First off, dont flame me unless it is actually called for. Secondly, some terms that I will use are probably not correct, but I am sure you can get the gist of what I am saying. Work with me here a little bit. Also, this is probably a repost. If so, whatever.

What if instead of simply making the housings and rotors for the 13B wider than a 12A, they (Mazda) made them larger diameter? Granted, this would not be cost effective at all as they would be 2 completely different engines and parts would cost considerably more. I mean, they basically just increased the "bore". What if they instead increased the "stroke"? Wouldnt increasing the "stroke" give the engine more low RPM torque? I would think that this would make the larger engines even more "streetable" and overall better for city/town driving.

Now, I understand that the 13B engine in general has more torque than a 12A. I also would have to assume that by increasing "stroke" that the red line would be lowered, possibly dramatically. My thinking behind this is that ON AVERAGE from what I HAVE SEEN the 12A seems to OVERALL last longer than a 13B. Lets stick with NA applications on this one. Someone on here mentioned that the possibility of 13B apex seal failure (versus 12A) could have something to do with the fact that it is wider than the 12A version. Pretty simple thinking really. Which would be easier to break - a 20 inch long thin piece of hardened steel, or a 30 inch one? Think of a small diameter drill bit here.

Did Mazda simply go with an all around good size, or was there more to it? Granted, this engine was primarily for a sports car although simple gearing changes in the transmission and rear end would overcome the difference in torque curve and red line. I would be willing to bet that if Mazda made a rotary powered full size truck, that it COULD have some sort of 20B or possibly a simpler 2 rotor design that was just a larger diameter. Large trucks which require low end grunt when towing large objects in general have relatively large engines in them and weight is a far less issue.

Ideas? Comments? Other theories or ideas on a related subject?

-Ian

 

How would they adjust the stroke? Enlarge the "cams" on the Eccentric shaft? Maybe I am not following.

 

Basically, The rotors would still be the same width only larger diameter. This would force more "swing" of the rotors, essentially making a larger stroke. If that makes any sense.

 

Makes sence to me, seems there would be alot more rotating weight though.

 

On most NEWER engines the stroke is already higher than the bore, so you increase the bore size.

On rotarys I guess you techincally could do that, but I wouldn't expect any huge gains from it and I would expect decrease reliability

 

i've brought this topic up before.

i think it would increase both the volumetric efficiency, and also fuel economy.

they're talking about making the renesis wider for the next rx7.... AND spinning it (ambiguously) 10k rpm. man, that's great for a race motor, but i'd rather see taller, thinner rotors (given the same weight to work with).

here's why:
port timing... when you have a wider rotor, you have to fill more volume with the same port size and timing. with a taller motor, you can have larger ports, and more air going into the engine at any given point. this would be REALLY handy for the renesis motors with the uber-restrictive exhaust port layout.

now in making the rotors taller, you have more inertia...i'm not hip to the math behind it, but i'd assume you wouldn't be able to safely spin the motor as fast as one with shorter rotors. but how about you take 2 motors and compare them...

300 hp, 180 tq, 10k rpm's
or
300 hp, 230 tq, 6k rpm's

i know revving a motor to 8,9, 10k sounds awesome, but in reality, if you could make the same amount of power at a lower rpm, wouldn't you? the one that makes the power sooner is going to be the faster car, all other things set aside.n "taller" rotors would mean more mechanical advantage over the e-shaft, which means torque.

the one thing i'd be concerned with is apex seal wear. a taller rotor would have the apex seal move a greater distance than a shorter one, BUT if the seal is shorter, it's stronger. less flex.

 

Taller rotors would make a little more torque, but what about the opposite - Smaller radial size (or whatever you wanna call it) but wider. This gives you a more square combustion chamber, perhaps only one spark plug is needed. This would promote more complete combustion - better fuel efficiency and more power. Then engine would also be able to rev higher from lower rotational inertia. You might have more apex seal problems, though. Both have their ups and downs. Mazda has mostly likely gone through this line of thought many times - the current design is probably what they came up with as the best compromise.

 

I always thought a "DUAL" apex seal would be a good idea, if one became damaged the other would keep her running.

 

...or just get a supercharger

 

Torque output, for a given combustion pressure and equal porting, is proportional to:

w x e x R

w=rotor width
e=eshaft offset
R=rotor radius at apex.

But so is displacement. So only way to get more torque is more displacement, or better porting.

Say for 2X torque:

1) just double w

2) If R is increased, e must be proportionally increased. so you use 1.41R and 1.41e. That doubles displacement, and torque, with original width.

---------

I would like to see a 3 rotor based on the new mazda race car. Or, another path for 1.5x more punch, increase w, e, and R by 15% each on the 13B.

 

i like the smaller diameter idea, what size was the NSU rotary engine? was it the same?

 

but the problem with that is if you had twice the width, you wouldn't be able to get twice the air in there. you'd have the port that was meant for 654cc's trying to flow twice that.

 

agree, 2 x w was just an example, and would require periferal ports. main point is displacement is key to broad torque curve, and balance of w vs R dimensions should be based on porting capabilities (and distortion of the e-shaft).

 

gotcha.

i gues most of us will never have the budget to build something and find out for ourselves, so let's see what mazda comes up with next

 

I've thought having a more square-faced (combustion chamber wise) rotor might help with the thermodynamics of the engine and help increase displacement without changing revability.

However, Id have no idea how to manufacture that, plus youd have longer apex seals. Could they handle being that long without warping?

 

Going with the idea of smaller diameter, wider rotor - I have also wondered if it would be possible to somehow connect (weld possibly) 2 rotors together to essentially make 1. Now, do this twice. As far as the intake flow, all you would need to do I think is to make it an 8 port setup. With the electronics of today, I could see it very possible. You could make the 5/6 ports open at roughly 2500-3000 RPM and then the 7/8 ports open up closer to the 4500 RPM area. Really, it wouldnt be that bad of a concept I dont think. The housings, e-shaft, and center iron would have to be different (where location of 7/8 ports would be). I am not extremely familiar with the 6 port setup (I just have a 12A). I believe that the 5/6 ports are on the outer irons (if I am wrong, you get the idea of what I am thinking).

 

I think the original idea was a bit better. Wouldn't the 'double rotor' have too much rotating mass, therefore slowing the entire combustion process down greatly?

Of course, I could be way the hell out of the ball park on that one

 

Jays83GSL - try the rotating asembly of a big block v8 for heavyness. Big *** crankshaft, rods, pistons....

It still makes some SERIOUS power, though, doesnt it? :P

The real issue is size and power/weight, but right now the principle is to improve thermal efficiency.

Actually, I've had another idea. Why not make the side housing where the spark plugs are hemispherical (two lumps, sorta like someones chest LOL) and do the same to the combustion chambers of the rotor, but where its not a lump have it closer. So, two combustion chambers and big quench zones.

Would be a bitch to machine, but it could work!

 

they did experiment with this, i think its on that one seriously old but huge, and informative page. http://www.monito.com/wankel/engines.html
Mazda had at least two like a 15a, and a 20g and some others

 

it ways they exist, but doesn't give any useful info on them.

 

Oh yeah! A little belt-driven boost from modern lysholm units is pure torque, at all rpm ranges.

 

no replacement for displacement. tres rotor

didn't they have something like 250+ lbs at 1800 rpm, or something dumb like that?

 

I hate to seem crued but I agree. There is no replacement for displacement though the only downside to increased displacement is the increased weight(unless you bore it out which seems very sketchy to me). But weight is relatively negated if you just look at the power to weight ratio. I am not sure a wider engine would imply greater thermodynamics. If it is just wider but same displacement then I do not think it will have increased intertnal surface area or outer for that matter.

Do higher rev engines use the more fuel then lower rpm engines, and if so are they less fuel effiecent than a larger engine of comparable performance.?

One more thing. A high Rpm engine with lower displacement should weigh less which will increase the power to weight ratio.

 

I personally would like to see an all side port 3 rotor based on 12A specs.

 

^ switch that over to renesis specs, except fix the bugs and you've got a deal