Why Do Rotaries Suck?

[nisskid]

i guess the problem is that rotor boys for ages have criticised piston engines for not revving as well, when in fact what we've learnt here is that to compare rpm between piston and rotors is completely useless.

i understand and agree with most of what gary is saying, i think the problem here is we look too much at comparing the technical areas of 2 different machines that are only relevant to each type of machine. all we can do is look at the outputs in application really and compare that. the problem is that Mazda obviously tried to use these technical specs to market their engines over piston engines, which we now know was stupid to do so. so in that aspect, Mazda were deceiving.

[GT-R32]

Ok i appreciate you saying that. I have however read through this whole thread, thats why i decided to post.

For those of you who still doubt what i'm trying to say, have a look at this youtube vid:

This video explains it well, and is much easier than trying to explain with words in a post.

Does anyone still think there is a step up gear ratio between the rotors and eccentric shaft?

The stationary gear pinion and the ring gear on the rotor keep the rotor where it should be for the Wankel cycle, much like the cylinder bore keeps a piston where it should be. A Wankel needs this 3:1 (no other) ratio to operate properly just as a piston engine needs a bore.

Precisely, great post. A rotary is all about how it turns that movement into power, a relative motion. It therefore pumps 654cc x2 per shaft revolution (and fires accordingly), equivalent to a 1308cc 2 stroke or a 2616cc 4 stroke piston.

[Jez13]

You can and in fact Mazda does. But is it correct to state that "a rotary engine does 9,000 rpm"? You will say, "well of course it is correct because the eccentric shaft does 9,000 rpm". Whereas I will say "no it isn't correct because the rotors are only doing 3,000 rpm". When we say "an engine does 9,000 rpm" the natural assumption is that all of the engine is doing 9,000 rpm". Now in a 2 stroke piston engine that is 100% correct, all of the engine is in fact doing 9,000 rpm. But in a rotary engine only the eccentric shaft is doing 9,000 rpm, nothing else. The rotors are only doing 3,000 rpm.

Keeping the above in mind, that means 100% of a 2 stroke piston engine is doing 9,000 rpm, but only 33% of a rotary engine is doing 9,000 rpm. In other words the vast majority of a rotary engine (ie; 67%) is only doing 3,000 rpm. So when someone says "a rotary engine does 9,000 rpm" I say rubbish because the whole engine is not doing 9,000 rpm. In fact I say double rubbish, because most of the engijne is only doing 3,000 rpm.

But wait, let me add even more weight to my argument. We are talking about an internal combustion engine here, so surely it is logical to count the rpm of the part that actualy carries out the combustion process. In a rotary engine that's the rotors and, like it or not, the fact is they are only doing 3,000 rpm.

This is usually the point at which the rotary huggers, start throwing in cycles not revolutions, orbits instead of rotations, no strokes in a rotary engine, elliptical motion, not up and down etc etc. But to me that's just being pedantic and deliberately narrowing the meaning of common use words, in some vane attempt to escape the truth. But the truth is right here, it's not out there.

Cheers

Gary

I think it makes perfect sense to measure rpms by the essentric shaft, just like pistons measure theirs by the crank. The e/shaft is the rotories equivalent of a crank shaft so the comparison is perfect. Who cares what is going on to make the shaft spin at that speed, it doesn't matter if its a windmill or a hampster on a bicycle - it should get measured where it counts. There is obviously just a difference in opinions on where rpms should be measured but lets face it, a rotor is not the same as a piston but both their primary objectives is to turn the driveshaft.

[GT-R32]

Where is power measured? Where is torque measured?

[Smity42]

I think it makes perfect sense to measure rpms by the essentric shaft, just like pistons measure theirs by the crank. The e/shaft is the rotories equivalent of a crank shaft so the comparison is perfect. Who cares what is going on to make the shaft spin at that speed, it doesn't matter if its a windmill or a hampster on a bicycle - it should get measured where it counts. There is obviously just a difference in opinions on where rpms should be measured but lets face it, a rotor is not the same as a piston but both their primary objectives is to turn the driveshaft.

well said!

[Birds]

And the truth shall set you free.................................!

[mad082]

i understand what you are saying there, and mostly agree, however if a rotor fanboy came up to me and started going on about how a rotary is better because it could rev to 9000rpm and how many piston engines can do that? i would instantly revert to SK's point of view. the point is that they are different.

[GT-R32]

I'd just refer to the dangerous harmonics that come into play above certain RPM that doesn't happen with piston engines.

[scotty014]

i understand what you are saying there, and mostly agree, however if a rotor fanboy came up to me and started going on about how a rotary is better because it could rev to 9000rpm and how many piston engines can do that? i would instantly revert to SK's point of view. the point is that they are different.

id reply back to them that my honda engine revs to that easily with all that vtec power!!!

edit: so does Tak's car on intial d, especially when he gets the new engine put in, it goes higher than 9000rpm .

[nisskid]

i understand what you are saying there, and mostly agree, however if a rotor fanboy came up to me and started going on about how a rotary is better because it could rev to 9000rpm and how many piston engines can do that? i would instantly revert to SK's point of view. the point is that they are different.

i think what gary's trying to say is that mazda have effectively done this, they have used these "specs" to market their engine against piston engines. pretty sure this is gary's beef with mazda more than anything.

[GT-R32]

i think what gary's trying to say is that mazda have effectively done this, they have used these "specs" to market their engine against piston engines. pretty sure this is gary's beef with mazda more than anything.

I don't think it's Mazda, I think it started with Felix Wankel himself, given NSU did the same thing with their rotaries.

[Birds]

i understand what you are saying there, and mostly agree, however if a rotor fanboy came up to me and started going on about how a rotary is better because it could rev to 9000rpm and how many piston engines can do that? i would instantly revert to SK's point of view. the point is that they are different.

Just tell him he's an absolute dumbass for even saying that an engine is better just because it revs higher. You need a defined variable for "better"...and engine rpm proves nothing. My air dremmel revs higher than 9000rpm...could it beat a car down the quartermile? Depends how long the air hose is and how fast you can run with it I guess.

[GT-R32]

Just tell him he's an absolute dumbass for even saying that an engine is better just because it revs higher. You need a defined variable for "better"...and engine rpm proves nothing. My air dremmel revs higher than 9000rpm...could it beat a car down the quartermile? Depends how long the air hose is and how fast you can run with it I guess.

LOL, exactly.

[jame88]

it is magic ,the rotor orbits the shaft at 9000 orbits per minute while it rotates at 3000 rpm.

That page demonstrates the Wankel cycle perfectly. Everyone should look at that and observe that the power is coming from the "orbital" motion rather than the "rotational" motion, before they again say a Wankel only does 3000rpm.

Ok now that we have all learned that part, onto the capacity debate.

I would define "capacity" as the amount of air an engine would pump in one "cycle" of its operation. Would anyone disagree?

Now lets define "cycle".

I would call a cycle when something is in a initial state, does something, and returns to its initial state.

Wikipedia defines a "cycle" as: "...one complete occurrence of the event which repeats"

Nobody here is arguing that a 4-stroke engine takes 2 complete rotations of the output shaft to complete its cycle.

Nobody here is arguing that a 2-stroke engine takes 1 complete rotations of the output shaft to complete its cycle.

At the end of each cycle, both engines are doing exactly what they were doing at the start of their respective cycles.

At the end of a 4-stroke cycle of a 2L engine, the engine has pumped in 2L and pumped out 2L.

At the end of a 2-stroke cycle of a 1L engine, the engine has pumped in 1L and pumped out 1L.

No arguments there?

Now lets play spot the difference (courtesy of RICE RACING)

In both of these pictures, the top left chamber has almost finished inletting, the bottom left chamber is about to exhaust and the right chamber has compressed the air/fuel, waiting for a spark (or two).

In terms of combustion, both of those pictures are in the same state, however they are one output shaft ratation away from each other.

After a 13b Wankel has completed 1 full rotation of the output shaft, the engine has pumped in 1.3L and pumped out 1.3L.

In 3 full rotations of the output shaft, it will have done this cycle 3 times.

Hopefully this has explained my point of view.

I'll be interested to see how people will debate this. Please back your arguments up with evidence.

[DRD-00F]

"...one complete occurrence of the event which repeats"

Shouldn't the side labeled (1) be back where it was for it to be a complete occurence which repeats?

[Jez13]

That page demonstrates the Wankel cycle perfectly. Everyone should look at that and observe that the power is coming from the "orbital" motion rather than the "rotational" motion, before they again say a Wankel only does 3000rpm.

Ok now that we have all learned that part, onto the capacity debate.

I would define "capacity" as the amount of air an engine would pump in one "cycle" of its operation. Would anyone disagree?

Now lets define "cycle".

I would call a cycle when something is in a initial state, does something, and returns to its initial state.

Wikipedia defines a "cycle" as: "...one complete occurrence of the event which repeats"

Nobody here is arguing that a 4-stroke engine takes 2 complete rotations of the output shaft to complete its cycle.

Nobody here is arguing that a 2-stroke engine takes 1 complete rotations of the output shaft to complete its cycle.

At the end of each cycle, both engines are doing exactly what they were doing at the start of their respective cycles.

At the end of a 4-stroke cycle of a 2L engine, the engine has pumped in 2L and pumped out 2L.

At the end of a 2-stroke cycle of a 1L engine, the engine has pumped in 1L and pumped out 1L.

No arguments there?

Now lets play spot the difference (courtesy of RICE RACING)

In both of these pictures, the top left chamber has almost finished inletting, the bottom left chamber is about to exhaust and the right chamber has compressed the air/fuel, waiting for a spark (or two).

In terms of combustion, both of those pictures are in the same state, however they are one output shaft ratation away from each other.

After a 13b Wankel has completed 1 full rotation of the output shaft, the engine has pumped in 1.3L and pumped out 1.3L.

In 3 full rotations of the output shaft, it will have done this cycle 3 times.

Hopefully this has explained my point of view.

I'll be interested to see how people will debate this. Please back your arguments up with evidence.

With every combustion the output shaft completes one cycle. SK has been trippling his numbers because it takes 3.9l of air for the first side of the rotor to return to its origional position (the other 2 sides following afterwards). If we measure the cycles by full turns of the e/shaft then 1.3l is correct.

[Sydneykid]

I'll be interested to see how people will debate this. Please back your arguments up with evidence.

No evidence required, you have already supplied enough in your post for me to prove that I'm correct. At this point I suggest you go back and actually read what you posted. Since you reused Rice's diagrams, I'm surprised that you didn't realise why he stopped the debate, his diagram confirmed my opinion.

It's actually quite simple, you measure capacity by eccentric shaft revolutions. I measure capacity by revolutions (oribits if you prefer) of the pumping medium, ie; the rotors. In one revolution (orbit if you prefer) of the 2 rotors a 13B pumps 3.9 litres.

So I define a cycle of a rotary engine as one complete revolution (orbit if you prefer) of the rotor. You on the other hand define a cycle of a rotary engine as one complete revolution of the eccentric shaft.

So who is right?

Well I base my argument on the fact that a 4 stroke 2 litre engine is always called a 2 litre even though it only pumps 1 litre for one complete cycle (revolution if you prefer) of its crankshaft.

In simple terms we don't dived a 4 strokes piston engine's capacity by 2, so why should we dived a rotary engine's capacity by 3?

Cheers

Gary

[jame88]

Has the "cycle" of an engine got to do with the combustion cycle or where things are in the engine?

If it was just about where things are in the engine, a 4-stroke piston cycle would only be one rotation of the shaft.

Wouldn't work very well though.

I do see your point, but i don't think the fact that it takes 3 combustion cycles for the rotor to do a complete rotation is somewhat irrelevant.

[GT-R32]

Now we're getting somewhere. We are beginning to understand the relative time scale - how the combustion is applied and ends in power and subsqeuently how it all relates for this unique engine. This is the only correct way to understand the it, considering the whole unit. Well done fellas.

[GT-R32]

... I'm correct.

Cheers

Gary

LOL

Get your hand off it mate. RICE Racing is miles ahead of you. I posted the link from 2008 where he cut up a rotor to educate people on its cycle. He's been saying all this stuff since the early 1990s. The thing is he understand relative time scale, considering the engiune as a whole unit, which you do not. He gave up arguing as you merely assume you are correct.