Why Do Rotaries Suck?

[mad082]

actually one rotation of the crank is 2.6L, two rotations of the crank is a full cycle and 5.2L

in a full combustion cycle of a rotor it actually takes in 3.9L for the 3 sides x 2 rotors that are full combusted, plus 2 extra sides on each rotor 1.3L by 2 remaining in the engine... so why don't we now call it a 6.5L by your reasoning

think about your second comment... your thinking about it the wrong way... capacity is max amount of air a single cylinder or rotor can hold times by the amount there are, not the whole engine at one given time

i would like to know how you justify this, when talking about a 4 stroke engine. because a 4 stroke engine will only take in air every 2 rotations. so it takes air/fuel in (down stroke) compresses the mixture (up stroke) and that is 1 rotation of the crank. then it fires the spark plug and ignites the mixture and pushes the piston down (down stroke) and then pushes the mixture out the exhaust valve on the next up stroke. so unless 4 stroke engines take in extra air on the compression stroke (which they don't) then it is 2.6L per full cycle.

[Sydneykid]

It appears you can't get past your beef with Mazda quoting the 13B as 1308cc.

Why should I? It's blatantty wrong.

I'll repeat once again, in order to accurately compare the Wankel to the 4/2 stroke piston, it has to be done on a displacement per shaft revolution basis.

OK, so over there is a 400 cc trail bike, it has CBR400 on the fuel tank and it says 400 cc on the for sale sign and it has 400 cc in the owners manual. What's the engine's pumping capacity? Obviously the answer is 400 cc's, unless someone is lying. It doesn't matter whether it's a 2 stoke or a 4 stroke, the engine is still 400 cc.

Now, over there is an RX8, it has 1.3 litres on the for sale sign and it has 1.3 litres in the owners manual. What's the engine pumping capacity? Obviously the answer should be 1.3 lites, but someone is lying, it's actually 3.9 litres. It doesn't matter whether it's a 2 stoke or a 4 stroke, the engine is still a 3.9 litre .

I don't see 400 cc 2 stroke trail bikes sold as "equivalent to 800 cc 4 stroke", so why should I tolerate 3.9 litres being sold as 1.3 litres? I don't, because it's simply not true.

Cheers

Gary

[Smity42]

The eliptical motion of a rotary isn't as smooth as it sounds. It's not circular, it's eliptical. The motion of a piston is in a way smoother, thanks to a few bearings on key components. The running motion of a human's legs is actually pretty similar to the motion of conrods, and just like the stride we take when our foot touches the ground, the change of direction for a piston is not an immediate one...but largely cushioned/buffered by the motion of the lobe. Also, displacement is very much at hand here...I don't doubt a smaller displacement rotary could produce much faster RPM than a 13B.

Thanks, very informative. Much the same as we can make a smaller piston engine rev faster than a bigger one in that way.

P.S. Can anyone find a good definition of 'displacement' or 'capacity' of an engine, that doesnt in any way mention pistons, so we can settle that particular aspect of the debate once and for all. I have tried but cannot find one. I am quite certain it won't equate to 1.3L though

[Birds]

Thanks, very informative. Much the same as we can make a smaller piston engine rev faster than a bigger one in that way.

Indeed my friend

Size is quite relative to speed...bigger components can't move as fast as smaller components. Dentist drills can spin at 70,000rpm if I recall correctly. Some ship engines produce 200rpm at maximum speed

[Smity42]

I guess I just wasn't taking into account the 'elliptical' motion of the the rotors and the way the pistons motions are smoothed out by other components. Guess I woulda just loved to see a 13B reving to 18,000

[GT-R32]

P.S. Can anyone find a good definition of 'displacement' or 'capacity' of an engine, that doesnt in any way mention pistons, so we can settle that particular aspect of the debate once and for all. I have tried but cannot find one. I am quite certain it won't equate to 1.3L though

There's no point. Gary has already said he doesn't do comparisons in that fashion (using an equaliser). This will never end.

Best to agree to disagree. Let's just call the 13B a 80ci engine (and ignore the fact it uses that air like a 2 stroke) and move on.

[Smity42]

There's no point. Gary has already said he doesn't do comparisons in that fashion (using an equaliser). This will never end.

Best to agree to disagree. Let's just call the 13B a 80ci engine and move on.

displacement/capacity however is a characteristic of an engine that should be independant of how the engine works. I'm willing to accept gary's defintion, and call it a 3.9L, on the caveat that you multiply it by ~2/3 to compare with a 4 stroke, in the same way you multiple a 2 stroke by ~2 to compare it to a 4 stroke.

But, is that a correct definition of displacement? I think it is, but am not certain.

[Granthem]

whats the full displacement of a 20b then

Its a full time job keeping up with this thread

Im still thinking Gary is correctomundo - and its no secret the rotors have big displacement, the turbos they spin clearly show that

Edited September 24, 2009 by Granthem

[ylwgtr2]

i have a CR500 two stroke engine here which the manufacture claims 491cc...it bore and stroke is 89mmx79mm...its exhaust port closes at around 47mm from TDC....i feel robbed of 32mm of combusting stroke and 198.61cc's:(

Edited September 24, 2009 by ylwgtr2

[bozodos]

good point about the turbo sizes, all that air has to fit in somewhere! (even with the rotary's lower inherent compression ratio).

After reading this thread, I can't look at an RX-7 the same way because I'm constantly thinking theres a 3.9 litre motor in there, and my boner goes down just that little bit more

[SimonR32]

I'm a simple guy, I measure an engine's capacity by how much it pumps. Actually I can't be that simple, because every engine manufacturer in the world agrees with me and measure their engines' capacities by how much they pump. Except one, why should we allow an exception for one engine manufacturer? I don't think we should.

Cheers

Gary

I dissagree, it's not the how much it pumps but the maximum volume able to be held...

"a. The ability to receive, hold, or absorb.

b. Abbr. c. A measure of this ability; volume."

[Birds]

displacement/capacity however is a characteristic of an engine that should be independant of how the engine works. I'm willing to accept gary's defintion, and call it a 3.9L, on the caveat that you multiply it by ~2/3 to compare with a 4 stroke, in the same way you multiple a 2 stroke by ~2 to compare it to a 4 stroke.

But, is that a correct definition of displacement? I think it is, but am not certain.

That is indeed the fair displacement comparison

Most of the world has recognised this and in motorsport categories where rotaries are allowed to compete...bar some other variables that influence it...the displacement of a rotary is approximately doubled to put it into fair displacement categories with 4 stroke engines.

i have a CR500 two stroke engine here which the manufacture claims 491cc...it bore and stroke is 89mmx79mm...its exhaust port closes at around 47mm from TDC....i feel robbed of 32mm of combusting stroke and 198.61cc's:(

LOL

Perhaps the most important thing to remember in all this displacement arguing...is you can call a 13B X displacement if you want...its actual displacement is so irrelevant in the real world it's not funny. The end power/torque outputs and the fuel economy of the engine are what its customers actually give a shit about. Who buys an RX7 and argues with the salesmen about the displacement of the engine? You want to know how fast it goes and how much it's going to cost you. I fail to see how Mazda can successfully LIE about 1.3 litres to make the engine seem attractive, when the thing has the fuel economy of a V6/V8? What's the point of lying about its displacement if fuel economy and power output are going to shut you down? There's no advantage to listing it as 1.3 over 2.6 or 3.9. The moment someone asks Mazda how many litres per 100km the thing uses, the lie is all over? Rotaries are their own engines, the displacement is whatever the inventor wants to attach to it. For all other areas where displacement IS a relevant factor, e.g. motorsport...we have formulas for working out the displacement advantage/disadvantage. End of story!

And that's all we need to know...the rest can be agreed to disagree.

[SimonR32]

i would like to know how you justify this, when talking about a 4 stroke engine. because a 4 stroke engine will only take in air every 2 rotations. so it takes air/fuel in (down stroke) compresses the mixture (up stroke) and that is 1 rotation of the crank. then it fires the spark plug and ignites the mixture and pushes the piston down (down stroke) and then pushes the mixture out the exhaust valve on the next up stroke. so unless 4 stroke engines take in extra air on the compression stroke (which they don't) then it is 2.6L per full cycle.

capacity is maximum volume not air ingested into the engine...

[GT-R32]

displacement/capacity however is a characteristic of an engine that should be independant of how the engine works. I'm willing to accept gary's defintion, and call it a 3.9L, on the caveat that you multiply it by ~2/3 to compare with a 4 stroke, in the same way you multiple a 2 stroke by ~2 to compare it to a 4 stroke.

But, is that a correct definition of displacement? I think it is, but am not certain.

http://www.answers.com/topic/engine-displacement

The above is a 720 degree, 4 stroke engine.

The rotary is the exception to the rule. A 13B is a 3-revolution (1080 degree) 3.9 litre engine. The displacement figure is where I agree with Gary. He does not standardise it by revolution, this then makes comparisons impossible (and incorrectly refers to it as a two stroke which is impossible). Thankfully the motorsport bodies out there understand the idea of relatives.

Standardise the 13B to a (4 stroke) 720 degree (2 revolution) rotation and you end up with 2.616L - where each rotor displaces .654 litres of air.

2 stroke? 1 revolution? It's a 1.3L.

[mad082]

capacity is maximum volume not air ingested into the engine...

well if you are talking capacity as total volume then no piston engine is what it is quoted and even when you said that a 2.6L engine is a 5.2L after a full cycle you are incorrect. it would be bigger because when the piston is at TDC there is still a gap between the piston and the head (combustion chamber).

however the motoring world usually talks about engine displacement (although they may use the word capacity) as the displacement is the amount of air able to be displaced by the piston (so the volume), and since a 4 stroke engine only actually only displaces this air every second stroke a 2.6L engine is just a 2.6L engine. and this is the exact reason why when 2 stroke motorbikes race against 4 stroke bikes they are usually put in with bikes of twice the capacity as they both displace the same amount of air.

[GT-R32]

however the motoring world usually talks about engine displacement (although they may use the word capacity) as the displacement is the amount of air able to be displaced by the piston (so the volume), and since a 4 stroke engine only actually only displaces this air every second stroke a 2.6L engine is just a 2.6L engine. and this is the exact reason why when 2 stroke motorbikes race against 4 stroke bikes they are usually put in with bikes of twice the capacity as they both displace the same amount of air.

This bloke understands it. 720 degrees for a 4 stroke TDC to BDC.

[mad082]

i should add that they will measure the displacement with 1 rotation of the crank (well half actually since they only need to measure 1 cylinder), but since it it doesn't expell any gas until the last half of the second rotation they don't take into account the second stroke, and why should they? it would be like me selling you 1L of oil but charging you for 2 because i had tipped a second litre of oil over the bottle to make it 2L

[SimonR32]

well if you are talking capacity as total volume then no piston engine is what it is quoted and even when you said that a 2.6L engine is a 5.2L after a full cycle you are incorrect. it would be bigger because when the piston is at TDC there is still a gap between the piston and the head (combustion chamber).

however the motoring world usually talks about engine displacement (although they may use the word capacity) as the displacement is the amount of air able to be displaced by the piston (so the volume), and since a 4 stroke engine only actually only displaces this air every second stroke a 2.6L engine is just a 2.6L engine. and this is the exact reason why when 2 stroke motorbikes race against 4 stroke bikes they are usually put in with bikes of twice the capacity as they both displace the same amount of air.

MAXIMUM VOLUME, not total volume... come on already

turn the cylinder to the point where you can get the most volume, record that volume and then times by the number of cylinders

[DRD-00F]

so... 1.3L x 3?

[ylwgtr2]

however the motoring world usually talks about engine displacement (although they may use the word capacity) as the displacement is the amount of air able to be displaced by the piston (so the volume), and since a 4 stroke engine only actually only displaces this air every second stroke a 2.6L engine is just a 2.6L engine. and this is the exact reason why when 2 stroke motorbikes race against 4 stroke bikes they are usually put in with bikes of twice the capacity as they both displace the same amount of air.

well why is that the two strokes problem to have to suffer a capacity disability?(or are we basing things on 720degrees now)And wheres my extra capacity as ive lost some from having an exhaust port!!!!!

Edited September 24, 2009 by ylwgtr2