so may be wrong

[Birds]

Agreed, mostly. However, it is slightly relevant in that the physical difficulty of getting a piston engine to rev is predominantly that the pistons have to change direction so fast. (There are of course many other factors involved)

If a car manufacturer sold me a car telling me it had a 4 stroke piston engine that reved to 24,000 RPM, I would marvel at the feat of engineering at getting pistons to move so fast. However if i found it was just a plain old 6,000 RPM engine with a built-in 4:1 (or 1:4, depending which way you look at it) gear inside the engine, I would feel cheated. However I would agree with you, it is still a 24,000 RPM engine.

Hmmm...on the topic of feeling cheated...if a car manufacturer sold me a car telling me it had a 4 stroke piston engine that revved to 24,000rpm...my technical mind would assume it has an extremely short stroke and a very wide bore, e.g. a motorbike engine. Or in our hypothetical example (and I say this because we can both agree the engine would be severely lacking in torque), a gear ratio to allow the output to rev that high. But that's looking at it from a technical mind. In the example of someone who knows nothing about engines, they would just assume they have an extremely rev happy engine. Their tacho would read so. They would acknowledge that a good launch need be had at say 15,000rpm instead of 4000rpm in a typical car. Their dyno chart would quite rightly and relevantly be plotted against 24,000rpm worth of spacing. The only thing that would hint to them their vehicle is different is the common knowledge that most piston engines don't rev that high. Just like rotary owners who don't actually know how rotaries work...just assume all the engine components are spinning at 9000rpm and that this is exactly the same as a piston engine's 9000rpm. They don't question it though, because it's common knowledge that rotaries generally rev higher than piston engines due to their different design.

I would not deny the engine revs to 24,000rpm. It's just that there are no piston engines (none that I know of) that adopt this sort of gear step up design like the rotary. It is appropriate for the rotary because it relies on an eccentric shaft just to operate, but also because the 1:3 ratio gives it a usable rev range for a road vehicle. I might be looking at the black and white of engine RPM here...but when you are seperating a vehicle into sections...whatever comes before the flywheel is the engine and the things it uses to create this output are part and parcel of the complete engine.

[Smity42]

agreed 100%, but you can see why it creates a bit of grey area with the built-in 1:3 ratio of the wankels

Edited September 23, 2009 by Smity42

[180Roman]

for the rotary to work properly the eccentric shaft ratio has to be 3:1

if you try to change the ratio to 6:1 that means that the lobe has to have a diameter of 6 times the eccentric shaft, which is not feasible

this will also reduce the torque even more

people who dis rotaries are just jealous that the rotary is such a clever design with a few moving parts

i don't think anyone on here has designed their own unique operating engine?

[DRD-00F]

sorry, my bad... i explained it incorrectly, maximum capacity of a indiviual bank times by the number of banks

a full cycle using you theory of a 2.6L RB is actually 5.2L

Mmmm yep true, I also explained it incorrectly. The correct way to say it would have been MAD082s way... Maximum capacity of each combustion chamber, times by the number of chambers.

RB26 = 0.428L x 6 = 2.568L

13B = ??? I don't exactly know so I'm not going to guess... the ausrotor people seem to think guessing is just hilarious.

[Smity42]

well then, is each individual rotor one combustion chamber or 3?

[DRD-00F]

Combustion happens on each side of the 3 sided rotor, so each individual rotor has 3 combustion chambers.

[Birds]

agreed 100%, but you can see why it creates a bit of grey area with the built-in 1:3 ratio of the wankels

Definitely, that's why we're having this whole debate right now...it's just a bit of a grey area that opens up for devils advocate and rotor/Mazda haters lol.

Another example is an electric motor...they almost always have step up motors that are just a part of the engine, not considered a gearbox, but change the amount of torque and the RPM to suit the application. It's just, until rotaries arrived on the scene, the whole step up thing being part of an engine is new to people who only know engines as piston enignes...or people who base the science of an engine solely on the piston engine.

[bozodos]

I think the main problem is your continuing acceptance of the Mazda lies Birds

[Smity42]

Combustion happens on each side of the 3 sided rotor, so each individual rotor has 3 combustion chambers.

Well, that definitely gives you 3.9L then. 6 combustion chambers * .652L (i think?) per chamber = 3.9L.

But since a rotory takes 1080 degrees compared to 720 degrees of a 4 stroke, that would make it ROUGHLY the equivalent of 2.6L 4 stroke (3.9 * 720/1080), which I think we are starting to come to an agreement in this thread on?

However, by this logic, shouldn't we double the capacity of a 2 stroke?

If you treat the rotor as a single combustion chamber, you get 2 combustion chambers * capacity of entire chamber (~1.3L) = 2.6L.

No matter what way you swing it, you can't get 1.3L

I'm going to stick with calling it 3.9L, roughly equivalent to a 2.4L 4-stroke or 1.3L 2-stroke

[DRD-00F]

However, by this logic, shouldn't we double the capacity of a 2 stroke?

Again, that's the problem with that logic (which you've already said in the same post anyway ).

It's uneven and creates problems between the engine types.

The MOST LOGICAL way to judge literage is for one full combustion to combustion cycle. In 2 strokes this is a simple down then up motion, for 4 strokes it's twice that, and for a wankel, it's completely different, but still measurable to a full cycle.